The Heart - Biofluidmechanics: An Introduction to Fluid Mechanics Macrocirculation and Microcirculation - page 107

depolarization. The atrial depolarization wave is a small wave, and it is hidden by the
much larger QRS complex that is occurring at the same time.
The normal ECG is characterized by the length of time of each wave, the time between
eac h wave (typically called the interval, e.g., the P-Q interval or the Q-T interval), and the
voltage associated with each wave. Many pathological heart conditions can be diagnosed
by changes in the ECG signal. For instance, an increased voltage associated with the QRS
complex can be attributed to an increased muscle mass. Decreased QRS voltage is typically
associated with a decrease in muscle mass potentially because of infarctions present within
the muscle mass. If the conduction system is blocked or damaged, then it is typical to see a
change in the onset time or the length associated with the QRS complex. Under these abnor-
mal conditions, the QRS complex can elongate to approximately 0.15 second or more. It is
also typical to see an inverted T wave when the QRS complex is significantly elongated.
4.3 THE CARDIAC CYCLE
The cardiac cycle describes all of the events that occur during one heartbeat and during
the latent time until the next heartbeat. It makes the most sense to describe these events
starting from the initiation of an action potential within the SA node (see Section 4.2 ). The
cardiac cycle consist of two phases: diastole and systole. Cardiac myocytes do not contract
during diastole, and this is when the heart chambers fill with blood. During systole, the
myocytes (particularly those within the ventricles) contract and eject blood from the heart.
To depict the cardiac cycle, the aortic pressure, the left ventricular pressure, the left atrium
pressure, the left ventricular volume, and the electrocardiogram are overlaid onto one
figure that is plotted against time ( Figure 4.7 , a similar figure can be drawn for the right side
of the heart, with lower pressures). Recall that the ECG P wave is associated with atrial depo-
larization. During this time, the AV valves are open and the atria eject blood into the ventri-
cles. During atrial contraction, the ventricles fill with blood, and therefore, there is an
increase in ventricular volume. Upon the onset of the QRS complex, there is a rapid increase
in ventricular pressure due to ventricle contraction. This is associated with the closing of the
AV valves and isovolumic contraction of the ventricles. As the QRS complex ends, the semi-
lunar valves open (due to the ventricular pressure surpassing the particular blood vessel
pressure) and the ventricles eject blood into the pulmonary and systemic circulation. The
duration of blood ejection is termed systole. The volume of blood in the ventricles reduces
from approximately 120 mL to approximately 45 mL, which is termed the residual ventricu-
lar volume. During systole, the T wave is recorded, and this is when the ventricles begin to
relax. At this time, the vascular pressure (aorta and pulmonary arteries) is still lower than
the ventricular pressure, so that blood continues to be ejected out of the heart for a few milli-
seconds. Toward the end of the T wave, the semilunar valves close and the ventricles enter
the isovolumic relaxation phase, which marks the beginning of diastole. After a few millise-
conds, the pressure in the ventricles returns to approximately 1 mmHg and the AV valves
open again. At this point, diastole continues until the AV valves close once again. During the
entire period of diastole, even though the atria are not contracting, both ventricles are filling
with blood. In fact, approximately 75% of the blood that enters the atria passes directly into
the ventricles without the aid of atrial contraction. During atrial contraction, the remaining