DYNAMICS OF PULMONARY CIRCULATION 



! 7I5 



during exercise; this effect may continue, to an un- 

 predictable degree, during mild exercise. 



In heart failure the ratio AQ/AV 02 is often abnor- 

 mally low, i.e., less than 600 ml increase in flow per 

 100 ml increase in oxygen uptake. A sti iking dissocia- 

 tion between AQ, and AV , follows the exhibition of 

 dinitrophenol, so that the cardiac output continues 

 at basal levels even though oxygen uptake increases 

 tremendously (216). 



Arteriovenous Oxygen Difference 



The pulmonary arteriovenous oxygen difference 

 increases during exercise. However, in contrast to the 

 roughly linear relation between cardiac output and 

 oxygen uptake during graded exercise, the relation 

 between the arteriovenous oxygen difference and 

 oxygen consumption is clearly hyperbolic (109, 382). 

 Whether the oxygen requirements of the tissue are met 

 predominantly by an increase in cardiac output or by 

 a greater extraction of oxygen from each unit of blood 

 perfusing the tissues seems to depend, at least in part, 

 on the type of exercise, the body position in which the 

 exercise is performed, and the ambient temperature 

 (7, 336). Parenthetically, it is of interest that during 

 graded exercise (up to 2000 ml/min/m 2 ) trained and 

 untrained subjects increase cardiac output and widen 

 arteriovenous differences for oxygen in an identical 

 fashion (149). 



Pulmonary Vascular Pressures 



Because of the difficulty in measuring intrathoracic 

 pressures, pulmonary vascular pressures are conven- 

 tionally referred to atmosphere. In only one study 

 were they also referred to esophageal pressures (109); 

 this study suggested that conventional luminal pres- 

 sures tend to underestimate slightly the transmural 

 pressures. Before considering the change in pulmo- 

 nary artery pressure during exercise, it is relevant to 

 recall that: a) pulmonary vascular pressures are 

 difficult to measure accurately during exercise since 

 respiratory swings are marked and the records are 

 apt to be distorted by artifacts, and b) especially 

 during severe exercise, shifts in mid-position of the 

 lung and changes in compliance confuse the recogni- 

 tion of the mechanisms involved in a change in pres- 

 sure (109). 



Until a few years ago, because of the practical 

 difficulties in measuring small changes in pressure 

 during exercise, it was uncertain if light (supine) 

 exercise elicited an increase in pulmonary arterial 



pressure (104, 208, 346). Indeed, an appreciable 

 increase was believed to occur only at levels of exer- 

 cise which tripled the cardiac output (89). However, 

 recent refinements in manometric methods, coupled 

 with the substitution of continuous pressure recording 

 for the tedious process of measuring and integrating 

 individual pressure pulses, have established that the 

 pulmonary arterial pressure increases (by 3-5 mm Hg) 

 even during light supine exercise (132, 370, 383). 



The behavior of the pulmonary arterial pressure 

 during light exercise is quite stereotyped (fig. 28) : at 

 the start of the exercise, the (luminal) pulmonary 

 arterial pressure increases abruptly by 3 to 5 mm Hg. 

 As exercise is continued, a plateau is reached, gener- 

 ally 1 to 2 mm Hg less than peak values (132, 382). 

 The increase in systolic pressure exceeds the increase 

 in diastolic pressure. As a rule, the higher the pre- 

 exercise level of the pulmonary arterial pressures, the 

 higher the values reached during exercise. Immedi- 

 ately after the exercise, the pulmonary arterial pres- 

 sure often falls below control, resting values (iog, 132, 

 382). 



The pulmonary arterial flow-pressure points ob- 

 tained by different investigators during graded exer- 

 cise are superimposed on the pressure-flow line of 

 figure 34. At the lower grades of exercise, the points 

 fall along the flow-pressure curve obtained in the 

 course of progressive curtailment of the pulmonary 

 vascular bed by balloon-occlusion. At the higher 

 grades of exercise the pulmonarv arterial pressure at 

 any given level of blood flow tends to exceed the 

 corresponding pressure during balloon-occlusion. 



Direct measurements of the left atrial pressure 

 during exercise in intact man or dog have not been 

 reported. On the other hand, in dogs exercised by 

 electrical stimulation of the extremities, the left atrial 

 pressure remains unchanged (125); unfortunately, the 

 level of exercise in these experiments is unknown. In 

 man the slight increments of the pulmonary arterial 

 pressure during mild exercise suggest that if the left 

 atrial pressure does increase, the increase cannot 

 exceed a few mm Hg. The pulmonary ''wedge" pres- 

 sure is unaffected by mild exercise but may increase 

 slightly during severer exercise (104). 



Pulmonary Blood Volume 



There is considerable indirect evidence to indicate 

 that the pulmonary blood volume increases during 

 supine exercise: the central blood volume increases 

 (48, 101), the pulmonary compliance decreases (279), 

 and, except for the muscles (250), the regional blood 



