Cardiovascular and Pulmonary Physiology
This chapter reviews basic cardiovascular and pulmonary physiology. A thorough understanding of normal physiology provides a basis for understanding the deficits in cardiovascular and pulmonary function (in the context of limitation of activity and social participation, as well as structure and function) and the adaptations to changing physiological and pathophysiological demands. This knowledge provides the foundation for conducting a thorough assessment and prescribing treatment.1–8
Cardiovascular Physiology
Control of the Heart
The mechanical activity of the heart is precisely regulated in accordance with the electrical activity of the heart to effect optimal cardiac output to the organs of the body.9,10 The electrical activity of the heart, based on electrocardiography, in both health and pathology, is described in detail in Chapter 12. The electrical and mechanical events of the cardiac cycle and their coupling (electromechanical coupling) are summarized in Figure 4-1. These events include the spread of the wave of electrical excitation throughout the myocardium; the resulting sequence of contraction of the atria and ventricles, followed by dynamic changes in blood pressure and volume in the heart chambers; the heart sounds; and the timing of these events. The cardiac cycle takes 0.8 second in a heart beating at 75 beats per minute. Ventricular systole or ejection takes about one-third of this time. Its onset and termination are marked, respectively, by the closing and opening of the atrioventricular valves (mitral and tricuspid). Diastole, or the period between successive ventricular systoles, in which the ventricles fill with blood, takes two-thirds of the 0.8 second of each cardiac cycle.
Volume and Pressure Changes
Changes in the ventricular volume curve and aortic pressure wave reflect changes in atrial and ventricular pressures during systole and diastole. The sequence of events appears in a flow chart in Figure 4-2. Pressure gradients within the heart are responsible for the opening and closing of the valves. Coordinated valve opening and closure are important to promote the forward movement of blood and prevent mechanical inefficiency of the heart pump resulting from valvular regurgitation of blood during ventricular contraction. Regurgitation of blood in the retrograde direction gives rise to heart murmurs that are audible on auscultation of the heart.
Peripheral Circulation
The purpose of the peripheral circulation, including the microcirculation at the tissue level, is to provide saturated oxygenated blood and remove partially desaturated blood. The microcirculation within each organ regulates the blood flow both exogenously, via the neurological system, and endogenously, via the humoral system, commensurate with the metabolic needs of that tissue bed (see Chapter 3). The four principal factors that determine the movement of fluid in the microcirculation are the following:
1. The capillary hydrostatic pressure from the blood pressure, which tends to move blood across the capillary membrane and out of the circulation into the interstitium
2. The capillary oncotic pressure from the proteins within the blood vessels, which tends to retain fluid in the circulation
3. The interstitial hydrostatic pressure, which tends to move fluid back into the circulation
4. The interstitial oncotic pressure, which tends to draw fluid out of the circulation and into the interstitium
The net forces acting on the capillary fluid are nearly in equilibrium, with a slight tendency for fluid to be filtered out of the systemic circulation into the interstitium. Box 4-1 illustrates the mean pressures that determine normal fluid dynamics across capillary membranes.
Transport of Oxygen
Once oxygen reaches the blood, it rapidly combines with hemoglobin to form oxyhemoglobin. A small proportion of oxygen is dissolved in the plasma. The use of the hemoglobin molecule as an oxygen carrier allows for greater availability and efficiency of oxygen delivery to the tissues in response to metabolic demand. Saturation of the oxygen-carrying sites on the hemoglobin molecule is curvilinearly related to the partial pressure of oxygen in the tissues. This relationship is called the oxyhemoglobin dissociation curve and is a sigmoid, or S-shaped, curve (Fig. 4-3). The hemoglobin of arterial blood is 99%, or almost completely saturated with oxygen. Under normal circumstances, arterial blood is mixed with a small proportion of venous blood from the coronary and pulmonary circulation, resulting in arterial saturation slightly less than 100%. The graph shows a range of partial pressures of oxygen that may exist in the tissues. At relatively high arterial oxygen pressures, the oxygen saturation is high. This reflects high association or low dissociation between oxygen and hemoglobin. Saturation does not fall significantly until the partial pressure of oxygen falls below 80 mm Hg. Even at PO2 levels of 40 to 50 mm Hg, arterial saturation is still 75%. This suggests that the oxyhemoglobin dissociation system has an enormous capacity to meet the varying needs of different tissues without severely compromising arterial saturation. A PO2 of less than 50%, for example, has a profound effect on arterial saturation. This demonstrates an adaptive response of hemoglobin dissociation to respond to low oxygen tissue pressures by greater dissociation of oxygen from hemoglobin as the need arises. As PO2 improves with increased supply of oxygen or decreased demand, the affinity between oxygen and hemoglobin increases, and arterial saturation increases. Thus oxygen is not released unless there is a need for greater oxygen delivery to the tissues.
Various conditions can increase or decrease hemoglobin’s affinity for oxygen and thereby cause a shift in the oxyhemoglobin dissociation curve (see Fig. 4-3). A shift to the right results in decreased oxygen affinity and greater dissociation of oxygen and hemoglobin. In this instance, for any given partial pressure of oxygen, there is a lower saturation than normal. This means that there is more oxygen available to the tissues. Shifts in the curve to the right occur with increasing concentration of hydrogen ions (i.e., decreasing pH), increasing PCO2, increasing temperatures, and increasing levels of 2,3-DPG (diphosphoglycerate), a byproduct of red blood cell metabolism. West suggests that “a simple way to remember these shifts is that an exercising muscle (increased metabolic demand), is acid, hypercapnic and hot, and it benefits from increased unloading of oxygen from its capillaries.”8
Cardiopulmonary Physiology
Ventilation
Ventilation is the process by which air moves into the lungs. The volume of air inhaled can be measured with a spirometer. The various lung capacities and volumes are defined in Chapter 9.