Echocardiography provides bedside and immediate insight in the post-operative ICU patient, whenever hemodynamic deterioration occurs. Both morphological and hemodynamic features can be diagnosed instantly and related to clinical practice. Furthermore, this tool is used as a functional hemodynamic monitoring device offering on line information on systolic function, preload and afterload of both the left and right sides of the heart.This review will provide an overview of the data available with respect to improving outcome utilising this particular technology.
by Dr Jan Poelaert
Doppler-echocardiography is now generally accepted as an invaluable tool to assess cardiac compromised patients. All morphological and functional aspects of the cardiac chambers, including valves and the respective connective tissue and major vessels, can be evaluated in a physiological approach. Furthermore, hemodynamic monitoring, revealing ventricular function, insufficient preload or excessive afterloading conditions [1], may often be fine-tuned utilising Doppler echocardiography [2]. In mechanically ventilated ICU patients, the non-invasive transthoracic mode is often the preferred technique of choice.
Nevertheless, the transoesophageal approach is ideal, with much better visualisation possibilities. Although the transoesophageal approach is somewhat more invasive, it remains a safe technique. Recent, still unpublished, guidelines from the European Society of Intensive Care Medicine strongly suggest only to use this technique with an advanced level of training in echocardiography.
Modern medicine is driven by endpoints and goals. Most often the outcome and length of stay in the ICU and in the hospital are part of these. A summary of the potential of Doppler-echocardiography to improve the outcome of critically ill patients is provided in this review.
From a morphological point of view, post-operative Doppler-echocardiography is particularly useful in those patients with hemodynamic instability, high PEEP-ventilation, necessity of high doses of inotopic or vasopressor drugs, or any situation where inadequate perfusion is present, even with normal cardiac output. One look permits the evaluation of the function of both the left and right ventricles by combining different views. Furthermore, tissue Doppler imaging allows the assessment of regional or more global evaluation of the function of these chambers [3,4]. The different tools of Doppler-echocardiography provide insight into functional hemodynamics, in conjunction with other more invasively obtained data, on the condition that these data are integrated and interpreted by the human brain using a physiological approach. In this way, it is perfectly possible to get information on systolic and diastolic function of the ventricles, the actual preloading conditions and even the afterload.
Echo-Doppler diagnostic tool
The unique combination of several echo-Doppler tools facilitates the correct interpretation of flows within a selected zone. These tools comprise:
- two-dimensional imaging, offering insight into morphology and function;
- colour Doppler, exemplifying scattering and directions of flows, within the selected area;
- Doppler, providing information on direction, intensity and duration of flows. In addition the morphology of the Doppler pattern itself can provide indications of the pathology present;
- myocardial Doppler imaging, demonstrating the relative motion, direction and intensity of the investigated myocardial wall segment. In particular, this technique is useful when analysing systolic and diastolic function of the left ventricle. Care should be taken that both systolic and diastolic characteristic Doppler waves are preload dependent.
The difficulty with Doppler-echocardiography concerns both acquiring the different images in conjunction with the correct interpretation: both practical issues and knowledge must be combined to come to a correct evaluation [5]. Hence, correct interpretation can only be achieved when the physiological meaning is fully understood, applied and integrated with other data. This necessitates a prolonged learning curve, although the user need not be acquainted with all facets to permit a practical approach [6,7].
Immediate bedside hemodynamic information
Performing a complete echocardiogram offers a full picture of the heart as cardiac muscle and the circulation pump. As with each other (invasive) hemodynamic monitoring tools, all tricks and flaws must be recognised to permit a comprehensive hemodynamic evaluation of a hemodynamically unstable patient.
In a hypotensive patient, a quick investigation of cardiac function at the level of the short axis view permits differentiation between a cardiac and a non-cardiac cause of hypotension [1]. A small left ventricle suggests hypovolemia [8] or a ventricle loaded with a high sympathomimetic intrinsic or extrinsic load [9,10]. In contrast, a dilated, barely contracting left ventricle needs inotropic support. Therefore, this initial short axis view and the correct interpretation has a direct impact on bedside management and hence outcome.
The myocardial performance index (MPI) is a variable of both systolic and diastolic function [11], which is, however, load dependent. The following formula allows calculation of this index:
MPI = (ICT + IRT)/ET
(ET, ejection time measured at the Doppler signal of the aortic flow; ICT, isovolumic contraction time; IRT, isovolumic relaxation time).
Although this index has a prognostic power in cardiology practice, this index adds little information on the ICU patient in view of the load dependency. This was shown both in an animal experimental [12] and in a clinical [13] setting.
Another, more useful load-dependent variable is the systolic velocity of the mitral annulus, assessed with tissue Doppler imaging. Velocities lower than 8 cm/s suggest decreased systolic function whereas velocities above 15 cm/s imply normal left ventricular systolic function. Both preload [4] and afterload [14] appear to have impact on the amplitude.
Rapid diagnosis of LV failure permits immediate intervention, which is, indirectly, related to improved outcome [15,16].
Right ventricle
A similar differentiation can be made with respect to the right ventricle. A normal right ventricle is depicted as a crescent-shaped structure. A dilated right ventricle (i.e. right ventricular diameter > 0,6 diameter of the left ventricle) suggests either right ventricular myocardial ischemia, or volume and/or pressure overload [1], with typical management approaches.
Assessment of flow across cardiac valves reveals transvalvular pressure gradients. Typically, a pressure gradient can be assessed from a tricuspid regurgitant flow in order to calculate right ventricular systolic pressure (RVSP) correctly if right atrial pressure can be estimated [17,18].
The knowledge of the presence of a dilated right ventricle in conjunction with increase RVSP may also be important in the direct management of ventilator settings [19], optimisation of preload [20], or reduction of afterloading conditions [21-24], with indirect impact on outcome.
Myocardial ischemia
The direct visualisation of the relative motion of the different wall segments provides an ideal window for detection of myocardial ischemia, on the condition that no other interfering factors occur and the regional wall motion abnormality is detected after previous normal motion of the segment in question. These two conditions suggest the difficulties which can be encountered when trying to detect myocardial ischemia with Doppler-echocardiography. Conversely, Doppler echocardiography is a perfect tool to confirm the localisation of an occluded coronary artery with respect to a malperfused myocardial region after a positive ECG or ST segment monitoring, which alerted the clinician. Newer technologies are currently being developed utilising vector-related technology to allow early diagnosis of myocardial ischemia.
Preload and fluid responsiveness
Preload is the first issue to be assessed whenever hypotension has to be managed, and has been related to improved outcome [25,26]. Clinically, the legs-up test is preferred for evaluating optimal preloading conditions: it does no harm and provides immediate information about the filling status.
From a short axis view, the left ventricular end-diastolic area (LVEDA) < 5.5 cm² was shown to be associated with low preloading conditions. Although a purely static variable of load, the legs-up test brings this LVEDA as a truely dynamic descriptor of fluid responsiveness.
Other variables are used in conjunction with mechanical ventilation and they rely on the variation of intra-thoracic pressures with ventilation. Both inferior [27] and superior vena cava [28] variations with ventilation can be utilised. Care should be taken that these variables only provide insight into right ventricular preload. Acute right ventricular failure in conjunction with a hyperdynamic left ventricle will be associated with an absence of ventilation-induced variation of the diameter. Commencing the echocardiographic investigation with the short axis view will already eliminate right ventricular dilatation. Variation of stroke volume exemplified by variations of the time-velocity-integral (TVI) will provide the same information [29], [Figure 1].
Conclusions
Doppler echocardiography provides immediate insight into the morphological and hemodynamic functional aspects of cardiac and circulation-related issues. The most important advantage is that appropriate use leads to direct action depending on the findings, even with a limited number of views [30]. The adage ‘do not harm your patient’ can be followed by introducing the TTE tool in conjunction with a really knowledgeable echocardiographer-intensivist. ICU clinicians responsible for the daily management of hemodynamically unstable patients should be convinced to utilise echocardiography as primary diagnostic and monitoring tool.
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The author
Jan Poelaert, MD, PhD
Department of Anesthesiology and Perioperative Medicine
Acute and Chronic Paintherapy
UZ Brussel, VUB
Laarbeeklaan 101
1090 Brussels
Belgium
