A new aspect of the carotid body function controlling hypoxic ventilatory decline in humans

Integrated respiratory chemoreflex-mediated regulation of cerebral blood flow in hypoxia: Implications for oxygen delivery and acute mountain sickness

NEW FINDINGS

What is the central question of this study? To what extent do hypoxia-induced changes in the peripheral and central respiratory chemoreflex modulate anterior and posterior cerebral oxygen delivery, with corresponding implications for susceptibility to acute mountain sickness? What is the main finding and its importance? We provide evidence for site-specific regulation of cerebral blood flow in hypoxia that preserves oxygen delivery in the posterior but not the anterior cerebral circulation, with minimal contribution from the central respiratory chemoreflex. External carotid artery vasodilatation might prove to be an alternative haemodynamic risk factor that predisposes to acute mountain sickness.

ABSTRACT

The aim of the present study was to determine the extent to which hypoxia-induced changes in the peripheral and central respiratory chemoreflex modulate anterior and posterior cerebral blood flow (CBF) and oxygen delivery (CDO₂ ), with corresponding implications for the pathophysiology of the neurological syndrome, acute mountain sickness (AMS). Eight healthy men were randomly assigned single blind to 7 h of passive exposure to both normoxia (21% O₂ ) and hypoxia (12% O₂ ). The peripheral and central respiratory chemoreflex, internal carotid artery, external carotid artery (ECA) and vertebral artery blood flow (duplex ultrasound) and AMS scores (questionnaires) were measured throughout. A reduction in internal carotid artery CDO₂ was observed during hypoxia despite a compensatory elevation in perfusion. In contrast, vertebral artery and ECA CDO₂ were preserved, and the former was attributable to a more marked increase in perfusion. Hypoxia was associated with progressive activation of the peripheral respiratory chemoreflex (P < 0.001), whereas the central respiratory chemoreflex remained unchanged (P > 0.05). Symptom severity in participants who developed clinical AMS was positively related to ECA blood flow (Lake Louise score, r = 0.546-0.709, P = 0.004-0.043; Environmental Symptoms Questionnaires-Cerebral symptoms score, r = 0.587-0.771, P = 0.001-0.027, n = 4). Collectively, these findings highlight the site-specific regulation of CBF in hypoxia that maintains CDO₂ selectively in the posterior but not the anterior cerebral circulation, with minimal contribution from the central respiratory chemoreflex. Furthermore, ECA vasodilatation might represent a hitherto unexplored haemodynamic risk factor implicated in the pathophysiology of AMS.

Time-dependent changes in cardiorespiratory functions of anesthetized rats exposed to sustained hypoxia

Although cardiovascular responses may be altered by respiratory changes under prolonged hypoxia, the relationship between respiratory and cardiovascular changes remains unknown. The aim of the present study is to clarify cardiorespiratory changes in anesthetized rats during and after hypoxic conditions using simultaneous recordings of cardiorespiratory variables with 20-sec recording intervals. After air breathing for 20 min (pre-exposure period), rats were subjected to 10% O₂ for 2 h (hypoxic exposure period) and then air for 30 min (recovery period). Minute ventilation (V_E), respiratory frequency, tidal volume, arterial blood pressure (BP), and heart rate (HR) were continuously monitored during the experimental period. Just after hypoxic exposure, V_E, BP, and HR exhibited an overshoot, undershoot, and overshoot followed by a decrease, respectively. During the remaining hypoxic exposure period, continuous high V_E and low BP were observed, whereas HR re-increased. In the recovery period, V_E, BP, and HR showed an undershoot, increase, and decrease followed by an increase, respectively. These results suggest that the continuation of enhanced V_E and re-increased HR, probably, due to carotid body excitation and accompanying sympathetic activation, during the late period of hypoxic exposure are protective responses to avoid worsening hypoxemia and further circulatory insufficiencies under sustained hypoxia.

On the existence of a central respiratory oxygen sensor

A commonly held view that dominates both the scientific and educational literature is that in terrestrial mammals the central nervous system lacks a physiological hypoxia sensor capable of triggering increases in lung ventilation in response to decreases in Po₂ of the brain parenchyma. Indeed, a normocapnic hypoxic ventilatory response has never been observed in humans following bilateral resection of the carotid bodies. In contrast, almost complete or partial recovery of the hypoxic ventilatory response after denervation/removal of the peripheral respiratory oxygen chemoreceptors has been demonstrated in many experimental animals when assessed in an awake state. In this essay we review the experimental evidence obtained using in vitro and in vivo animal models, results of human studies, and discuss potential mechanisms underlying the effects of CNS hypoxia on breathing. We consider experimental limitations and discuss potential reasons why the recovery of the hypoxic ventilatory response has not been observed in humans. We review recent experimental evidence suggesting that the lower brain stem contains functional oxygen sensitive elements capable of stimulating respiratory activity independently of peripheral chemoreceptor input.

Integration of Central and Peripheral Respiratory Chemoreflexes

A debate has raged since the discovery of central and peripheral respiratory chemoreceptors as to whether the reflexes they mediate combine in an additive (i.e., no interaction), hypoadditive or hyperadditive manner. Here we critically review pertinent literature related to O2 and CO2 sensing from the perspective of system integration and summarize many of the studies on which these seemingly opposing views are based. Despite the intensity and quality of this debate, we have yet to reach consensus, either within or between species. In reviewing this literature, we are struck by the merits of the approaches and preparations that have been brought to bear on this question. This suggests that either the nature of combination is not important to system responses, contrary to what has long been supposed, or that the nature of the combination is more malleable than previously assumed, changing depending on physiological state and/or respiratory requirement.

The effects of lower body positive and negative pressure on the hypoxic ventilatory decline

PURPOSE

Lower body negative pressure (LBNP) augments the acute hypoxic ventilatory response (AHVR) in humans, presumably through altered central integration of baro- and chemoreceptor afferents. This study investigated the effects of LBNP and lower body positive pressure (LBPP) on hypoxic ventilatory decline (HVD) in humans.

METHODS

Nine individuals (4 females and 5 males) were tested in a supine position with the lower body supported inside a hypo/hyperbaric chamber. During each test the participant was exposed in a random order to LBNP at -37.5mmHg, LBPP at +37.5mmHg and to ambient pressure (LBAP) at 0mmHg. Blood pressure, expired gases and haemoglobin O(2) saturation were continuously recorded. Hypoxia was administered in a single step to a PET O₂ of 50mmHg for 20min. For all tests PET CO₂ was maintained at the pre-hypoxic resting level.

RESULTS

The peak ventilation was significantly greater during LBNP (36.0+/-10.8Lmin(-1)) than during ambient pressure (29.4+/-8.1Lmin(-1); p=0.032). However, peak ventilation was not significantly different between LBPP and ambient pressure. The HVD was not significantly different across the three conditions (p=0.144). Both mean arterial pressure and pulse pressure were not affected by 37.5mmHg of either LBPP (p=0.941) or LBNP (p=0.275). Baroreflex slope was decreased by both hypoxia and LBNP.

CONCLUSION

These data suggest that LBNP increases AHVR through an effect on the baroreflex, while LBPP has no effect on AHVR. Since LBNP increases AHVR without affecting HVD, these findings support that the mechanism accounting for the HVD includes afferent output originating from the peripheral rather than the central chemosensitive tissues.

The Ventilatory Response to Hypoxia in Mammals: Mechanisms, Measurement, and Analysis

The respiratory response to hypoxia in mammals develops from an inhibition of breathing movements in utero into a sustained increase in ventilation in the adult. This ventilatory response to hypoxia (HVR) in mammals is the subject of this review. The period immediately after birth contains a critical time window in which environmental factors can cause long-term changes in the structural and functional properties of the respiratory system, resulting in an altered HVR phenotype. Both neonatal chronic and chronic intermittent hypoxia, but also chronic hyperoxia, can induce such plastic changes, the nature of which depends on the time pattern and duration of the exposure (acute or chronic, episodic or not, etc.). At adult age, exposure to chronic hypoxic paradigms induces adjustments in the HVR that seem reversible when the respiratory system is fully matured. These changes are orchestrated by transcription factors of which hypoxia-inducible factor 1 has been identified as the master regulator. We discuss the mechanisms underlying the HVR and its adaptations to chronic changes in ambient oxygen concentration, with emphasis on the carotid bodies that contain oxygen sensors and initiate the response, and on the contribution of central neurotransmitters and brain stem regions. We also briefly summarize the techniques used in small animals and in humans to measure the HVR and discuss the specific difficulties encountered in its measurement and analysis.

Arterial [H+] and the ventilatory response to hypoxia in humans: influence of acetazolamide-induced metabolic acidosis

In this study, we investigated possible separate effects of H+ ions and CO2 on hypoxic sensitivity in humans. We also examined whether hypoxic sensitivity, conventionally defined as the ratio of (hypoxic - normoxic) ventilation over (hypoxic - normoxic) Hb oxygen saturation can also be estimated by taking the ratio (hypoxic - normoxic) ventilation over (logPa(O2) hypoxia - logPa(O2) normoxia), enabling one to measure the hypoxic response independently from potential confounding influences of changes in position of the Hb oxygen saturation curve. We used acetazolamide to induce a metabolic acidosis. To determine the acute hypoxic response (AHR), we performed step decreases in end-tidal Po2 to approximately 50 Torr lasting 5 min each at three different constant end-tidal Pco2 levels. Nine subjects ingested 250 mg of acetazolamide or placebo every 8 h for 3 days in a randomized double-blind crossover design. The metabolic acidosis was accompanied by a rise in ventilation, a substantial fall in Pa(CO2), and a parallel leftward shift of the ventilatory CO2 response curve. In placebo, CO2 induced equal relative increases in hypoxic sensitivity (O2-CO2 interaction) regardless of the way it was defined. Acetazolamide shifted the response line representing the relationship between hypoxic sensitivity and arterial [H+] ([H+](a)) to higher values of [H+](a) without altering its slope, indicating that it did not affect the O2-CO2 interaction. So, in contrast to an earlier belief, CO2 and H+ have separate effects on hypoxic sensitivity. This was also supported by the finding that infusion of bicarbonate caused a leftward shift of the hypoxic sensitivity-[H+](a) response lines in placebo and acetazolamide. A specific inhibitory effect of acetazolamide on hypoxic sensitivity was not demonstrated.

Sudden death after major head and neck surgery

PURPOSE OF REVIEW

This review will discuss the mortality after major ear, nose and throat surgery, particularly sudden death. It will also discuss the postoperative follow-up of patients.

RECENT FINDINGS

Sudden death is a rare event after major ear, nose and throat surgery, and occurs mainly during the first three postoperative days.

SUMMARY

In more recent studies, the mortality rate after neck dissection was below 4%, which is at a lower value than reported in previous studies. Sudden deaths have been described, however, mainly during the first three postoperative days. Alcoholism and perioperative hypotension are two predictive factors for cardiac complications. Careful follow-up of these patients during the early postoperative period should be performed to reduce the mortality by shortening the delay of care.

Time-dependent modulation of carotid body afferent activity during and after intermittent hypoxia

The ventilatory response to several minutes of hypoxia consists of various time-dependent phenomena, some of which occur during hypoxia (e.g., short-term depression), whereas others appear on return to normoxia (e.g., posthypoxic frequency decline). Additional phenomena can be elicited by acute, intermittent hypoxia (e.g., progressive augmentation, long-term facilitation). Current data suggest that these phenomena originate centrally. We tested the hypothesis that carotid body afferent activity undergoes time-dependent modulation, consistent with a direct role in these ventilatory phenomena. Using an in vitro rat carotid body preparation, we found that 1) afferent activity declined during the first 5 min of severe (40 Torr Po(2)), moderate (60 Torr Po(2)), or mild (80 Torr Po(2)) hypoxia; 2) after return to normoxia (100 Torr Po(2)) and after several minutes of moderate or severe hypoxia, afferent activity was transiently reduced compared with prehypoxic levels; and 3) with successive 5-min bouts of mild, moderate, or severe hypoxia, afferent activity during bouts increased progressively. We call these phenomena sensory hypoxic decline, sensory posthypoxic decline, and sensory progressive augmentation, respectively. These phenomena were stimulus specific: similar phenomena were not seen with 5-min bouts of normoxic hypercapnia (100 Torr Po(2) and 50-60 Torr Pco(2)) or hypoxic hypocapnia (60 Torr Po(2) and 30 Torr Pco(2)). However, bouts of either normoxic hypercapnia or hypocapnic hypoxia resulted in sensory long-term facilitation. We suggest time-dependent carotid body activity acts in parallel with central mechanisms to shape the dynamics of ventilatory responses to respiratory chemostimuli.

Peripheral chemoreceptor control of ventilation following sustained hypoxia in young and older adult humans

The rate and duration of peripheral chemoreceptor resensitization following sustained hypoxia was characterized in young and older (74-year-old) adults. In addition, cerebral blood velocity (CBV) was measured in young subjects during and following the relief from sustained hypoxia. Following 20 min of sustained eucapnic hypoxia (50 mmHg), subjects were re-exposed to brief (1.5 min) hypoxic pulses (50 mmHg), and the magnitude of the ventilatory response was used to gauge peripheral chemosensitivity. Five minutes after the relief from sustained hypoxia, ventilation (V(E)) increased to 40.3 +/- 4.5% of the initial hypoxic ventilatory response, and by 36 min V(E) increased to 100%, indicating that peripheral chemosensitivity to hypoxia was restored. The V(E) response magnitude plotted versus time demonstrated that V(E), hence peripheral chemosensitivity, was restored at a rate of 1.9% per minute. Cerebral blood flow (CBF, inferred from CBV) remained constant during sustained hypoxia and increased by the same magnitude during the hypoxic pulses, suggesting that CBF has a small, if any, impact on the decline in V(E) during hypoxia and its subsequent recovery. To address the issue of whether hypoxic pulses affect subsequent challenges, series (continuous hypoxic pulses at various recovery intervals) and parallel (only 1 pulse per trial) methods were used. There were no differences in the ventilatory responses between the series and parallel methods. Older adults demonstrated a similar rate of recovery as in the young, suggesting that ageing in active older adults does not affect the peripheral chemoreceptor response.

Adaptation in the respiratory control system

Exposure to hypoxia, whether for short or prolonged periods or for repeated episodes, produces alterations in the ventilatory responses. This review presents evidence that these adaptations are likely to be mediated by adaptations in the respiratory chemoreflexes, particularly the peripheral chemoreflex, and proposes models of respiratory control explaining the observed changes in ventilation. After a brief introduction to the respiratory control system, a graphical model is developed that illustrates the operation of the system in the steady state, which will be used later. Next, the adaptations in ventilatory responses to hypoxia that have been observed are described, and methods of measuring the alterations in the chemoreflexes that might account for them are discussed. Finally, experimental data supporting the view that changes in the activity of the peripheral chemoreflex can account for the ventilatory adaptations to hypoxia are presented and incorporated into models of chemoreflex behaviour during exposures to hypoxia of various durations.

Repeated hypoxic exposures change respiratory chemoreflex control in humans

1. A group of seven volunteers (5 male, 2 female) were exposed to 20 min isocapnic (eucapnic) hypoxia once daily for 14 consecutive days. Their chemoreflexes were measured before and after each exposure. The same volunteers repeated the exposures with air substituted for the hypoxic gas mixture in a pseudorandom crossover design. 2. On day 1 an initial ventilatory response to hypoxia and subsequent decline was discernible in two volunteers, but the mean response for all volunteers at this stage was not significant. However, the response gradually increased, and by day 14 was discernible in six volunteers making the mean response for all volunteers significant. No change was observed over the 14 days of air exposure. 3. Only the chemoreflex threshold measured in iso-oxic (hypoxic) modified rebreathing tests changed significantly, and only for the series of exposures to hypoxia. 4. Over 14 days, the mean +/- S.E.M. threshold for all volunteers fell proportionately, from 42 +/- 1.1 mmHg on day 1 to 39 +/- 1.0 mmHg on day 14. By contrast, the mean +/- S.E.M. threshold, for all volunteers and all days, rose from 40 +/- 0.4 mmHg before to 42 +/- 0.5 mmHg after the hypoxic exposures. 5. We conclude that the enhancement of the initial ventilatory response to hypoxia induced by repeated hypoxic exposure is produced by a decrease in chemoreflex threshold. However, the decline in the ventilatory response during a single exposure is produced by an increase in the chemoreflex threshold. Since threshold changes were only found for hypoxic (iso-oxic) modified rebreathing tests, we conclude that only the peripheral chemoreflex changed.