The Number of Nitrogen Breathing Epochs Influences the Heart Rate and Blood Pressure Response to Hypoxia

Abstract

Peripheral chemosensitivity in humans is commonly assessed using hypoxia to activate the carotid body chemoreceptors. Breathing 2–6 breaths of 100% nitrogen for up to 10 nitrogen epochs has become a popular method to determine carotid body chemosensitivity. With this method, the hypoxic ventilatory response (HVR), the hypoxic heart rate response (HHR), and the hypoxic blood pressure response (HBPR) are used as indicators of carotid body chemosensitivity. However, it is not known if fewer nitrogen epochs elicit a similar HVR, HHR, and HBPR as 10 epochs.PurposeWe tested the hypothesis that 10 epochs of nitrogen breathing would elicit similar HVR, HHR, and HBPR responses when compared to 4 and 6 nitrogen epochs.MethodsNine young, healthy participants (age: 25 ± 2 y, BMI: 26 ± 3 kg/m2, 3 females) participated in one carotid body chemosensitivity trial. Participants rested in the supine position for 10 minutes before 5 minutes of baseline data were collected. Then, participants were exposed to 10 nitrogen epochs with 3 minutes of room air breathing separating each epoch. Each nitrogen epoch consisted of breathing 2–6 breaths of 100% nitrogen. The number of nitrogen breaths was randomized with the goal of eliciting a broad range of arterial oxygen saturation (SaO2) values (70–99%). Minute ventilation (pneumotach), heart rate (ECG), blood pressure (Finometer), and SaO2 (finger pulse oximeter) were continuously recorded throughout the trial. The mean of the three highest consecutive ventilation values, the peak heart rate, the greatest mean arterial pressure, and the lowest SaO2 were determined within the 2 minutes following each nitrogen epoch. HVR, HHR, and HBPR data are reported as the slope of the linear regression line of ventilation, heart rate, and mean arterial pressure vs. SaO2, respectively, for the first 4, 6, and 10 nitrogen epochs. These slopes were compared using one‐way repeated measures ANOVA.ResultsThe range of SaO2 values for 4, 6, and 10 nitrogen epochs was identical (71% to 98%). We did not observe differences in HVR between 4 epochs (0.10 ± 0.08 L/min/SaO2), 6 epochs (0.06 ± 0.11 L/min/SaO2), or 10 epochs (0.09 ± 0.07 L/min/SaO2) (p = 0.23). HHR was lower with 10 epochs (0.69 ± 0.37 bpm/SaO2) vs. both 4 epochs (1.00 ± 0.72 bpm/SaO2, p = 0.04) and 6 epochs (0.88 ± 0.54 bpm/SaO2, p < 0.01), while we did not observe any differences in the HHR between 4 and 6 epochs (p = 0.20). The HBPR was lower with 10 epochs (0.45 ± 0.25 mmHg/SaO2) vs. both 4 epochs (0.95 ± 0.37 mmHg/SaO2, p < 0.01) and 6 epochs (0.78 ± 0.44 mmHg/SaO2, p = 0.01). The HBPR was also different between 4 and 6 epochs (p = 0.05).ConclusionThese data indicate that fewer nitrogen epochs elicit a similar HVR as 10 epochs. However, the slopes for the HHR and HBPR are lowered with an increasing number of nitrogen epochs.Support or Funding InformationThe study was supported by the University at Buffalo Innovative Micro‐Programs Accelerating Collaboration in Themes (IMPACT) program.