Electrocardiographic changes during pressure breathing.

It was recognized many years ago that the form of the electrocardiogram could be modified by a voluntary increase of breathing pressure, as during the Valsalva manceuvre (Kahn, 1909). During the early part of the second world war further studies were undertaken by German workers (Zettel and Fink, 1939; Muller, 1941), and it was found that it was altered even by the small fluctuations of breathing pressure associated with the wearing of a gas mask. At the altitudes of interest to modern aviation physiology, quite high positive breathing pressures may be necessary for survival, and such pressures produce dramatic alterations (Howard et al., 1953; R.C.A.F., 1954), including occasional abnormalities of rhythm, and a consistent change in the standard limb leads interpreted as clockwise rotation (Howard et al., 1953) or more cautiously as right axis deviation (R.C.A.F., 1954). The alteration in electrical axis is of particular interest: it offers a new technique for analysing the theoretical background to certain features of the electrocardiogram, and may also provide objective evidence of the success with which various counterpressure garments maintain normal anatomical relationships during pressure breathing. The present paper reports some measurements of the manifest electrical axis made during pressure breathing (with and without trunk counter-pressure) and during venous occlusion. Findings are correlated with other circulatory and respiratory changes observed during pressure breathing. The concept of a clockwise rotation of the heart in the frontal plane is confirmed, and it is concluded the electrocardiogram may indicate at least the order of this change.