PHYSIOLOGY OF THE INTACT HEART

To understand the mechanical performance of the intact heart, it is important to have knowledge of the phases of the cardiac cycle and determinants of ventricular function.

Cardiac Cycle

The cardiac cycle is the sequence of electrical and mechanical events during the course of a single heartbeat. Figure 18-1 illustrates (1) the electrical events of a single cardiac cycle represented by the electrocardiogram (ECG) and (2) the mechanical events of a single cardiac cycle represented by left atrial and left ventricular pressure pulses correlated in time with aortic flow and ventricular volume.^[1]

The cardiac cycle begins with initiation of the heartbeat. Intrinsic to the specialized cardiac pacemaker tissues is automaticity and rhythmicity. The sinoatrial (SA) node is usually the pacemaker; it can generate impulses at the greatest frequency and is the natural pacemaker.

Electrical Events and the Electrocardiogram

Electrical events of the pacemaker and the specialized conduction system are represented by the ECG at the body surface (also see Chapter 34 ). It is the result of electrical potential differences generated by the heart at sites of the surface recording. The action potential initiated at the SA node is propagated to both atria by specialized conduction tissue, and it leads to atrial systole (contraction) and the P wave of the ECG. At the junction of the interatrial and interventricular septa, specialized atrial conduction tissue converges at the atrioventricular (AV) node, which is connected distally to the His bundle. The AV node is an area of relatively slow conduction, and a delay between atrial and ventricular contraction normally occurs at this locus. The PR interval can be used to measure the delay between atrial and ventricular contraction at the level of the AV node. From the distal His bundle, an electrical impulse is propagated through large left and right bundle branches and finally to the Purkinje system fibers, which are the smallest branches of the specialized conduction system. Finally, electrical signals are transmitted from the Purkinje system to the individual ventricular cardiomyocytes. The spread of depolarization to the ventricular myocardium is manifested as the QRS complex on the ECG. Depolarization is followed by ventricular repolarization and appearance of the T wave on the ECG.^[2]

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Figure 18-1 The electrical and mechanical events during a single cardiac cycle are depicted. Shown are pressure curves of aortic blood flow, ventricular volume, venous pulse, and the electrocardiogram. (From Berne RM, Levy MN: The cardiac pump. In Cardiovascular Physiology, 8th ed. St Louis, Mosby, 2001, pp 55–82.)

Mechanical Events

The mechanical events of a cardiac cycle begin with return of the blood to the right and left atria from the systemic and pulmonary circulation, respectively. As blood accumulates in the atria, atrial pressure increases until it exceeds the pressure within the ventricle, and the AV valve opens. Blood first flows passively into the ventricular chambers, and such flow accounts for approximately 75% of total ventricular filling.^[3] The remainder of the blood flow is by active atrial contraction or systole, known as the atrial "kick." The onset of atrial systole is coincident with depolarization of the sinus node and the P wave. While the ventricles fill, the AV valves are displaced upward and ventricular contraction (systole) begins with closure of the tricuspid and mitral valves, which corresponds to the end of the R wave on the ECG. The first part of ventricular systole is known as isovolumic or isometric contraction. The electrical impulse traverses the AV region and passes through the right and left bundle branches into the Purkinje fibers. This leads to contraction of ventricular myocardium and a progressive increase in intraventricular pressure. When intraventricular pressure exceeds pulmonary artery and aortic pressure, the pulmonic and aortic valves open and ventricular ejection occurs, which is the second part of ventricular systole.

Ventricular ejection can be further separated into the rapid ejection phase and the reduced ejection phase. During the rapid ejection phase, forward flow is maximal, and pulmonary artery and aortic pressure is maximally developed. In the reduced ejection phase, flow and great artery pressure taper with progression of systole. Pressure in both ventricular chambers falls as blood is ejected from the heart, and ventricular diastole begins with closure of the pulmonic and aortic valves. The initial period of ventricular diastole consists of the isovolumic/isometric relaxation phase. This phase is concomitant with repolarization of the ventricular myocardium and corresponds to the end of the T wave on the ECG. The final portion of ventricular diastole involves a rapid decrease in intraventricular pressure until it falls below that of the right and left atria, at which point the AV valve reopens, ventricular filling occurs, and the cycle repeats itself.