Accepted Manuscript Ultrasonography for Haemodynamic Monitoring Jens M. Poth, M.D Daniel R. Beck, M.D Karsten Bartels, M.D

PII:

S1521-6896(14)00074-3

DOI:

10.1016/j.bpa.2014.08.005

Reference:

YBEAN 823

To appear in:

Best Practice & Research Clinical Anaesthesiology

Received Date: 5 June 2014 Revised Date:

5 August 2014

Accepted Date: 27 August 2014

Please cite this article as: Poth JM, Beck DR, Bartels K, Ultrasonography for Haemodynamic Monitoring, Best Practice & Research Clinical Anaesthesiology (2014), doi: 10.1016/j.bpa.2014.08.005. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Ultrasonography for Haemodynamic Monitoring

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Jens M. Poth, M.D. 1, Daniel R. Beck, M.D. 2, Karsten Bartels, M.D. 2

Affiliations: 1

Developmental Lung Biology, Cardiovascular Pulmonary Research Laboratories,

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Departments of Pediatrics and Medicine, University of Colorado Denver,

Department of Anesthesiology, Eastern Colorado Veterans Affairs Medical Center and

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University of Colorado Denver, USA

Address to which correspondence should be sent: Karsten Bartels, M.D.

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University of Colorado Hospital Department of Anesthesiology|12401 E. 17th Avenue Leprino Office Building, 7th Floor, MS B-113 Aurora, CO 80045

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Phone: 303-724-0166 Email: [email protected]

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Words (including references): 6158 Figures: 6 Tables: 1 Words including figures & table (250 for each): 7908

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Abstract Echocardiography has become an indispensable tool in the evaluation of medical and

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surgical patients. As ultrasound (US) machines have become more widely available and significantly more compact, there has been an exponential growth in the use of

transthoracic (TTE), transoesophageal echocardiography (TOE), and other devices in the perioperative setting. Here, we review recent findings relevant to the use of perioperative

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US, with a special focus on haemodynamic management of the surgical patient.

Key Words: Ultrasound techniques; Hemodynamic Monitoring; Transoesophageal Echocardiography; Goal-directed Therapy; Ultrasound Education

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Practice Points:

Ultrasound-based techniques have gained wide-spread acceptance for the evaluation of hemodynamic disturbances Understanding the basic principles of medical ultrasound is required to judge

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possibilities and limitations inherent to this technology. Transoesophageal ultrasound, transthoracic ultrasound, oesophageal Doppler,

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and handheld US machines are available for assessment of volume status and cardiac function.

Research Agenda:

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Future studies should focus on the impact of ultrasound-based technologies to better diagnose and treat hemodynamic instability. Meaningful clinical outcomes, such as 30-day mortality should be collected.

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Comparative studies between high-fidelity / high-cost versus basic / low-cost devices are desirable

Best teaching practices for achievement of US-based competency should be

determined according to specific uses. More widespread use of US-based devices

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begets more widespread education.

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Introduction In an attempt to make haemodynamic monitoring less invasive and to acquire additional

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relevant information not obtained with other monitoring approaches, ultrasound devices are increasingly being used in perioperative medicine.(1) The field is rapidly evolving as technology advances. Here, we describe basic principles of ultrasonography and how it can

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be used for haemodynamic monitoring in the perioperative setting.

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Basic Principles of Ultrasound

Sound is the mechanical deformation of a medium that propagates as a wave. Sound waves have the following unique characteristics:(2, 3)

• Frequency f (Hertz, Hz) – Sound waves with frequencies >20kHz are termed US, as

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they are not audible to the human ear. Most medical US devices used for imaging have frequencies ranging from 2 to 18 MHz.

• Propagation velocity – The velocity of propagation is dependent on the tissue or

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medium the sound travels through, e.g. 1540 m/s in average human soft tissue.

• Wavelength λ (mm) – Wavelength (λ), propagation velocity (c), and frequency (f) are

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directly related: c = λf

While US signals with longer wavelengths penetrate deeper into the tissue, shorter wavelengths result in better image resolution. These basic considerations guide the choice of the transducer used during an examination.

• Amplitude (decibel, dB) – the difference between peak and trough of the sound wave excursions. The amplitude of an audible sound can be appreciated as its loudness.

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• Acoustic impedance – The product of the tissue-specific US velocity and the density of the tissue US waves are reflected back to the transducer, when they reach an interface of tissues with

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different acoustic impedances. The piezoelectric crystal within a brightness mode (B-mode) US transducer acts both as sender and receiver of the US waves. Prior to image display the return signal undergoes complex processing, including filtering, compression, and

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amplification.

Imaging Modalities B-mode / M-mode

In B-mode US, reflections of a single, distinct path of sound are displayed as dots. The brightness of these dots represents the strength of the sound echo(4). When a series of

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sequential B-mode images is used to display moving structures within a single path of the US beam, the resulting image is referred to as motion mode (M-mode) (figure 1).(2, 4)

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While M-mode images only provide a one-dimensional view, their high frame rate permits

2D US

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superior imaging of fast moving structures due to high temporal resolution.

In two-dimensional (2D) US, multiple B-mode images are aligned to create a 2D image of anatomical structures (figure 2). The resulting tomographic image of a single sector scan is a “frame”, the frequency with which these frames are created determines the temporal resolution and is referred to as “frame rate”. Hence, if the image sector is narrowed to the structure of interest, less time is needed to scan the width of the image, resulting in a higher

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image frame rate and better temporal resolution (figure 2). This is desirable for the analysis of fast-moving structures.(2, 4)

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Doppler US

If US is aimed at a moving target, reflection of sound waves will be dependent on the

direction of the US beam relative to the direction of the moving target (figure 3). In the

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depicted example, sound waves are reflected at a higher frequency than emitted, causing a decrease in the wavelength of the reflected echo signal. The difference between the

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originally transmitted frequency (ft) and the signal received back at the transducer (fs) is the Doppler shift. The velocity of the interrogated target can be calculated according to the Doppler equation:

c( f s − f t ) 2 f t (cos θ )

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v=

v is the velocity of blood flow, c the speed of sound (1540 m/s in average human soft tissue) and θ is the incident angel between the trajectory of the target and the US beam. The beam

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should be parallel to the direction of blood flow (cos 0° = 1), since an incident angel of 90° does not allow recording of any Doppler shift at all (cos 90° = 0).(2, 3) For practical

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purposes, one should recall that if the there is a significant angle of incident and it is not corrected for, the true velocity of the object of interest will be underestimated.

3D

Three-dimensional echocardiographic (3D) imaging was introduced in the 1970s with initial devices being rather cumbersome to use and limited mostly to research applications (Figure 4). Current 3D echocardiographic images are obtained in real-time using matrix-

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array transducer technology.(5) Technologic advances have reduced the sizes of 3D US probes dramatically. Some advantages of 3D US technology include improved imaging for the assessment of cardiac chamber volumes and mass, the assessment of regional left

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ventricular wall motion, and presentation of realistic views of heart valves and structural defects.(6-8) While a comprehensive overview of 3D echocardiography is not within the scope of this review, it should be noted that this technology is rapidly gaining traction. In

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fact, its potential to transform perioperative imaging has been likened to the effects of the

Haemodynamic Monitoring

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introduction of the stethoscope into clinical practice in the 19th century.(9)

US for haemodynamic monitoring facilitates decision making in acute care environments: TTE and TOE allow the differentiation between non-cardiac and cardiac causes of

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haemodynamic instability. Valvular pathologies and abnormalities in ventricular function can be assessed. During non-cardiac surgery, the American Heart Association (AHA) and the

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American College of Cardiology (ACC) recommend the use of echocardiography in the “evaluation of acute, persistent and life-threatening haemodynamic disturbances in which

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ventricular function and its determinants are uncertain and have not responded to treatment” (level of evidence I).(10) The guidelines also recommend the use of echocardiography during “surgical procedures in patients at increased risk of myocardial ischemia […] or haemodynamic disturbances” (level of evidence IIa).(10) For the purpose of this review, we will focus on the indications for perioperative TOE and on technical aspects of CO and volume status assessment using US.

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While TOE is obviously more invasive than TTE, TOE provides better resolution and image quality in certain situations (e.g. following sternotomy and cardiac surgery) and might be the only feasible option intra-operatively. That being said, TTE, if feasible, is usually the

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more desirable option for rapid assessment of a haemodynamically unstable patient. Since many principles of image assessment are similar in TOE and TTE, we have opted not to

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provide a separate discussion of both modalities.

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TOE – Imaging

The utility of TOE for haemodynamic monitoring has been highlighted in several guidelines by the ASA, ASE, the Society of Cardiovascular Anesthesiologists (SCA), ACC and by the corresponding European societies (European Association of Echocardiography, European Association of Cardiothoracic Anesthesiologists (EACTA)).(10-13) ASE and SCA also

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released a consensus statement on non-comprehensive, basic perioperative transoesophageal echocardiography (PTE)(12), focusing on the identification of the cause of haemodynamic instability in surgical patients. In addition, similar published

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recommendations exist for the use of echocardiography in emergency rooms and in the

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neonatal intensive care unit.(14, 15)

Current recommendations and guidelines for perioperative TOE The intraoperative use of TOE is recommended during cardiac and thoracic aortic surgery. (11, 13) In non-cardiac surgery, ASA and ESA suggest the use of TOE only during specific procedures – such as during neurosurgery with a high risk for air embolism – or in other

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specific conditions (table 1). TOE aims at identifying the underlying aetiology of haemodynamic instability and guiding therapeutic interventions.(12)

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Ventricular function: Global, systolic LV function can be visually estimated. According to current SCA

recommendations, this basic qualitative assessment is not precise, but sufficient for the

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identification of patients who might benefit from inotropic therapy.(12) The SCA

recommends using the transgastric (TG) midpapillary short axis (SAX) view, as well as the

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mid-oesophageal (MOE) four-chamber, the MOE two-chamber and the MOE long axis (LAX) views for the monitoring of LV function.(12)

For a more precise quantification of global LV systolic function, ASE and ESC recommend calculating the ejection fraction (EF), quantifying end-systolic and end-diastolic volumes

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(ESV and EDV, respectively) using the modified Simpson’s method.(16) Here, the ventricle is conceptualized as a series of stacked disks. The diameters of these disks and the long axis of the ventricle are then measured on the MOE four-chamber and two-chamber views.(16,

SV *100 EDV

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EF% =

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17) While the difference in ESV and EDV defines the stroke volume (SV), EF is defined as:

CO can then be calculated by multiplying SV by heart rate (HR). Smith et al. demonstrated that 2D TOE measurements of EF correlated well with values measured by LV angiography (correlation coefficient r = 0.85 for EF).(18) Transthoracic 3D measurements of ventricular

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volumes and EF show an even better correlation with reference standards (such as MRI or SPECT), because no assumptions about ventricular geometry are necessary.(19, 20) SV can also be evaluated by Doppler-TOE: Utilizing Doppler, the velocity of blood flow

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passing a known cross-sectional area (CSA) within a period of time is measured (velocitytime integral). The VTI describes the distance that the blood column has moved forward

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during that given period of time (systole). SV can then be calculated as follows:

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SV = VTI * CSA

Several assumptions have to be made for this: First, flow has to be laminar and second, the flow profile has to be flat. Generally, any intra-cardiac site for which CSA and VTI can be recorded is suitable for Doppler-based calculation of SV, but the LV outflow tract (LVOT) is

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used for its relatively circular shape and the presence of laminar rather than turbulent flow in most clinical scenarios. The cross-sectional dimensions of the LVOT are measured using 2D imaging (e.g., from the MOE aortic valve (AV) LAX view, see figure 5a).(17) Blood flow

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5b).(2, 17)

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velocity at the site of diameter measurement is performed using pulsed Doppler (figure

To detect myocardial ischemia, regional wall motion analysis can be performed using more or less extensive scoring systems, which are described in detail elsewhere.(16, 17) While these comprehensive scoring systems use a 16- or 17-segment model of the LV,(16) visualization of the six midpapillary segments from the TG midpapillary SAX view may offer a reasonable impression of LV wall motion in an orienting exam.(12) In fact, the wall

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motion index at the level of papillary muscles (WMIp) was not significantly different from the WMI based on the comprehensive 16-segment model in patients after cardiac surgery.(21)

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Qualitative assessment of global RV function can be performed in a similar fashion as for the LV and is recommended in every patient with refractory hypotension. SCA and ESC also recommend monitoring RV function routinely in patients undergoing liver transplantation,

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especially those with pulmonary hypertension.(12, 13) Monitoring RV function in the

detrimental to the failing RV.(22)

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haemodynamically unstable patient is paramount, because volume therapy can be

Precise quantification of RV-EF is challenging given the complex geometric architecture of the RV compared to the LV. Using the MOE four-chamber view and TG views, data can be collected for quantification of RV function.(17) Current guidelines recommend 2D

EDA − ESA *100 EDA

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FAC% =

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Fractional Area Change (FAC) as an estimate of RV systolic function(23) defined as:

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with ESA being end-systolic area and the lower reference value for FAC% being 35%. FAC% was shown to correlate fairly well with EF% measured by MRI (r=0.8).(24) Alternative measures of global RV systolic function include the Tei index, tricuspid annular plane systolic excursion (TAPSE), longitudinal RV strain, and estimation of RV EF by 3D echocardiography.(22, 25)

Intravascular volume status: 11

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Hypovolaemia is a common cause of cardio-circulatory instability in the operating theatre and the intensive care unit. A central concept in the care of critically ill patients and patients undergoing surgery is to predict fluid responsiveness: Will a patient’s haemodynamic

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situation improve (i.e. increase in SV and CI) with fluid administration or not? If certain preconditions are met (closed chest, controlled ventilation with sufficiently high tidal volumes, regular heart rhythm, normal intra-abdominal pressure) Systolic Pressure

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Variation (SPV), arterial Pulse Pressure Variation (PPV) and Stroke Volume Variation (SVV) represent “dynamic” parameters that more reliably predict fluid responsiveness.(26-29)

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Central venous pressure (CVP) and LV-EDA do not predict fluid responsiveness, as they are static parameters that are dependent not only on volume status. Other variables impacting CVP and LV-EDA include for example cardiac compliance (i.e., diastolic ventricular function) as well as intrathoracic pressure. (26, 29, 30) Consequently, LV-EDAI (LV-EDA indexed to

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body surface area) does not correlate with fluid responsiveness.(31) Other studies confirmed the inferiority of LV-EDA in predicting fluid responsiveness in comparison to dynamic parameters.(32, 33) Different systematic reviews also concluded that LV-EDA is

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inferior compared to dynamic parameters such as PPV.(34, 35)

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US can also be utilized to measure dynamic parameters: As outlined above, SV can be measured using the VTI over a given CSA (e.g. the LVOT). Assuming that the CSA is constant, ventilation-induced variability in SV will be reflected in changes of blood flow velocity (∆Vpeak). Feissel et al. demonstrated that ∆Vpeak – determined by TOE – can be utilized to accurately predict fluid responsiveness.(33) The authors found a threshold value of 12% ∆Vpeak to discriminate between responders and non-responders (sensitivity 100%, specificity 89%). Similarly, fluid responders were identified using echocardiography-based

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approaches in other mechanically ventilated patient populations. Respiratory variability of peak aortic blood flow velocity predicted fluid responsiveness in a study of children after ventricular septal repair.(36) A study by Renner et. al in infants undergoing congenital

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heart surgery identified respiratory variation in peak blood flow velocity as most predictive of fluid responsiveness.(37) Interestingly, in both latter studies central venous pressure did not correlate.

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Measuring respiratory variation in inferior vena cava (IVC) diameter is dependent on

ventilation-induced changes in pleural pressure causing partial collapse of the IVC. Moretti

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and Pizzi focused on ventilated (VT = 8ml/kg body weight), sedated and paralyzed patients and consequently observed a good positive predictive value of IVC distensibility for fluid responsiveness, with a cut-off value of 16% identifying responders.(38) The superior vena cava (SVC) can also be assessed.(39) Here, a threshold of >36% in SVC collapsibility

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identified fluid responders. Of note, this study was conducted in mechanically ventilated patients, showing no signs of spontaneous breathing activity. In spontaneously breathing patients, passive leg-raising (PLR)-induced changes in VTI, SV

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or CO demonstrated to be useful to predict fluid responsiveness: Using TTE, Lamia et al.

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found a PLR-induced change of 12.5% or more in VTIAo (VTI over the aortic valve) or SVI (SV index) to reliably predict volume responsiveness (sensitivity 77%, specificity 100%).(40) Maizel et al. found a PLR-induced change in SV of 12% (as measured by TTE) to accurately predict fluid responsiveness (positive predictive value 85%); other studies confirmed these findings.(41, 42)

Valvular function:

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For a basic assessment of valvular regurgitation visual inspection of the regurgitant jet area, vena contracta width, as well as flow reversal in receiving or originating cardiovascular chambers can be used amongst other criteria.(12) Stenotic lesions can be grossly evaluated

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by continuous-wave Doppler using an imaging plane parallel to blood flow. (see Doppler section above). An orienting assessment of valvular function should be part of every basic

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echocardiographic examination.(43)

Pulmonary Embolism, Pericardial Effusion and Thoracic Trauma:

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Haemodynamically relevant pulmonary embolism (PE) is one reason of cardio-circulatory compromise. In the intraoperative or emergency setting, TOE might be the only feasible, yet reliable tool to detect the presence of haemodynamically relevant emboli. (44-46) Signs of RV failure and motion abnormalities of the RV free wall permit diagnosis of PE in patients

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with hypotension or shock.(47),(48) Cardiac contusion and pericardial effusion with tamponade can also lead to haemodynamic compromise. TTE and TOE permit rapid diagnosis of pericardial effusions and can effectively guide treatment of tamponade

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(pericardiocentesis).(12, 49)

Oesophageal Doppler TTE and TOE allow the evaluation of cardiac morphology and function in great detail, yet they are mostly designed for intermittent imaging and monitoring. Newer devices permitting continuous hemodynamic TOE monitoring in the intensive care unit have been introduced into clinical practice.(50) While appealing in concept, the ability to improve clinical outcomes remains a focus of on-going studies. Traditional Transoesophageal

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Doppler US (hereafter TD) lacks the complete diagnostic capabilities of TTE/TOE, but it allows the continuous evaluation of several key haemodynamic parameters. Current miniature TD/TOE probes are now capable of basic cardiac imaging, but can remain in situ

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for continuous TD monitoring (“haemodynamic TEE (hTEE)”).(50-52) More studies will have to be completed to fully appreciate this new technology. Currently, the National

Institute for Health and Clinical Excellence, NICE (United Kingdom), recommends using

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traditional TD in high-risk patients undergoing major surgery.(53) Similarly, the Agency for Healthcare Research and Quality (AHRQ), which evaluates medical procedures for the

monitoring during major surgery.(54)

Effect on outcomes

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Medicare and Medicaid services in the United States, endorsed TD-guided haemodynamic

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Several studies have demonstrated that TOE has an immediate impact on surgical decisionmaking in cardiac surgeries (reviewed in (11, 12)). Khoury et al. investigated the impact of

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US on decision making in the critically ill patient:(55) Several indications triggered US imaging in this setting, the most frequent one being haemodynamic instability. TOE was

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performed whenever TTE was inconclusive or of poor quality. In the patients suffering from haemodynamic instability, TOE allowed the discrimination of hypovolaemia from other causes (here most often MR). In this study by Khoury et al., US-based findings led to an adjustment in medical therapy (such as initiation of antibiotics or volume therapy) or triggered surgical interventions (such as valve replacement) in 60% of patients studied with TTE or TOE.

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While TTE/TOE has not been compared to other monitoring devices for the sole purpose of fluid management, TD has been studied more extensively. A recent Cochrane Systematic Review could not find any advantage for perioperative (here, 24h before to 6h after

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surgery) GDHT (Goal-directed haemodynamic therapy) in reducing mortality. However, GDHT reduced perioperative complications (renal impairment, respiratory failure, wound infections) and LoS.(56) Other systematic reviews confirm these findings and report less

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post-operative complications.(57-60) For additional reading we refer the reader to Dr.

Clinical Anesthesiology.

Learning US techniques

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Tony Roche’s review on this topic in this current issue of Best Practice and Research –

Since introduction to clinical practice, echocardiography has long been a protected tool of

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the select few with access to the technology and adequate clinical experience. In order to ensure patient safety, and to reassure the public of competency and training, medical

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specialists have more recently moved to formalize this skill with training guidelines and exams to test competency. It was first limited to the cardiologist, who along with the ASE

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have continued to set forth standards for best practice based on training experience allowing independent practice, currently named Level 2 training consisting of at least 6 months of dedicated time, 150 TTE exams performed, and at least 300 exams reviewed.(61) These skills are then verified via a written examination, the Adult Special Competency in Echocardiography Examination (ASCeXAM), given first in 1996 to ensure competency and comprehensive knowledge. In the 1990s, echocardiography found its way into the operating room as demand for real-time interpretation and evaluation of structural and

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haemodynamic information was deemed beneficial for cardiac surgery – here, TOE was performed mostly by cardiac anaesthesiologists. Early institutional curricula for echocardiography education were published(62) and have now progressed to formalized

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training guidelines with an established path for basic practice (50 TOE exams performed, 150 reviewed) using TOE as a intraoperative monitor of basic cardiac function and volume status(43) and advanced perioperative echocardiography (150 exams performed, 300

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reviewed) with a comprehensive diagnostic examination to guide surgical

interventions.(63) In 1998, the first formal exam in advanced perioperative TOE was

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offered, currently known as the Advanced Perioperative TOE Examination (PTEeXAM)(64) and subsequently the Basic Perioperative TOE exam, Basic PTEeXAM.

As we have moved into the 21st century, echocardiography has now been passed on to the

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bedside with intensivists, emergency room physicians, internists, and medical students taking advantage of this technology (figure 6), but not always with formal training or competency pathways. Recent data from multiple fields of medicine has come to show that

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short-term echocardiographic education can produce high levels of sensitivity and

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specificity as compared to advanced users to answer clinically focused questions affecting haemodynamic parameters, such as ventricular function, volume status, or cardiac tamponade.(65-67) For example, integrated ultrasound curricula have been successfully implemented into all 4 years of medical school at the University of South Carolina. Here, mastery of basic US examination skills in multiple organs were demonstrated using standardized patients, integrated US-based examination questions throughout the medical school curriculum, as well as demonstration of hands-on skills such as a FAST exam during

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more senior rotations.(68) While rigorous studies demonstrating benefit on clinical outcomes of limited echocardiographic exams are currently lacking, there appear to be at

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least signs that they may be beneficial in certain situations.(66)

The ideal training process for teaching limited or focused echocardiography has ranged from the traditional didactic to novel approaches using internet based technology,

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simulation, healthy volunteers, and on the job training. High fidelity simulation with guided feedback is superior to traditional didactic training,(69) and this has been validated for

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teaching focused basic echocardiography as well.(70, 71) Following the lead from cardiology and anaesthesiology, many relevant societies have adapted by setting standards, guidelines, and training recommendations.(72) Organization such as the American College of Chest Physicians offer comprehensive hands-on program targeted towards practicing

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clinicians (http://www.chestnet.org/Education/Advanced-Clinical-Training/Certificate-ofCompletion-Program/Critical-Care-Ultrasonography). Integration of dedicated echocardiographic training modules into medical education curricula will continue to

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advance utilization to this powerful diagnostic tool.

Conclusion

US use for medical imaging has evolved from being a technology utilized by only a few specialists to becoming one of the most heavily relied on bedside diagnostic modalities in the perioperative care of critically ill patients. A basic understanding of the fundamental principles of medical US as well as knowledge of the devices available is of utmost importance to the practicing clinician. Technologies in broad use by anaesthesiologists and

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intensivists include TOE, TTE, TD, as well as handheld US machines. The technologic capabilities of US imaging, including 3D applications will continue to grow, while size and price of many applications should shrink. On-going efforts to ensure the quality of provider

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education will be necessary to ensure that the obtained images will lead to valid

conclusions. In order to demonstrate tangible benefits from the use of these devices, further

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studies with meaningful clinical outcomes will be required.

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Conflict of interest: none.

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1974 Dec;7(6):544-53.

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Figure legends: Figure 1:

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Image 1: M-mode image depicting regurgitant flow in the LVOT secondary to aortic regurgitation. The EKG tracing indicated the end of systole with the completion of the Twave, upon which the reguritant flow present in the M-mode image. Note the dotted line in the 2D image above (midoesophageal aortic valve long axis). This line indicates the origin of

Figure 2 (A+B):

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the M-mode image.

Midoesophageal mitral commissural view. Note the narrowed image sector in image 2B compared to 2A, which results in a significantly higher frame rate (58Hz vs. 35 Hz) and

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higher temporal resolution.

Schematic principle of Doppler-based velocity measurements, here while interrogating

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regurgitant flow through an incompetent mitral valve. Note that the regurgitant trans-

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mitral flow is not flowing directly towards the US transducer. Hence, its velocity would be underestimated if no correction for the angle of incident θ were performed.

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Figure 4 Photograph of the five degree of freedom arm including signal conditioners and stand. Reproduced with permission from: Dekker DL, Piziali RL, Dong E, Jr.: A system for

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ultrasonically imaging the human heart in three dimensions. Computers and biomedical research, an international journal. 7:544-553, 1974. Copyright Elsevier 1974.(73)

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Figure 5 (A+B):

For the cardiac output measurement using US, first the area of the left-ventricular outflow

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tract (LVOT) is estimated through measurement of its diameter (5A). The volume-time integral (VTI) describes the distance that the blood column has moved forward in the LVOT

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during a given period of time and is measured using pulsed wave Doppler (5B).

A pocket sized, battery-powered US device is used by a medical student to evaluate a

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patient following cardiac surgery. Photograph: Karsten Bartels, M.D.

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Table 1 – Indications and contraindications for perioperative transoesophageal echocardiography10; 11; 12 requiring access to cardiac chambers • Consider for CABG Thoracic Aortic Surgery

• All procedures under general anesthesia

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• Specific conditions

• When TTE not feasible

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Critical Care /post-op

• Severe haemodynamic instability, threatened or present

• Patients undergoing major surgery with high cardiac risk (heart failure, coronary disease, valve disease)

• Severe haemodynamic disturbance, threatened or present • Relevant cardiac disease suspected

• Relative contraindication: When potential risk higher than expected benefit

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Oral, oesophageal or gastric disease Avoid complications in

• Consider other imaging modalities (TTE) • Consultation of gastroenterologist • Reduce probe size • Limit examination • Avoid unnecessary probe manipulation • Use most experienced operator

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Contraindications patients at risk

• Neurosurgery with high risk for embolism • Liver transplantation • Lung transplantation • Major vascular surgery (incl. trauma)

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• Select major surgeries

Non-cardiac Surgery

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• All cardiac procedures

Cardiac Surgery

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