PHYSIOLOGIC CONSEQUENCES OF CONGENITAL HEART DISEASE 



479 



importance and probability ot occurrence of such lui- 

 desirable side effects must of course be taken into ac- 

 count in any complete evaluation of an adaptive 

 mechanism, especially when individual cases are con- 

 cerned. Burchell and co-workers (49) observed that 

 polycythemia was not a uniformly necessary require- 

 ment for the well-being of the hypoxemic patient 

 suffering from congenital heart disease of the cyanotic 

 type. 



Numerous inv'estigators have shown that indi\iduals 

 who are chronically hypoxic from prolonged residence 

 at high altitudes have an increased ventilation which is 

 greater the higher the altitude (154). There is also 

 evidence that individuals who are hypoxemic because 

 of circulatory shunts also ventilate more than normal 

 (138). The hyperpnea of the altitude dweller consti- 

 tutes for him an important adaptation. If at 15,000 

 feet, for example, an individual did not increase his 

 ventilation above that for sea level, his alveolar (and 

 arterial) oxygen tension would be about 40 mm of 

 mercury. The increase in ventilation that usually oc- 

 curs in the individual acclimatized to this altitude is 

 such as to increase his arterial oxygen tension by about 

 20 mm of mercury and his arterial saturation to about 

 90 per cent, an increase which is really significant as 

 an adaptive mechanism. The individual with shunt 

 hypoxemia, howex'er, can increase the oxygenation of 

 his arterial blood but little by hyperpnea. His pul- 

 monary venous blood will be about 98 per cent 

 saturated even with normal ventilation, and although 

 increasing the ventilation will raise the oxygen tension 

 of his blood, it will alter the percentage of saturation 

 only insignificantly because of the flatness of the dis- 

 sociation curve in this region (139). 



Husson & Otis (139) have pointed out that the 

 presence of a right-to-left shunt introduces a problem 

 of carbon dio.xide elimination as well as a problem of 

 oxygen uptake. With a normal resting ventilation the 

 pulmonary venous carbon dioxide tension will be 

 normal but the arterial carbon dioxide tension will be 



higher and the arterial pH lower than normal because 

 of the admixture of venous blood. A normal arterial 

 carbon dioxide tension and pH can be maintained 

 only by hyperventilation of the proper magnitude. 

 With no increase in ventilation, a normal pH but ele- 

 vated carbon dioxide tension could be maintained by 

 an increase in the alkaline reserve. They observed, as 

 did Morse & Cassels (i 78), that the average congenital 

 cyanotic individual appears to be in a state of meta- 

 bolic acidosis that is only partially compensated for by 

 increased ventilation. Husson & Otis (139) noted that 

 the acidosis observed in some congenital cyanotic in- 

 dividuals may be considered as an advantageous 

 adaptation insofar as delivery of oxygen to the tissue is 

 concerned, although it may be disadvantageous in 

 other regards. 



SUMMARY 



The application of existing and the development 

 and application of new hemodynamic techniques have 

 greatly improved the understanding of the hemody- 

 namic alterations associated with various congenital 

 cardiac defects and the compensatory mechanisms that 

 frequently maintain, to a surprising degree, the func- 

 tions of the circulatory system despite the defect or 

 defects present. This information, in addition to con- 

 tributing importantly to the accuracy of diagnosis and 

 the efficacy of treatment of congenital heart disease, 

 has also contributed significantly to the understanding 

 of the physiology of the normal circulatory system and 

 its reactions to stress. 



The authors are indebted to many of their professional and 

 technical colleagues at the Mayo Clinic who assisted in the 

 collection and analysis of the data upon which this chapter is 

 mostly based. Deserving of particular mention are Drs. H. B. 

 Burchell, H.J. C. Swan, J. T. .Shepherd, Mr. William Sutterer, 

 Miss Lucille Cronin, and Mrs. Jean Frank. 



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