Arterial Oxygen Saturation vs. Alveolar Oxygen Tension as a Measure of Venous Admixture and Diffusion Difficulty in the Lung

Abstract

A method is described for diagnosis and semi-quantitative evaluation of the severity of (a) anatomical right to left blood shunting past normal lung, (b) shunt-like effect due to distribution difficulty and (c) diffusion difficulty. Arterial O2 saturation, determined by recording oximeter, is plotted against alveolar O2 tension, automatically sampled and analyzed, as the patient or animal breathes each of several concentrations of O2 in N. With normally functioning lungs, the curve relating arterial O2 saturation to alveolar O2 tension ("saturation-tension" curve) lies slightly to the right of an in vitro O2 dissociation curve. In certain forms of pulmonary or congenital heart disease, the curve shape and position is altered in a fashion characteristic of the nature and severity of the defect. These effects were studied in animals with experimental reversible pulmonary lesions of known severity, produced by means of pneumatic "occlusion units" applied several days prior to the experiment to a pulmonary artery and to a bronchus. Artery occlusion produced an effect like a multiple lobectomy, with resulting diffusion difficulty, due to decrease in area of functioning lung. Bronchus occlusion with entrapment of a low O2 mixture in the affected lobes produced an effect resembling right to left blood shunting past normal lung. An approximate measure of fraction of right to left shunting is obtained, using a conventional procedure, by substituting in a mixing equation the value of arterial saturation at a measured alveolar Oo tension near 100 mm. Diffusing capacity is evaluated semiquantitatively in terms of the shift of the saturation-tension curve during exercise, as follows: (a) shift of the curve to the right at an arterial saturation near or below 80% is due chiefly to increase in alveolo-end capillary oxygen diffusion gradient and only slightly to increase in venous admixture gradient; (b) under these conditions, increase in end-capillary gradient is accompanied by an approximately equal increase in mean alveolo-capillary gradient; (c) value of the ratio: "increase in O2 consumption/mm shift" falls with increasing exercise to a minimal value approximately equal the maximal DO2. Curves were constructed using theoretical data of Riley et al. for the conditions of rest and exercise in a hypothetical patient with decreased pulmonary reserve, in order (a) to visualize the venous admixture and endcapillary diffusion gradients which together comprise the total alveolo-arterial Oo pressure gradient, and (b) to analyze the factors involves in the shift of the saturation-tension curve to the right during exercise.