M. SIEKIERKA1, M. TAFIL-KLAWE1, W. ADAMCZYK1,
J.J. KLAWE2, P. ZLOMANCZUK1


LOW AMPLITUDE DAILY CHANGES IN REFLEX VENTILATORY
RESPONSE TO PROGRESSIVE ISOCAPNIC HYPOXIA


1Department of Physiology and 2Department of Hygiene and Epidemiology,
Collegium Medicum in Bydgoszcz, Nicolaus Copernicus University in Torun, Poland


  Daily changes in reflex ventilatory responses to hypoxia were studied in 56 young healthy men participated. A rebreathing technique - progressive isocapnic hypoxia - was used to measure hypoxic chemoreflex reactivity. The measurements were repeated every 3 hours during a 24-hour period. The ventilatory response to hypoxia - analyzed as the relationships (slope) MV/SaO2 (minute ventilation/arterial oxygen blood saturation) and P0.1/SaO2 (mouth occlusion pressure/oxygen arterial blood saturation) remained relatively stable throughout the examined period exhibiting low-amplitude differences from one another with the maximum variation at 12:00 hours.

Key words: daily rhythm, hypoxia, rebreathing technique



INTRODUCTION

Many physiological variables exhibit daily oscillations. Most of them are circadian and dependent on endogenous circadian pacemaker located in the suprachiasmatic nuclei of anterior hypothalamus (1). Respiratory variability, and hence respiratory instability, is not a random process but depends on chemoreflex properties, i.e., spontaneous changes in breathing pattern are influenced by chemoreflex regulation (2-5). Stephenson at al (6) showed that nocturnal decline in the hypercapnic ventilatory response in the absence of sleep was primarily a result of the right shift of the chemoreflex response curve (6). Chemosensitivity, as a slope of the suprathreshold ventilatory response to hypercapnia, was not markedly reduced at night. Raschke (7) and Raschke and Moller (8), in a study in 10 male subjects (the study protocol allowed subjects to sleep before some of the rebreathing tests), observed that body temperature and ventilatory chemoreflex responses (hypoxic and hypercapnic) varied in parallel, both peaking in the early evening and reaching minimum in the morning (7, 8). The mechanisms underlying the effects of hypoxia on the daily patterns of breathing remain unclear.

The aim of the present study was to analyze daily variations in hypoxic sensitivity in human subjects, in normal conditions of moderate activity during the day and a short sleep time at night (04:00-6:00). Daily changes in the timing system and in the control of breathing are particularly important in the pathogenesis of nocturnal sleep-related breathing disorders.


MATERIAL AND METHODS

The study was approved by a local Ethics Committee and informed consent was obtained from all study participants. Fifty six male subjects, all healthy nonsmokers aged 18-28, were enrolled in the study. All subjects were asked to abstain from food, alcohol, or caffeinated drinks for at least 12 h before the experiment. They were placed for 24 h on a modified constant-routine protocol (10). They were isolated from the external environment, stayed in a room with ambient temperature of 20-22°C, artificial lightning and small meal every 3 h with a total energy content of 1500 kJ. The subjects were permitted to sleep between 04:00 and 06:00. During the day they presented moderate physical activity, could move, read, watch movies.

Each subject was placed on a repetitive 3-h test cycle. Each cycle began with a 10-min seated rest followed by a rebreathing test (9). A computerized experimental setup for ventilatory studies during changes in inspiratory gas mixtures consisting of a closed circuit with a built-in CO2 scrubber (soda lime) to maintain automatically a constant end-tidal PCO2 and with an electromagnetic valve closing every 5 breaths at 100 ms from the beginning of inspiratory effort to measure mouth occlusion pressure (P0.1) was used in the study (MES, Cracow, Poland). The rebreathing test took 16 min. Oxygen arterial blood saturation (SaO2) was monitored by a finger pulse oximeter (Trident, Poland). Total of 8 rebreathing tests were performed during the 24 h period of the constant-routine protocol. Hypoxic ventilatory drive was determined by minute ventilation (MV). Slopes of the regression curves MV/SaO2 and P0.1/SaO2 were analyzed. All data were expressed as means ±SD. Whitney-Mann U test was used for the statistical analysis. P<0.05 was accepted as the level of statistical significance.


RESULTS

Daily changes in the ventilatory response to progressive isocapnic hypoxia, presented as the mean values of the MV/SaO2 and P0.1/SaO2 slopes, are illustrated in Fig. 1, Fig. 2 and Fig. 3. In all cases the maximum level of hypoxic reactivity occurred at 12.00 am. The values at other times of the day are relatively stable, with the possible tendency to show ultradian rhythmicity, particularly in the hypoxic ventilatory response presented as the P0.1/SaO2 slope (Fig. 3).

Fig. 1. Circadian changes in hypoxic ventilatory responses presented as the mean values of the MV/SaO2 slope. The 12:00 slope was significantly different from those at 0:00, 06:00, 18:00, and 21:00 hours (P<0.05).

Fig. 2. Circadian changes in hypoxic ventilatory response presented as the mean values of minute ventilation (MV) at SaO2 of 80%. Minute ventilation at 12:00 was significantly different from those at 0:00, 18:00, and 21:00 hours (P<0.05).

Fig. 3. Circadian changes in hypoxic ventilatory response presented as the mean values of the P0.1/SaO2 slope. There were significant differences in mouth occlusion pressure between 12:00 vs. 00:00 and vs. 21:00, and between 15:00 vs. 18:00 (P<0.05).


DISCUSSION

Mills et al (10) proposed protocol of "constant routine". It is a protocol aiming to explore daily patterns in constant behavioral and environmental conditions. This approach has been employed to the studies of daily rhythms of respiratory function in humans (6, 11, 12). The imposed prolonged wakefulness of the "constant routine" protocol is a small disadvantage compared to the interpretative problems which would arise by changes in the state of arousal, different caloric intake, and the level of activity (13). Raschke (7) and Raschke and Moller (8) demonstrated daily rhythmicity in O2 sensitivity, applying a protocol that allowed subjects to sleep. Stephenson et al (6) showed daily variations in the CO2 threshold (but not in the CO2 sensitivity) in subjects who did not sleep. Short sleep loss has been shown to have minimal effects on human or canine respiratory chemoreflex (14). We studied the subjects applying a little modification of "constant routine" protocol, as our modified protocol allowed subjects to sleep between 04:00 and 06:00 (one sleep cycle) and to be moderately active during the day. In our study, the hypoxic ventilatory response remained relatively stable in the course of 24 hours, showing a maximum at mid-day (12:00). A tendency to ultradian rhythmicity is similar to another ultradian biological rhythms observed in humans (15-17). We conclude that the respiratory response to hypoxia in the condition of moderate activity remains relatively stable during 24 hours, except for a noon peak. The stability of hypoxic responsiveness may be influenced by changes in arousal during normal activity, as opposed to the "constant routine protocol". We presume that change from wakefulness to night sleep of normal duration is accompanied by depression of respiratory responsiveness, and increased propensity for respiratory instability at night may potentially have to do with the circadian timing system.

Acknowledgments: This study was supported by KBN grant 2P05B02626.


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Author’s address: M. Tafil-Klawe, Department of Physiology, Collegium Medicum in Bydgoszcz, Kar³owicza 24 St., 85-092 Bydgoszcz, Poland; phone: +48 604 547374.