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[Initial heart rate transient at dynamic exercise performed in apnea. Influence of the variation rate of previous
P P Soares1, A C da Nóbrega, C G de Araújo
1Hospital Universitário Clementino Fraga Filho-UFRJ, Instituto Biomédico-UFF, Rio de Janeiro.
Purpose:
To study the influence of inspiratory flow on the heart rate (HR) responses during the four-second exercise test (4sET) as a method to estimate cardiac vagal activity.
Methods:
Fifteen healthy subjects performed the 4sET under two experimental situations: a) with maximal individual inspiratory flow (MIF) and b) with a slow inspiratory flow adjusted to reach total lung capacity in 4s (SIF). Vagal activity was estimated from B/C ratio obtained in ECG tracings, where B is the last R-R interval before the onset of exercise and C is the shortest R-R interval during exertion.
Results:
Similar inspiratory capacities were reached, with significantly different inspiratory flows [mean (se)] [MIF = 2.23 (0.28) 1.s-1 and SIF = 0.59 (0.06) 1.s-1; t = 6.69; p < 0.0001]; as a result, it was observed a greater B/C ratio in MIF [1.36 (0.05)] as compared to SIF [1.13 (0.04); t = 4.42; p = 0.0006]. The inspiratory flow influenced HR responses in the 4sET. The biphasic transient induced by inspiration, with a HR acceleration followed by a relative bradycardia, has not been completed before exercise in the 4sET with SIF.
Conclusion:
Lower values of B/C in the 4sET with SIF seemed to be due to overlapping effects of apnea and exertion on HR during cycling. On the other hand, when the inspiration was performed at MIF, the oscillatory effect of respiration upon HR was completed before the onset of exercise. Therefore, the tachycardia observed during the 4sET performed after individual maximal inspiratory flow represents the isolated effect of dynamic exercise.