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The normal heart is composed of two ventricles that circulate blood through the pulmonary and systemic capillary beds in series, providing a means of delivering oxygen and nutrients to the tissues and removing carbon dioxide and metabolic waste. Deoxygenated blood is received in the right atrium and pumped from the right ventricle through the pulmonary capillary bed, where carbon dioxide is exchanged for oxygen in the alveoli. Pulmonary venous return to the left atrium is circulated through the systemic capillary bed by the left ventricle (Figure 1-1).
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The term “cardiac cycle” refers to the events that occur in 1 heartbeat. By convention, the cycle is said to begin with the onset of systole. Notably, the events in the right and left heart occur in parallel, in the same sequence and at roughly the same time, although at different pressures. These events are tied to the electrical activities of the heart in a process described as electromechanical coupling. Figure 1-2 depicts the events of the cardiac cycle, including the surface electrocardiogram (ECG) and intracardiac pressures and volumes.
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In diastole, the ventricles fill with blood, which occurs in two phases. In the first phase, the ventricle fills passively as the ventricular myocardium relaxes and blood is drawn into the ventricles through the atrioventricular (AV) valves as the pressure in the ventricle drops below that of the atria. Next, depolarization of the sinus node, located in the superior portion of the right atrium, initiates atrial contraction. On the surface ECG, atrial systole inscribes a P wave. As the atria contract, additional blood is pumped into the ventricles—the second phase of diastole. Meanwhile, the wave of depolarization travels along the atrial wall toward the ventricles. It ultimately reaches the AV node near the medial aspect of the tricuspid valve. The AV node funnels the impulse into the bundle of His, which rapidly conducts the impulse into the ventricles.
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On the surface ECG, ventricular depolarization results in the inscription of the QRS complex. As the ventricles start to contract, the ventricular pressure begins to rise, and the AV valves close when the pressure in the ventricle exceeds that of the atria. A period called isovolumetric contraction follows, as the ventricular pressure increases without a change in volume, as both the AV valves and the semilunar valves are closed. As the pressure in the ventricles increases, it soon exceeds ...