Cardiac Physiology

semilunar valves close, starting the isovolumic phase of diastole. A third sound called S3 is

rarely heard. It is the sound of the blood in the atria, the sound of blood sloshing in the

ventricles, or the tensing of the chordae tendineae. It may be normal in young people but, in

older people, it represents the possibility of congestive heart failure. An S4 sound stems from

contraction of the atria into a stiff ventricle and is abnormal. What is occasionally referred to as

an S7 is the combination of an S3 and S4 in the same person.

A heart murmur is the sound of turbulent blood flow. It can be graded on a scale of 1 through

6, with 6 being the loudest murmur. Certain maneuvers like the Valsalva (bearing down)

maneuver and deep breathing can change the quality of the murmur and can help identify the

source of the murmur. Murmurs can be diastolic or systolic in nature and can represent

narrowed flow through a valve or the backflow through a leaky valve.

CARDIAC PHYSIOLOGY

The heart beats at a regular pace because of its autorhythmicity. There are some important

aspects of cardiac function that you should know about. For example, the cardiac output or CO

of the heart is the stroke volume times the heart rate in contractions per minute or beats per

minute (bpm). The stroke volume is the end diastolic volume minus the end systolic volume and

is about 70 milliliters. The range in cardiac output is about 4-8 liters per minute or about 5.25

liters per minute on average. There are things that affect this number.

Factors affecting heart rate include hormones, autonomic input, fitness levels, and age. Factors

affecting the stroke volume include heart size, fitness levels, preload, afterload, gender,

duration of contraction, and contractility of the heart muscle. The ejection fraction is another

important measurement in the heart function. This number, the EF, is the SV divided by the end

diastolic volume. The average EF is about 58 percent, with a range of 55-70 percent. The

cardiac reserve is the difference between the maximum cardiac output and the resting cardiac

output and is a measure of the reserve or potential of the heart to pump blood during exercise.