How well is the heart performing its job of pumping blood? (Ventricular function is the term cardiologists use to describe the pumping efficiency of the heart.) A cardiologist must be able to answer this question before deciding whether cardiac surgery will be safe or helpful. For example, coronary artery surgery may prolong life in patients with three-vessel disease and mild depression of ventricular function. If ventricular function is normal, bypass surgery will not improve survival; if it is severely depressed, bypass surgery will be too dangerous. In a disease such as myocardiopathy, measurement of ventricular function is often the only way a cardiologist has of knowing that disease is in fact present and how serious it is.
After many false starts and a great deal of research, it is now possible to measure ventricular function very accurately.
The heart pumps about 6 liters of blood a minute. This volume is called cardiac output. Cardiac output is simple to measure, but it isn’t a good index of ventricular function. A patient with a normal cardiac output can still die of heart failure.
Stroke volume is the amount of blood pumped out of the heart with each beat and is easy to calculate using simple arithmetic. If the heart is pumping 6,000 cc of blood per minute and beating 80 times a minute, the stroke volume will be 75 cc. This is a step further but it still doesn’t give an accurate idea of cardiac function.
After many false starts, a reliable measurement of ventricular function has emerged. This measurement is called the ejection fraction.
Everyone involved in the care of cardiac patients must know what the ejection fraction is, what the normal values are, and how these values are used (Fig. 23-1
The ventricles pump out only part of the blood they contain with each beat. Obviously, they can’t eject all the blood since that would leave a vacuum. If there are 125 cc in the left ventricle at the end of diastolic filling, about 75 cc will be ejected by a healthy ventricle—in other words, 60%. Thus the ejection fraction is the amount of blood pumped out with each beat compared to the amount of blood that was in the
ventricle to start with. To put it another way, “What percentage of the blood in the ventricle gets pumped out with each beat?”
FIGURE 23-1 Measuring ejection fraction. Outline of the ventricle in systole and diastole.
|Normal ejection fraction
|Depressed ejection fraction
|Seriously depressed ejection fraction
||30% or below
|Critically depressed ejection fraction
||20% or below
The ejection fraction is the best predictor of survival in coronary artery disease. It is the most important yardstick of risk when surgery is being considered. It is, overall, the most reliable prognostic measurement in any type of heart disease.
It is a curious fact that as reliable as the ejection fraction is, it often doesn’t correlate with symptoms. Thus a patient with a very depressed ejection fraction may be able to perform surprisingly well on a treadmill or in real life. Even though the heart has been severely damaged, the vascular system can often adapt amazingly well for a time.
How the Ejection Fraction is Measured
The outline of the ventricle is obtained during a whole heart cycle, so that the total size at the end of diastolic filling can be compared with size at the end of systolic emptying. This can be done during cardiac catheterization (when dye is injected into the ventricle) by two-dimensional (2-D) echocardiography, or by radioisotope methods (the MUGA scan).
The maximum and minimum ventricular sizes are then overlaid and compared; the ejection fraction can be calculated by simple measurements. Experienced cardiologists can estimate the ejection fraction with acceptable accuracy by simply looking at the two outlines.
The ejection fraction can also be measured from the 2-D echocardiogram. The cardiologist observes the beating heart in several projections and estimates the ejection fraction. There are no exact formulas: the cardiologist must assess the force of contraction in the various parts of the left ventricle and try to average them into an overall ejection fraction. While there is considerable variation between observers with this method, it works surprisingly well and in most cases correlates satisfactorily with other methods.