Effects of different levels of end-tidal PO2 on ventilation during isocapnia in humans

Recovery from acute hypoxia: A systematic review of cognitive and physiological responses during the 'hypoxia hangover'

Recovery of cognitive and physiological responses following a hypoxic exposure may not be considered in various operational and research settings. Understanding recovery profiles and influential factors can guide post-hypoxia restrictions to reduce the risk of further cognitive and physiological deterioration, and the potential for incidents and accidents. We systematically evaluated the available evidence on recovery of cognitive and basic physiological responses following an acute hypoxic exposure to improve understanding of the performance and safety implications, and to inform post-hypoxia restrictions. This systematic review summarises 30 studies that document the recovery of either a cognitive or physiological index from an acute hypoxic exposure. Titles and abstracts from PubMed (MEDLINE) and Scopus were searched from inception to July 2022, of which 22 full text articles were considered eligible. An additional 8 articles from other sources were identified and also considered eligible. The overall quality of evidence was moderate (average Rosendal score, 58%) and there was a large range of hypoxic exposures. Heart rate, peripheral blood haemoglobin-oxygen saturation and heart rate variability typically normalised within seconds-to-minutes following return to normoxia or hyperoxia. Whereas, cognitive performance, blood pressure, cerebral tissue oxygenation, ventilation and electroencephalogram indices could persist for minutes-to-hours following a hypoxic exposure, and one study suggested regional cerebral tissue oxygenation requires up to 24 hours to recover. Full recovery of most cognitive and physiological indices, however, appear much sooner and typically within ~2-4 hours. Based on these findings, there is evidence to support a 'hypoxia hangover' and a need to implement restrictions following acute hypoxic exposures. The severity and duration of these restrictions is unclear but should consider the population, subsequent requirement for safety-critical tasks and hypoxic exposure.

Lack of influence of dexmedetomidine on rat glomus cell response to hypoxia, and on mouse acute hypoxic ventilatory response

Background and Aims

There is a lack of basic science data on the effect of dexmedetomidine on the hypoxic chemosensory reflex with both depression and stimulation suggested. The primary aim of this study was to assess if dexmedetomidine inhibited the cellular response to hypoxia in rat carotid body glomus cells, the cells of the organs mediating acute hypoxic ventilatory response (AHVR). Additionally, we used a small sample of mice to assess if there was any large influence of subsedative doses of dexmedetomidine on AHVR.

Material and Methods

In the primary study, glomus cells isolated from neonatal rats were used to study the effect of 0.1 nM (n = 9) and 1 nM (n = 13) dexmedetomidine on hypoxia-elicited intracellular calcium [Ca2%]i influx using ratiometric fluorimetry. Secondarily, whole animal unrestrained plethysmography was used to study AHVR in a total of 8 age-matched C57BL6 mice, divided on successive days into two groups of four mice randomly assigned to receive sub-sedative doses of 5, 50, or 500 μg.kg-1 dexmedetomidine versus control in a crossover study design (total n = 12 exposures to drug with n = 12 controls).

Results

There was no effect of dexmedetomidine on the hypoxia-elicited increase in [Ca2%]i in glomus cells (a mean ± SEM increase of 95 ± 32 nM from baseline with control hypoxia, 124 ± 41 nM with 0.1 nM dexmedetomidine; P = 0.514). In intact mice, dexmedetomidine had no effect on baseline ventilation during air-breathing (4.01 ± 0.3 ml.g-1.min-1 in control and 2.99 ± 0.5 ml.g-1.min-1 with 500 μg.kg-1 dexmedetomidine, the highest dose; P = 0.081) or on AHVR (136 ± 19% increase from baseline in control, 152 ± 46% with 500 μg.kg-1 dexmedetomidine, the highest dose; P = 0.536).

Conclusion

Dexmedetomidine had no effect on the cellular responses to hypoxia. We conclude that it unlikely acts via inhibition of oxygen sensing at the glomus cell. The respiratory chemoreflex effects of this drug remain an open question. In our small sample of intact mice, hypoxic chemoreflex responses and basal breathing were preserved.

Time Domains of the Hypoxic Ventilatory Response and Their Molecular Basis

Ventilatory responses to hypoxia vary widely depending on the pattern and length of hypoxic exposure. Acute, prolonged, or intermittent hypoxic episodes can increase or decrease breathing for seconds to years, both during the hypoxic stimulus, and also after its removal. These myriad effects are the result of a complicated web of molecular interactions that underlie plasticity in the respiratory control reflex circuits and ultimately control the physiology of breathing in hypoxia. Since the time domains of the physiological hypoxic ventilatory response (HVR) were identified, considerable research effort has gone toward elucidating the underlying molecular mechanisms that mediate these varied responses. This research has begun to describe complicated and plastic interactions in the relay circuits between the peripheral chemoreceptors and the ventilatory control circuits within the central nervous system. Intriguingly, many of these molecular pathways seem to share key components between the different time domains, suggesting that varied physiological HVRs are the result of specific modifications to overlapping pathways. This review highlights what has been discovered regarding the cell and molecular level control of the time domains of the HVR, and highlights key areas where further research is required. Understanding the molecular control of ventilation in hypoxia has important implications for basic physiology and is emerging as an important component of several clinical fields. © 2016 American Physiological Society. Compr Physiol 6:1345-1385, 2016.

Volatile anaesthetic depression of the carotid body chemoreflex-mediated ventilatory response to hypoxia: directions for future research

In assessing whether volatile anaesthetics directly depress the carotid body response to hypoxia it is necessary to combine in meta-analysis studies of when it is "functionally isolated" (e.g., recordings are made from its afferent nerve). Key articles were retrieved (full papers in English) and subjected to quantitative analysis to yield an aggregate estimate of effect. Results from articles that did not use such methodology were assessed separately from this quantitative approach, to see what could be learned also from a nonquantitative overview. Just 7 articles met the inclusion criteria for hypoxia and just 6 articles for hypercapnia. Within these articles, the anaesthetic (mean dose 0.75, standard deviation (SD) 0.40 minimum alveolar concentration, MAC) statistically significantly depressed carotid body hypoxic response by 24% (P = 0.041), but a similar dose (mean 0.81 (0.42) MAC) did not affect the hypercapnic response. The articles not included in the quantitative analysis (31 articles), assessed qualitatively, also indicated that anaesthetics depress carotid body function. This conclusion helps direct future research on the anaesthetic effects on putative cellular/molecular processes that underlie the transduction of hypoxia in the carotid body.

Ventilatory responses to isocapnic and poikilocapnic hypoxia in humans

We examined the hypoxic ventilatory response (HVR) including breathing frequency (f(R)) and tidal volume (V(T)) responses during 20 min of step isocapnic (IH) and poikilocapnic (PH) hypoxia (45 Torr). We hypothesized an index related to [Formula: see text] (pHPR) may be more robust during PH. Peak HVR was suppressed during PH (P<0.001), and mediated by V(T) during PH and both V(T) and f(R) during IH. The relative magnitude of HVD remained similar between conditions indicating a suppressive role of hypocapnia in development of the HVR unrelated to the degree of subsequent HVD, implying a primarily O(2) dependant mechanism. Post-hypoxic frequency decline was observed following both IH (3.4+/-3.7 bpm, P<0.05) and PH (3.6+/-3.1 bpm, P<0.01), despite no f(R) response during exposure to PH. Use of pHPR improved the signal to noise ratio during PH, though failed to detect the peak ventilatory response, and therefore may not be appropriate when describing peak responses.

Specific carotid body chemostimulation is sufficient to elicit phrenic poststimulus frequency decline in a novel in situ dual-perfused rat preparation

Time-dependent ventilatory responses to hypoxic and hypercapnic challenges, such as posthypoxic frequency decline (PHxFD) and posthypercapnic frequency decline (PHcFD), could profoundly affect breathing stability. However, little is known about the mechanisms that mediate these phenomena. To determine the contribution of specific carotid body chemostimuli to PHxFD and PHcFD, we developed a novel in situ arterially perfused, vagotomized, decerebrate rat preparation in which central and peripheral chemoreceptors are perfused separately (i.e., a nonanesthetized in situ dual perfused preparation). We confirmed that 1) the perfusion of central and peripheral chemoreceptor compartments was independent by applying specific carotid body hypoxia and hypercapnia before and after carotid sinus nerve transection, 2) the PCO(2) chemoresponse of the dual perfused preparation was similar to other decerebrate preparations, and 3) the phrenic output was stable enough to allow investigation of time-dependent phenomena. We then applied four 5-min bouts (separated by 5 min) of specific carotid body hypoxia (40 Torr PO(2) and 40 Torr PCO(2)) or hypercapnia (100 Torr PO(2) and 60 Torr PCO(2)) while holding the brain stem PO(2) and PCO(2) constant. We report the novel finding that specific carotid body chemostimuli were sufficient to elicit several phrenic time-dependent phenomena in the rat. Hypoxic challenges elicited PHxFD that increased with bout, leading to progressive augmentation of the phrenic response. Conversely, hypercapnia elicited short-term depression and PHcFD, neither of which was bout dependent. These results, placed in the context of previous findings, suggest multiple physiological mechanisms are responsible for PHxFD and PHcFD, a redundancy that may illustrate that these phenomena have significant adaptive advantages.

Time course of air hunger mirrors the biphasic ventilatory response to hypoxia

Determining response dynamics of hypoxic air hunger may provide information of use in clinical practice and will improve understanding of basic dyspnea mechanisms. It is hypothesized that air hunger arises from projection of reflex brain stem ventilatory drive (“corollary discharge”) to forebrain centers. If perceptual response dynamics are unmodified by events between brain stem and cortical awareness, this hypothesis predicts that air hunger will exactly track ventilatory response. Thus, during sustained hypoxia, initial increase in air hunger would be followed by a progressive decline reflecting biphasic reflex ventilatory drive. To test this prediction, we applied a sharp-onset 20-min step of normocapnic hypoxia and compared dynamic response characteristics of air hunger with that of ventilation in 10 healthy subjects. Air hunger was measured during mechanical ventilation (minute ventilation = 9 ± 1.4 l/min; end-tidal Pco2 = 37 ± 2 Torr; end-tidal Po2 = 45 ± 7 Torr); ventilatory response was measured during separate free-breathing trials in the same subjects. Discomfort caused by “urge to breathe” was rated every 30 s on a visual analog scale. Both ventilatory and air hunger responses were modeled as delayed double exponentials corresponding to a simple linear first-order response but with a separate first-order adaptation. These models provided adequate fits to both ventilatory and air hunger data ( r2 = 0.88 and 0.66). Mean time constant and time-to-peak response for the average perceptual response (0.36 min−1 and 3.3 min, respectively) closely matched corresponding values for the average ventilatory response (0.39 min−1 and 3.1 min). Air hunger response to sustained hypoxia tracked ventilatory drive with a delay of ∼30 s. Our data provide further support for the corollary discharge hypothesis for air hunger.

Selected contribution: High-altitude natives living at sea level acclimatize to high altitude like sea-level natives

Sea-level (SL) natives acclimatizing to high altitude (HA) increase their acute ventilatory response to hypoxia (AHVR), but HA natives have values for AHVR below those for SL natives at SL (blunting). HA natives who live at SL retain some blunting of AHVR and have more marked blunting to sustained (20-min) hypoxia. This study addressed the question of what happens when HA natives resident at SL return to HA: do they acclimatize like SL natives or revert to the characteristics of HA natives? Fifteen HA natives resident at SL were studied, together with 15 SL natives as controls. Air-breathing end-tidal Pco(2) and AHVR were determined at SL. Subjects were then transported to 4,300 m, where these measurements were repeated on each of the following 5 days. There were no significant differences in the magnitude or time course of the changes in end-tidal Pco(2) and AHVR between the two groups. We conclude that HA natives normally resident at SL undergo ventilatory acclimatization to HA in the same manner as SL natives.

Hypoxic and hypercapnic drives to breathe generate equivalent levels of air hunger in humans

Anecdotal observations suggest that hypoxia does not elicit dyspnea. An opposing view is that any stimulus to medullary respiratory centers generates dyspnea via "corollary discharge" to higher centers; absence of dyspnea during low inspired Po(2) may result from increased ventilation and hypocapnia. We hypothesized that, with fixed ventilation, hypoxia and hypercapnia generate equal dyspnea when matched by ventilatory drive. Steady-state levels of hypoxic normocapnia (end-tidal Po(2) = 60-40 Torr) and hypercapnic hyperoxia (end-tidal Pco(2) = 40-50 Torr) were induced in naive subjects when they were free breathing and during fixed mechanical ventilation. In a separate experiment, normocapnic hypoxia and normoxic hypercapnia, "matched" by ventilation in free-breathing trials, were presented to experienced subjects breathing with constrained rate and tidal volume. "Air hunger" was rated every 30 s on a visual analog scale. Air hunger-Pet(O(2)) curves rose sharply at Pet(O(2)) <50 Torr. Air hunger was not different between matched stimuli (P > 0.05). Hypercapnia had unpleasant nonrespiratory effects but was otherwise perceptually indistinguishable from hypoxia. We conclude that hypoxia and hypercapnia have equal potency for air hunger when matched by ventilatory drive. Air hunger may, therefore, arise via brain stem respiratory drive.

A simple breathing circuit to maintain isocapnia during measurements of the hypoxic ventilatory response

We report the development and testing of a simple breathing circuit that maintains isocapnia in human subjects during hypoxic hyperpnea. In addition, the circuit permits rapid switching between two gas mixtures with different partial pressures of oxygen. Eleven volunteers breathed repeated cycles of exposure to air (2 min of 21% O(2), balance N(2)) and hypoxia (2 min of 8.3+/-0.1% O(2), balance N(2)). Hypoxia induced significant increases in minute ventilation, breathing frequency and tidal volume (P < 0.05) that were consistent over repeated cycles of hypoxia (P > 0.1, one-way ANOVA). The system successfully maintained isocapnia in all subjects, with an average change in end-tidal CO(2) of only -0.2 mmHg during hyperventilation in hypoxia (range 0.4 to -0.8 mmHg). This system may be suitable for repeated tests of the hypoxic ventilatory response (HVR) and may prove useful for exploring intra- and inter-individual variability of HVR in humans.

Ventilatory response to 2-h sustained hypoxia in humans

We used two protocols to determine if hypoxic ventilatory decline (HVD) involves changes in slope and/or intercept of the isocapnic HVR (hypoxic ventilatory response, expressed as the increase in VI per percentage decrease in SaO2). Isocapnia was defined as 1.5 mmHg above hyperoxic PET(CO2). HVD was recorded in protocol I during two sequential 25 min exposures to isocapnic hypoxia (85 and 75% SaO2, n=7) and in protocol II during 14 min of isocapnic hypoxia (90% SaO2, FIO2=0.13, n=15), extended to 2 h of hypoxia with CO2-uncontrolled in eight subjects. HVR was measured by the step reduction to sequentially lower levels of SaO2 in protocol I and by 3 min steps to 80% SaO2 at 8, 14 and 120 min in protocol II. The intercept of the HVR (VI predicted at SaO2=100%) decreased after 14 and 25 min in both protocols (P<0.05). Changes in slope were observed only in protocol I at SaO2=75%, suggesting that the slope of the HVR is more sensitive to depth than duration of hypoxic exposure. After 2 h of hypoxia the HVR intercept returned toward control value (P<0.05) with still no significant changes in the HVR slope. We conclude that HVD in humans involves a decrease in hyperoxic ventilatory drive that can occur without significant change in slope of the HVR. The partial reversal of the HVD after 2 h of hypoxia may reflect some components of ventilatory acclimatization to hypoxia.

Comparison of effects of sustained isocapnic hypoxia on ventilation in men and women

Sleep-related respiratory disturbances are more common in men than in premenopausal women. This might, in part, be due to different susceptibilities to the respiratory depressant effects of hypoxia. Therefore, we compared ventilation during 10 min of baseline room-air breathing and 20-min sustained isocapnic hypoxia (fractional inspired O2 = 11%, arterial saturation of O2 approximately 80%) followed by 10 min of breathing 100% O2 in 10 normal men and in 10 women in the follicular phase of the menstrual cycle. Control measurements were made during two transitions from room air (10 min) to 100% O2 (10 min) and averaged. Inspired minute ventilation (VI) after 2 min of hypoxia was the same in men and women [131 +/- 6.1% baseline for men, 136 +/- 7.7% baseline for women; not significant (NS)] and declined to the same level after 20 min (115 +/- 5.0% baseline for men, 116 +/- 6.6% baseline for women; NS) associated with a similar decline in inspiratory time and tidal volume. Breathing frequency did not change. VI decreased transiently during subsequent 100% O2 breathing in both men and women, associated with reduced frequency and duty cycle and increased expiratory time. The fall in VI was significantly greater than that observed during control hyperoxia experiments in men but not in women. We conclude that ventilatory responses to sustained isocapnic hypoxia do not differ between awake healthy men and women in the follicular phase of their menstrual cycle. However, after termination of isocapnic hypoxia, men appear to depress their ventilation to a greater degree than women.

Extended models of the ventilatory response to sustained isocapnic hypoxia in humans

The purpose of this study was to examine extensions of a model of hypoxic ventilatory decline (HVD) in humans. In the original model (model I) devised by R. Painter, S. Khamnei, and P. Robbins (J. Appl. Physiol. 74: 2007-2015, 1993), HVD is modeled entirely by a modulation of peripheral chemoreflex sensitivity. In the first extension (model II), a more complicated dynamic is used for the change in peripheral chemoreflex sensitivity. In the second extension (model III), HVD is modeled as a combination of both the mechanisms of Painter et al. and a component that is independent of peripheral chemoreflex sensitivity. In all cases, a parallel noise structure was incorporated to describe the stochastic properties of the ventilatory behavior to remove the correlation of the residuals. Data came from six subjects from a study by D.A. Bascom, J.J Pandit, I.D. Clement, and P.A. Robbins (Respir. Physiol. 88: 299-312, 1992). For model II, there was a significant improvement in fit for two out of six subjects. The reasons for this were not entirely clear. For model III, the fit was again significantly improved in two subjects, but in this case the subjects were those who had the most marked undershoot and recovery of ventilation at the relief of hypoxia. In these two subjects, the chemoreflex-independent component contributed approximately 50% to total HVD. In the other four subjects, the chemoreflex-independent component contributed approximately 10% to total HVD. It is concluded that in some subjects, but not in others, there may be a component of HVD that is independent of peripheral chemoreflex sensitivity.

Problems with determining the hypoxic response in humans using stepwise changes in end-tidal PO2

This study examined whether the form of the acute ventilatory response to different levels of end-tidal PO2 (PETO2) could be determined from a progressive series of steps in PETO2 at constant end-tidal PCO2. Seven levels of PETO2 were employed each lasting 2 min. The steps were performed in both ascending and descending order to test whether the ventilatory responses were affected by the order of the hypoxic exposures. These exposures were carried out both with and without a prior 20 min period of isocapnic hypoxia (PETO2 = 50 mmHg). Each protocol was undertaken 6 times in each of 6 subjects. With prior exposure to hypoxia, the order in which the steps were performed affected the ventilatory response (P < 0.005, ANOVA). Without prior exposure to hypoxia, this finding did not quite reach significance (P < 0.056), unless one particular abnormal subject was excluded (P < 0.001). It is concluded that the order of the hypoxic exposures in this particular test affects the form of the hypoxic response.