Electrocardiographic features of experimental cardiac tamponade in closed chest dogs

To determine the effect of cardiac tamponade on the ECG in a closed-chest model, alpha-chloralose anesthetized dogs were studied 1-7 days after insertion of a catheter in the pericardial sac. T wave inversion was the first change observed. As pericardial volumes increased, QRS complex voltage on surface ECG leads decreased (peak: 53 ± 11%) (mean ± SD) whereas the amplitude of bipolar ventricular electrograms increased. In 7 of 14 dogs the mean QRS axis shifted leftward and superiorly; no appreciable change occurred in the other 7. P wave voltage did not change until large pericardial volumes were present. Heart rate progressively increased with a significant change occurring at 55 ± 27 ml. At the point of a precipitous decline of systolic pressure there was a sudden slowing of heart rate (151 ± 24 vs 84 ± 33, P < 0.001). At this point, fluid was evacuated, atropine administered or atrial pacing initiated: evacuation of fluid was followed by a faster heart rate, although 5 min after relief of tamponade, heart rate was still slower than control (151 ± 24 vs 108 ± 22, P < 0.001). Atropine reversed bradycardia (77 ± 32 to 220 ± 40, P < 0.001). Atrial pacing (twice control diastolic threshold at a cycle length of 285 msec) performed on 11 dogs resulted in atrial capture with 1 : 1 conduction in 4 dogs, atrial capture with 2 : 1 atrioventricular block in three and atrial fibrillation in 4. Electrical alternans occurred in 4 dogs, all following administration of atropine when large pericardial volumes were present. Thus, in the closed-chest, anesthetized dog, the surface ECG shows a progressive reduction of QRS complex voltage whereas P wave voltage remains relatively unchanged; however, voltages recorded with the ventricles are increased. Left axis shift of the QRS complex and electrical alternans are observed frequently, especially when heart rate is rapid and large pericardial volumes are present. T wave inversion and ST junctional changes are also found. Tachycardia occurs in the early phases of cardiac tamponade whereas bradycardia occurs in the late phases, the latter being reversed by atropine.