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Background  The electrocardiographic differentiation between atrial ( APD s) and ventricular ( VPD s) premature depolarizations is important. P wave prematurity and normal  QRS  and T wave morphology generally are used as discriminating criteria for  APD s.    Hypothesis/Objectives  The aim of this study was to determine whether P, Q, R, S, and T wave amplitude,  PQ  interval,  QRS  and P wave duration and P and T wave morphology differ between  APD s and sinus beats. To determine the relationship between the  RR  coupling interval and the change in S wave amplitude between sinus beats and  APD s.    Methods  Case–control study. From a modified base‐apex configuration of 30 horses with  APD s at rest, sinus beat and  APD  associated preceding  RR  interval, P,  PQ  and  QRS  duration and P, R, S, and T wave amplitudes were measured. Linear mixed models and logistic regression were used to determine the effect of  APD s on the  ECG  variables studied.    Results  In comparison to sinus beats,  APD s were associated with a significant ( P  < .001) change in P amplitude (−0.03 ± 0.01 mV) and increase in S (0.20 ± 0.02 mV) and T (0.08 ± 0.03 mV) amplitude.  PQ  (−20.3 ± 5.2 ms) and  RR  (−519 ± 14 ms) interval and P duration (−21.1 ± 3.0 ms) decreased ( P  < .001).  APD s were significantly associated with a singular positive P wave ( OR : 11.0,  P  < .001) and were more likely to have a monophasic positive T wave ( OR : 9.2,  P  < .001). A smaller  RR  coupling interval was associated with an increased relative difference in S amplitude ( P  < .01).    Conclusions  Atrial premature depolarizations may lead to changes in  QRS  and T wave morphology. Knowledge of these changes is important to avoid interpreting certain  APD s as  VPD s.

Atrial Premature Depolarization‐Induced Changes in QRS and T Wave Morphology on Resting Electrocardiograms in Horses