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Figure 1 For each strip the top tracing is the surface lead electrocardiogram and the bottom tracing is left atrial pressure (LAP). Atrial pacing (AOO): note small atrial pacemaker deflections preceding each narrow QRS complex and normal appearing LAP tracing. Dual-chamber pacing (DOO): note atrial deflection, wide QRS, and normal appearing LAP tracing. Ventricular pacing (VOO): note absence of P-wave, wide QRS, abnormal and elevated LAP waves. See Discussion section in the text. NSR = normal sinus rhythm with narrow QRS and normal A and V waves on left atrial tracing.