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The effect of atrial-ventricular versus ventricular pacing and contraction were studied in seven open-chest dogs. Cardiac output, left ventricular, left atrial, right atrial and pulmonary artery pressures were recorded. The right or left ventricular apical areas were consistently superior as ventricular pacing sites. Appearance of cannon A waves within the pre- or ejection period produced a significant decrease in left ventricular and systemic blood pressure, and cardiac output with a concomitant increase in right atrial, ventricular and pulmonary pressures. Prominent "v" waves were also observed during these periods. Reducing the basic driving cycle length from 400 to 300 msec caused a marked deterioration of all hemodynamic parameters with the appearance of mechanical alternans. Random VA conduction or ventricular pacing in the presence of complete AV and VA heart block appeared to offer a more favorable hemodynamic result than constant 1:1 VA conduction. It is concluded that maintenance of a physiologic AV interval permitting atrial contraction to appear outside of pre- or ejection period of ventricular systole is an important determinant or ventricular function during cardiac pacing.

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