PH'l'SIOLOGIG CONSEQUENCES OF CONGENITAL HEART DISEASE 



427 



respectively, and Csao, and Cvq, the oxygen contents 

 of systemic-artery and mixed venous blood, respec- 

 tively. Since the oxygen saturation of inferior vena 

 caval blood is usually higher than that in the superior 

 vena cava, it is the practice in this laboratory, for 

 patients with shunts at atrial level, to take the average 

 of the oxygen saturation of blood from the inferior 

 and superior venae cavae for the mixed venous satura- 

 tion for determining systemic blood flow. It is usually 

 not possible to obtain the oxygen saturation of pul- 

 monary-vein blood for determination of pulmonary 

 blood flow. If there is no right-to-left shunt present, 

 systemic arterial oxygen saturation may, for practical 

 purposes, be considered the same as that in the pul- 

 monary veins. If a right-to-left shunt is present and 

 pulmonary ventilation is assumed to be normal, the 

 pulmonary venous oxygen saturation can be taken 

 to equal 98 per cent of the oxygen capacity of the 

 i)lood plus 0.3 volume per cent for that in physical 

 solution while breathing 20 per cent oxygen. 



For many years the application of the Fick principle 

 has provided the classic indirect means of measuring 

 blood flow. Other methods have been judged on the 

 basis of their agreement with Fick measurements. 

 Careful workers have recognized the importance of 

 "the steady state" as being necessary for reliable 

 determinations. Until recently, however, apparently 

 few doubts have been entertained regarding the 

 validity of the conventional application of Fick's 

 law. In the past few years, provocative articles have 

 appeared in the literature concerning the justifica- 

 tion of some of the assumptions made. 



Under the influence of the cardiac cycle, the respira- 

 tory cycle, and other causes, the lung volume and 

 pulmonary blood volume periodically change. Since 

 in practice the oxygen uptake can be measured only 

 at the mouth or nose, the variable buffer capacity 

 for oxygen of the aheolar space and the pulmonary 

 blood may interfere with the accurate short-term 

 measurement of the uptake as referred to the blood 

 flowing to and from the lungs. Fishman et al. (104) 

 have pointed out that in order to avoid this type of 

 error it is essential that the amount of oxygen within 

 this Ijuff^er reservoir be the same at the beginning and 

 at the end of the period of observation. 



Visscher & Johnson (252) pointed out that the 

 usual procedure of sampling blood at a constant rate 

 may not yield a sample of the same oxygen content 

 as the average content of all the blood that flows 

 past the sampling site in the collection period. This 

 undesirable situation can occur whenever both the 

 rate of flow and the oxygen content of the blood 



passing the sampling site vary during the .sampling 

 period. A more general and rigid description of the 

 factors that may cause errors in conventional applica- 

 tions of the Fick principle has been given by Stow 

 (232). It is certain that the criteria for accurate 

 measurement of blood flow by the Fick principle are 

 seldom if ever completely fulfilled in studies of intact 

 animals or men. However, it is believed that in most 

 circumstances the errors are not so large as to in- 

 validate the method (269). 



The volume of a shunt may be easily estimated 

 when it is unidirectional by obtaining the difference 

 in the calculated systemic and pulmonary blood 

 flows. When the shunt is bidirectional, calculation of 

 the actual volume of blood crossing the defect in the 

 right-to-left and left-to-right directions is practically 

 impo,ssible. In the presence of unidirectional shunts 

 the proportion of pulmonary venous blood contrib- 

 uting to pulmonary-artery flow, that is, left-to-right 

 shunt, and of systemic venous blood contributing to 

 svstemic blood flow mav be estimated as follows: 



Cpa^)., — Cv 



Q'.LR = % left-to-right shunt = — - 



Cpvii, — Cv 



L.pV(i, — Cv 



i2? ■ 



:vo. 



and 



Cpv(i, — Csao;> 



Q'.KL = % right-to-lcft-shunt = : -— X 100 



Cpvfi, — Cv(i. 



in which Qo^lr 's expressed as a percentage of the 

 pulmonary flow and Qpjrl as a percentage of systemic 

 flow, and the various C"s with subscripts have been 

 defined previously. 



It frequently is not possii:)le to measure the oxygen 

 saturation of blood in the pulmonary veins. If there 

 is no right-to-left shunt the oxygen saturation of 

 systemic arterial blood is assumed to be the same as 

 that in the pulmonary veins. When there is a right- 

 to-left shunt resulting in dcsaturation of systemic 

 arterial blood, and if pulmonary disease as a source 

 of unsaturation has been ruled out, then it is permis- 

 sible, if pulmonary ventilation is adequate, to assume 

 that the pulmonary-vein blood was normally saturated 

 (98 Tt) prior to dilution by the shunt. There are 

 inaccuracies in these calculations, but they do seem 

 to indicate the magnitude of intracardiac shunts with 

 sufficient accuracy for most practical purposes. 



CALCULATION OF VASCULAR RESISTANCE. VaSCular 



resistance may be described as impedance to blood 

 flow through a given portion of the circulation; this 

 is usually a total circuit, that is, pulmonary or sys- 



