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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



the shape and quantitative values of the cardiac out- 

 put curve. These are /) the effectiveness of the heart 

 as a pump, 2) the pressure against which the heart 

 must pump, and 3) factors that affect the pressure on 

 the outside of the heart. 



EFFECTIVENESS OF THE HEART AS A PUMP. When the 



effectiveness of the heart as a pump increases, the 

 cardiac output curve correspondingly increases, as 

 illustrated by the upper curve of figure 2. That is, 

 for any given right atrial pressure the cardiac output 

 is considerably greater from the "hypereffective" 

 heart than from the normal heart. There are only 

 three major causes of a hypereffective heart; these 

 are: /) stimulation of the heart by the sympathetic 

 nervous system, 2) inhibition of the parasympathetic 

 nerves to the heart, and 3) hypertrophy of the heart. 

 In each one of these instances the heart contracts with 

 increased force or increased rate and consequently 

 pumps more blood for any given input pressure than 

 does the normal heart. 



Figure 2 also illustrates the effect of a hypoeffective 

 heart on the cardiac output curve. That is, for any 

 given input right atrial pressure, the hypoeffective 

 heart pumps far less blood than does the normal heart. 

 Among the different factors that can cause a hypo- 

 effective heart are a) myocardial infarction, b) coro- 

 nary sclerosis, c) myocarditis, d) valvular heart 

 disease, e) congenital heart disease, /) cardiac ar- 

 rhythmias, g ) hypothermia, h) abnormal electrolytes 

 in the extracellular fluids, i) parasympathetic stimula- 

 tion, j) sympathetic inhibition, or any other factor 

 that reduces the ability of the heart to pump blood. 



Obviously, there can be all degrees of hypereffec- 

 tiveness or hypoeffectiveness of the heart. Therefore, 

 an infinite number of curves would be required to 

 depict the various degrees of pumping effectiveness of 

 the heart under all different conditions. Yet, at any- 

 given instant in the life of the heart, there is only one 

 of these curves which depicts its instantaneous 

 pumping effectiveness. 



ALTERATION OF THE LOAD AGAINST WHICH THE HEART 



must pump. It is quite obvious that the greater the 

 resistance against which the heart must pump blood, 

 the less effectively can it pump (171). Consequently, 

 increased resistive load in either the pulmonary or 

 systemic circulation decreases the cardiac output 

 curve, while decreased resistive load increases the 

 cardiac output curve. These effects are also illustrated 

 in figure 2. 



Obviously, cardiac load can vary through all differ- 



ent degrees at different times. Therefore, once again a 

 vast "family" of cardiac output curves is required to 

 depict the effects of varying loads on the function of 

 the heart-lung segment. This family of curves is 

 almost identical with that which depicts the func- 

 tional characteristics of the hypereffective versus the 

 hypoeffective heart. 



ALTERATIONS IN PRESSURE ON THE OUTSIDE OF THE 



heart. The degree of filling of the heart is determined 

 by the effective filling pressures of the cardiac cham- 

 bers, that is, by the differences between the pressures 

 inside the cardiac chambers and the pressure on the 

 outside of the heart. Ordinarily, the heart is sur- 

 rounded by the negative pressure of the intrathoracic 

 cavity and this negativity contributes greatly to the 

 effective filling pressure. However, at times the nega- 

 tive pressure becomes altered or completely lost. For 

 instance, opening the chest immediately shifts the 

 heart from negative pressure surroundings to zero 

 pressure surroundings (atmospheric pressure). And, 

 even more important, pericardial fluid (145, 157), 

 pericardial constriction (25), intrapleural fluid, pneu- 

 mothorax ( 1 86), or mediastinal compression can all 

 cause intensive positive pressure on the outside of the 

 heart, thereby markedly reducing the effective filling 

 pressures of the cardiac chambers. 



In circuit analysis, an increase in the external pres- 

 sure on the heart is an extremely important considera- 

 tion for the following very simple reason: The 

 resultant changes in filling pressures are not compen- 

 sated by simultaneous changes in pressure in the 

 systemic circulation. As a consequence, venous return 

 ordinarily becomes greatly reduced. 



Figure 3 illustrates the effects of opening the chest 

 and of cardiac tamponade on the cardiac output 

 curves. Simply opening the chest shifts the entire 

 curve approximately 5 mm Hg to the right because of 

 loss of the normal 5 mm Hg negative pressure in the 

 intrathoracic cavity. This means that for the heart to 

 pump an equivalent amount of blood after opening 

 the chest as before, the right atrial pressure must be 

 increased 5 mm Hg, thus explaining the detrimental 

 effect of opening the chest on blood flow in the circu- 

 lation. 



The lowest curve of figure 3 illustrates even more 

 drastic depression of the cardiac output curve, this 

 time caused by severe cardiac tamponade. Because 

 in this condition the intrapericardial pressure rises as 

 the heart fills to greater volumes, the curve is not 

 simply shifted to the right as one might have expected, 

 but its slope and its maximum value are also reduced, 



