Beat-to-beat variability in QRS potentials recorded with an orthogonal lead system in horses with second degree partial A-V block.

Beat-to-beat variation in the amplitude of the QRS waveform from 13 horses with second degree partial atrioventricular block was examined using an orthogonal lead (X,Y,Z) system. The peak positive deflections (XR, YR and ZR) and peak negative deflections (XQS, YS and ZQ) were regressed against PR and RR intervals. XQS was used to indicate that the peak negative deflection in X could have been a Q or S wave. XR and YR were directly related to PR (P less than 0.001) and XQS and YS were inversely related to PR (P less than 0.001). YR was also directly related to RR (P less than 0.001) but not so strongly as to PR. YS was inversely related to RR (P less than 0.001). Neither ZQ nor ZR were related to PR or RR. The findings in this study suggest that vagal tone can affect intraventricular conduction in the horse.