DYNAMICS OF PULMONARY CIRCULATION 



l68 5 



PULMONARY 

 ARTERY 



SYSTEMIC 

 ARTERY 



(Cq) 



Qs c a _ c c 



Q Cy — C c ' 



fig. 16. Schematic representation of the lung to illustrate 

 the components of the venous admixture and the calculation of 

 the venous admixture as a fraction of the cardiac output. 

 Q, = venous admixture (anatomical plus "virtual"); Q = 

 cardiac output; C a , C c , C; = oxygen content of arterial, end- 

 capillary, and mixed venous blood, respectively. Furthermore, 

 by administering enriched-oxygen mixtures, the total venous 

 admixture (Qs/Q) can be subdivided into its anatomical and 

 "virtual" portions. 



Attempts have also been made to measure anatomical 

 venous admixture in other ways, e.g., the simultaneous 

 intravenous injection of T-1824 and Kr S5 ; unfor- 

 tunately, such methods are most reliable when the 

 anatomical venous admixture is large, i.e., greater 

 than 1 5 per cent of the cardiac output (155). 



In recent years, relationships between pulmonary 

 capillary perfusion and other gas-exchanging param- 

 eters have been clarified in many different ways: 

 a) the determination of alveolar-arterial gradients 

 for nitrogen (67, 232); b) the quantification of the 

 role played by parameters other than ventilation and 

 perfusion in determining virtual venous admixture 

 (319); c) the comparison of anatomical dead spaces 

 with physiological and alveolar dead spaces (347, 

 377); d) the analysis of the pulmonary elimination 

 of intravenously injected radioactive tracers (173); 

 and e) by the creation of new and more elaborate 

 models (52, 131). With the growth of understanding 

 of these interplays has come the fuller appreciation 

 of the extent to which they may affect conventional 

 tests of pulmonary performance and calculations of 

 pulmonary resistance. 



PULMONARY VASCULAR PRESSURES 



Recording 



The characteristics of adequate manometric systems 

 as well as the limitations of the cardiac catheter in 



reproducing the intravascular and intracardiac 

 pressure pulses are considered elsewhere in this 

 volume. However, it should be emphasized that 

 with modern, hi-fidelity recorders and manometers, 

 it is generally the catheter attached to sensing ele- 

 ment, rather than the manometric system, which 

 limits the capacity of the apparatus to duplicate 

 faithfully the pressure pulse. It is also noteworthy 

 that even though blood pressure recorded from the 

 end of a catheter in the pulmonary artery is suffi- 

 ciently exact for most physiologic purposes, it fails to 

 measure the lateral pressure in the vessel by a small, 

 but variable, amount. 



Hydrostatic Reference Level 



For the measurement of absolute pressures within 

 the thorax, correction is made for the hydrostatic 

 pressure difference between the intrathoracic site 

 from which pressure is being recorded and the exter- 

 nally-placed sensing element of the manometer 

 (169). For this purpose, the plane of the sensing 

 element is set in relationship to both the heart and 

 to some thoracic landmark. In practice, different 

 hydrostatic zero levels have been adopted : most 

 popular are levels 5 cm below the angle of Louis 

 (253) and 10 to 12 cm above the tabletop (103). 

 While the different reference levels do complicate 

 the comparison of data from different laboratories, 

 each is a perfectly reliable standard for comparing 

 consecutive measurements in a single animal. 



Several uncertainties creep into the use of fixed 

 external references to obtain absolute values of 

 intrathoracic blood pressures, particularly in patients 

 who are dyspneic from cardiac or pulmonary disease. 

 Thus, in subjects with large hearts or unusual con- 

 figurations of the chest, it may be difficult to estimate 

 precisely the difference between the external reference 

 plane and the intracardiac site of reference (253); 

 moreover, even in normal subjects, the heart changes 

 position during each cardiac cycle. Consequently, it 

 seems reasonable to view pulmonary vascular and 

 intracardiac pressures which are measured in this 

 way as accurate only to within a few mm Hg. 



Unfortunately, many intuitively attractive solutions 

 to the problem of "zeroing" are not feasible: the 

 tip of the cardiac catheter, as localized by X ray, 

 cannot, per se, serve as the zero reference plane; nor 

 is it a simple matter to "zero" an intracardiac 

 manometer which is built into the tip of a cardiac 

 catheter. Nonetheless, despite these difficulties in- 

 herent in the choice of reference levels for absolute 



