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Am J Physiol Heart Circ Physiol (September 4, 2003). doi:10.1152/ajpheart.00504.2003
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Submitted on June 2, 2003
Accepted on September 3, 2003

THE CONTRIBUTION OF THE RESPIRATORY RHYTHM TO SINUS ARRHYTHMIA IN NORMAL UNANESTHETIZED SUBJECTS DURING MECHANICAL HYPERVENTILATION WITH POSITIVE PRESSURE

H. E. Cooper1*, T. H. Clutton-Brock2, and M. J. Parkes1

1 School of Sport and Exercise Sciences, University of Birmingham, Birmingham, United Kingdom
2 Depart,emt of Anaesthesia and Intensive Care, University of Birmingham, Birmingham, United Kingdom

* To whom correspondence should be addressed. E-mail: H.E.Cooper{at}Bham.ac.uk.

The precise contribution the CO2 dependent respiratory rhythm makes 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 hperventilation with positive pressure. In normocapnia (41 ±1 mmHg) the respiratory rhythm was always detectable from airway pressure and inspiratory EMG 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 msec). These results show that hypocapnia caused by mechanical hyperventilation reveals a greater contribution to sinus arrhythmia from the respiratory rhythm 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.




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