Impact of acute hypoxia on heart rate and blood pressure variability in conscious dogs

To examine whether the impacts of hypoxia on autonomic regulations involve the phasic modulations as well as tonic controls of cardiovascular variables, heart rate, blood pressure, and their variability during isocapnic progressive hypoxia were analyzed in trained conscious dogs prepared with a permanent tracheostomy and an implanted blood pressure telemetry unit. Data were obtained at baseline and when minute ventilation ((V) over dot I) first reached 10 ((V) over dot I10), 15 ((V) over dot I15), and 20 ((V) over dot I20) 1/min during hypoxia. Time-dependent changes in the amplitudes of the high-frequency component of the R-R interval (RRIHF) and the low-frequency component of mean arterial pressure (MAPLF) were analyzed by complex demodulation. In a total of 47 progressive hypoxic runs in three dogs, RRIHF decreased at (V) over dot I15 and (V) over dot I20 and MAPLF increased at (V) over dot I10 and (V) over dot I15 but not at (V) over dot I20, whereas heart rate and arterial pressure increased progressively with advancing hypoxia. We conclude that the autonomic responses to isocapnic progressive hypoxia involve tonic controls and phasic modulations of cardiovascular variables; the latter may be characterized by a progressive reduction in respiratory vagal modulation of heart rate and a transient augmentation in low-frequency sympathetic modulation of blood pressure.