Gender Differences in the Arterial Pressure Response to Apnea

Background: Gender-based differences in basic physiology receive intense scrutiny within the scientific community. These differences can potentially explain the mechanisms of various health outcome disparities among the genders. It has been widely documented that women and men respond differently to physiological stress—especially in the response to sympathetic stimulation. Women are considered “cardiac responders” (increasing heart rate and contractility) while men are considered “vessel responders” (increasing systemic vascular resistance). Up to this point, no study has tested this hypothesis with apnea. This study tested the hypothesis that women receiving metoprolol will decrease their arterial pressure and heart rate response to hypoxic voluntary apnea compared to men. Methods: For this pilot study, 2 men and 4 women were recruited. Each of the women was in the early follicular phase of her menstrual cycle (day 1-4 post menses). No women were taking hormonally active medications. Each subject was exposed to 12% oxygen via Douglas bag for 5 minutes. The subject then initiated 3 voluntary apneas. Beat-to-beat arterial pressure (Finometer) and O2 saturation (Nellcor pulsoximeter) were measured. Each subject repeated the experimental condition with (a) a full blocking dose of intravenous metoprolol (average=0.1 mg/kg) and (b) the same dose of normal saline. Results: Males and females exhibited similar responses in systolic, diastolic and mean arterial pressure to voluntary hypoxic apnea with and without metoprolol (all P [greater than] 0.05, repeated measures two-way ANOVA). Conclusions: This preliminary study has shown that women in the early follicular phase of menstruation respond similarly to men with hypoxic apnea in terms absolute arterial pressure. This potentially implicates the role of estrogen and progesterone in this phenomenon. Future studies will quantify the role of menstrual cycle phase and comparison to other physiological stressors.