The nature of the increased pulmonary vascular resistance in mitral stenosis.

It has been known for many years that in certain cases of mitral stenosis the pulmonary arterial pressure may be disproportionately high. More recently it has been shown that this pressure, which is due to an increase in the pulmonary vascular resistance, may rise still further on effort (Bayliss et al., 1950; Eliasch et al., 1952). There is considerable debate as to the mechanism and significance of this increase in pulmonary vascular resistance for, as the author of a recent annotation in the Lancet remarked, " it is not fully understood." In view of its theoretical and practical importance we decided to reinvestigate this problem with especial reference to the role of the autonomic nervous system. In a previous paper we reported the results of injecting adrenergic-blocking agents into the pulmonary artery in cases of mitral stenosis (Mackinnon et al., 1956), concluding that they had no effect. In this paper we report the effects of hexamethonium bromide and of atropine. We find that hexamethonium will frequently lower the high resistance, but will bring forward evidence that this is an indirect action secondary to lowering the pulmonary capillary or venous pressure and independent of the autonomic system. We find that atropine has no consistent effect on pulmonary dynamics.