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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



sistance of the small precapillary vessels (102). On 

 the other hand is the fact that, somewhere en route to 

 the pulmonary veins, the pulmonary arterial pressure 

 pulse is damped out so that the pulmonary venous 

 pressure pulse ordinarily reflects only left atrial 

 events. The problem then devolves into deciding the 

 degree to which the flow pattern in the pulmonary 

 capillaries resembles the pattern of instant-to-instant 

 changes in the pulmonary vascular pressure gradient 

 and of flow in the pulmonary artery. 



If it is assumed that the pattern of pulmonary 

 capillary blood flow is uniform throughout the lung, 

 direct inspection of the surface capillaries of the lung 

 should afford some insight into the nature of the 

 pulmonary capillary flow. In 1733, while examining 

 the exposed frog lung, Stephen Hales observed that 

 not only was blood "sensibly accelerated at each 

 svstole in the finest capillaries, but also in their cor- 

 responding capillary veins, tho' not in their larger 

 trunks" (178). This was the first declaration that 

 pulmonary capillary flow — at least in the frog — was 

 pulsatile. However, 200 years later, observations on 

 transilluminated lung of the cat indicated that the 

 pattern of capillary flow in the mammalian lung was 

 distinctly different from that of the amphibian lung; 

 thus, instead of pulsatility, YVearn and co-workers 

 stressed intermittency, a phenomenon attributable to 

 the opening and closing of pulmonary precapillary 

 vessels (419). The rarity of pulsatile flow in the sur- 

 face capillaries of the mammalian lung has since been 

 confirmed by others (21). 



Opposed to these direct observations on the mam- 

 malian lung are the results obtained by plethysmo- 

 graphic techniques in man (37, 254, 418). Not only 

 do they picture pulmonary capillary flow as regularly 

 and vigorously pulsatile, but they also depict a flow 

 pattern in the pulmonary capillaries which corre- 

 sponds more closely to the instantaneous changes in 

 the blood pressure gradient across the lungs than to 

 the flow pulse in the pulmonary artery (fig. 29) 

 (270). Moreover, unless one postulates species 

 differences among mammals, these results in man 

 also challenge the notion that examination of the 

 surface capillaries of the lung is a useful index of the 

 pattern of flow in the bulk of the pulmonary capil- 

 laries. 



Even though the majority of the plethysmographic 

 studies agree that pulmonary capillary blood flow is 

 pulsatile in man, they are not entirely consistent 

 with respect to the form of the capillary flow pulse. 

 For example, not only do the published records 

 differ with respect to the amplitude of peak flow, 



but they also display different contours for the flow 

 pulse: some, but not all, the records indicate that 

 flow is interrupted for much of each cardiac cycle; 

 often, pulmonary capillary blood flow seems to 

 reverse; finally, the reported flow patterns generally 

 vary from beat-to-beat. Undoubtedly, at least part of 

 this variability is attributable to practical difficulties 

 inherent in the plethysmographic techniques (342). 

 In addition, as may be gleaned from figure 35, in- 

 accuracies are inescapable in the matching and 

 analvsis of the air and nitrous oxide records. 



ECG 



AIR 



© 



-kv_X 



feo%N 2 -vi 



(20% 2 \/*S^ 



4.5 — 



1.5 — 



D 1 



18 — 

 12 

 6 — 



fig. 35. The pattern of the pulmonary capillary blood flow 

 according to the pncumocardiographic (A) and plethysmo- 

 graphic (B and C) methods. From above downward, the elec- 

 trocardiogram (ECG), the air record (AIR), the nitrous oxide 

 record (8o*>c NjO, 20% O.), the difference between the air 

 record and the nitrous oxide record (V N „ ), and the rate of 

 pulmonary capillary blood flow (Q c ) in liters per minute. A: 

 actual pneumocardiograms obtained from an anesthetized, 

 curarized dog during arrested respiration. Mechanical events 

 are eliminated in the process of point-by-point subtraction of 

 the nitrous oxide pneumocardiogram from the air pneumo- 

 cardiogram, yielding a record of instantaneous changes in air- 

 way pressures due only to the volumes of nitrous oxide removed 

 by the perfusing blood (V N -, ). Differentiation of the volume- 

 uptake record provides a record of the rate of uptake of nitrous 

 oxide and, therefore, of the pulmonary capillary blood flow 

 (Qc)- Except for the unexplained dip in the Qc record, pulmo- 

 nary capillary flow appears to be continuous and largely con- 

 fined to the systolic portion of the cardiac cycle. B and C: 

 hypothetical plethysmographic records to compare strongly 

 pulsatile but continuous capillary flow (B) with weakly pulsa- 

 tile but continuous capillary flow (C). Not illustrated is the 

 possibility of interrupted capillary flow, i.e., that capillary flow- 

 may actually stop (drop to zero) for part of each cardiac cycle. 

 [After Morkin el al. (298).] 



