Atrial inotropic responses to brief vagal stimuli: frequency-force interactions

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Abstract
The effect of brief vagal stimulus burst on atrial contractile force was assessed in paced- and unpaced-heart preparations by recording the changes in pressure (AP) generated in a balloon in the [dog] right atrial appendage or by a miniature strain gauge on the atrium. One and 3 stimuli per vagal burst depressed AP up to 62 and 90%, respectively. The response varied with the time in the cardiac cycle at which the stimulus was given. Identical stimuli produced a greater inotropic depression at longer cardiac cycle durations. The differences between the magnitudes of the AP responses at 2 different mean heart rates were relatively constant when the stimulus intensity was increased from 1 to 99 stimuli per burst. A low-level stimulus caused a greater depression in contractile force when the heart rate was allowed to change than with the identical stimulus when the heart was paced, although this difference in paced- and unpaced-heart responses tended to disappear when the stimulus intensity (burst width) was increased. The composite frequency-force relationship was relatively flat over normal heart periods, but fell off by 15-20% at the extremes. An unexpected interaction between the vagal inotropic effect and the frequency-force relationship is shown such that a given vagal stimulus causes a greater depression of atrial contractile force at lower heart rates.

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