RELATIVE IMPORTANCE OF ACTIVATION SEQUENCE COMPARED TO ATRIOVENTRICULAR SYNCHRONY IN LEFT VENTRICULAR FUNCTION

This study evaluated the relative hemodynamic importance of a normal left ventricular (LV) activation sequence compared to atrioventricular (AV) synchrony with respect to systolic and diastolic function. Twelve patients with intact AV conduction and AV sequential pacemakers underwent radionuclide studies at rest and Doppler echocardiographic studies at rest and during submaximal exercise, comparing atrial demand pacing (AAI) to sequential AV sensing pacing (DDD) and ventricular demand pacing (VVI). Studies at rest were performed at a constant heart rate between pacing modes, and the exercise study was performed at a constant heart rate and work load. Cardiac output was higher during AAI than during both DDD and VVI (6.2 ± 1 vs 5.6 ± 1 and 5.3 ± 1 liters/min, p < 0.05). LV ejection fraction was likewise higher during AAI (55 ± 12 vs 49 ± 11 vs 51 ± 13, p < 0.05). VVI with or without AV synchrony was associated with a paradoxical septal motion pattern, resulting in a 25% impairment of regional septal ejection fraction. In addition, LV contraction duration was more homogenous during AAI. Peak filling rate during AAI and VVI was higher than during DDD (2.86 ± 1 and 2.95 ± 1 vs 2.25 ± 1 end-diastolic volume/s; p < 0.05). During VVI, the time to peak filling was significantly shorter than during both AAI and DDD (165 ± 34 vs 239 ± 99 and 224 ± 99 ms; p < 0.05). Doppler echocardiographic evaluation of cardiac systolic function (peak aortic blood velocity, mean aortic blood acceleration and systolic ventricular time integral) during rest and submaximal exercise showed improved LV performance with AAI compared to both DDD and VVI. VVI with or without AV synchrony resulted in an altered activation pattern of the left ventricle, associated with delayed onset, asynchronous contraction, interventricular septal motion abnormalities and an impaired LV systolic function. These changes persisted during submaximal exercise. Peak filling rate depended more on a normal LV activation pattern than on AV synchrony. It is concluded that normal ventricular activation is a prerequisite for optimal LV function. Pacing systems allowing a normal ventricular activation pattern (AAI or DDD with a long AV delay) are preferred for patients who have intact AV conduction but require artificial pacing.