Time course of cardiovascular neural regulation during programmed 20-sec apnea in rats.
OBJECTIVE
Long-term hypoxia results in hemodynamic breakdown in patients in the intensive care unit; however, intermittent hypoxia causes hypertension in individuals with sleep apnea. The objective of this study was to explore the sequential cardiovascular neural alterations in response to acute hypoxic apnea.


METHODS
The authors conducted a prospective, randomized animal study.


METHODS
The study was conducted in a university animal laboratory.


METHODS
A programmable apnea model was developed in anesthetized rats, in which a 20-sec period of apnea was produced and subsequently divided into the early (2.0 to 7.9 secs), middle (8.0 to 13.9 secs), and late apneic phases (14.0 to 19.9 secs) and immediate reventilatory phase (20.0 to 25.9 secs).


RESULTS
Evident hypoxia developed during 20-sec apnea. Arterial pressure increased in the early apneic phase and returned to control level in the middle phase. Significant hypotension developed in the late apneic phase and deteriorated in the reventilatory phase. Interbeat interval increased mildly along the apneic event. The increase of pressure in the early and middle phases was inhibited by propranolol (1.0 mg/kg intravenously [iv]) but was provoked by phentolamine (2.5 mg/kg iv). The decrease of pressure in the late and reventilatory phases was reversed, at least in part, by phentolamine. Atropine (0.5 mg/kg iv) did not produce discernible effects in the arterial pressure. The increase of interbeat interval was suppressed by propranolol. Power spectral analysis of arterial pressure variability demonstrated significant increases of the low-frequency (sympathetic vasomotor activity) and normalized high-frequency (cardiac sympathetic modulation) power after reventilation.


CONCLUSIONS
Although the sympathetic activity is excited during and after a hypoxic apnea, the immediate pressor effect is related to an inotropic response of cardiac sympathetic regulation, whereas the negative chronotropic and subsequent depressor effect is associated with a failure in the cardiovascular response to sympathetic excitation.
Sun
Tzong-Bor
TB
Institute of Medical Sciences,. Tzu Chi University, and the Center for Hyperbaric Oxygen Therapy, Buddhist Tzu Chi General Hospital, Hualien, Taiwan.
Yang
Cheryl C H
CC
Lai
Ching-Jung
CJ
Kuo
Terry B J
TB
D000704:Analysis of Variance; D000818:Animals; D001049:Apnea / Q000503:physiopathology*; D001340:Autonomic Nerve Block; D001341:Autonomic Nervous System / Q000503:physiopathology; D001794:Blood Pressure; D002319:Cardiovascular System / Q000503:physiopathology*; D006339:Heart Rate; D000860:Hypoxia / Q000503:physiopathology*; D008297:Male; D011897:Random Allocation; D051381:Rats; D017207:Rats, Sprague-Dawley; D012815:Signal Processing, Computer-Assisted; D020181:Sleep Apnea, Obstructive / Q000503:physiopathology; D013564:Sympathetic Nervous System / Q000503:physiopathology*