Terminology of cardiac function

The following terms are essential to understanding the work done by the heart, and for cardiac response to exercise: cardiac cycle, stroke volume, ejection fraction, and cardiac output(Q).

Cardiac cycle

The cardiac cycle includes all the mechanical and electrical events that occur during one heartbeat. In mechanical terms, it consists of all heart chambers undergoing a relaxation phase(diastole) and a contraction phase(systole). During diastole, the chambers fill with blood. During systole, the ventricles contract and expel blood into the aorta and pulmonary arteries. The diastolic phase is approximately twice as long as the systolic phase. Consider an individual with a heart rate of 74 beats/min. At this heart rate, the entire cardiac cycle takes 0.81s to complete(60s divided by 74 beats). Of the total cardiac cycle at this rate, diastole accounts for 0.50s, or 62% of the cycle, and systole accounts for 0.31s, or 38%. As the heart rate increases, these time intervals shorten proportionately.

Refer to the normal ECG. One cardiac cycle spans  the time between one systole and the next. Ventricular contraction(systole) begins during the QRS complex and ends in T-wave. Ventricular relaxation(diastole) occurs during the T wave and continues until the next contraction. Although the heart is continually working, it spends slightly more time in the diastole(~2/3 of the cardiac cycle) than in systole(~1/3 of the cardiac cycle).

The pressure inside the heart chambers rises and falls during each cardiac cycle. When the atria are relaxed, blood from the venous circulation fills the atria. About 70% of the blood filling the atria during this time passively flows directly through the mitral and tricuspid valves into the ventricles. When the atria contract, the atria push the remaining 30% of their volume into the ventricles.

During ventricular diastole, the pressure inside the ventricles is low, allowing the ventricles to passively fill with blood. After atrial contraction, pressure inside the ventricles increases slightly due to the increase in blood volume delivered from the atria. As the ventricles contract, pressure inside the ventricles rises sharply. This increase in ventricular pressure forces the atrioventricular valves(i.e. tricuspid and mitral valves) closed, preventing any backflow of blood from the ventricles to the atria. The closing of the atrioventricular valves results in the first heart sound. Furthermore, when ventricular pressure exceeds the pressure in the pulmonary artery and the aorta, the pulmonary and aortic valves open, allowing blood to flow into the pulmonary and systemic circulations, respectively. Following ventricular contraction, pressure inside the ventricles falls and the pulmonary and aortic valves close. The closing of these valves corresponds to the second heart sound.

The interactions of various events of the heart are illustrated in the picture below, called a Wiggers diagram after the physiologist who created it. The diagram integrates information from the electrical conduction signals(ECG), heart sounds from the heart valves, pressure changes within the heart chambers, and left ventricular volume.

Stroke volume

During systole, most, but not all, of the blood in the ventricles is ejected. This amount is the stroke volume(SV) of the heart – the volume of blood pumped per beat(contraction). This is depicted below in the part a of the figure. To understand stroke cycle, consider the amount of blood in the ventricle before and after contraction. At the end of diastole, just before contraction, the ventricle has completed filling. The volume of blood it now contains is called the end-diastolic volume(EDV). At rest in a normally active adult, this value is approximately 100ml. At the end of systole, just after contraction, the ventricle has completed its ejection phase, but not all the blood is pumped out of the heart. The volume of blood remaining in the ventricle is called the end-systolic volume(ESV) and is approximately 40ml under resting conditions. Stroke volume is the volume of blood that was ejected and is merely the difference between the volume originally there and the amount remaining in the ventricle after contraction. So stroke volume is simply the difference between EDV and ESV; that is, SV = EDV – ESV ( example: SV = 100ml – 40ml = 60ml).

Ejection fraction

The fraction of the blood pumped out of the left ventricle in relation to the amount of blood that was in the ventricle before contraction is called the ejection fraction(EF). We determine this value, as seen in the figure b, by dividing stroke volume by EDV( 60ml/100ml = 60%). The EF, generally expressed as a percentage averages about 60% at rest. Thus, 60% of the blood in the ventricle at the end of diastole is ejected with the next contraction, and 40% remains in the ventricle. Ejection fraction is often used clinically as an index of the pumping ability of the heart.

Cardiac output

Cardiac output(Q), as shown in the figure c, is the total volume of blood pumped by the ventricle per minute, or simply the product of HR and the SV. The SV at rest in the standing posture averages between 60 and 80 ml of blood in most adults. Thus, at an RHR of 70 beats/min, the resting cardiac output will vary between 4.2 and 5.6 L/min. The average adult body contains about 5L of blood, so this means that the equivalent of our total blood volume is pumped through our hearts about once every minute.