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Contribution of the respiratory rhythm to sinus arrhythmia in normal unanesthetized subjects during positive-pressure mechanical hyperventilation

¹School of Sport and Exercise Sciences and ²Department of Anaesthesia and Intensive Care, University of Birmingham, Birmingham B15 2TT, United Kingdom

Submitted 2 June 2003 ; accepted in final form 3 September 2003

The precise contribution of the CO₂-dependent respiratory rhythm to sinus arrhythmia in eupnea is unclear. The respiratory rhythm and sinus arrhythmia were measured in 12 normal, unanesthetized subjects in normocapnia and hypocapnia during mechanical hyperventilation with positive pressure. In normocapnia (41 ± 1 mmHg), the respiratory rhythm was always detectable from airway pressure and inspiratory electromyogram activity. The amplitude of sinus arrhythmia (138 ± 21 ms) during mechanical hyperventilation with positive pressure was not significantly different from that in eupnea. During the same mechanical hyperventilation pattern but in hypocapnia (24 ± 1 mmHg), the respiratory rhythm was undetectable and the amplitude of sinus arrhythmia was significantly reduced (to 40 ± 5 ms). These results show a greater contribution to sinus arrhythmia from the respiratory rhythm during hypocapnia caused by mechanical hyperventilation than previously indicated in normal subjects during hypocapnia caused by voluntary hyperventilation. We discuss whether the respiratory rhythm provides the principal contribution to sinus arrhythmia in eupnea.

hypocapnia; eupnea

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