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 fluctua- tions 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 electro- cardiogram, and may also provide objective evidence of the success with which various counter- pressure 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. METHODSElectrocardiographic Technique.Simultaneous recordings of the three principal leads were obtained in some observations, and in these it was possible to follow fluctuations in the electrical axis from beat to beat.In the remaining observations, single channel recording was used, tracings for different leads being obtained by repeating the pressure breathing after the elapse of a few minutes.The two methods gave closely similar results, although with the second method it was only possible to measure the mean axis over the course of one respiratory cycle.The standard technique (Wood, 1950) has been used to calculate the momentary manifest electrical axis of the heart, the maximum upright QRS deflection being determined in leads I, II, III, and CV, which measures the potential of a point on the back (one inch to the left of the seventh thoracic vertebra) relative to Wilson's neutral electrode.Potential changes in the frontal plane have been displayed on a conventional triaxial reference system (Bayley, 1943).Approximate values for the horizontal plane have been calculated from the resultant of leads I and CV, and for the sagittal plane from the resultant of leads II .cos 30°and CV.It seems clear from the detailed analysis of Frank (1954) that any attempt to estimate rotation in the horizontal and sagittal planes will be an approximation, and the above method of calculation was adopted on the grounds of simplicity rather than theoretical accuracy.Muscle action potentials showed a marked increase during the course of pressure breathing.Their effect was minimized by the use of an appropriate filter throughout the experiments, but despite this, the precision of potential measurements was somewhat reduced during pressure breathing.Subjects and Pressure Breathing Equipment.The subjects were four healthy young men, aged 20-26.