Origins of heart rate variability: relationship of heart rate burst morphology to work duration and load

We are developing a lexicon of specific heart period changes, or lexons, th at recur frequently and whose physiological meaning can be read into ambula tory electrocardiogram (ECG). The transient, reversible "burst" of tachycar dia induced by exercise initiation can also be seen on ambulatory EGG. We h ypothesized that burst morphology depended on the work that preceded it and on baroreceptor activation. Ten subjects with mean age 38 yr (range 17-69 yr) underwent two protocols of semisupine cycling in which load and duratio n were varied. Burst duration increased with longer cycling times (median v alues of 18.0, 25.5, and 23.7 s with 1, 3, and 5 s of cycling, respectively ; P = 0.033). Burst shape as assessed by heart period exponential decay con stant and burst magnitude did not change. To assess the impact of workload, subjects cycled for 5 s at loads of 0, 25, 50, and 75 W. No significant di fferences were seen in burst duration, burst magnitude, or burst shape. Tac hycardia preceded hypotension by 4.6 +/- 2.2 s, which is inconsistent with baroreceptor involvement in the onset of burst tachycardia. Because burst m orphology is a nearly quantal response to the initiation of exercise, the p resence of a burst on an ambulatory ECG implies the onset of exercise.