The Effect of Acetylcholine Upon Respiratory Gas Exchange in Mitral Stenosis

Abstract

Most patients with mitral stenosis show hemodynamic evidence of an increased resistance to blood flow through the lungs. Structural abnormalities in the small pulmonary arteries are seen in such patients, with increased thickness of the vessel wall and narrowing of the lumen. The increased resistance is also in part due to active vasoconstriction in many patients, and this can be released by acetylcholine (1-3). Soderholm and Werko (4) also observed that the oxygen saturation of arterial blood decreased during the infusion of acetylcholine in patients with mitral stenosis. They suggested that this was probably due to the increased perfusion of poorly ventilated alveoli which had previously been poorly perfused.The present studies were undertaken in order to investigate the above possibility further by the measurement of blood gas tensions. Observations have been made while the patients breathed a gas mixture containing from 45 to 50 per cent oxygen.this level of oxygen being selected for two reasons. Firstly, the magnitude of the difference between alveolar and arterial oxygen tension (A-a difference) is greater, and hence it can be measured with a relatively greater precision. Secondly, with this concentration of inspired oxygen the "membrane component" is negligible. A subsidiary purpose of the study was the comparison of the pulmonary hemodynamics in these patients when they breathed air and when they breathed 45 to 50 per cent oxygen. METHODSAll the patients had rheumatic heart disease wvith mitral stenosis, and their personal details are given in Table I. Patients were studied in the supine position, unsedated and having taken no food since a light breakfast 5 hours earlier.A double-lumen cardiac catheter was passed into the lung so that the terminal orifice was wedged in the * In receipt of a research grant from the Medical Research Council. periphery while the proximal orifice lay in the pulmonary artery. The tip of another single-lumen catheter was introduced into the middle of the right atrium. An inlwelling needle was placed in the brachial artery. Intravascular pressures were recorded by capacitance manometers and a direct writing recorder. Mean pressures were obtained by planimetric integration over at least 2 respiratory cycles.The percentage oxygen saturation of blood samples was estimated by a spectrophotometric method (5), and the blood oxygen capacity was measured photometrically.(;as analysis was by the Scholander micro-method. Inspired gas mixtures other than air were supplied from cylinders, the patient breathing from an anesthesia hag which served as a reservoir.The oxygen tension of arterial blood was measured polarographically with a Clark electrode incorporated in an assembly with a magnetic stirrer (6). Each rep)orted value of oxygen tension represents the mean of estimates on 3 blood samples. Carbon dioxide tension w as measured by a modification of the interpolation method of Astrup (7) using a MNetrohm EA 125 glass electrode. All measurements were made in a water bath at 380 C. Two sy...

The Effect of Acetylcholine on Pulmonary Circulation and Gas Exchange in Cases of Mitral Stenosis

“…There was no definite relationship between the change in total pulmonary resistance on the one hand and the variation in arterial oxygen saturation on the other, nor was the calculated venous admixture or alveolo-arterial oxygen tension difference significantly related to changes in resistance values. These findings do not confirm those of Bishop et al (2), but it must be emphasized that the test situation was very different in the two investigations. Bishop et al studied their cases during oxygen breathing, while this series was studied during air breathing.…”

“…Among those within the low pressure group, 3 out of 13 cases thus responded. The paper of Bishop et al (2) shows exactly the same tendency. This observation may be of importance, because in cases with pulmonary disease, no such dividing line has been observed.…”