Physiologic Studies of Paroxysmal Hyperpnea in Cyanotic Congenital Heart Disease

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Abstract
Four children with tetralogy of Fallot were studied with an isothermal thermistor flowmeter. Respiratory pumping effect on vena caval flow was demonstrated. Corresponding fluctuations in arterial saturation were recorded in the patients with more severe cyanosis. Evidence from other work confirms the disadvantage of hyperpnea in tetralogy increased oxygen consumption, increased cardiac output, and lowered arterial saturation, due to the relatively fixed or decreased pulmonary blood flow. Because of the right-to-left shunt, arterial blood in these patients usually will have low pO2 and pH and relative hypercapnia, creating a chronic stimulus for hyperpnea. The puzzle is the paroxysmal nature of marked hyperpnea (cyanotic or "anoxic" spells) which these patients demonstrate at times. Earlier theories attempting to explain the onset and mechanism of these paroxysms are reviewed. The following sequence is postulated. During prolonged sleep, oxygen requirements are reduced, and can be met by the relatively fixed, inadequate pulmonary blood flow. Arterial pO2, pCO2, and pH became nearly normal. During the first hour, the respiratory control mechanism is thereby relatively vulnerable, and a sudden increase in cardiac output or decrease in pulmonary blood flow may initiate a vicious cycle of increased arterial pCO2, decreased arterial pO2 or pH; hyperpnea; increased systemic venous return with fixed or decreased pulmonary flow; increased R-L shunting leading to increased arterial pCO2, decreased arterial pO2 or pH, etc. Exercise, bowel movements, or crying could produce sudden alterations in arterial composition to initiate these spells and we found in a previous clinical study that these events frequently initiated paroxysmal hyperpnea.