An endogenous circadian rhythm of respiratory control in humans

Abstract

Many physiological and behavioural functions have circadian rhythms - endogenous oscillations with a period of approximately 24 h that can occur even in the absence of sleep. We determined whether there is an endogenous circadian rhythm in breathing, metabolism and ventilatory chemosensitivity in humans. Ten healthy, adult males were studied throughout 4 days in a stable laboratory environment. After two initial baseline days (16 h wakefulness plus 8 h sleep) that served to achieve a steady state, subjects were studied under constant behavioural and environmental conditions throughout 41 h of wakefulness. Ventilation, metabolism and the magnitude of the hypercapnic ventilatory response (HCVR) were measured every 2 h. Individuals' data were aligned according to circadian phase (core body temperature minimum; CBTmin) and averaged. In the group average data, there was a significant and large amplitude circadian variation in HCVR slope (average of +/-0.4 l min-1 mmHg-1; corresponding to +/-12.1 % of 24 h mean), and a smaller amplitude rhythm in the HCVR x-axis intercept (average of +/-1.1 mmHg; +/-2.1 % of 24 h mean). Despite a significant circadian variation in metabolism (+/-3.2 % of 24 h mean), there were no detectable rhythms in tidal volume, respiratory frequency or ventilation. This small discrepancy between metabolism and ventilation led to a small but significant circadian variation in end-tidal PCO2 (PET,CO2; +/-0.6 mmHg; +/-1.5 % of 24 h mean). The circadian minima of the group-averaged respiratory variables occurred 6-8 h earlier than CBTmin, suggesting that endogenous changes in CBT across the circadian cycle have less of an effect on respiration than equivalent experimentally induced changes in CBT. Throughout these circadian changes, there were no correlations between HCVR parameters (slope or x-axis intercept) and either resting ventilation or resting PET,CO2. This suggests that ventilation and PET,CO2 are little influenced by central chemosensory respiratory control in awake humans even when at rest under constant environmental and behavioural conditions. The characteristic change in PET,CO2 during non-rapid eye movement sleep was shown to be independent of circadian variations in PET,CO2, and probably reflects a change from predominantly behavioural to predominantly chemosensory respiratory control. This study has documented the existence and magnitude of circadian variations in respiration and respiratory control in awake humans for the first time under constant behavioural and environmental conditions. These results provide unique insights into respiratory control in awake humans, and highlight the importance of considering the phase of the circadian cycle in studies of respiratory control.

Figure 1

Circadian rhythms of respiration detected with a constant routine protocol Shown are the group mean levels (±s.e.m.) of end-tidal P_CO2 (P_ET,CO2), ventilation (V_E), CO₂ production (V_CO2), the slope of the line that describes the hypercapnic ventilatory response (HCVR), plasma cortisol concentration and core body temperature (CBT). Individuals’ data (n = 10) were aligned with respect to the reference circadian rhythm – core body temperature minimum (CBT_min) – and averaged. The s.e.m. of all data are shown, but are small and indistinguishable from the mean at most time points for P_ET,CO2, cortisol and CBT. Wider variation occurred in the other variables. The ordinate is expressed as the percentage deviation from the 24 h mean (left) and in absolute units (right). Data from 24 h collected during the constant routine and centred around the CBT_min are ‘double plotted’ for ease of visualising circadian rhythms (i.e. the 24 h data on the left are reproduced on the right). The abscissa is expressed in degrees (with CBT_min assigned a phase of 0 deg) and in relative clock hour. The shaded bars on the abscissa represent the time of the subjects’ usual sleep episodes (though sleep did not occur during this constant routine). Small differences occur between the values in Table 1 and the impression from this figure because the values in the table were derived from the regression analysis, whereas the actual mean data are plotted in this figure.

Figure 2

Relationship between variables throughout a constant routine protocol Relationship among pairs of variables across 36 h of a constant routine protocol in a single representative subject (V_E, P_ET,CO2, HCVR slope and x-axis intercept, V_CO2, plasma cortisol concentration and CBT). Whenever significant correlations occurred (P < 0.05) these are indicated with the regression line of the relationship. These graphs show that there were significant positive correlations between HCVR slope and HCVR x-axis intercept, and between V_E and V_CO2, and there was a significant negative correlation between V_E/V_CO2 and V_CO2, indicating that metabolism increases more than V_E, inducing an increase in P_ET,CO2. These data also show that across the circadian cycle, ventilation and metabolic rate were not correlated with CBT, cortisol concentration or HCVR. In addition, there were no significant correlations between HCVR and either P_ET,CO2 or V_E (not shown).

Figure 3

Relationship between resting and hypercapnic ventilatory responses throughout a constant routine protocol The relationship between the HCVR (lines), spontaneous resting P_ET,CO2 plotted at resting ventilation (V_E, ○), and estimated P_CO2 at the central chemoreceptors plotted at a ventilation expected during rest when hyperoxic (□, see Discussion for details). Data are plotted across 36 h of a constant routine protocol in a single representative subject (same subject as in Fig. 2). The intersection between the HCVR and the x-axis represents the x-intercept. The graph shows that, as the HCVR slope increases, the x-axis intercept increases such that the HCVR response line appears to ‘pivot’ around a specific P_ET,CO2 well above both the resting P_ET,CO2 and the estimated P_CO2 at the central chemoreceptors. We found that in all subjects, the estimated P_CO2 at the central chemoreceptors was similar to the HCVR intercept but substantially lower (-5.9 mmHg) than the P_CO2 required to explain the actual level of resting ventilation (minus 20 % adjustment for hyperoxia). In the typical subject, equilibrium P_CO2 was less than the HCVR intercept in 11 out of 18 measurement periods, and the hyperoxic resting ventilation level at equilibrium P_CO2 was substantially higher than predicted from the HCVR line in 17 out of 18 periods. These data suggest that CO₂ at the central chemoreceptors at rest is not sufficient to explain the level of ventilation at rest.