Notes on a case of congenital heart disease with cyanotic episodes.

Recent papers on cyanotic congenital heart disease (1-8) contain references to patients that suddenly become cyanotic, lose consciousness, and suffer dyspnea or other signs of cardio-respiratory distress. When such episodes are the result of exercise or of other factors which give rise to an increase in oxidations there might be some decrease in arterial saturation, even though the intracardiac shunt remains constant in the relative amount of venous blood which mixes into the arterial stream. If we imagine a case with large intracardiac shunt in which blood in the pulmonary vein is 100% saturated and two-thirds of the arterial stream is venous blood, and if we were to assume further the resting mixed venous saturation to be 50%o, we would have an arterial saturation of 67%o. During exercise as violent as such a patient could sustain, it is doubtful that the saturation mixed venous blood could get below 20%o. Under these conditions, the arterial saturation would be 47%o. Even if the mixed venous blood had no oxygen at all, the saturation of the arterial blood would be 33%o. If the shunt were smaller, and only onethird of the aortic stream were venous blood, arterial saturation would vary from 83%o to 73%o as the saturation of the mixed venous blood is reduced from 50% to 20%o, by increased oxidations. It is obvious that the activity needed to produce the changes in arterial saturation indicated above is considerable, and that some other mechanism is necessary to account for the occasional unheralded episodes of cyanosis which have been described. It is possible that these episodes have a vasomotor basis but no mechanism that would account for them has been described. Could constriction of the pulmonary blood vessels be responsible for 1 This work was supported by a grant from the Life Insurance Medical Research Fund. 2 Technical assistance by Mrs. Kay Allen is gratefully acknowledged. changes in the size of the pulmonary stream and hence for changes in the size of the shunt? In the tetralogy of Fallot the resistance is made by a strong stenosis of the pulmonary artery and is probably fixed. Similarly, the structural arteriolar changes that are seen in the lungs in the Eisenmenger complex (3) result in a high resistance to pulmonary blood flow which is also probably fixed. Moreover it is extremely doubtful that the pulmonary bed is able, in response to reflex vasomotor activity, to constrict and dilate sufficiently to produce appreciable changes in its peripheral resistance (9). The direct constrictor effect of low oxygen in the inspired air is of course irrelevant (10). Another possibility is that alterations in the systemic arterial pressure may passively affect the pulmonary blood flow so as to lead to changes in arterial saturation of a larger order than can be explained as resulting from increased oxidations with a constant shunt. In the case to be described here the sudden onset of episodes of cyanosis seems to be related to an instability in the regulation of arterial pressure dating from a recent illness. Low arterial pressure was associated with low arterial saturation and high arterial pressure with a saturation that was nearly normal.