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Heart rate variability responses to hypoxic and hypercapnic exposures in different mouse strains

¹Lovelace Respiratory Research Institute, Albuquerque, New Mexico; ²Department of Anesthesiology, Chiba University School of Medicine, Chiba, Japan; ³Department of Medical Physics, Brody School of Medicine, East Carolina University, Greenville, North Carolina; ⁴Bloomberg School of Public Health, Johns Hopkins University, Baltimore, Maryland; and ⁵Department of Medicine, Division of Pulmonary, Allergy, and Critical Care Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania

Submitted 11 January 2005 ; accepted in final form 9 May 2005

Heart rate variability (HRV) is a well-characterized, noninvasive means of assessing cardiac autonomic nervous system activity. This study examines the basic cardiac responses to hypoxic and hypercapnic challenges in seven strains of commonly used inbred mice (A/J, BALB/cJ, C3H/HeJ, C57BL/6J, CBA/J, DBA/2J, and FVB/J). Adult male mice, 8–12 wk of age, were chronically instrumented to a femoral artery catheter for the continuous measurement of systemic arterial blood pressure and heart rate. Mice were exposed to multiple 4-min periods of hypoxia (10% O₂), hypercapnia (5% CO₂), and combined hypoxia/hypercapnia (10% O₂ + 5% CO₂). HRV was derived from pulse intervals of the blood pressure tracings. Hypoxia induced increases in high-frequency HRV power and decreased low-frequency (LF) HRV power in most strains. Hypercapnia led to decreased high-frequency HRV power and increased LF HRV power in most strains. Strain differences were most notable in regard to the concomitant exposures of hypoxia and hypercapnia, with FVB/J mice mirroring their own response to hypercapnia alone, whereas CBA/J mice mirrored their own responses to hypoxia. As blood pressure is most likely the driving factor for heart rate changes via the baroreflex pathway, it is interesting that LF, considered to reflect cardiac sympathetic activity, was negatively correlated with heart rate, suggesting that LF changes are driven by baroreflex oscillation and not necessarily by absolute sympathetic or parasympathetic activity to the heart. These findings suggest that genetic background can influence the centrally mediated cardiovascular responses to basic hypoxic and hypercapnic challenges.

cardiovascular; sleep apnea; blood pressure; frequency domain

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