Model based analysis of sleep disordered breathing in congestive heart failure

Of Mice and Men and Plethysmography Systems: Does LKB1 Determine the Set Point of Carotid Body Chemosensitivity and the Hypoxic Ventilatory Response?

Our recent studies suggest that the level of liver kinase B1 (LKB1) expression in some way determines carotid body afferent discharge during hypoxia and to a lesser extent during hypercapnia. In short, phosphorylation by LKB1 of an as yet unidentified target(s) determines a set point for carotid body chemosensitivity. LKB1 is the principal kinase that activates the AMP-activated protein kinase (AMPK) during metabolic stresses, but conditional deletion of AMPK in catecholaminergic cells, including therein carotid body type I cells, has little or no effect on carotid body responses to hypoxia or hypercapnia. With AMPK excluded, the most likely target of LKB1 is one or other of the 12 AMPK-related kinases, which are constitutively phosphorylated by LKB1 and, in general, regulate gene expression. By contrast, the hypoxic ventilatory response is attenuated by either LKB1 or AMPK deletion in catecholaminergic cells, precipitating hypoventilation and apnea during hypoxia rather than hyperventilation. Moreover, LKB1, but not AMPK, deficiency causes Cheyne-Stokes-like breathing. This chapter will explore further the possible mechanisms that determine these outcomes.

LKB1 is the gatekeeper of carotid body chemosensing and the hypoxic ventilatory response

The hypoxic ventilatory response (HVR) is critical to breathing and thus oxygen supply to the body and is primarily mediated by the carotid bodies. Here we reveal that carotid body afferent discharge during hypoxia and hypercapnia is determined by the expression of Liver Kinase B1 (LKB1), the principal kinase that activates the AMP-activated protein kinase (AMPK) during metabolic stresses. Conversely, conditional deletion in catecholaminergic cells of AMPK had no effect on carotid body responses to hypoxia or hypercapnia. By contrast, the HVR was attenuated by LKB1 and AMPK deletion. However, in LKB1 knockouts hypoxia evoked hypoventilation, apnoea and Cheyne-Stokes-like breathing, while only hypoventilation and apnoea were observed after AMPK deletion. We therefore identify LKB1 as an essential regulator of carotid body chemosensing and uncover a divergence in dependency on LKB1 and AMPK between the carotid body on one hand and the HVR on the other.

Exercise intolerance in volume overload heart failure is associated with low carotid body mediated chemoreflex drive

Mounting an appropriate ventilatory response to exercise is crucial to meeting metabolic demands, and abnormal ventilatory responses may contribute to exercise-intolerance (EX-inT) in heart failure (HF) patients. We sought to determine if abnormal ventilatory chemoreflex control contributes to EX-inT in volume-overload HF rats. Cardiac function, hypercapnic (HCVR) and hypoxic (HVR) ventilatory responses, and exercise tolerance were assessed at the end of a 6 week exercise training program. At the conclusion of the training program, exercise tolerant HF rats (HF + EX-T) exhibited improvements in cardiac systolic function and reductions in HCVR, sympathetic tone, and arrhythmias. In contrast, HF rats that were exercise intolerant (HF + EX-inT) exhibited worse diastolic dysfunction, and showed no improvements in cardiac systolic function, HCVR, sympathetic tone, or arrhythmias at the conclusion of the training program. In addition, HF + EX-inT rats had impaired HVR which was associated with increased arrhythmia susceptibility and mortality during hypoxic challenges (~ 60% survival). Finally, we observed that exercise tolerance in HF rats was related to carotid body (CB) function as CB ablation resulted in impaired exercise capacity in HF + EX-T rats. Our results indicate that: (i) exercise may have detrimental effects on cardiac function in HF-EX-inT, and (ii) loss of CB chemoreflex sensitivity contributes to EX-inT in HF.

Autonomous control of ventilation through closed-loop adaptive respiratory pacing

Mechanical ventilation is the standard treatment when volitional breathing is insufficient, but drawbacks include muscle atrophy, alveolar damage, and reduced mobility. Respiratory pacing is an alternative approach using electrical stimulation-induced diaphragm contraction to ventilate the lung. Oxygenation and acid-base homeostasis are maintained by matching ventilation to metabolic needs; however, current pacing technology requires manual tuning and does not respond to dynamic user-specific metabolic demand, thus requiring re-tuning of stimulation parameters as physiological changes occur. Here, we describe respiratory pacing using a closed-loop adaptive controller that can self-adjust in real-time to meet metabolic needs. The controller uses an adaptive Pattern Generator Pattern Shaper (PG/PS) architecture that autonomously generates a desired ventilatory pattern in response to dynamic changes in arterial CO₂ levels and, based on a learning algorithm, modulates stimulation intensity and respiratory cycle duration to evoke this ventilatory pattern. In vivo experiments in rats with respiratory depression and in those with a paralyzed hemidiaphragm confirmed that the controller can adapt and control ventilation to ameliorate hypoventilation and restore normocapnia regardless of the cause of respiratory dysfunction. This novel closed-loop bioelectronic controller advances the state-of-art in respiratory pacing by demonstrating the ability to automatically personalize stimulation patterns and adapt to achieve adequate ventilation.

Rostral ventrolateral medullary catecholaminergic neurones mediate irregular breathing pattern in volume overload heart failure rats

KEY POINTS

A strong association between disordered breathing patterns, elevated sympathetic activity, and enhanced central chemoreflex drive has been shown in experimental and human heart failure (HF). The aim of this study was to determine the contribution of catecholaminergic rostral ventrolateral medulla catecholaminergic neurones (RVLM-C1) to both haemodynamic and respiratory alterations in HF. Apnoea/hypopnoea incidence (AHI), breathing variability, respiratory-cardiovascular coupling, cardiac autonomic control and cardiac function were analysed in HF rats with or without selective ablation of RVLM-C1 neurones. Partial lesion (∼65%) of RVLM-C1 neurones reduces AHI, respiratory variability, and respiratory-cardiovascular coupling in HF rats. In addition, the deleterious effects of central chemoreflex activation on cardiac autonomic balance and cardiac function in HF rats was abolished by ablation of RVLM-C1 neurones. Our findings suggest that RVLM-C1 neurones play a pivotal role in breathing irregularities in volume overload HF, and mediate the sympathetic responses induced by acute central chemoreflex activation.

ABSTRACT

Rostral ventrolateral medulla catecholaminergic neurones (RVLM-C1) modulate sympathetic outflow and breathing under normal conditions. Heart failure (HF) is characterized by chronic RVLM-C1 activation, increased sympathetic activity and irregular breathing patterns. Despite studies showing a relationship between RVLM-C1 and sympathetic activity in HF, no studies have addressed a potential contribution of RVLM-C1 neurones to irregular breathing in this context. Thus, the aim of this study was to determine the contribution of RVLM-C1 neurones to irregular breathing patterns in HF. Sprague-Dawley rats underwent surgery to induce volume overload HF. Anti-dopamine β-hydroxylase-saporin toxin (DβH-SAP) was used to selectively lesion RVLM-C1 neurones. At 8 weeks post-HF induction, breathing pattern, blood pressures (BP), respiratory-cardiovascular coupling (RCC), central chemoreflex function, cardiac autonomic control and cardiac function were studied. Reduction (∼65%) of RVLM-C1 neurones resulted in attenuation of irregular breathing, decreased apnoea-hypopnoea incidence (11.1 ± 2.9 vs. 6.5 ± 2.5 events h-1 ; HF+Veh vs. HF+DβH-SAP; P < 0.05) and improved cardiac autonomic control in HF rats. Pathological RCC was observed in HF rats (peak coherence >0.5 between breathing and cardiovascular signals) and was attenuated by DβH-SAP treatment (coherence: 0.74 ± 0.12 vs. 0.54 ± 0.10, HF+Veh vs. HF+DβH-SAP rats; P < 0.05). Central chemoreflex activation had deleterious effects on cardiac function and cardiac autonomic control in HF rats that were abolished by lesion of RVLM-C1 neurones. Our findings reveal that RVLM-C1 neurones play a major role in irregular breathing patterns observed in volume overload HF and highlight their contribution to cardiac dysautonomia and deterioration of cardiac function during chemoreflex activation.

An integrated mathematical model of the human cardiopulmonary system: model development

Several cardiovascular and pulmonary models have been proposed in the last few decades. However, very few have addressed the interactions between these two systems. Our group has developed an integrated cardiopulmonary model (CP Model) that mathematically describes the interactions between the cardiovascular and respiratory systems, along with their main short-term control mechanisms. The model has been compared with human and animal data taken from published literature. Due to the volume of the work, the paper is divided in two parts. The present paper is on model development and normophysiology, whereas the second is on the model's validation on hypoxic and hypercapnic conditions. The CP Model incorporates cardiovascular circulation, respiratory mechanics, tissue and alveolar gas exchange, as well as short-term neural control mechanisms acting on both the cardiovascular and the respiratory functions. The model is able to simulate physiological variables typically observed in adult humans under normal and pathological conditions and to explain the underlying mechanisms and dynamics.

A mathematical model approach quantifying patients' response to changes in mechanical ventilation: evaluation in volume support

This paper presents a mathematical model-approach to describe and quantify patient-response to changes in ventilator support. The approach accounts for changes in metabolism (V̇O2, V̇CO2) and serial dead space (VD), and integrates six physiological models of: pulmonary gas-exchange; acid-base chemistry of blood, and cerebrospinal fluid; chemoreflex respiratory-drive; ventilation; and degree of patients' respiratory muscle-response. The approach was evaluated with data from 12 patients on volume support ventilation mode. The models were tuned to baseline measurements of respiratory gases, ventilation, arterial acid-base status, and metabolism. Clinical measurements and model simulated values were compared at five ventilator support levels. The models were shown to adequately describe data in all patients (χ(2), p > 0.2) accounting for changes in V̇CO2, VD and inadequate respiratory muscle-response. F-ratio tests showed that this approach provides a significantly better (p < 0.001) description of measured data than: (a) a similar model omitting the degree of respiratory muscle-response; and (b) a model of constant alveolar ventilation. The approach may help predict patients' response to changes in ventilator support at the bedside.

Periodic breathing with no heart beat

A protocol was originally designed to study breathing control during and following cardiac arrest in humans, taking advantage of the period of pulseless ventricular fibrillation (PVF) produced while testing a newly implanted cardioverter-defibrillator device. A patient aged in his 60s with New York Heart Association class III heart failure (HF) (left ventricular ejection fraction of 25%) who was originally part of this study displayed permanent periodic breathing (PB) and was then excluded from the final data analysis; his response is presented in this report. The 8- to 9-s PVF was incidentally produced during the ascending phase of a PB cycle, followed by another 12-s recovery period of low BP. PVF and its recovery had no effect on PB characteristics (period or amplitude). This occurred despite a profound change in Paco2, cerebral blood flow, and perfusion of the carotid bodies. It is concluded that PB in patients with HF could be produced by primary oscillations originating from the central pattern generator.

Acetazolamide improves loop gain but not the other physiological traits causing obstructive sleep apnoea

There is some evidence to suggest that acetazolamide may improve obstructive sleep apnoea (OSA).However, how acetazolamide affects the key traits causing OSA remains uncertain. We aimed to investigate the effect of acetazolamide on the traits contributing to OSA and its severity. Acetazolamide (500 mg twice daily) was administered for 1 week to 13 OSA subjects. Pharyngeal anatomy/collapsibility, loop gain (LG), upper-airway muscle responsiveness (gain) and the arousal threshold were determined using multiple 3 min 'CPAP pressure drops': pharyngeal anatomy/collapsibility was quantified as the ventilation at CPAP=0. LG was defined as the ratio of the ventilatory overshoot to the preceding reduction in ventilation. Upper-airway gain was taken as the ratio of the increase in ventilation to the increase in ventilatory drive across the drop. Arousal threshold was quantified as the level of ventilatory drive associated with arousal. The apnoea-hypopnoea index (AHI)was assessed on separate nights using standard polysomnography. Acetazolamide reduced the median [interquartile range] LG (3.4 [2.4-5.4] versus 2.0 [1.4-3.5]; P <0.05) and NREM AHI (50 [36-57] versus 24 [13-42] events h-1; P <0.05), but did not significantly alter pharyngeal anatomy/collapsibility, upper-airway gain, or arousal threshold. There was a modest correlation between the percentage reduction in LG and the percentage reduction in AHI (r =0.660, P =0.05). Our findings suggest that acetazolamide can improve OSA, probably due to reductions in the sensitivity of the ventilatory control system. Identification of patients who may benefit from reductions in LG alone or in combination with other therapies to alter the remaining traits may facilitate pharmacological resolution of OSA in the future.

Computational models for the study of heart-lung interactions in mammals

The operation and regulation of the lungs and the heart are closely related. This is evident when examining the anatomy within the thorax cavity, in the brainstem and in the aortic and carotid arteries where chemoreceptors and baroreceptors, which provide feedback affecting the regulation of both organs, are concentrated. This is also evident in phenomena such as respiratory sinus arrhythmia where the heart rate increases during inspiration and decreases during expiration, in other types of synchronization between the heart and the lungs known as cardioventilatory coupling and in the association between heart failure and sleep apnea where breathing is interrupted periodically by periods of no-breathing. The full implication and physiological significance of the cardiorespiratory coupling under normal, pathological, or extreme physiological conditions are still unknown and are subject to ongoing investigation both experimentally and theoretically using mathematical models. This article reviews mathematical models that take heart-lung interactions into account. The main ideas behind low dimensional, phenomenological models for the study of the heart-lung synchronization and sleep apnea are described first. Higher dimensions, physiology-based models are described next. These models can vary widely in detail and scope and are characterized by the way the heart-lung interaction is taken into account: via gas exchange, via the central nervous system, via the mechanical interactions, and via time delays. The article emphasizes the need for the integration of the different sources of heart-lung coupling as well as the different mathematical approaches.

Hypocapnia-dependent facilitation of augmented breaths: observations in awake vs. anesthetized rats

We investigated whether commonly used injectable laboratory anesthetics alter the regulation of augmented breaths (ABs) in different respiratory backgrounds. Male rats were studied on three separate experimental days, receiving one of three injections in randomized order: ethyl carbamate ('urethane'; 1.2mgkg(-1)), ketamine/xylazine (ket/xyl; 80/10mgkg(-1)), or normal saline. Following each of the three interventions, breathing was monitored during 15min exposures to normoxia (room air), hypoxia (10% O(2)) and hypoxia+CO(2) (10% O(2), 5% CO(2)). Urethane anesthesia completely eliminated ABs from the breathing rhythm in room air conditions (p<0.001), and decreased the hypocapnia-dependent component of this response (p<0.001). ket/xyl left the normal incidence of ABs in room air breathing intact but significantly suppressed the hypoxia-induced facilitation of ABs (p=0.0015). These results provide the first clear evidence that laboratory anesthesia can profoundly alter the regulation of ABs including the hypocapnia-dependent component of their facilitation.

An integrative model of respiratory and cardiovascular control in sleep-disordered breathing

While many physiological control models exist in the literature, none thus far has focused on characterizing the interactions among the respiratory, cardiovascular and sleep-wake regulation systems that occur in sleep-disordered breathing. The model introduced in this study integrates the autonomic control of the cardiovascular system, chemoreflex and state-related control of respiration, including respiratory and upper airway mechanics, along with a model of circadian and sleep-wake regulation. The integrative model provides realistic predictions of the physiological responses under a variety of conditions including: the sleep-wake cycle, hypoxia-induced periodic breathing, Cheyne-Stokes respiration in chronic heart failure, and obstructive sleep apnoea (OSA). It can be used to investigate the effects of a variety of interventions, such as isocapnic and hypercapnic and/or hypoxic gas administration, the Valsalva and Mueller maneuvers, and the application of continuous positive airway pressure on OSA subjects. By being able to delineate the influences of the various interacting physiological mechanisms, the model is useful in providing a more lucid understanding of the complex dynamics that characterize state-cardiorespiratory control in the different forms of sleep-disordered breathing.

The role of the central chemoreceptors: a modeling perspective

After introducing the respiratory control system, a previously developed model of the respiratory chemoreflexes, based on rebreathing test data, is briefly described. This model is used to gain insights into the respiratory chemoreflex characteristics of a selection of individuals, and so discover the role of their central chemoreceptors. The chemoreflex model characteristics for each individual were estimated by adjusting the model parameters so that its predictions fit their rebreathing test results. To gain a steady state description of the control of breathing at rest the chemoreflex model is combined with a model of the cerebrovascular reactivity and converted from P(CO)₂ to [H(+)] chemoreceptor inputs. This description is used to illustrate how acid-base and cerebrovascular reactivity factors affect the environment of the central chemoreceptors and determine their role in breathing control. Finally, a dynamic model incorporating the chemoreflex model, acid-base and cerebrovascular reactivity is used to show the role of the central chemoreceptors in stabilizing breathing during sleep at altitude.

Control of breathing: two types of delays studied in an integrated model of the respiratory system

We use a recently developed mathematical model that integrates a reduced representation of the brainstem respiratory neural controller together with peripheral gas exchange and transport to study numerically the dynamic response of the respiratory system to several physiological stimuli. We compare between the system responses with two major sources of delay: circulatory transport vs. neural feedback dynamics, and we show that the dynamics of the neural feedback processes dictates the dynamic response to hypoxia and hypercapnia. The source of the circulatory delay (blood velocity vs. distance from the lungs to chemoreceptors) was found to be important. Our model predicts that periodic breathing is associated with the ventilatory "afterdischarge" (slow recovery of ventilation) after a brief perturbation of CO(2). We also predict that there could be two possible mechanisms for the appearance of periodic breathing and that circulatory delay is not a necessary condition for this to occur in certain cases.

Integration of cerebrovascular CO2 reactivity and chemoreflex control of breathing: mechanisms of regulation, measurement, and interpretation

Cerebral blood flow (CBF) and its distribution are highly sensitive to changes in the partial pressure of arterial CO(2) (Pa(CO(2))). This physiological response, termed cerebrovascular CO(2) reactivity, is a vital homeostatic function that helps regulate and maintain central pH and, therefore, affects the respiratory central chemoreceptor stimulus. CBF increases with hypercapnia to wash out CO(2) from brain tissue, thereby attenuating the rise in central Pco(2), whereas hypocapnia causes cerebral vasoconstriction, which reduces CBF and attenuates the fall of brain tissue Pco(2). Cerebrovascular reactivity and ventilatory response to Pa(CO(2)) are therefore tightly linked, so that the regulation of CBF has an important role in stabilizing breathing during fluctuating levels of chemical stimuli. Indeed, recent reports indicate that cerebrovascular responsiveness to CO(2), primarily via its effects at the level of the central chemoreceptors, is an important determinant of eupneic and hypercapnic ventilatory responsiveness in otherwise healthy humans during wakefulness, sleep, and exercise and at high altitude. In particular, reductions in cerebrovascular responsiveness to CO(2) that provoke an increase in the gain of the chemoreflex control of breathing may underpin breathing instability during central sleep apnea in patients with congestive heart failure and on ascent to high altitude. In this review, we summarize the major factors that regulate CBF to emphasize the integrated mechanisms, in addition to Pa(CO(2)), that control CBF. We discuss in detail the assessment and interpretation of cerebrovascular reactivity to CO(2). Next, we provide a detailed update on the integration of the role of cerebrovascular CO(2) reactivity and CBF in regulation of chemoreflex control of breathing in health and disease. Finally, we describe the use of a newly developed steady-state modeling approach to examine the effects of changes in CBF on the chemoreflex control of breathing and suggest avenues for future research.

Hypocapnia increases the prevalence of hypoxia-induced augmented breaths

Augmented breaths promote respiratory instability and have been implicated in triggering periods of sleep-disordered breathing. Since respiratory instability is well known to be exacerbated by hypocapnia, we asked whether one of the destabilizing effects of hypocapnia might be related to an increased prevalence of augmented breaths. With this question in mind, we first sought to determine whether hypoxia-induced augmented breaths are more prevalent when hypocapnia is also present. To do this, we studied the breath-by-breath ventilatory responses of a group of freely behaving adult rats in a variety of different respiratory background conditions. We found that the prevalence of augmented breaths was dramatically increased during hypocapnic-hypoxia compared with room air conditions. When hypocapnia was prevented during exposure to hypoxia by adding 5% CO2 to the inspired air, the rate of occurrence of augmented breaths was no greater than that observed in room air. The addition of CO2 alone to room air had no effect on the prevalence of augmented breaths. We conclude that in spontaneously breathing rats, hypoxia promotes the generation of augmented breaths, but only in poikilocapnic conditions, where hypocapnia develops. Our results, therefore, reveal a means by which CO2 exerts a stabilizing influence on breathing, which may be of particular relevance during sleep in conditions commonly associated with respiratory instability.

Ventilatory and cerebrovascular responses to hypercapnia in patients with obstructive sleep apnoea: effect of CPAP therapy

The purpose of this study was to assess whether the cerebrovascular response to hypercapnia is blunted in OSA patients and if this could alter the ventilatory response to hypercapnia before and after CPAP therapy. We measured the cerebrovascular, cardiovascular and ventilatory responses to hypercapnia in 8 patients with OSA (apnoea-hypopnoea index=101+/-10) before and after 4-6 weeks of CPAP therapy and in 10 control subjects who did not undergo CPAP therapy. The cerebrovascular and ventilatory responses to hypercapnia were not different between OSA and controls at baseline or follow-up. The cardiovascular response to hypercapnia was significantly increased in the OSA group by CPAP therapy (mean arterial pressure response: 1.30+/-0.16 vs. 2.04+/-0.36 mmHg Torr(-1); p=0.007). We conclude that in normocapnic, normotensive OSA patients without cardiovascular disease, the ventilatory, cerebrovascular, and cardiovascular responses to hypercapnia are normal, but the cardiovascular response to hypercapnia is heightened following 1 month of CPAP therapy.

Mechanisms of periodic breathing during exercise in patients with chronic heart failure

BACKGROUND

Periodic breathing (PB) in heart failure (HF) is attributed to many factors, including low cardiac output delaying the time it takes pulmonary venous blood to reach the central and peripheral chemoreceptors, low lung volume, lung congestion, augmented chemoreceptor sensitivity, and the narrow difference between eupneic carbon dioxide tension and apneic/hypoventilatory threshold.

METHODS AND RESULTS

We measured expired gases, ventilation, amplitude, and duration of PB in 23 patients with PB during progressive exercise tests done with 0 mL, 250 mL, or 500 mL of added dead space. Periodicity of PB remained constant despite heart rate, oxygen consumption, and minute ventilation increasing. Within each PB cycle, starting from the beginning of exercise, the largest (peak) tidal volume approached maximum observed tidal volume, while the smallest (nadir) tidal volume increased as exercise power output increased. PB ceased when nadir tidal volume reached peak tidal volume. End-tidal carbon dioxide increased with added dead space, and PB ceased progressively earlier during the exercise done with increased dead space.

CONCLUSION

Circulatory delay does not contribute to the PB observed in exercising HF patients. The pattern of gradually increasing nadir tidal volume during exercise and the effect of dead space on both PB ceasing and end-tidal carbon dioxide suggest that low tidal volume and carbon dioxide apnea threshold are important contributors to PB that occurs during exercise in HF.

Exposure to retinoic acid at the onset of hindbrain segmentation induces episodic breathing in mice

Hyperpnoeic episodic breathing (HEB), a cyclic waxing and waning of breathing, has been widely reported in pre-term neonates, patients with Joubert syndrome and adults (Cheyne-Stokes respiration) with congestive heart failure and brainstem infarction. We now provide a developmental mouse model of neonatal HEB. We used retinoic acid (RA) (0.5-10 mg/kg of maternal weight) to alter embryonic development of the respiratory neuronal network at the onset of hindbrain segmentation (7.5 days post-coitum). HEB was observed in vivo after RA treatment during post-natal days 1-7 but not in control animals. HEB persisted after reduction of the chemoafferent input by hypocapnic hyperoxia (100% O(2)). A large increase and decrease of the rhythm resembling an HEB episode was induced in vitro by stimulating the parafacial respiratory oscillator in treated but not in control neonates. Post-natal localization of the superior cerebellar peduncle and adjacent dorsal tegmentum was found to be abnormal in the pons of RA-treated juvenile mice. Thus, early developmental specifications in the rostral hindbrain are required for the development of neurones that stabilize the function of the respiratory rhythm generator, thereby preventing HEB during post-natal maturation.

Measuring the ventilatory response to hypoxia

After defining the current approach to measuring the hypoxic ventilatory response this paper explains why this method is not appropriate for comparisons between individuals or conditions, and does not adequately measure the parameters of the peripheral chemoreflex. A measurement regime is therefore proposed that incorporates three procedures. The first procedure measures the peripheral chemoreflex responsiveness to both hypoxia and CO(2) in terms of hypoxia's effects on the sensitivity and ventilatory recruitment threshold of the peripheral chemoreflex response to CO(2). The second and third procedures employ current methods for measuring the isocapnic and poikilocapnic ventilatory responses to hypoxia, respectively, over a period of 20 min. The isocapnic measure is used to determine the time course characteristics of hypoxic ventilatory decline and the poikilocapnic measure shows the ventilatory response to a hypoxic environment. A measurement regime incorporating these three procedures will permit a detailed assessment of the peripheral chemoreflex response to hypoxia that allows comparisons to be made between individuals and different physiological and environmental conditions.

Cardiorespiratory and cerebrovascular responses to acute poikilocapnic hypoxia following intermittent and continuous exposure to hypoxia in humans

We tested the hypothesis that intermittent hypoxia (IH) and/or continuous hypoxia (CH) would enhance the ventilatory response to acute hypoxia (HVR), thereby altering blood pressure (BP) and cerebral perfusion. Seven healthy volunteers were randomly selected to complete 10–12 days of IH (5-min hypoxia to 5-min normoxia repeated for 90 min) before ascending to mild CH (1,560 m) for 12 days. Seven other volunteers did not receive any IH before ascending to CH for the same 12 days. Before the IH and CH, following 12 days of CH and 12–13 days post-CH exposure, all subjects underwent a 20-min acute exposure to poikilocapnic hypoxia (inspired fraction of O2, 0.12) in which ventilation, end-tidal gases, arterial O2 saturation, BP, and middle cerebral artery blood flow velocity (MCAV) were measured continuously. Following the IH and CH exposures, the peak HVR was elevated and was related to the increase in BP ( r = 0.66 to r = 0.88, respectively; P < 0.05) and to a reciprocal decrease in MCAV ( r = 0.73 to r = 0.80 vs. preexposures; P < 0.05) during the hypoxic test. Following both IH and CH exposures, HVR, BP, and MCAV sensitivity to hypoxia were elevated compared with preexposure, with no between-group differences following the IH and/or CH conditions, or persistent effects following 12 days of sea level exposure. Our findings indicate that IH and/or mild CH can equally enhance the HVR, which, by either direct or indirect mechanisms, facilitates alterations in BP and MCAV.