[Elementary changes in the duration of successive sinus cycles; the concept of sinus extrasystole]

The authors studied the duration of successive sinus cycles in 35 patients confined to bed using a computerized system to trace graphs of heart rate (tachograms) with a resolution greater than +/- 2,5 ms due to precise detection of the upstroke of the R wave. A coupled visualization system excluded variations due to changes in the PR interval. Several types of modification were demonstrated: 1) slow fluctuations of high amplitude of the heart rate probably related to the autonomic nervous system or circulating catecholamines, 2) smaller variations of heart rate, from cycle to cycle with different forms of distribution: progressive increment or decrement over 3 to 4 cycles in 10 ms steps. This may be a system of intrasinus regulation for controlling variations of periods of 10 ms from one cycle to another. In both forms of distribution, the total amplitude of variation for a complete period of 3 to 4 cycles ranged from 30 to 75 ms. In some cases, sinus complexes were recorded with a prematurity of about 50 ms with respect to the preceding cycle and followed by an equivalent compensatory pause with respect to the last non-extrasystolic cycle. When the coupling interval of these premature complexes varied, the compensatory pauses remained unchanged. These results suggest a very precise system of control over the sinus rhythm, the mechanisms of which could be very varied: either cyclical variations of intrasinus conduction time from the same zone of automaticity to the atrium, or a sequential system of automaticity of slightly different frequencies obeying an unknown law; or again, electronic variations between two or more pacemakers of comparable frequency within the sinus node.(ABSTRACT TRUNCATED AT 250 WORDS)