Anaesthesia, 1992, Volume 41, pages 316-319

Nasal CPAP after coronary artery surgery

A . N . THOMAS, J. P. R Y A N , B. R. H . D O R A N



Summary Two groups of 14 patients were compared a f e r coronary artery bypass surgery where the left internal mammary artery had been used as a conduit. One group received nasal continuous positive airway pressure for I h, the other group acted as a control. Mean pulmonary shunt fraction was 16.3% before, 12.6% during and 15.7% after continuous positive airways pressure. In the control group the shunt fraction fell jirorn 17.3% to 16.8%. The reduction in shunt fraction was significantly greater with nasal continuous positive airways pressure than in the control group ( p = 0.016). There was a significant reduction ( p = 0.025) in respiratory rate from 18.3 to 16.7 breath.min-’ during continuous positive airway pressure. Other measured cardiorespiratory variables did not differ significantly between the groups. Visual analogue scores showed no significant diflerence in chest pain or mask conlfort betw,een the groups. The ease of breathing score was, however, significantly better in the continuous positive airways pressure group, 7.5 ( S D 1.8) cm and control 5.6 ( S D 2.6) cm. Key words

Coronary artery surgery, lung, postoperative care, equipment. Nasal CPAP. Coronary artery bypass surgery (CABS) commonly results in the development of pulmonary atelectasis in the postoperative period [ I]. This is more pronounced when the pleura has been opened during dissection of the left internal mammary artery [2]. The use of positive end expiratory pressure (PEEP) during mechanical ventilation results in a reduction in the alveolar-arterial (A-a) oxygen gradient. Unfortunately, the beneficial effects of PEEP do not persist following extubation [3]. The aim of this study was to measure the effect of nasally applied continuous positive airway pressure (N-CPAP) on shunt fraction and oxygen delivery in a group of spontaneously breathing, extubated patients after CABS in whom, in addition to the use of saphenous vein conduits, the left internal mammary artery has also been used, resulting in opening of the left pleura. The patient acceptability of N-CPAP was also assessed. Method Twenty-eight male patients who had undergone elective CABS without significant complications were studied. All patients consented to take part in the study, which was approved by the hospital ethics committee. Patients were studied if they had stable angina that was not present at rest. They were not studied if they had a history of valvular

heart disease, recent nasal obstruction, or significant obstructive or restrictive pulmonary disease. None of the patients studied had significant abnormalities of the lung field as measured by clinical examination or chest X ray and patients were only included if the physicians responsible for their care felt that the benefits of pulmonary artery catheterisation outweighed any potential disadvantages. The patient’s height, weight and age were recorded before operation. Patients were also divided by history into current, ex (more than 6 months) o r nonsmokers and their left ventricular function was classed as good, moderate or poor according to the left ventricular end-diastolic pressure (LVEDP), the ejection fraction and the appearance of the left ventricle during angiography. Anaesthesia was induced with fentanyl and midazolam and repeated doses of these drugs were given during and after the procedure. Muscle relaxation was provided with pancuronium or vecuronium and enflurane was used in the maintenance of anaesthesia. Myocardial protection was achieved with moderate hypothermia, topical cooling and cold aortic root crystalloid cardioplegia solution. Saphenous and internal mammary artery conduits were used as previously described. Patients received controlled ventilation postoperatively with PEEP if required, until their clinical condition allowed them to breathe spontaneously. Cardiac index, mean

A.N. Thomas, FFARCS, J.P. Ryan, FFARCS FFARCSI, DCH, Senior Registrars, B.R.H. Doran, MB. BS. FFARCS. Consultant. B.J. Pollard, BPharm, MB,ChB, FFARCS, Senior Lecturer, Department of Anaesthesia, Manchester Royal Infirmary, Oxford Road, Manchester M I 3 9WL. Correspondence should be addressed to D r J.P. Ryan please. 0003-2409/92/040316 + 04 $03.00/0

@ 1992 The Association of Anaesthetists of G t Britain and Ireland


Nasal C P A P after coronary artery surgery


Fig. 1. Nasal CPAP circuit: I , Downs adjustable Row generator: 2 , Concha therm humidifier; 3, CPAP valve; 4,Pressure monitor line; 5 , Adam pillows.

arterial pressure (MAP) and systemic vascular resistance were maintained within acceptable levels by the use of infusions of dopamine, dobutamine, nitroglycerin or noradrenaline as clinically indicated. On the morning after surgery patients were randomly allocated to receive conventional oxygen therapy ( I4 patients) or 5 c m H z 0 N-CPAP (14 patients). The number of coronary grafts fashioned, the duration of the operative procedure, the cross-clamp time and the bypass time were noted, as was the duration of postoperative mechanical ventilation and intubation and the time from extubation to the start of the study. All patients breathed air-enriched oxygen through a conventional oxygen mask; the oxygen was mixed with room air using an entrainment device (entrainment nebuliser; Respiratory Care Inc. USA) and humidified with a cold water humidifier (Kendall Aquapak 03700). Patients were seated in bed at an angle of between 45 and 90". Indwelling pulmonary and radial or brachial artery catheters were used to record baseline values for MAP, central venous and pulmonary artery occlusion pressures (CVP. PAOP). Cardiac index was calculated from the mean of three thermodilution measurements. Pulmonary and radial artery blood samples were analysed for oxygen and carbon dioxide tensions and pH using a Ciba Corning 288 blood gas analyser. Haemoglobin concentration and the percentage of oxyhaemoglobin were measured using an Instrumentation Laboratory 482 Co-oximeter. A 10 ml gas sample was taken from in front of the patient's nose during peak inspiration and the inspired oxygen concentration was measured using the blood gas analyser. After baseline measurements had been made. patients in the N-CPAP group had CPAP applied for 60 min. using a system which we have previously described (Fig. I). Patient connections


were made using Adams pillows (Puritan Bennett Corp.) held in position with the standard headgear assembly provided. The fresh gas flow through the system was adjusted to provide a continuous leak through the CPAP valve and the pressure within the system was monitored continuously. A fuel cell oxygen analyser was used to measure the oxygen concentration within the system so that it could be maintained as near as possible to that breathed by the patient prior to CPAP. Patients were encouraged to breathe through their noses during their pre-operative visit and when the N-CPAP mask was first applied. Patients in the control group continued to breathe added oxygen from a facemask in a routine fashion. ,Sixty minutes after starting N-CPAP, the baseline measurements were repeated in both the control and study groups. The inspired oxygen in the N-CPAP group was sampled from the CPAP system just proximal to the patient connection. Patients in the two groups were also asked to complete visual analogue scores, the nature of which had been explained during their pre-operative visit. These scores assessed chest pain (severe pain = 0. no pain = 10). comfort of the mask (very comfortable = 0, very uncomfortable = 10) and ease of breathing (very easy = 10, very hard = 0). Once the measurements had been repeated, N-CPAP was discontinued and patients returned to their conventional oxygen therapy. Fifteen minutes after this, baseline measurements were again repeated in the CPAP group. All measurements in both groups were made before the patients received physiotherapy. For parametric data, comparison between the two groups was made using unpaired Student's t-tests; withingroup comparison of this type of data was made using paired Student's r-tests. Categorical variables (for example, smoking history and left ventricular function) were compared between the two groups using Chi-squared tests. Visual analogue scores were subject to logistic transformation before the two groups were compared using unpaired Student's r-tests. Significance was set at the conventional 5% level for all tests. Results

Patients in the control and CPAP groups did not differ significantly with respect to height, weight, age, smoking history or pre-operative left ventricular function (Table I). Left ventricular function was not assessed in one patient because of a technical difficulty during catheterisation. There was also no significant difference between the two groups with respect to the operative procedure, as measured by the number of grafts performed, the cross-

Table 1. Preoperative data in the control and study groups.

Patient details Smoking history

(mean SD) ~-

Age (years)



Height (cm)





(10) 59



I77 (6) I72









Ventricular function _

Ex-smoker Non-smoker LVEDP (over 6 months) (mmHg) Good Moderate





2 0


12 (2)



2 ( = 13)

13 ( 6 )




A . N . Thomas et al.


Table 2. Operative and postoperative data in the CPAP and control groups expressed as mean (SD). Postoperative time (h)

Operative time (min) Group

Number of grafts

Cardiopulmonary bypass

Cross clamp

5.0 (1.4) 4.8 (1.9)

96 (37) 108 (28)

49 (16)

Total operation





230 (59) 236



9.0 (3.6) 9.8 (4.0)

11.5 (3.6) 11.3 (3.5)

5.4 (4.1) 5.0 (4.0)


CPAP Control

I , End of surgery to spontaneous respiration; 2, end of surgery to extubation; 3, extubation to start of study.

level. Fifteen minutes after discontinuation of CPAP the shunt fraction was significantly worse than during CPAP and was not significantly different from that recorded before CPAP. There was no change in oxygen delivery or extraction with the application or discontinuation of CPAP. This was because patients' haemoglobin was well saturated with oxygen before CPAP so that the reduction in shunt fraction could not result in a significant improvement in oxygen content. There was a small but significant (p = 0.025) reduction in respiratory rate during N-CPAP while the respiratory rate actually increased in the control group; this difference between the two groups was significant (p = 0.038). Table 4 shows the cardiovascular variables measured in the two groups. There was no significant difference between the two groups with respect to any of these variables before and during the application of CPAP. Within the study group no variable changed significantly with either the application or discontinuation of CPAP. Visual analogue scores in Table 5 showed no significantdifference between the two groups with respect to chest pain or mask comfort. There was, however, a significant difference (p = 0.007) in favour of the N-CPAP mask with respect to the ease of breathing score (7.5 (SD 1.8) cm in the N-CPAP group and 5.6 (SD 2.6) cm in the control group). One patient found the nasal mask more uncomfortable than the conventional oxygen mask. He also found it more difficult to breathe through the mask and had an increase in shunt fraction during CPAP.

Table 3. Mean values (SD) for oxygenation variables and respiratory rate in control and CPAP groups during the study period. Control Time


% inspired 0,





46 (8) 16.3 (5.7) 423 (93) 159 (31) 18.3 (4.5)

46 (8) 12.6 (6.2) 430 (83) 155 (33) 16.7 (3.3)

(1 1) % shunt

DO, index (ml.min-'.m2) VO, index (mlmin- 'd) Respiratory rate (min-')


16.8 (7.4) 412 (80) 132 (36) 22.0 (7.5)


clamp or bypass times or the total duration of the operative procedure (Table 2). The postoperative course before the study, as measured by the period of post-operative ventilation and intubation, and the time from extubation to the start of the study did not differ between the two groups (Table 2). Table 3 shows the calculated values for oxygenation in the two groups as well as their respiratory rates. The shunt fraction before CPAP did not differ significantly between the two groups. However, there was a significant reduction in shunt fraction in patients using CPAP, without there being any such reduction in patients in the control group. The mean absolute reduction in shunt fraction was 3.76% (SD 4.0) in the N-CPAP group and only 0.52% (SD 2.0) in the control group, a difference that was significant (p = 0.016). After one hour, the shunt fraction was lower in the CPAP group than in the control group. However, this difference did not reach significance at the conventional 5%

Discussion Our results show that in patients who have had CABS in which the left pleura has been opened during internal mammary artery dissection there is a significant reduction in shunt fraction during breathing via an N-CPAP system.

Table 4. Mean (SD) values for cardiovascular variables in the control and CPAP groups. Control Time (min) Cardiac index; I.min-'.sq.m-' MAP; mmHG HR; minutes CVP mmHG PAOP; mmHG







3.1 (0.4) 78 (11.7) 90

3.1 (0.6) 80 ( 1 1.4) 90


2.9 (0.4) 82 (4.0) 90 (14) 7.2 (3.8) 9.15 (3.9)

2.8 (0.3) 83


3.0 (0.5) 81 (4.4) 88 (14) 7.4 (3.6) 8.4 (3.9)

7.4 (4.0) 8.6 (2.9)

7.1 (3.3) 8.0 (3.5)

(4.5) 89 (14) 7.01 (3.4) 8.23 (3.9)

Nasal CPAP after coronary artery surgery Table 5. Mean (SD) visual analogue scores in cm for control and

study groups.

Chest pain (10 = no chest pain)




5.3 (2.3) 6.2 (1.7) 5.6 (2.6)


Comfort (10 = very comfortable) Ease of breathing (10 = very easy)

6.7 (1.9) 7.5 (1.8)

Although this reduction in shunt fraction was associated with no demonstrable haemodynamic effects, the improvement was modest and was not sustained after CPAP was discontinued. We have also demonstrated that after 60 min use the CPAP mask is at least as well tolerated as a conventional facemask and that there was a small but significant reduction in respiratory rate during N-CPAP. These results are in broad agreement with previously published work that has shown a significantly lower A-a gradient in patients treated with PEEP during mechanical ventilation [4]. This improvement was not, as in our study, sustained after the therapy was withdrawn. A reduction in A-a gradient has also been demonstrated during treatment with N-CPAP in patients with pulmonary atelectasis [5] and in patients with pneumonia [6]. Pinilla et al. [7] have shown a reduction in A-a gradient after saphenous vein grafting was used for coronary revascularisation in patients treated with a combination of mask and N-CPAP. This improvement was again temporary and did not result in any significant improvement in pulmonary atelectasis. In apparent contrast to these studies, suggesting that the beneficial effects of PEEP are not sustained, other authors have shown a more rapid recovery in functional residual capacity [8] and reduction in A-a gradient [9] in patients treated with CPAP on a regular basis on each postoperative day after upper abdominal surgery. The system’s patient acceptability would make it suitable for this type of application, particularly as the mask can be used during physiotherapy. One potential disadvantage of N-CPAP, which we have described, is the loss of airway pressure during mouth breathing and this may limit its usefulness in patients with severely impaired pulmonary oxygen transfer where a full


facemask will, if tolerated by the patient, produce a more continuous level of CPAP. We conclude that the use of nasal CPAP is a simple, tolerable and effective method of treating hypoxaemia in adult patients after coronary artery bypass surgery and warrants further study.

Acknowledgments The authors acknowledge the support of the nursing, surgical and technical staff of the Manchester Royal Infirmary, in particular M r D. Edwards, Physiological Measurement Technician. They also wish to thank Mr B. Faragher for statistical advice and Mr M. Boulsted and Mr M. Twitchett from Puritan Bennett for their support.

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Nasal CPAP after coronary artery surgery.

Two groups of 14 patients were compared after coronary artery bypass surgery where the left internal mammary artery had been used as a conduit. One gr...
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