Eur J Anaesthesiol 2014; 31:626–634
ORIGINAL ARTICLE
Distal infrared thermography and skin temperature after ultrasound-guided interscalene brachial plexus block A prospective observational study Semera Asghar, Lars S. Bjerregaard, Lars H. Lundstrøm, Jørgen Lund, Morten T. Jenstrup and Kai H.W. Lange BACKGROUND Increases in skin temperature may be used as an early predictor of the success of interscalene brachial plexus block (IBPB), but we lack detailed information on the thermographic response. OBJECTIVE To investigate and characterise the thermographic response after IBPBs. DESIGN Prospective observational study. SETTING University hospital and private hospital. PATIENTS Twenty-nine male and 17 female patients scheduled for ambulatory shoulder surgery. Exclusion criteria were age less than 18 years, body weight more than 120 kg and any coagulation abnormality. INTERVENTIONS Infrared thermographic imaging of the hand before and at 1 min intervals for 30 min after an ultrasound-guided IBPB with 20 ml ropivacaine 7.5 mg ml1. Cooling of both hands was performed to standardise measurements. MAIN OUTCOME MEASURES Thermographic changes in skin temperature on the dorsum of the hand.
RESULTS Forty-four blocks were successful and two were failures. Four thermographic patterns were observed after successful blocks: the increase in skin temperature was restricted to the thumb (n ¼ 5); increase in skin temperature of the thumb and the second digit (n ¼ 11); increase in skin temperature of the thumb, the second and fifth digits (n ¼ 4); and an increase in skin temperature in all parts of the hand (n ¼ 24). All successful blocks demonstrated a significant (P < 0.0001) increase in median (range) of distal skin temperature of the thumb of 6.68C (0.7 to 17.2) by 30 min, which was already significant (P < 0.0001) by 5 min. By contrast, skin temperature decreased significantly (P < 0.0001) in the hand after failed blocks and in the contra-lateral non-blocked hand by 1.58C (6.2 to 4.2). CONCLUSION Successful IBPB resulted in four thermographic patterns. Skin temperature always increased on the thumb within 30 min and this increase achieved statistical significance at 5 min after the block. Published online 18 September 2014
Introduction Brachial plexus blocks are widely used for upper limb regional anaesthesia, but a reliable clinical tool to determine early block success is lacking.1 Block success is ultimately defined by the clinical outcome: surgical anaesthesia or absence of pain on emergence from general anaesthesia. There is a need for a quick, reliable and non-invasive test or biomarker to distinguish a failed from a successful block in time to permit appropriate further measures to be taken in the operating room.
Previous studies have shown that blocking sympathetic nerves leads to vasodilatation, increased peripheral blood flow and an increase in distal skin temperature (Ts).2–5 An increase in distal Ts could therefore serve as an objective surrogate marker to evaluate brachial plexus block success. Evidence of sympathectomy does not necessarily indicate a somatic sensory-motor block adequate for surgery. Detailed descriptions of infrared thermography after individual blocks of the musculocutaneous, radial, median and
From the Department of Anaesthesiology and Intensive Care, Nordsjællands Hospital and Copenhagen University Hospital (SA, LSB, LHL, KHWL), Department of Anaesthesiology, Aleris Hamlet Hospital (JL, MTJ), Copenhagen, Denmark Correspondence to Kai H.W. Lange, Department of Anaesthesiology and Intensive Care, Nordsjællands Hospital, Dyrehavevej 29, DK-3400 Hillerød, Denmark Tel: +45 48 29 42 01; fax: +45 48 29 42 13; e-mail: [email protected] 0265-0215 ß 2014 Copyright European Society of Anaesthesiology
DOI:10.1097/EJA.0000000000000152
Copyright © European Society of Anaesthesiology. Unauthorized reproduction of this article is prohibited.
Infrared thermography and interscalene brachial plexus block 627
ulnar nerves have recently become available.6 Individual blocks of the median and ulnar nerves resulted in large and characteristic increases in distal Ts. In contrast, individual blocks of both the radial and the musculocutaneous nerves did not produce any changes in distal Ts. Because sympathetic fibres join the plexus in a complex and uncertain manner, the thermographic response after brachial plexus blocks may not be straightforward. Details of the thermographic response after interscalene brachial plexus block (IBPB) are limited. Hermanns et al.4 measured Ts at five different points on the upper limb using a non-contact thermometer. They concluded that increases in distal Ts after IBPB are small and late, and therefore of limited clinical value. However, conventional thermometers have less thermographic resolution compared with the two-dimensional thermographic images obtained by infrared thermographic cameras. We hypothesised that by using high-resolution thermographic cameras, we would observe marked increases in Ts that could serve as a surrogate marker for block success in patients receiving IBPB under ultrasoundguidance. The aim of our study was to map in detail the thermographic responses in the hand after ultrasound-guided IBPB using infrared thermography. We characterised changes in Ts in terms of distribution, magnitude and time course.
Methods Ethical approval for this study was provided by the Committees on Biomedical Research Ethics of the Capital Region of Denmark (protocol number H-C2008-047, initially approved 15 May 2008, amendment approved 13 December 2011). Written informed consent was obtained from each patient. Adults scheduled for day-case shoulder surgery were included. Inclusion criteria were American Society of Anesthesiology physical status I to III, age 18 to 90 years and weight 50 to 120 kg. Exclusion criteria were age less than 18 years, international normalised ratio more than 1.4, platelet count less than 80 109 l1, coagulopathy, medication with vitamin K antagonists, high-dose or fractionated heparin treatment, allergy to local anaesthetics, infection at the site of needle insertion, peripheral neurological disease, Raynaud’s phenomenon and patient refusal. Routine monitoring included continuous ECG, peripheral pulse oximetry (ear lobe) and non-invasive arterial blood pressure measurement on the thigh. An intravenous cannula was inserted into a superficial vein of the contra-lateral antecubital fossa. The patients were in the supine position in the recovery ward at a room temperature of 218C. Direct sunlight was avoided, all bandages and clothing were removed from the shoulders, forearms and hands, and the patients were allowed to acclimatise for 10 min with the palms of the hands resting on 58C cold
packs (Nexucare Cold Hot Maxi Pack; 3M Healthcare, Neuss, Germany). The palms of the hands were kept on the cold packs during the entire measurement period. Both hands were cooled in an attempt to standardise measurements and to augment the magnitude of Ts differences between the hands due to the IBPB. Interscalene brachial plexus block
With the use of a high-frequency linear ultrasound transducer (HFL 38/13–6 MHz, S-ICU Ultrasound System; Sonosite Inc, Bothell, Washington, USA, or LOGIQ e Ultrasound, GE Healthcare, Milwaukee, Wisconsin, USA) we identified the C5 and C6 roots of the brachial plexus between the anterior and middle scalene muscles whenever possible. After disinfection of the injection site with chlorhexidine 0.5% in ethanol 83%, we performed the IBPB in the manner originally described by Winnie7 and later modified for ultrasound imaging by Chan,8 injecting a single bolus injection of 20 ml ropivacaine 7.5 mg ml1 (Naropin 7.5 mg ml1; Astra-Zeneca A/S, Albertslund, Denmark). Using an in-plane, posterior approach, the needle tip (Stimuplex D 22 G 35 mm, 158, or Stimuplex D 22 G 50 mm, 158; B. Braun Melsungen AG, Melsungen, Germany) was positioned peri-neurally between the C5 and C6 anterior rami of the spinal nerves just before they merge to form the superior trunk, and the peri-neural spread of local anaesthetic was observed. We performed multiple needle aspirations to avoid accidental intra-vascular injection. Both experienced and less experienced anaesthesiologists performed the blocks. Block assessment
We trained a group of four nurses to make standardised evaluations of the IBPB. They performed sensory and motor examinations of both upper limbs, prior to and 30 min after, performing the IBPB, and were unaware of the side blocked. At the latter examination a light sheet was positioned to cover the neck of the patient and antecubital fossa, and the assessments were made through the sheet when necessary. The nurse assessed sensory function according to the innervated skin areas shown in Fig. 1. Cold sensation was assessed by applying a liquid-containing cooled (58C) 20 ml vial to the skin innervated by the axillary (lateral distal deltoid muscle) and musculocutaneous (radial distal forearm) nerves. The sensation was recorded as either cold or not cold. The motor function of the deltoid muscle was examined by the ability to abduct the arm above 908 against resistance. The biceps brachii muscle was examined by the ability to flex the elbow joint from neutral position against resistance.9 The force was recorded as either normal or markedly reduced by simultaneous comparison with the contra-lateral arm. A block was defined as successful when both cold stimulation was recorded as ‘not cold’ and muscle force was recorded as ‘markedly reduced’ in all tests as described above. This clinical evaluation was
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628 Asghar et al.
Fig. 1
C3 C4
C5
C6
C8
C7
Cutaneous innervation of the upper limb.
used as our outcome measure in our further assessments. Patients with failed IBPB had the block repeated before surgery. In the operating room the patients were either sedated with propofol or had general anaesthesia with propofol 10 mg ml1 [B. Braun Melsungen AG: 1 to 2.5 mg kg1 (induction), 6 mg kg1 h1 (maintenance)] together with remifentanil 0.06 mg ml1 (GlaxoSmithKline, Brentford, UK; 0.25 mg kg1 h1). Doses were adjusted according to national guidelines (www.dasaim.dk). The airway was managed by inserting a laryngeal mask airway [Ambu AuraOnce # 3–4 ADULT (50 to 70 kg); Ambu Inc, Glen Burnie, Maryland, USA] in patients who had general anaesthesia. Using a visual–analogue scale (VAS) (0 to 100 mm) the patients were asked about post-operative pain on return to the recovery ward.10,11
bed. Thermovision A320 provides a two-dimensional thermal image with a thermal resolution of less than 0.078C, an accuracy of 2% and a picture resolution of 320 240 pixels. Because the emissive factor of the skin is 0.98, the measured temperature values were treated as Ts values. Temperature assessment
From pilot studies we had observed several different thermographic patterns on the hand and forearm after IBPB. From these studies it seemed that the thumb and the second digit showed the most consistent increases in Ts, whereas no changes in Ts were observed above the elbow. Selective block of either the musculocutaneous or radial nerve had been shown not to increase Ts in the forearm.6 Furthermore, in a study wherein we selectively blocked the axillary nerve,12 we could not detect any change in Ts in the area innervated by the axillary nerve measured by infrared thermography (unpublished data). We therefore defined specific areas of interest (AOI) according to Fig. 2 on both hands. In brief, the dorsum of each hand extending from the wrist and including all digits (AOI) was outlined and an additional area of interest (AOI-r) was identified, representing the radial nerve innervation area. This area included the dorsum of the thumb and second digit, extending over part of the radial dorsum of the hand to the wrist. Each AOI was outlined using a specific software package (ThermaCAM Researcher 22.9 Pro; FLIR Systems) and the mean Ts was automatically calculated as the mean of the total pixels inside each. We performed similar calculations for three spot Ts measurements on each hand, defined as circular areas of 2 mm in diameter distally on the thumb, second and fifth digits (SPOT1, SPOT2 and SPOT5), just radial and proximal to the nail bed. These areas were chosen because changes in Ts are likely to be largest and most rapid on the distal part of the digits.6 We analysed the thermographic images and calculated Ts as described above for both hands. Identical measurements of the contra-lateral hand were used as control in our assessments. Statistical methods
Infrared thermographic imaging
We recorded an infrared thermographic image (Thermovision A320; FLIR Systems, Danderyd, Sweden) of the dorsum of both hands at baseline (after acclimatisation) and at 1 min intervals for 30 min, starting immediately after performing the block (time ¼ 0). Infrared thermography is based on the principle that all physical objects above absolute zero (08K) emit electromagnetic radiation according to the black body radiation law (Planck law). The emitted radiation increases with increasing temperature, and infrared thermographic cameras use this property to generate two-dimensional thermographic images of the target objects. The camera was calibrated and fixed in a standardised position (1.30 m) vertically above the
To estimate the sample size needed to detect a significant difference in Ts (paired Student t- test), we used data from our previous study.6 The following values were used: DTs ¼ 2.58C, SD ¼ 3.8, power (b) ¼ 80% and a (two-sided) ¼ 5%, giving 20 patients. However, the main objective of the study was to describe different thermographic patterns. We therefore chose to include 46 patients to be confident that the main thermographic patterns were identified. The Wilcoxon signed-rank test was used to analyse changes in mean Ts between time ¼ 0 and time ¼ 30 min within the blocked and non-blocked hands. Two-way repeated-measures analysis of variance was used for analysing the effects of time (time ¼ 0, 5, 10, 15, 20, 25 and 30 min) and block
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Infrared thermography and interscalene brachial plexus block 629
Fig. 2
AOI AOI
SPOT 1
SPOT 1 AOI-r
AOI-r
SPOT 2 SPOT 5
SPOT 2 SPOT 5
Thermographic image illustrating areas of interest (AOI) and spot measurements (dorsum of the hand). AOI: area including all parts of the hand to the wrist. AOI-r: area representing part of the radial nerve innervation area, including the thumb and second digit extending to the wrist. SPOT1, SPOT2 and SPOT5: 2 mm circular areas distally on the thumb, second and fifth digits just radial and proximal to the nail bed.
(block or control) on Ts, and time with location within the blocked side (SPOT1, SPOT2, SPOT5, AOI and AOI-r) on Ts. The Mauchly test of sphericity was assessed and the Greenhouse–Geisser correction was applied, if the assumption about sphericity was violated. Because of multiple testing, all P values were Bonferroni corrected. Results are presented as median (range) and timed data in figures as mean (SD). P < 0.05 was considered statistically significant. Statistical Package for the Social Sciences (SPSS) software (version 19.0.0; SPSS Inc, Chicago, Illinois, USA) was used for analyses.
Results
post-operatively. VAS remained 0 (rest) for all successful blocks until discharge from the hospital at about 2 h after the end of surgery and without any supplementary analgesics. Thermographic imaging of the blocked hands revealed four different patterns associated with a successful block and one pattern associated with block failure (Fig. 3). All 44 successfully blocked hands had an increase in Ts of the thumb from time ¼ 0 within the blocked hand [Figs 3 and 4; DTs SPOT1 0 to 30 min: 6.68C (0.7 to 17.2), Table 1
Patient characteristics
The clinical study was conducted from 4 November 2011 to 17 April 2012. All 46 patients completed the study (Table 1). Forty-four of the IBPB were evaluated as successful and two as failed. In the two patients with a failed block, the IBPB was repeated successfully preoperatively. Six patients had IBPB with general anaesthesia and 38 had propofol sedation.
Male : female Age (years) Height (cm) Weight (kg) BMI (kg m2) ASA (1 : 2) Acute : elective
All 44 patients with successful IBPB block reported VAS ¼ 0 (rest), when returning to the recovery ward
ASA physical status classification. Values are given as count or median (range). ASA, American Society of Anaesthesiologists; IBPB, interscalene brachial plexus block.
Successful IBPB (n U 44) 47.5 176 81.5 26.1
28 : 16 (19.0 to 69.0) (152 to 192) (53.0 to 145.0) (19.0 to 41.5) 29 : 15 3 : 41
Failed IBPB (n U 2) 59.5 170 98.5 34.1
1:1 (55 to 64) (166 to 174) (98.0 to 99.0) (32.3 to 36.0) 0:2 0:2
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630 Asghar et al.
Fig. 3
(a1)
(a2)
40.0 °C
35
30
(b1)
(b2) 25
20.0 °C
(c1)
(c2)
(d1)
(d2)
(e1)
(e2)
Infrared thermographic images before and 30 min after performing interscalene brachial plexus blocks. Five different patterns are illustrated. Image 1: before the block; Image 2: 30 min after the block. Panel A: Skin temperature (Ts) increase confined to the thumb. Panel B: Ts increase restricted to the thumb and second digit. Panel C: Ts increase including the thumb, second and fifth digits. Panel D: Ts increase in all parts of the hand. Panel E: Ts decrease after block failure.
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Infrared thermography and interscalene brachial plexus block 631
Fig. 4
15
10
Successful IBPB SPOT 1
∆ Ts (°C)
SPOT 2 Successful IBPB
5
SPOT 5 AOI-r AOI Contralateral SPOT 1
0 Contralateral
SPOT 2 SPOT 5 AOI-r
–5
AOI
0
5
10
15
20
25
30
Time (min) Time course of mean skin temperature changes (DTs) after a successful interscalene brachial plexus block. Mean (SD) for each measured area (AOI, AOI-r, SPOT1, SPOT2, SPOT5 [see Fig. 2 for a detailed explanation of the measured areas]) is given for both the successfully blocked side and the contra-lateral side.
P < 0.0001]. In five patients, the increase in Ts was confined to the thumb [Fig. 3, panel A, DTs SPOT1 0 to 30 min: 8.28C (3.1 to 11.7), P < 0.0001]. In 11 patients the increase in Ts from time ¼ 0 within the hand was restricted to the thumb, the second digit and the skin on part of the radial dorsal hand [Fig. 3, panel B, DTs AOI-r 0 to 30 min: 5.58C (1.8 to 8.7), P < 0.0001]. In four patients, the increase in Ts included the thumb, second and fifth digits and the skin on part of the radial and ulnar dorsal hand with no increase in Ts of the third and fourth digits [Fig. 3, panel C, DTs AOI-r combined with DTs SPOT5 0 to 30 min (area-weighted summation): 7.88C (2.0 to 14.7), P < 0.0001]. In 24 patients, Ts increased from time ¼ 0 within the blocked hand in the thumb and all digits extending to the skin of the wrist (and forearm) [Fig. 3, panel D, DTs AOI 0 to 30 min: 3.18C (1.1 to 12.2), P < 0.0001]. In the two patients with failed blocks Ts decreased from time ¼ 0 in the blocked hand [Fig. 3, panel E, DTs AOI 0 to 30 min: 1.9 and 4.28C]. Ts decreased from time ¼ 0 in the contra-lateral hand [Fig. 3, all panels and Fig. 5, DTs AOI 0 to 30 min: 1.58C (6.2 to 4.2), P < 0.0001]. To analyse Ts changes in the distal part of the digits, we plotted the time course of the changes (compared with time ¼ 0) in SPOT1, SPOT2, SPOT5, AOI and AOI-r in all 44 successful blocks (Fig. 4). There were highly significant effects of time and block on Ts (P < 0.0001). There was also a highly significant interaction between
time and block on Ts (P < 0.0001). For the blocked hand, there were highly significant effects of time and location on Ts (P < 0.0001) as well as a highly significant interaction between time and location on Ts (P < 0.0001). As all successful blocks demonstrated Ts increases in SPOT1 within the same hand after 30 min, we further analysed the increases at SPOT1 over time within the same hand. Ts at SPOT1 increased significantly (P < 0.0001) by 2.58C (1.1 to þ9.0) after 5 min and all successful blocks demonstrated a significant (P < 0.0001) increase at SPOT1 after 9 min of 4.18C (0.5 to 13.0). Although only results from the dorsum of the hands are reported, thermographic imaging of the palms showed similar (symmetrical) thermographic responses (data not presented).
Discussion The main purpose of the present study was to examine in detail the thermographic responses in the hand after ultrasound-guided IBPB. We found five patterns: four similar patterns associated with a successful block and one quite different pattern associated with block failure. The common finding in the successful blocks was a significant increase in Ts of the thumb, but Ts also increased in the second and fifth digits, and in more than 50% of the successful blocks Ts was increased in all parts of the hand. Our interpretation is that in successful IBPB, Ts increases first in the thumb and later this extends to the rest of the fingers – sometimes including only the second digit, often including all digits and sometimes,
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632 Asghar et al.
Fig. 5
Successful IBPB Contralateral successful IBPB Failed IBPB Contralateral failed IBPB
40
(n = 44) 35
Ts (°C)
30
(n = 44) 25
(n = 2)
20
(n = 2)
0 Baseline
0
5
10
15
20
25
30
Time (min)
Time course of mean skin temperature (Ts) measured at SPOT1 (a 2-mm circular area distally on the thumb and just radial and proximal to the nail bed) after performing an interscalene brachial plexus block (IBPB). Data are mean (SD).
quite unexpectedly, including only the thumb and the second and fifth digits. Although we only had two failed blocks, our measurements taken from both hands, strongly suggest that a failed IBPB block is associated with no increase in Ts of the thumb (and second digit). The other main purpose of the study was to describe the magnitude and timing of the changes in Ts after successful IBPB. We found that distal Ts increased significantly within 5 min with the fastest onset in the thumb. The increase in distal Ts was of the order of 78C after 30 min. We believe this finding to be novel, as we could not find previous reports on the magnitude of temperature change. In a previous study, Hermanns et al.4 measured Ts at five different points after IBPB using a non-contact thermometer. They concluded that increases in distal Ts were small, occurred late and consequently, were of limited clinical value. However, conventional thermometers do not provide a two-dimensional thermographic image, which is important for revealing thermographic patterns. Furthermore, Hermanns et al. did not measure Ts on the digits but chose three points located close to the wrist, one point in the forearm and one point over the shoulder region. Analysing the detailed thermographic images in our study, it is obvious that the increase in Ts is substantially faster and larger at
the digit tips as opposed to more proximal measurements (Fig. 4). These findings are also supported by our previous study reporting on changes in distal Ts after individual blocks of four major nerves in the upper limb.6 We speculate that the increases in Ts observed in the other parts of the hand (and forearm) are caused by increased venous return from the opening of arteriovenous anastomoses located at the fingertips. The certain test for block success is, and will always be, the clinical outcome such as surgical anaesthesia or absence of pain on emergence. However, the major limitation of these clinical outcome measures is that a failed block will not be detected before the operating room, necessitating anaesthetic intervention. For this reason, surrogate outcomes such as pinprick and thermography may have clinical value prior to surgery. Both pinprick and thermography are non-invasive, but infrared thermography is not operator dependent and therefore not vulnerable to inter-observer bias. A further advantage is that, as opposed to pinprick, thermographic imaging does not require a co-operative patient, and this may be of value in specific groups such as children, and demented adults. The existence of five different thermographic patterns after IBPB is surprising. The brachial plexus is formed by
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Infrared thermography and interscalene brachial plexus block 633
the anterior rami of the spinal nerves from C5 to T1, whereas sympathetic fibres ascend in the sympathetic chain from the T1 level and below.13,14 High thoracic epidural anaesthesia probably leads to increased Ts in all parts of the hand, although this has not been investigated in detail.15,16 Peripheral median nerve block leads to increased Ts in the first to fourth digits and peripheral ulnar nerve block leads to increased Ts in the fourth and fifth digits.6 However, this knowledge does not allow us to predict the thermographic patterns after a proximal brachial plexus block, as we have demonstrated. The thermographic patterns with sparing of the third and fourth digits (Fig. 3, panel C2) are difficult to explain from our existing knowledge of the anatomy. Ulnar sparing is a well known feature of the IBPB. We expected the existence of a thermographic pattern with increases in Ts in the first to fourth digits and no increase in the fifth. Although we did not find such a pattern, it may well exist. We did not assess sensory or motor function of the radial, ulnar or median nerves. It would be of interest to link such results to the observed thermographic patterns in future studies. We have no good explanation for the much lower baseline and starting point temperature in the two patients with failed blocks. It should be noted that we performed measurements for only 30 min after performing the block. We do not know whether the thermographic patterns remain the same beyond this time, but the changes that occur after 30 min are unlikely to be of clinical use. We used a relatively large volume (20 ml) and a high concentration of ropivacaine (7.5 mg ml1) because this was part of the routine in the hospital where the blocks were performed. In recent years IBPB has been shown to be effective with much smaller volumes (5 to 10 ml) and post-operative analgesia can be obtained with much lower concentrations.17,18 The effects of changes in volume and concentrations in local anaesthetic on the thermographic response after IBPB are still unknown.19,20 It is also possible that a misplaced IBPB may spread centrally and cause sympathectomy with little somato-sensory effects. Moreover, a large volume of local anaesthetic could spread centrally to the sympathetic chain prior to fibres joining the brachial plexus. There are further limitations in the present study. Distal Ts is subject to major fluctuations due to changes in environmental temperature, sympathetic nervous activity (stress, anxiety) and local inflammation, for example. To reduce the impact of these factors and to standardise measurements, we tried to cool both hands by placing them on cooled packs (58C). This procedure itself may have introduced fluctuations. Moreover, the method did not provide adequate and constant cooling, as the cooled packs increased in temperature over time. It is also obvious that Ts declined slightly in the contra-lateral hand after the acclimatisation period (Figs 4 and 5), which suggests a too short acclimatisation period, or
inadequate cooling or both. However, considering the data, we think that our conclusions, with respect to the different thermographic patterns, are valid. By cooling the hands, we may have underestimated the rapidity of changes in Ts and overestimated the size of the change in Ts. In subjects that were not cooled, the increase in distal Ts after median and ulnar nerve blocks was 78C to 88C.6 Future clinical studies should omit the cooling procedure because it most probably just introduces noise to the measurements and is inconvenient in clinical practice. Our study was not a randomised, placebocontrolled study, which would have been a superior design. Instead we used the contra-lateral hand as control and used blinded nurses to clinically evaluate block success. Also, as we only had two block failures, we cannot make firm conclusions on the thermographic patterns of failed blocks. In conclusion, successful IBPB resulted in four different thermographic patterns in the hand. Ts always increased in the thumb. In more than 50% of the patients Ts increased in the whole hand. The increase in distal Ts (SPOT1) was 78C after 30 min and statistically significant within 5 min after siting the block. These results may be useful in future studies investigating the role of distal Ts of the thumb in predicting IBPB success.
Acknowledgements relating to this article Assistance with the study: We would like to thank the nursing staff at the Aleris-Hamlet Hospital, Copenhagen, Denmark and the nursing staff at the Department of Anaesthesiology, Nordsjællands Hospital and Copenhagen University Hospital, Hillerød, Denmark for their assistance with the study. Financial support and sponsorship: This work was supported by the Department of Anaesthesiology, Nordsjællands Hospital, Hillerød, Denmark and by local grants from Nordsjællands Hospital, Hillerød, Denmark. Conflicts of interest: none. Presentation: Preliminary data were presented as a poster presentation at the European Society of Anaesthesiology (ESA) Euroanaesthesia Congress, 1–4 June 2013, Barcelona, Spain.
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Hopf HB, Weissbach B, Peters J. High thoracic segmental epidural anaesthesia diminishes sympathetic outflow to the legs, despite restriction of sensory blockade to the upper thorax. Anesthesiology 1990; 73:882–889. Magnusdottir H, Kirno K, Ricksten SE, et al. High thoracic epidural anaesthesia does not inhibit sympathetic nerve activity in the lower extremities. Anesthesiology 1999; 91:1299–1304. Gautier P, Vandepitte C, Ramquet C, et al. The minimum effective anaesthetic volume of 0.75% ropivacaine in ultrasound-guided interscalene brachial plexus block. Anesth Analg 2011; 113:951– 955. Renes SH, van Geffen GJ, Rettig HC, et al. Minimum effective volume of local anaesthetic for shoulder analgesia by ultrasound-guided block at root C7 with assessment of pulmonary function. Reg Anesth Pain Med 2010; 35:529–534. Plante T, Rontes O, Bloc S, et al. Spread of local anaesthetic during an ultrasound-guided interscalene block: does the injection site influence diffusion? Acta Anaesthesiol Scand 2011; 55:664–669. Abrahams MS, Panzer O, Atchabahian A, et al. Case report: limitation of local anaesthetic spread during ultrasound-guided interscalene block. Description of an anatomic variant with clinical correlation. Reg Anesth Pain Med 2008; 33:357–359.
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ORIGINAL ARTICLE
Distal infrared thermography and skin temperature after ultrasound-guided interscalene brachial plexus block A prospective observational study Semera Asghar, Lars S. Bjerregaard, Lars H. Lundstrøm, Jørgen Lund, Morten T. Jenstrup and Kai H.W. Lange BACKGROUND Increases in skin temperature may be used as an early predictor of the success of interscalene brachial plexus block (IBPB), but we lack detailed information on the thermographic response. OBJECTIVE To investigate and characterise the thermographic response after IBPBs. DESIGN Prospective observational study. SETTING University hospital and private hospital. PATIENTS Twenty-nine male and 17 female patients scheduled for ambulatory shoulder surgery. Exclusion criteria were age less than 18 years, body weight more than 120 kg and any coagulation abnormality. INTERVENTIONS Infrared thermographic imaging of the hand before and at 1 min intervals for 30 min after an ultrasound-guided IBPB with 20 ml ropivacaine 7.5 mg ml1. Cooling of both hands was performed to standardise measurements. MAIN OUTCOME MEASURES Thermographic changes in skin temperature on the dorsum of the hand.
RESULTS Forty-four blocks were successful and two were failures. Four thermographic patterns were observed after successful blocks: the increase in skin temperature was restricted to the thumb (n ¼ 5); increase in skin temperature of the thumb and the second digit (n ¼ 11); increase in skin temperature of the thumb, the second and fifth digits (n ¼ 4); and an increase in skin temperature in all parts of the hand (n ¼ 24). All successful blocks demonstrated a significant (P < 0.0001) increase in median (range) of distal skin temperature of the thumb of 6.68C (0.7 to 17.2) by 30 min, which was already significant (P < 0.0001) by 5 min. By contrast, skin temperature decreased significantly (P < 0.0001) in the hand after failed blocks and in the contra-lateral non-blocked hand by 1.58C (6.2 to 4.2). CONCLUSION Successful IBPB resulted in four thermographic patterns. Skin temperature always increased on the thumb within 30 min and this increase achieved statistical significance at 5 min after the block. Published online 18 September 2014
Introduction Brachial plexus blocks are widely used for upper limb regional anaesthesia, but a reliable clinical tool to determine early block success is lacking.1 Block success is ultimately defined by the clinical outcome: surgical anaesthesia or absence of pain on emergence from general anaesthesia. There is a need for a quick, reliable and non-invasive test or biomarker to distinguish a failed from a successful block in time to permit appropriate further measures to be taken in the operating room.
Previous studies have shown that blocking sympathetic nerves leads to vasodilatation, increased peripheral blood flow and an increase in distal skin temperature (Ts).2–5 An increase in distal Ts could therefore serve as an objective surrogate marker to evaluate brachial plexus block success. Evidence of sympathectomy does not necessarily indicate a somatic sensory-motor block adequate for surgery. Detailed descriptions of infrared thermography after individual blocks of the musculocutaneous, radial, median and
From the Department of Anaesthesiology and Intensive Care, Nordsjællands Hospital and Copenhagen University Hospital (SA, LSB, LHL, KHWL), Department of Anaesthesiology, Aleris Hamlet Hospital (JL, MTJ), Copenhagen, Denmark Correspondence to Kai H.W. Lange, Department of Anaesthesiology and Intensive Care, Nordsjællands Hospital, Dyrehavevej 29, DK-3400 Hillerød, Denmark Tel: +45 48 29 42 01; fax: +45 48 29 42 13; e-mail: [email protected] 0265-0215 ß 2014 Copyright European Society of Anaesthesiology
DOI:10.1097/EJA.0000000000000152
Copyright © European Society of Anaesthesiology. Unauthorized reproduction of this article is prohibited.
Infrared thermography and interscalene brachial plexus block 627
ulnar nerves have recently become available.6 Individual blocks of the median and ulnar nerves resulted in large and characteristic increases in distal Ts. In contrast, individual blocks of both the radial and the musculocutaneous nerves did not produce any changes in distal Ts. Because sympathetic fibres join the plexus in a complex and uncertain manner, the thermographic response after brachial plexus blocks may not be straightforward. Details of the thermographic response after interscalene brachial plexus block (IBPB) are limited. Hermanns et al.4 measured Ts at five different points on the upper limb using a non-contact thermometer. They concluded that increases in distal Ts after IBPB are small and late, and therefore of limited clinical value. However, conventional thermometers have less thermographic resolution compared with the two-dimensional thermographic images obtained by infrared thermographic cameras. We hypothesised that by using high-resolution thermographic cameras, we would observe marked increases in Ts that could serve as a surrogate marker for block success in patients receiving IBPB under ultrasoundguidance. The aim of our study was to map in detail the thermographic responses in the hand after ultrasound-guided IBPB using infrared thermography. We characterised changes in Ts in terms of distribution, magnitude and time course.
Methods Ethical approval for this study was provided by the Committees on Biomedical Research Ethics of the Capital Region of Denmark (protocol number H-C2008-047, initially approved 15 May 2008, amendment approved 13 December 2011). Written informed consent was obtained from each patient. Adults scheduled for day-case shoulder surgery were included. Inclusion criteria were American Society of Anesthesiology physical status I to III, age 18 to 90 years and weight 50 to 120 kg. Exclusion criteria were age less than 18 years, international normalised ratio more than 1.4, platelet count less than 80 109 l1, coagulopathy, medication with vitamin K antagonists, high-dose or fractionated heparin treatment, allergy to local anaesthetics, infection at the site of needle insertion, peripheral neurological disease, Raynaud’s phenomenon and patient refusal. Routine monitoring included continuous ECG, peripheral pulse oximetry (ear lobe) and non-invasive arterial blood pressure measurement on the thigh. An intravenous cannula was inserted into a superficial vein of the contra-lateral antecubital fossa. The patients were in the supine position in the recovery ward at a room temperature of 218C. Direct sunlight was avoided, all bandages and clothing were removed from the shoulders, forearms and hands, and the patients were allowed to acclimatise for 10 min with the palms of the hands resting on 58C cold
packs (Nexucare Cold Hot Maxi Pack; 3M Healthcare, Neuss, Germany). The palms of the hands were kept on the cold packs during the entire measurement period. Both hands were cooled in an attempt to standardise measurements and to augment the magnitude of Ts differences between the hands due to the IBPB. Interscalene brachial plexus block
With the use of a high-frequency linear ultrasound transducer (HFL 38/13–6 MHz, S-ICU Ultrasound System; Sonosite Inc, Bothell, Washington, USA, or LOGIQ e Ultrasound, GE Healthcare, Milwaukee, Wisconsin, USA) we identified the C5 and C6 roots of the brachial plexus between the anterior and middle scalene muscles whenever possible. After disinfection of the injection site with chlorhexidine 0.5% in ethanol 83%, we performed the IBPB in the manner originally described by Winnie7 and later modified for ultrasound imaging by Chan,8 injecting a single bolus injection of 20 ml ropivacaine 7.5 mg ml1 (Naropin 7.5 mg ml1; Astra-Zeneca A/S, Albertslund, Denmark). Using an in-plane, posterior approach, the needle tip (Stimuplex D 22 G 35 mm, 158, or Stimuplex D 22 G 50 mm, 158; B. Braun Melsungen AG, Melsungen, Germany) was positioned peri-neurally between the C5 and C6 anterior rami of the spinal nerves just before they merge to form the superior trunk, and the peri-neural spread of local anaesthetic was observed. We performed multiple needle aspirations to avoid accidental intra-vascular injection. Both experienced and less experienced anaesthesiologists performed the blocks. Block assessment
We trained a group of four nurses to make standardised evaluations of the IBPB. They performed sensory and motor examinations of both upper limbs, prior to and 30 min after, performing the IBPB, and were unaware of the side blocked. At the latter examination a light sheet was positioned to cover the neck of the patient and antecubital fossa, and the assessments were made through the sheet when necessary. The nurse assessed sensory function according to the innervated skin areas shown in Fig. 1. Cold sensation was assessed by applying a liquid-containing cooled (58C) 20 ml vial to the skin innervated by the axillary (lateral distal deltoid muscle) and musculocutaneous (radial distal forearm) nerves. The sensation was recorded as either cold or not cold. The motor function of the deltoid muscle was examined by the ability to abduct the arm above 908 against resistance. The biceps brachii muscle was examined by the ability to flex the elbow joint from neutral position against resistance.9 The force was recorded as either normal or markedly reduced by simultaneous comparison with the contra-lateral arm. A block was defined as successful when both cold stimulation was recorded as ‘not cold’ and muscle force was recorded as ‘markedly reduced’ in all tests as described above. This clinical evaluation was
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628 Asghar et al.
Fig. 1
C3 C4
C5
C6
C8
C7
Cutaneous innervation of the upper limb.
used as our outcome measure in our further assessments. Patients with failed IBPB had the block repeated before surgery. In the operating room the patients were either sedated with propofol or had general anaesthesia with propofol 10 mg ml1 [B. Braun Melsungen AG: 1 to 2.5 mg kg1 (induction), 6 mg kg1 h1 (maintenance)] together with remifentanil 0.06 mg ml1 (GlaxoSmithKline, Brentford, UK; 0.25 mg kg1 h1). Doses were adjusted according to national guidelines (www.dasaim.dk). The airway was managed by inserting a laryngeal mask airway [Ambu AuraOnce # 3–4 ADULT (50 to 70 kg); Ambu Inc, Glen Burnie, Maryland, USA] in patients who had general anaesthesia. Using a visual–analogue scale (VAS) (0 to 100 mm) the patients were asked about post-operative pain on return to the recovery ward.10,11
bed. Thermovision A320 provides a two-dimensional thermal image with a thermal resolution of less than 0.078C, an accuracy of 2% and a picture resolution of 320 240 pixels. Because the emissive factor of the skin is 0.98, the measured temperature values were treated as Ts values. Temperature assessment
From pilot studies we had observed several different thermographic patterns on the hand and forearm after IBPB. From these studies it seemed that the thumb and the second digit showed the most consistent increases in Ts, whereas no changes in Ts were observed above the elbow. Selective block of either the musculocutaneous or radial nerve had been shown not to increase Ts in the forearm.6 Furthermore, in a study wherein we selectively blocked the axillary nerve,12 we could not detect any change in Ts in the area innervated by the axillary nerve measured by infrared thermography (unpublished data). We therefore defined specific areas of interest (AOI) according to Fig. 2 on both hands. In brief, the dorsum of each hand extending from the wrist and including all digits (AOI) was outlined and an additional area of interest (AOI-r) was identified, representing the radial nerve innervation area. This area included the dorsum of the thumb and second digit, extending over part of the radial dorsum of the hand to the wrist. Each AOI was outlined using a specific software package (ThermaCAM Researcher 22.9 Pro; FLIR Systems) and the mean Ts was automatically calculated as the mean of the total pixels inside each. We performed similar calculations for three spot Ts measurements on each hand, defined as circular areas of 2 mm in diameter distally on the thumb, second and fifth digits (SPOT1, SPOT2 and SPOT5), just radial and proximal to the nail bed. These areas were chosen because changes in Ts are likely to be largest and most rapid on the distal part of the digits.6 We analysed the thermographic images and calculated Ts as described above for both hands. Identical measurements of the contra-lateral hand were used as control in our assessments. Statistical methods
Infrared thermographic imaging
We recorded an infrared thermographic image (Thermovision A320; FLIR Systems, Danderyd, Sweden) of the dorsum of both hands at baseline (after acclimatisation) and at 1 min intervals for 30 min, starting immediately after performing the block (time ¼ 0). Infrared thermography is based on the principle that all physical objects above absolute zero (08K) emit electromagnetic radiation according to the black body radiation law (Planck law). The emitted radiation increases with increasing temperature, and infrared thermographic cameras use this property to generate two-dimensional thermographic images of the target objects. The camera was calibrated and fixed in a standardised position (1.30 m) vertically above the
To estimate the sample size needed to detect a significant difference in Ts (paired Student t- test), we used data from our previous study.6 The following values were used: DTs ¼ 2.58C, SD ¼ 3.8, power (b) ¼ 80% and a (two-sided) ¼ 5%, giving 20 patients. However, the main objective of the study was to describe different thermographic patterns. We therefore chose to include 46 patients to be confident that the main thermographic patterns were identified. The Wilcoxon signed-rank test was used to analyse changes in mean Ts between time ¼ 0 and time ¼ 30 min within the blocked and non-blocked hands. Two-way repeated-measures analysis of variance was used for analysing the effects of time (time ¼ 0, 5, 10, 15, 20, 25 and 30 min) and block
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Infrared thermography and interscalene brachial plexus block 629
Fig. 2
AOI AOI
SPOT 1
SPOT 1 AOI-r
AOI-r
SPOT 2 SPOT 5
SPOT 2 SPOT 5
Thermographic image illustrating areas of interest (AOI) and spot measurements (dorsum of the hand). AOI: area including all parts of the hand to the wrist. AOI-r: area representing part of the radial nerve innervation area, including the thumb and second digit extending to the wrist. SPOT1, SPOT2 and SPOT5: 2 mm circular areas distally on the thumb, second and fifth digits just radial and proximal to the nail bed.
(block or control) on Ts, and time with location within the blocked side (SPOT1, SPOT2, SPOT5, AOI and AOI-r) on Ts. The Mauchly test of sphericity was assessed and the Greenhouse–Geisser correction was applied, if the assumption about sphericity was violated. Because of multiple testing, all P values were Bonferroni corrected. Results are presented as median (range) and timed data in figures as mean (SD). P < 0.05 was considered statistically significant. Statistical Package for the Social Sciences (SPSS) software (version 19.0.0; SPSS Inc, Chicago, Illinois, USA) was used for analyses.
Results
post-operatively. VAS remained 0 (rest) for all successful blocks until discharge from the hospital at about 2 h after the end of surgery and without any supplementary analgesics. Thermographic imaging of the blocked hands revealed four different patterns associated with a successful block and one pattern associated with block failure (Fig. 3). All 44 successfully blocked hands had an increase in Ts of the thumb from time ¼ 0 within the blocked hand [Figs 3 and 4; DTs SPOT1 0 to 30 min: 6.68C (0.7 to 17.2), Table 1
Patient characteristics
The clinical study was conducted from 4 November 2011 to 17 April 2012. All 46 patients completed the study (Table 1). Forty-four of the IBPB were evaluated as successful and two as failed. In the two patients with a failed block, the IBPB was repeated successfully preoperatively. Six patients had IBPB with general anaesthesia and 38 had propofol sedation.
Male : female Age (years) Height (cm) Weight (kg) BMI (kg m2) ASA (1 : 2) Acute : elective
All 44 patients with successful IBPB block reported VAS ¼ 0 (rest), when returning to the recovery ward
ASA physical status classification. Values are given as count or median (range). ASA, American Society of Anaesthesiologists; IBPB, interscalene brachial plexus block.
Successful IBPB (n U 44) 47.5 176 81.5 26.1
28 : 16 (19.0 to 69.0) (152 to 192) (53.0 to 145.0) (19.0 to 41.5) 29 : 15 3 : 41
Failed IBPB (n U 2) 59.5 170 98.5 34.1
1:1 (55 to 64) (166 to 174) (98.0 to 99.0) (32.3 to 36.0) 0:2 0:2
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630 Asghar et al.
Fig. 3
(a1)
(a2)
40.0 °C
35
30
(b1)
(b2) 25
20.0 °C
(c1)
(c2)
(d1)
(d2)
(e1)
(e2)
Infrared thermographic images before and 30 min after performing interscalene brachial plexus blocks. Five different patterns are illustrated. Image 1: before the block; Image 2: 30 min after the block. Panel A: Skin temperature (Ts) increase confined to the thumb. Panel B: Ts increase restricted to the thumb and second digit. Panel C: Ts increase including the thumb, second and fifth digits. Panel D: Ts increase in all parts of the hand. Panel E: Ts decrease after block failure.
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Infrared thermography and interscalene brachial plexus block 631
Fig. 4
15
10
Successful IBPB SPOT 1
∆ Ts (°C)
SPOT 2 Successful IBPB
5
SPOT 5 AOI-r AOI Contralateral SPOT 1
0 Contralateral
SPOT 2 SPOT 5 AOI-r
–5
AOI
0
5
10
15
20
25
30
Time (min) Time course of mean skin temperature changes (DTs) after a successful interscalene brachial plexus block. Mean (SD) for each measured area (AOI, AOI-r, SPOT1, SPOT2, SPOT5 [see Fig. 2 for a detailed explanation of the measured areas]) is given for both the successfully blocked side and the contra-lateral side.
P < 0.0001]. In five patients, the increase in Ts was confined to the thumb [Fig. 3, panel A, DTs SPOT1 0 to 30 min: 8.28C (3.1 to 11.7), P < 0.0001]. In 11 patients the increase in Ts from time ¼ 0 within the hand was restricted to the thumb, the second digit and the skin on part of the radial dorsal hand [Fig. 3, panel B, DTs AOI-r 0 to 30 min: 5.58C (1.8 to 8.7), P < 0.0001]. In four patients, the increase in Ts included the thumb, second and fifth digits and the skin on part of the radial and ulnar dorsal hand with no increase in Ts of the third and fourth digits [Fig. 3, panel C, DTs AOI-r combined with DTs SPOT5 0 to 30 min (area-weighted summation): 7.88C (2.0 to 14.7), P < 0.0001]. In 24 patients, Ts increased from time ¼ 0 within the blocked hand in the thumb and all digits extending to the skin of the wrist (and forearm) [Fig. 3, panel D, DTs AOI 0 to 30 min: 3.18C (1.1 to 12.2), P < 0.0001]. In the two patients with failed blocks Ts decreased from time ¼ 0 in the blocked hand [Fig. 3, panel E, DTs AOI 0 to 30 min: 1.9 and 4.28C]. Ts decreased from time ¼ 0 in the contra-lateral hand [Fig. 3, all panels and Fig. 5, DTs AOI 0 to 30 min: 1.58C (6.2 to 4.2), P < 0.0001]. To analyse Ts changes in the distal part of the digits, we plotted the time course of the changes (compared with time ¼ 0) in SPOT1, SPOT2, SPOT5, AOI and AOI-r in all 44 successful blocks (Fig. 4). There were highly significant effects of time and block on Ts (P < 0.0001). There was also a highly significant interaction between
time and block on Ts (P < 0.0001). For the blocked hand, there were highly significant effects of time and location on Ts (P < 0.0001) as well as a highly significant interaction between time and location on Ts (P < 0.0001). As all successful blocks demonstrated Ts increases in SPOT1 within the same hand after 30 min, we further analysed the increases at SPOT1 over time within the same hand. Ts at SPOT1 increased significantly (P < 0.0001) by 2.58C (1.1 to þ9.0) after 5 min and all successful blocks demonstrated a significant (P < 0.0001) increase at SPOT1 after 9 min of 4.18C (0.5 to 13.0). Although only results from the dorsum of the hands are reported, thermographic imaging of the palms showed similar (symmetrical) thermographic responses (data not presented).
Discussion The main purpose of the present study was to examine in detail the thermographic responses in the hand after ultrasound-guided IBPB. We found five patterns: four similar patterns associated with a successful block and one quite different pattern associated with block failure. The common finding in the successful blocks was a significant increase in Ts of the thumb, but Ts also increased in the second and fifth digits, and in more than 50% of the successful blocks Ts was increased in all parts of the hand. Our interpretation is that in successful IBPB, Ts increases first in the thumb and later this extends to the rest of the fingers – sometimes including only the second digit, often including all digits and sometimes,
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632 Asghar et al.
Fig. 5
Successful IBPB Contralateral successful IBPB Failed IBPB Contralateral failed IBPB
40
(n = 44) 35
Ts (°C)
30
(n = 44) 25
(n = 2)
20
(n = 2)
0 Baseline
0
5
10
15
20
25
30
Time (min)
Time course of mean skin temperature (Ts) measured at SPOT1 (a 2-mm circular area distally on the thumb and just radial and proximal to the nail bed) after performing an interscalene brachial plexus block (IBPB). Data are mean (SD).
quite unexpectedly, including only the thumb and the second and fifth digits. Although we only had two failed blocks, our measurements taken from both hands, strongly suggest that a failed IBPB block is associated with no increase in Ts of the thumb (and second digit). The other main purpose of the study was to describe the magnitude and timing of the changes in Ts after successful IBPB. We found that distal Ts increased significantly within 5 min with the fastest onset in the thumb. The increase in distal Ts was of the order of 78C after 30 min. We believe this finding to be novel, as we could not find previous reports on the magnitude of temperature change. In a previous study, Hermanns et al.4 measured Ts at five different points after IBPB using a non-contact thermometer. They concluded that increases in distal Ts were small, occurred late and consequently, were of limited clinical value. However, conventional thermometers do not provide a two-dimensional thermographic image, which is important for revealing thermographic patterns. Furthermore, Hermanns et al. did not measure Ts on the digits but chose three points located close to the wrist, one point in the forearm and one point over the shoulder region. Analysing the detailed thermographic images in our study, it is obvious that the increase in Ts is substantially faster and larger at
the digit tips as opposed to more proximal measurements (Fig. 4). These findings are also supported by our previous study reporting on changes in distal Ts after individual blocks of four major nerves in the upper limb.6 We speculate that the increases in Ts observed in the other parts of the hand (and forearm) are caused by increased venous return from the opening of arteriovenous anastomoses located at the fingertips. The certain test for block success is, and will always be, the clinical outcome such as surgical anaesthesia or absence of pain on emergence. However, the major limitation of these clinical outcome measures is that a failed block will not be detected before the operating room, necessitating anaesthetic intervention. For this reason, surrogate outcomes such as pinprick and thermography may have clinical value prior to surgery. Both pinprick and thermography are non-invasive, but infrared thermography is not operator dependent and therefore not vulnerable to inter-observer bias. A further advantage is that, as opposed to pinprick, thermographic imaging does not require a co-operative patient, and this may be of value in specific groups such as children, and demented adults. The existence of five different thermographic patterns after IBPB is surprising. The brachial plexus is formed by
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Infrared thermography and interscalene brachial plexus block 633
the anterior rami of the spinal nerves from C5 to T1, whereas sympathetic fibres ascend in the sympathetic chain from the T1 level and below.13,14 High thoracic epidural anaesthesia probably leads to increased Ts in all parts of the hand, although this has not been investigated in detail.15,16 Peripheral median nerve block leads to increased Ts in the first to fourth digits and peripheral ulnar nerve block leads to increased Ts in the fourth and fifth digits.6 However, this knowledge does not allow us to predict the thermographic patterns after a proximal brachial plexus block, as we have demonstrated. The thermographic patterns with sparing of the third and fourth digits (Fig. 3, panel C2) are difficult to explain from our existing knowledge of the anatomy. Ulnar sparing is a well known feature of the IBPB. We expected the existence of a thermographic pattern with increases in Ts in the first to fourth digits and no increase in the fifth. Although we did not find such a pattern, it may well exist. We did not assess sensory or motor function of the radial, ulnar or median nerves. It would be of interest to link such results to the observed thermographic patterns in future studies. We have no good explanation for the much lower baseline and starting point temperature in the two patients with failed blocks. It should be noted that we performed measurements for only 30 min after performing the block. We do not know whether the thermographic patterns remain the same beyond this time, but the changes that occur after 30 min are unlikely to be of clinical use. We used a relatively large volume (20 ml) and a high concentration of ropivacaine (7.5 mg ml1) because this was part of the routine in the hospital where the blocks were performed. In recent years IBPB has been shown to be effective with much smaller volumes (5 to 10 ml) and post-operative analgesia can be obtained with much lower concentrations.17,18 The effects of changes in volume and concentrations in local anaesthetic on the thermographic response after IBPB are still unknown.19,20 It is also possible that a misplaced IBPB may spread centrally and cause sympathectomy with little somato-sensory effects. Moreover, a large volume of local anaesthetic could spread centrally to the sympathetic chain prior to fibres joining the brachial plexus. There are further limitations in the present study. Distal Ts is subject to major fluctuations due to changes in environmental temperature, sympathetic nervous activity (stress, anxiety) and local inflammation, for example. To reduce the impact of these factors and to standardise measurements, we tried to cool both hands by placing them on cooled packs (58C). This procedure itself may have introduced fluctuations. Moreover, the method did not provide adequate and constant cooling, as the cooled packs increased in temperature over time. It is also obvious that Ts declined slightly in the contra-lateral hand after the acclimatisation period (Figs 4 and 5), which suggests a too short acclimatisation period, or
inadequate cooling or both. However, considering the data, we think that our conclusions, with respect to the different thermographic patterns, are valid. By cooling the hands, we may have underestimated the rapidity of changes in Ts and overestimated the size of the change in Ts. In subjects that were not cooled, the increase in distal Ts after median and ulnar nerve blocks was 78C to 88C.6 Future clinical studies should omit the cooling procedure because it most probably just introduces noise to the measurements and is inconvenient in clinical practice. Our study was not a randomised, placebocontrolled study, which would have been a superior design. Instead we used the contra-lateral hand as control and used blinded nurses to clinically evaluate block success. Also, as we only had two block failures, we cannot make firm conclusions on the thermographic patterns of failed blocks. In conclusion, successful IBPB resulted in four different thermographic patterns in the hand. Ts always increased in the thumb. In more than 50% of the patients Ts increased in the whole hand. The increase in distal Ts (SPOT1) was 78C after 30 min and statistically significant within 5 min after siting the block. These results may be useful in future studies investigating the role of distal Ts of the thumb in predicting IBPB success.
Acknowledgements relating to this article Assistance with the study: We would like to thank the nursing staff at the Aleris-Hamlet Hospital, Copenhagen, Denmark and the nursing staff at the Department of Anaesthesiology, Nordsjællands Hospital and Copenhagen University Hospital, Hillerød, Denmark for their assistance with the study. Financial support and sponsorship: This work was supported by the Department of Anaesthesiology, Nordsjællands Hospital, Hillerød, Denmark and by local grants from Nordsjællands Hospital, Hillerød, Denmark. Conflicts of interest: none. Presentation: Preliminary data were presented as a poster presentation at the European Society of Anaesthesiology (ESA) Euroanaesthesia Congress, 1–4 June 2013, Barcelona, Spain.
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