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Ann Thorac Surg 1995;60:1166-1168
© 1995 The Society of Thoracic Surgeons
Division of Thoracic Surgery, Department of Surgery, University of Iowa Hospitals and Clinics, Iowa City, Iowa
Left ventricular function is one of the major determinants of mortality and morbidity in patients undergoing heart operations and, consequently, a matter of considerable concern to surgeons assessing patients for operation. A recent review, for example, included 134 references dealing with coronary bypass grafting in patients with moderate or severe left ventricular dysfunction [1].
In current practice the ejection fraction is used to assess ventricular function. Values of 0.20 or less are taken as evidence of severe left ventricular dysfunction and may be a reason to recommend against operation. Yet, who among us has not taken such a patient to the operating room only to find a low pulmonary artery pressure and normal cardiac output upon insertion of a pulmonary artery catheter and to then have the patient survive the procedure with little inotropic support required?
The current popularity of the ejection fraction as an index of ventricular contractility is due to its relative ease of measurement by ventricular cineangiography, isotope imaging, or echocardiography. The calculations involve a number of mathematic assumptions regarding such matters as ventricular geometry and can be criticized on that account, especially when applied to the right ventricle or to the diseased left ventricle with asymmetric contraction. Of more concern, however, is the frequent failure to consider the possible influence of a number of physiologic factors that are well known to affect the ejection fraction as a measure of myocardial contractility.
The ability of the heart to function as a pump depends on a complex interrelationship between the heart rate, preload, ventricular compliance, and afterload in addition to myocardial contractility itself. All have important effects on myocardial performance and on the ``contractility indices'' devised to assess it. The ventricular ejection fraction is, unfortunately, no exception.
Physiologists have long appreciated that the extent of shortening of cardiac muscle depends as much on its preload, afterload, and frequency of contraction as on its contractile state [2]. For example, in our laboratory [3] using an isolated right ventricular ``Starling'' preparation in which heart rate, afterload, and preload were all controlled (Fig 1
), we could vary the ejection fraction over a wide range simply by varying the preload (Fig 2). Obviously, the contractile state of the myocardium remained unchanged in this acute experimental model. In an even more basic preparation, we could vary the percent shortening of an isolated papillary muscle (equivalent to the ejection fraction) simply by changing its loading conditions (Fig 3).
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Such observations do not invalidate the ejection fraction as a contractility index. They merely demonstrate that it is neither better nor worse than other contractility indices in terms of the influence of other factors. Judgment is therefore required in evaluating the meaning of an ejection fraction measurement in a given patient. Was the patient anxious and hypertensive when it was made? Was he or she in congestive heart failure or having a tachyarrhythmia? Is there a history of congestive heart failure, which would serve as corroborative evidence of chronic ventricular dysfunction? Is angina the major symptom, indicating the possibility that reversible ischemia may be causing ventricular dysfunction? Or was the measurement made shortly after an infarction when an element of myocardial stunning might still be present?
There are some situations, such as when the patient has a ventricular septal defect or mitral valve regurgitation, when the left ventricular afterload cannot be determined because part of the stroke volume escapes into the right ventricle or left atrium. In those instances the investigator should admit that the ejection fraction cannot be used to assess left ventricular function. Comparison of the ejection fraction measured preoperatively with that measured after coronary bypass is another instance where caution in interpretation is warranted. Often the patient is receiving drugs such as propranolol that affect contractility preoperatively but not postoperatively or the patient's heart rate or arterial pressure is different in the two situations.
A review of the numerous articles published in which the ejection fraction has been used to assess the results of operation reveals, unfortunately, that often little attention is given to these matters. For example, radionuclide angiography has been used to assess right ventricular function before and after mitral valve replacement [8], to assess left ventricular function before and after coronary bypass in patients requiring counterpulsation [9], to predict surgical mortality after myocardial revascularization [10], and to compare the results of coronary angioplasty and bypass grafting [11]. In none of these studies were hemodynamic correlates reported, yet the uncritical reader might infer something about myocardial contractility when, in fact, the reported changes in ventricular function might have resulted from changes in some other variable, such as the reduction in right ventricular afterload that certainly occurred in patients having mitral valve replacement.
Certainly it is difficult if not impossible to control all of the relevant variables in clinical situations, and one often must use less than ideal data in making clinical judgments. In such instances the possible influences of these uncontrolled variables must at least be reported and taken into account. Within limits, ejection fraction should increase with increases in preload and decrease with increases in afterload or heart rate. For example, a patient with a low ejection fraction and severe mitral regurgitation is likely to respond poorly to mitral valve replacement as the ``pop-off valve'' is closed, whereas a patient with the same ejection fraction and severe aortic stenosis may do very well after lowering of the afterload by aortic valve replacement.
In conclusion, because the ejection fraction is so commonly used in making clinically important decisions regarding the operability of individual patients or the merit of specific operative techniques, surgeons are urged to familiarize themselves with the shortcomings and assumptions inherent in the method and to interpret these measurements with appropriate skepticism.
Footnotes
Address reprint requests to Dr Behrendt, Division of Cardiothoracic Surgery, Department of Surgery, University of Iowa Hospitals and Clinics, Iowa City, IA 52242.
References
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L/Li) of cat papillary muscle to preload, expressed as initial length, at three different afterloads. Muscle lengths were normalized by expressing them as (Li - LN)/LN, where Li is initial length and LN is the length observed at a stress of 0.5 g/mm2. The marked dependence of percentage shortening on both initial length and afterload is evident.