This Doppler echocardiographic study of patients with a dual chamber pacemaker was undertaken to assess the changes in mitral and aortic flow induced by passing from the double stimulation to the atrial detection mode. Thirteen patients totally dependent on ventricular pacing were examined and mitral and aortic blood flow recorded by pulsed wave Doppler. The chronology of left atrial contraction as assessed by the Doppler mitral A wave was measured with respect to the ventricular stimulation. The A wave was recorded on average 177 ms after the right atrial stimulation artefact. For an average AV delay of 168.8 ms and an identical pacing frequency, the passage from the double stimulation to the atrial detection mode led to left atrial contraction occurring on average 70 ms earlier with respect to ventricular stimulation, reflecting prolongation of the programmed AV delay related to the delay in detection of the sinus atrial wave. This earlier atrial systole shortened the total duration of mitral flow from 363 to 317 ms, decreased the early diastolic mitral flow and increased the atrial end diastolic flow; the stroke volume and cardiac output calculated from the aortic velocity time integral decreased significantly from 73 +/- 11 ml to 67 +/- 10 ml and 5.4 +/- 1.11/mn to 4.9 +/- 1.01/mn respectively. The initial parameters were restored (average 74 +/- 9 ml and 5.5 +/- 1.11/mn respectively) when the AV delay in the atrial detection mode was reduced by a value close to that of the calculated increase.(ABSTRACT TRUNCATED AT 250 WORDS)