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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



blood volume on the patent side also increases, the 

 pulmonary circulation time decreases and the pul- 

 monary vascular resistance decreases (42, 101). Be- 

 cause of the configuration of the trachea in the dog, 

 the completeness of the unilateral interruption of 

 blood flow is more readily checked by bronchospi- 

 rometry in man (fig. 42); when the right ventricular 

 output has been shown to be completely diverted to 

 one lung in man, the pulmonary arterial pressure has 

 been found to increase by 30 to 40 per cent (5 to 

 7 mm Hg), and the calculated resistance to fall to 40 

 or 50 per cent of the initial value (42, 53, 101). The 

 increase in pulmonary arterial pressure seems en- 

 tirely attributable to the passive consequences of an 

 augmented pulmonary blood flow and volume. 



EFFECTS OF EXERCISE ON PULMONARY CIRCULATION 



Exercise is a practical expedient for increasing pul- 

 monary blood flow. However, by comparison with 

 unilateral occlusion of one pulmonary artery, it suffers 

 the disadvantage of simultaneously evoking changes 

 in the ventilation, in the performance of the heart, 

 and in a variety of circulatory parameters. 



Experimentally, dog and man have been exercised 

 in various different ways: electrical stimulation, 

 ergometer, push-pedal, and treadmill. The workload 

 imposed by the exercise, as well as the efficiency with 

 which the exercise is performed, varies with the type 

 of exercise, the position in which it is performed, and 

 the familiarity with the exercise (7, 12). For practical 

 reasons, the work load is generally inferred from the 

 increase in oxygen uptake rather than measured 

 directly (216). 



Pulmonary Blood Flow 



The first measurements of the cardiac output 

 during exercise were made by foreign gas methods 

 (240). Since then, with few exceptions (3), the foreign 

 gas methods have been superceded by indicator- 

 dilution methods and by applications of the Fick 

 principle. 



For accuracy, the use of the Fick method during 

 exercise requires that the oxygen uptake at the mouth 

 provide a precise measure of the oxygen uptake by- 

 pulmonary capillary blood, and that the pulmonary 

 arteriovenous oxygen difference be constant. These 

 criteria are most apt to be satisfied when respiration 

 and circulation become stable, i.e., when minute 

 ventilation, respiratory exchange ratio, oxygen up- 



take, heart rate, arteriovenous oxygen difference, and 

 cardiac output no longer vary with time. Unfortu- 

 nately, all these parameters do not stabilize simulta- 

 neously (108, 109, 136). Thus, the oxygen uptake at 

 the mouth, and the arteriovenous oxygen difference 

 mav level off after 1 min of heavy exercise (up to 1200 

 ml min nr), whereas the respiratory exchange ratio 

 and the ventilation require much longer to reach a 

 plateau. As long as the respiratory gas exchange is 

 unstable, it is difficult to be sure that the measure- 

 ment of the oxygen uptake at the mouth provides a 

 reliable value for the numerator of the Fick equation, 

 i.e., of the oxygen taken up by blood perfusing the 

 pulmonary capillaries. On the other hand, when 

 both the respiration and circulation become stable — 

 usually within 3 min in normal subjects performing 

 light, supine exercise (O2 uptake up to 400 ml min/ 

 m' 2 ) — the prospect of an accurate measurement of 

 pulmonary blood flow is increased (212). Obviously, 

 during heavy exercise (O2 uptake greater than 1000 

 ml min m 2 ), it may become difficult to achieve a 

 steady state; indeed, as exhaustion is approached, the 

 Fick method may become completely unreliable. It 

 is apparent that these considerations do not support 

 the practice of applying the Fick principle to the 

 measurement of the cardiac output during brief 

 periods of heavy exercise (108, 109). It would be re- 

 assuring if this use of the Fick principle were validated 

 by another independent method, such as the Stewart- 

 Hamilton, which, in principle, requires a briefer 

 steady state. 



Blood Flow and Oxygen Uptake 



In the unanesthetized dog (12) and in normal man 

 (104, 149, 335), an increment in oxygen uptake (AY „) 

 of 100 ml is usually associated with an increment in 

 cardiac output (AQ.) of 600 to 800 ml. However, both 

 lower (101, 109, 258) and higher (101 ) ratios of AQ. / 

 AVo, have also been observed. One likely explanation 

 for at least part of the variability is that different 

 degrees of approach to the '"basal" state were achieved 

 prior to exercise in the different studies: it has been 

 shown that in those studies in which a serious attempt 

 is made to achieve a basal pre-exercise state, the 

 ratio AQ/AVoj may even exceed 1 liter of cardiac 

 output per 100 ml of oxygen uptake (132); conversely, 

 when a nonbasal state exists prior to the exercise, the 

 ratio AQ./AV „ may fall below 600 ml (109). This 

 point is emphasized by the solid line of figure 12, 

 which indicates that for a given level of oxygen uptake 

 the cardiac output during excitement is higher than 



