This study compared the results of AAIR and DDR pacing modes, using echocardiography and cardiopulmonary exercise testing. Fourteen patients underwent an echocardiographic evaluation of cardiac output (CO) during AAI and DDD pacing at rates of 60 and 100 bpm. Subsequently, the oxygen consumption at he anaerobic threshold was measured during exercise testing under AAIR mode and DDR mode with and AV delay of 120 ms. At rest and at low heart rates AAI pacing appears to maintain a higher CO than does D pacing. However, at higher heart rates there is no significant difference between the two pacing modes. On exertion, AAIR pacing does not appear to offer any advantage over DDR pacing.