Interrelationship of some electrocardiogram amplitudes, time intervals and respiration in the horse

The relationship of the scalar and vector magnitudes of the QRS waveform to RR, PR and respiration was examined in 10 horses in regular rhythm using an orthogonal lead system. There were no significant changes in the durations of RR and PR with respiration. The amplitudes of the scalar variables XR and YR were directly related and that of XQ (or XS) and ZR were inversely related to PR in this group of horses. The peak spatial vector magnitude and its H-plane angle were directly related to PR and inversely related to RR. The effect of inspiration on the QRS was a decrease in YR and ZR, an increase in YS and peak spatial vector magnitude and a reduction in its angle of elevation. Examined over the 10 horses these changes were only small (1.8 to 3.8 per cent) and considered to be not significant clinically. The possible mechanisms of the relationship to PR and RR and the effect of respiration on the QRS waveform are discussed. The results suggest the possibility of a vagal effect on intraventricular conduction. The effect of respiration was shown to be independent of this effect and it might be due to changes in lung electrical conductivity during respiration.