Department of Anaesthesia
St Michael's Hospital, University of Toronto
Echocardiography provides real time images of the beating heart (2D) or time motion images (M Mode) which are images along one line over time. Doppler assessments can be done along one axis in pulsed wave (velocity at one point) or in continuous wave (velocity along the whole interrogation axis). Colour Doppler provides a qualitative assessment of blood flow velocity and direction over a particular area.
Transesophageal echocardiography is fast becoming an essential tool in critical care and in the operating room. It provides a fast, non invasive assessment of heart function which is complimentary and often superior to invasive monitoring. With invasive, 2D and Doppler evaluation of heart function, a very precise haemodynamic picture can be outlined which often aids in determining therapeutic interventions.
There are 2 major techniques available to image the heart with ultrasound: transthoracic (TTE) and Transesophageal (TEE). In critical care and the operating room, TTE has limited applications. Access to the external chest wall is limited in the operating room and the technique requires constant probe application which is unsuitable for prolonged monitoring. Furthermore, during mechanical ventilation, lung inflation and its position between the heart and the chest wall causes loss of imaging. TEE however, provides clear and easily accessible pictures of the heart and mediastinal contents through the oesophagus and the stomach. It is an ideal monitoring tool as the probe can be left in situ during surgery without loss of image. With an omniplane probe, the plane can be changed from 0° to 180° in increments of 1 degree providing a large variety of interrogation angles not previously possible with the biplane probe (0° and 90°).
The standard images obtained with the TEE positioned in the oesophagus are the 4 and 5 chamber views. The 4 chamber view includes the left atrium (LA), the left ventricle (LV), the right ventricle (RV) and the right atrium (RA). The 5 chamber view is obtained by pulling the probe up slightly to include the LV outflow tract. A higher position will provide clear pictures of the aortic valve and while rotating to approximately 40°, a cross section of the aortic valve including the coronary arteries can be seen. Further rotation to approximately 120° will show a longitudinal cross section of the ascending aorta, aortic valve and LVOT. It is not possible to image the distal segment of the ascending aorta and the arch as the left main stem bronchus obstructs these structures. In the four chamber view, the mitral valve, right upper pulmonary vein, the left atrial appendage, the tricuspid valve and the inter-atrial septum can easily be evaluated. The descending aorta can be imaged from the arch to the diaphragm. Both the right and left pleural spaces along the mediastinum can also be imaged. Pleural effusions can be seen as well as consolidated or collapsed lung.
The probe can be pushed down further into the stomach and flexed superiorly to abut against the superior aspect of the stomach and the inferior aspect of the diaphragm. The hearts inferior wall sits on the left hemidiaphragm providing an excellent opportunity to obtain cross sections of the LV in its length as well as apical views. The transgastric LV short axis midpapillary view is a standard in monitoring and evaluating LV contractility as it contains walls supplied by the LAD, RCA and circumflex arteries. The LV end diastolic area (EDA) and end systolic area (ESA) can be traced by planimetry and provide a rough estimate of ventricular size and volume.
Critical Care Echocardiography.
Evaluation by transthoracic echo is limited by body habitus, available windows and lung inflation during mechanical ventilation obstructing ultrasound visualization. The excellent quality images obtained with TEE make it a reliable tool in assessing cardiac performance in the critically ill. There are some contraindications to this technique however, these include esophageal trauma, varices, tumours or recent surgery. Furthermore, some patients with chronic heart disease may have grossly distorted anatomy precluding adequate visualisation of various structures. Patients with prosthetic valves may also be difficult due to ultrasound scatter by the prosthetic devices.
The following are some of the indications for performing TEE in the ICU:
The TEE has become an invaluable tool in the assessment of acute haemodynamic decompensation in the ICU. The most common findings include loss of myocardial contractility either hypokinesis or akinesis. This can readily be assessed in the transgastric short axis view. Another commonly observed finding is that of hypovolemia. It is not uncommon in ventilated critically ill patients to see PCWP which do not accurately reflect the volume status of the patient as ventricular compliance and ventilatory parameters may influence this pressure. The LVEDA and the LVESA can provide a rapid estimation of the patients volume status. Normal LVEDA (in the transgastric midpapillary short axis view) in adults range from 10 to 20cm2 (figure 1). As a general rule, an EDA of less than 14cm2 and/or an ESA of less than 8cm2 (figure 2) usually indicate hypovolemia. When the LV area is obliterated in systole (virtualisation of the ESA), there is usually severe hypovolemia. A thorough understanding of the patients underlying pathophysiology however is helpful. Patients with chronic congestive heart failure for example will have a chronically dilated LV and may demonstrate hypovolemia at higher EDAs. Patients with septic syndrome usually demonstrate a low ESA secondary to a very low systemic vascular resistance. This parameter becomes less useful in assessing volume status however, the combination of low ESA and EDA is more helpful. In cardiac surgical patients we have sometimes found a small LVEDA indicating an under filled LV in patients not responding to volume administration. A quick examination of the right side sometimes indicates RV failure. At other times, pulmonary hypertension precluded adequate filling of the left side. It is sometimes very difficult without the TEE to completely understand a patients pathophysiology.
Cardiac output determination can be accomplished by using Doppler echocardiography. In the transgastric view through the apex, pulsed waved Doppler can be performed at the LVOT. This yields a velocity contour of the ejected blood volume per stroke. Integrating this over time (velocity time integral: Vti) yields the distance a column of blood would have traveled (in cm) per ejection. Knowing the LVOT diameter, the surface area (pr2) can be calculated and multiplied by the Vti and the heart rate to yield a cardiac output. This method has limitations and possesses the same error range as thermodilution. Indeed, arrhythmias and variable stroke volumes will lead to erroneous calculations. Furthermore, an error in determining the LVOT diameter will result in significant errors.
Cardiac tamponade is readily assessed by TEE. Classic echo signs of tamponade include a fluid collection surrounding the heart, compression of cardiac chambers including diastolic collapse and septal shift into the LV. These classic signs of cardiac tamponade are frequently absent in the post cardiac surgery patient. Echo signs of cardiac tamponade can be very subtle or absent. Fluid collections are often absent. More frequently clots can be observed compressing the RA, the RV and occasionally the LA. These clots may be small however their haemodynamic consequence may be profound. The diagnosis of tamponade in this situation is very difficult. Any patient who requires an increasing amount of inotropes with rising filling pressures (PAP, PCWP and RAP) without a dramatic change in LV wall motion abnormalities despite having no echo signs of clots or fluid should be presumed to have tamponade. In the cardiac surgical patients with tamponade, the TEE is not sensitive.
In patients with trauma, the TEE is useful in diagnosing myocardial injury by identification of wall motion abnormalities. Traumatic valvular incompetence can also be readily assessed. Most important of all, the TEE provides a rapid and non invasive way of assessing traumatic aortic injury. Mediastinal haematoma, aortic intramural haematoma, pleural fluid, tamponade, dissection, transection and aneurysms can be identified. This technique is limited by the loss of imaging due to the left main stem bronchus of the distal ascending aorta and the aortic arch. Signs suggestive of aortic injury include: 1) an increased distance (>3-4mm) between the probe and the aorta in the chest (at least 2cm distal to the arch), 2) the double contour sign, where the aortic wall appears thicker with two bright edges, 3) mediastinal or pleural fluid collection, 4) valvular incompetence, 5) intimal flap, 6) loss of aortic continuity.
TEE is much more suited to the assessment of endocarditis and sources of thrombi than TTE. The proximity of the probe to the heart and the clear pictures obtained allow for a high degree of sensitivity. The left atrium is directly anterior to the probe as is the left atrial appendage. Thrombi can readily been seen. In the left atrial appendage, thrombus is suspected if there is loss of trabeculation. Furthermore, left atrial appendage velocities (in PWD) which are inferior to 20cm/sec suggest a predisposition to clot formation. Vegetations can be well visualised on the mitral and aortic valves. The tricuspid is less easily visualised and requires more skill whereas the pulmonary valve is anterior to the aortic valve and is in most cases very difficult to image.
Valvular assessment for incompetence or stenosis requires a high degree of skill and a thorough knowledge of Doppler echo. Colour Doppler is used to assess incompetence and readily demonstrates jet size and direction. Pulsed wave and continuous wave Doppler are used to assess valvular areas utilising the continuity equation or PISA. Planimetry may also be used. Most methods suffer from various limitations. A thorough discussion on valvular area and incompetence using Doppler is beyond the scope of this review.
Advanced Doppler of the mitral valve and pulmonary veins have been used to assess diastolic function in the LV as well as estimating left ventricular pressures. Pulmonary vein S and D wave velocity ratios (S/D) (figure 3) have been used to estimate LVEDP. As the LVEDP rises, the S/D ratio decreases. Limitations include ventilatory variations and alterations in ratio with heart rate and age. Mitral valve Doppler E and A wave velocities (E/A) have been used to assess diastolic function and in combination with the S/D ratio to determine preload, can give a good estimate of disease progression and severity. The mitral E wave deceleration time has also been used as an estimate of LVEDP. Most of these methods have been validated in stable cardiac surgical patients and in ambulatory patients in the catheterisation lab. In the critical care setting however, we have found no correlation whatsoever. These parameters do not predict preload. This is likely due to the profound haemodynamic disturbances seen in these patients.
Intra operative Echocardiography
TEE has become a commonly used tool in the cardiac and vascular operating rooms. Because of the real time visualisation of the LV in its short axis view, it has become an invaluable monitor of contractility / ischaemia and LV preload. In the transgastric short axis view, all coronary distributions are represented. Acute development of akinesis in one of the segments of a coronary distribution indicates ischaemia. The appearance of a wall motion abnormality usually precedes changes seen on invasive monitoring and the EKG. Tracing the endocardium (planimetry) in diastole (end diastolic area) provides a good indication of LV filling. The same can be done in systole yielding an estimate of ejection fraction {(EDA-ESA)/EDA}. The smaller the EDA, the smaller the preload becomes. Furthermore, kissing of the papillary muscles or when they touch each other in systole indicates hypovolemia and obliteration (virtualisation) of the end systolic cavity indicates severe hypovolemia. Acute dilatation of the LV usually indicates decompensation (e.g.: during aortic cross clamping for AAA surgery).
Other more advanced uses for TEE in the operating room include assessment of valvular replacement or more importantly repair. Assessing the degree of valvular incompetence or stenosis and its influences on other cardiac structures and function can easily be performed keeping in mind that haemodynamics play a large role in altering the apparent severity of the disease (e.g.: mitral regurgitation will be worse with higher systolic blood pressures). Morphological changes need to be assessed and are indispensable in directing repairs. An extensive knowledge of Doppler and function is required to perform this.
