DYNAMICS OF PULMONARY CIRCULATION 



'7°3 



flow need not be measuring only the flow through 

 anatomic pulmonary capillaries. The physiologic 

 measurements may also be including the flow through 

 other small pulmonarv vessels that participate in the 

 uptake of the inert gas. However, this distinction be- 

 tween the anatomic and the physiologic pulmonary 

 capillary is more meaningful with respect to relating 

 the gas-exchanging characteristics of the small pul- 

 monary vessels to their hemodynamic behavior than 

 with respect to the measurement of the cardiac 

 output. 



The principle underlying the use of inert gases to 

 measure pulmonary blood flow was enunciated by 

 Bornstein in 1910 (343). Unfortunately, he chose an 

 insoluble gas, i.e., nitrogen, as the test gas. In 191 2, 

 Krogh & Lindhard (240) substituted the soluble 

 inert gas, nitrous oxide, for nitrogen and devised an 

 experimental protocol, involving respiratory maneu- 

 vers, to obtain the values needed for the equation, 



Q c = vw>n,,o-Fa N!0 



in which Qc = pulmonary capillary blood flow 



per minute 

 V N;0 = volume of N>0 absorbed per 



minute (BTPS) 

 XN2O = Ostwald's coefficient of solubility 



of nitrous oxide in blood at 37 C 

 Fa N20 = mean fraction of N2O in alveolar 



gas during the test (BTPS). 

 Since the coefficient of solubility (X) is constant, 

 the variables involved in the calculation of the flow 

 are two: /) the volume of N 2 absorbed per minute 

 (V N! o); and 2) the mean alveolar fraction of N2O 

 during the test (Fa v ,,,)- Subsequently, it was shown 

 that there are several practical limitations to the 

 Krogh and Lindhard method; these include: a) the 

 need to complete the test before recirculation of the 

 test gas; in normal man the pulmonary recirculation 

 time is of the order of 1 1 ± 3 sec (74, 343); b) the 

 difficulty in obtaining simultaneous measurements of 

 the different variables involved in the equation; c) 

 the dilemma of distinguishing the uptake of the gas 

 by the pulmonary tissues from the uptake by the 

 pulmonary capillary blood; and d) the unsubstanti- 

 ated use of "correction factors" (273, 343). Despite 

 these reservations, the inert-gas methods do provide 

 accurate measurements of pulmonary capillary blood 

 flow (Q c ) in resting subjects if proper precautions 

 are taken. However, during exercise and in chronic 

 pulmonary disease they become less reliable. The 

 current consensus appears to be that despite the 



attractive simplicity of these tests, their most reliable 

 use, even in resting patients with normal lungs, is for 

 consecutive measurements of Q c . 



Interest in the use of soluble, inert gases to measure 

 pulmonary capillary blood flow lagged once the direct 

 Fick and Stewart-Hamilton methods were stand- 

 ardized into clinically useful techniques. However, it 

 revived when Lee and DuBois substituted the body 

 plethysmograph for the spirometer to measure the 

 rate of uptake of nitrous oxide: this ingenious mod- 

 ification of the Krogh and Lindhard method promised 

 not only to provide the usual measure of the rate of 

 capillary blood flow per minute but also of the rate 

 of flow at any instant (254). Unfortunately, there are 

 practical difficulties inherent in the use of the bodv 

 plethysmograph for the measurement of instantaneous 

 flow. These limitations have led to the development 

 of modified plethysmography: techniques in man 

 (37, 418) and a modified cardiopneumographic 

 method in the dog (185, 298). 



A ature of Pulmonarv Capillary Blood Flow 



Whether pulmonary capillary flow is steady or 

 pulsatile is critical for the understanding of both 

 pulmonary hemodynamics and gas exchange (275, 

 319). For example, if the linear velocity of the blood 

 flow through the alveolar capillaries were to vary 

 during the cardiac cycle without compensatory 

 changes in other parameters, e.g., diffusing capacity 

 and capillary blood volume, the equilibration between 

 alveolar gas and capillary blood might well be dis- 

 turbed (143). 



A standard of reference for assessing the nature of 

 the pulmonary capillary blood flow is the prevalent 

 idea that blood flow through the systemic capillaries 

 is ordinarily continuous and devoid of major oscilla- 

 tions from the mean. This idea is consistent with two 

 features of the systemic circulation: a) the interplay 

 between arterial distensibility and arteriolar re- 

 sistance, so that the systemic "windkessel" maintains 

 flow during diastole; and b) the varying path lengths 

 between the root of the aorta and the capillaries. Of 

 these two influences, the windkessel effect is held to 

 be the more important. 



It is much more difficult to predict the nature of 

 the pulmonary capillary flow. On the one hand, 

 marked surges of capillary flow following systole 

 (pulsatile flow) might be expected to occur on at 

 least two accounts: /) the relatively small capacity of 

 the pulmonary arterial tree as compared to the 

 systemic arterial tree; and 2) the relatively low re- 



