Hypercapnic ventilatory response in patients with and without obstructive sleep apnea: do age, gender, obesity, and daytime PaCO(2) matter?

Airway Collapsibility during Drug-Induced Sleep Endoscopy Relates with Clinical Features of Obstructive Sleep Apnea

INTRODUCTION

Increased pharyngeal collapsibility leads to obstructive sleep apnea (OSA). Positive airway pressure titration during drug-induced sleep endoscopy (DISE-PAP) provides objective collapsibility metrics, the pharyngeal opening pressure (PhOP), and active pharyngeal critical pressure (PcritA ). We examined the interrelationships between risk factors of OSA, airway collapsibility measures, and clinical manifestations of the disease.

METHODS

This is a cross-sectional analysis of consecutive OSA patients undergoing DISE-PAP. Nasal PAP was increased stepwise until inspiratory flow limitation was abolished, signifying PhOP. PcritA was derived from the resulting titration pressure-flow relationships. Clinical data including demographics, anthropometrics, sleep studies, and patient-symptom questionnaires were obtained from the electronic medical record. Multivariate regression was used to evaluate the relationship between risk factors, airway collapsibility, and clinical data.

RESULTS

On average, the 164 patients meeting inclusion criteria were middle-aged (54.2 ± 14.7 years), overweight/obese (BMI 29.9 ± 4.5 kg/m2 ), male (72.6%), White (79.3%) and had severe OSA (AHI 32.0 ± 20.5 events/hour). Mean PhOP was 7.5 ± 3.3 cm H2 O and mean PcritA was 0.80 ± 3.70 cm H2 O. Younger age (Standardized β = -0.191, p = 0.015) and higher BMI (Standardized β = 0.176, p = 0.028) were associated with higher PhOP, but not PcritA . PhOP and PcritA were both associated with AHI, supine AHI, and SpO2 nadir. Higher PhOP was associated with higher snoring scores (Standardized β = 0.246, p = 0.008), but not other patient-reported outcomes.

CONCLUSION

Objective assessment of passive and active airway mechanics during DISE relates with clinical risk factors for OSA. Quantitative measures of collapsibility provide accessible and meaningful data, enhancing the standard sleep surgery evaluation.

LEVEL OF EVIDENCE

4 Laryngoscope, 134:1978-1985, 2024.

International Consensus Statement on Obstructive Sleep Apnea

BACKGROUND

Evaluation and interpretation of the literature on obstructive sleep apnea (OSA) allows for consolidation and determination of the key factors important for clinical management of the adult OSA patient. Toward this goal, an international collaborative of multidisciplinary experts in sleep apnea evaluation and treatment have produced the International Consensus statement on Obstructive Sleep Apnea (ICS:OSA).

METHODS

Using previously defined methodology, focal topics in OSA were assigned as literature review (LR), evidence-based review (EBR), or evidence-based review with recommendations (EBR-R) formats. Each topic incorporated the available and relevant evidence which was summarized and graded on study quality. Each topic and section underwent iterative review and the ICS:OSA was created and reviewed by all authors for consensus.

RESULTS

The ICS:OSA addresses OSA syndrome definitions, pathophysiology, epidemiology, risk factors for disease, screening methods, diagnostic testing types, multiple treatment modalities, and effects of OSA treatment on multiple OSA-associated comorbidities. Specific focus on outcomes with positive airway pressure (PAP) and surgical treatments were evaluated.

CONCLUSION

This review of the literature consolidates the available knowledge and identifies the limitations of the current evidence on OSA. This effort aims to create a resource for OSA evidence-based practice and identify future research needs. Knowledge gaps and research opportunities include improving the metrics of OSA disease, determining the optimal OSA screening paradigms, developing strategies for PAP adherence and longitudinal care, enhancing selection of PAP alternatives and surgery, understanding health risk outcomes, and translating evidence into individualized approaches to therapy.

The Elevated Central Chemosensitivity in Obstructive Sleep Apnea Patients with Hypertension

PURPOSE

Hypertension is a common comorbidity in obstructive sleep apnea (OSA), in which dysfunction of the autonomic nervous system plays an integral part. Chemoreflex is essential for ventilatory control and cardiovascular activity. This study aimed to determine whether central chemosensitivity was increased in OSA patients with hypertension and the potential role of the autonomic nerve activity in this relationship.

PATIENTS AND METHODS

A total of 77 men with OSA were included in this cross-sectional study. We measured hypercapnic ventilatory response (HCVR) by the rebreathing method under isoxic hyperoxia to test the central ventilatory chemosensitivity since hyperoxia silences the peripheral chemoreceptors' response to CO₂. To elevate the autonomic nerve activity, time-domain, frequency-domain, and non-linear variables of heart rate variability were calculated over 5-min records. Univariate and multivariate linear regression analyses were used to find the determinants of HCVR.

RESULTS

The median HCVR was 2.3 (1.8, 3.3), 2.1 (1.6, 3.0), and 3 (2.2, 3.7) L/min/mmHg in all participants, OSA patients, and OSA patients with hypertension, respectively. Hypertension was significantly associated with elevated HCVR after adjusting for age, central obesity, OSA severity, daytime sleepiness, and diabetes mellitus. Compared with OSA patients, OSA patients with hypertension had higher body mass index, worse nocturnal hypoxia, and lower time-domain variables and frequency-domain variables. After adjusting for age, apnea-hypopnea index, central obesity, and beta-blocker usage, approximate entropy was independently negatively associated with HCVR in OSA patients with hypertension.

CONCLUSION

This study demonstrated elevated central chemosensitivity in OSA patients with hypertension. Compared with OSA patients, OSA patients with hypertension had attenuated parasympathetic nerve activity. This study preliminarily illustrated that elevated central chemosensitivity might be associated with weak adaptability of the cardiac autonomic nervous system in OSA patients with hypertension.

Gender Differences in Risk Factors Associated With Pulmonary Artery Systolic Pressure, Heart Failure, and Mortality in Blacks: Jackson Heart Study

Background

Pulmonary hypertension is prevalent in black individuals, especially women. Elevated pulmonary artery systolic pressure (PASP) is associated with significant morbidity and mortality.

Methods and Results

We developed linear and proportional hazards models to examine potential gender‐related differences in risk factors for elevated PASP (estimated by transthoracic echocardiography) and PASP‐associated clinical outcomes (incident heart failure admissions and mortality) in JHS (Jackson Heart Study) participants. JHS is a prospective observational cohort study of heart disease in blacks from the Jackson, Mississippi, metropolitan area. The study cohort included participants with measurable transtricuspid gradients (n=3286) at the time of first/baseline examination, 2000–2004. The median age (interquartile range) of patients at baseline was 57.8 years (18.6 years) with 67.5% being women. The median PASP at baseline was higher in women (men: 26 mm Hg [interquartile range 8], women: 27 mm Hg [interquartile range 9]. In multivariate linear regression analyses with PASP, significant gender interactions were noted for age, chronic lung disease, pulse pressure, and obstructive spirometry. In exploratory analyses stratified by gender, body mass index, and obstructive and restrictive spirometry patterns were associated with PASP in women, and chronic lung disease was associated with PASP in men. Age and pulse pressure had stronger associations with PASP in women compared with men. There was a significant interaction between gender and PASP for heart failure admissions but not mortality.

Conclusions

Specific cardiopulmonary risk factors are associated with elevated PASP in women and men. Women with elevated PASP have a higher risk of incident heart failure admissions. Future research is needed to understand associated gender‐specific mechanisms that can help identify targeted prevention and management strategies for patients with elevated PASP.

Is Obesity Hypoventilation Syndrome A Postmenopausal Disorder?

Previous studies have assessed the role of gender and menopause in Obstructive Sleep Apnea (OSA). It is well known that menopause is a major risk factor for OSA. However, analogous studies on obesity Hypoventilation Syndrome (OHS) are limited. Recent studies have suggested that OHS is more prevalent in postmenopausal women. Moreover, women with OHS seem to have excess comorbidities, including hypothyroidism, hypertension, pulmonary hypertension, and diabetes mellitus, compared to men. In the present perspective, we discuss recent data on the prevalence and comorbidities associated with OHS in women, as well as the use of noninvasive ventilation in women with OHS, and try to answer the question, “Is OHS a disorder of postmenopausal women?”

Ventilatory response to CO2 in patients with epilepsy

OBJECTIVE

Severe periictal respiratory depression is thought to be linked to risk of sudden unexpected death in epilepsy (SUDEP) but its determinants are largely unknown. Interindividual differences in the interictal ventilatory response to CO₂ (hypercapnic ventilatory response [HCVR] or central respiratory CO₂ chemosensitivity) may identify patients who are at increased risk for severe periictal hypoventilation. HCVR has not been studied previously in patients with epilepsy; therefore we evaluated a method to measure it at bedside in an epilepsy monitoring unit (EMU) and examined its relationship to postictal hypercapnia following generalized convulsive seizures (GCSs).

METHODS

Interictal HCVR was measured by a respiratory gas analyzer using a modified rebreathing technique. Minute ventilation (V_E ), tidal volume, respiratory rate, end tidal (ET) CO₂ and O₂ were recorded continuously. Dyspnea during the test was assessed using a validated scale. The HCVR slope (ΔV_E /ΔETCO₂ ) for each subject was determined by linear regression. During the video-electroencephalography (EEG) study, subjects underwent continuous respiratory monitoring, including measurement of chest and abdominal movement, oronasal airflow, transcutaneous (tc) CO₂ , and capillary oxygen saturation (SPO₂ ).

RESULTS

Sixty-eight subjects completed HCVR testing in 151 ± (standard deviation) 58 seconds, without any serious adverse events. HCVR slope ranged from -0.94 to 5.39 (median 1.71) L/min/mm Hg. HCVR slope correlated with the degree of unpleasantness and intensity of dyspnea and was inversely related to baseline ETCO₂ . Both the duration and magnitude of postictal tcCO₂ rise following GCSs were inversely correlated with HCVR slope.

SIGNIFICANCE

Measurement of the HCVR is well tolerated and can be performed rapidly and safely at the bedside in the EMU. A subset of individuals has a very low sensitivity to CO₂ , and this group is more likely to have a prolonged increase in postictal CO₂ after GCS. Low interictal HCVR may increase the risk of severe respiratory depression and SUDEP after GCS and warrants further study.

Inherent vs. Induced Loop Gain Abnormalities in Obstructive Sleep Apnea

Unstable ventilatory chemoreflex control, quantified as loop gain, is recognized as one of four key pathophysiological traits that contribute to cause obstructive sleep apnea (OSA). Novel treatments aimed at reducing loop gain are being investigated, with the intention that future OSA treatment may be tailored to the individual's specific cause of apnea. However, few studies have evaluated loop gain in OSA and non-OSA controls and those that have provide little evidence to support an inherent abnormality in either overall chemical loop gain in OSA patients vs. non-OSA controls, or its components (controller and plant gain). However, intermittent hypoxia may induce high controller gain through neuroplastic changes to chemoreflex control, and may also decrease plant gain via oxidative stress induced inflammation and reduced lung function. The inherent difficulties and limitations with loop gain measurements are discussed and areas where further research are required are highlighted, as only by understanding the mechanisms underlying OSA are new therapeutic approaches likely to emerge in OSA.

Analysis of Hypoxic and Hypercapnic Ventilatory Response in Healthy Volunteers

Introduction

A previous study has suggested that the Human Leukocyte Antigen (HLA) allele DQB1*06:02 affects hypoxic ventilatory response (HVR) but not hypercapnic ventilatory response (HCVR) in an Asian population. The current study evaluated the relationship in Caucasians and Asians. In addition we assessed whether gender or polymorphisms in genes participating in the control of breathing affect HVR and HCVR.

Methods

A re-breathing system was used to measure HVR and HCVR in 551 young adults (56.8% Caucasians, 30% Asians). HLA-DQB1*06:02 and tagged polymorphisms and coding variants in genes participating in breathing (PHOX2B, GPR4 and TASK2/KCNK5) were analyzed. The associations between HVR/HCVR and HLA-DQB1*06:02, genetic polymorphisms, and gender were evaluated using ANOVA or frequentist association testing with SNPTEST.

Results

HVR and gender are strongly correlated. HCVR and gender are not. Mean HVR in women was 0.276±0.168 (liter/minute/%SpO2) compared to 0.429±0.266 (liter/minute/%SpO2) in men, p<0.001 (55.4% higher HVR in men). Women had lower baseline minute ventilation (8.08±2.36 l/m vs. 10.00±3.43l/m, p<0.001), higher SpO2 (98.0±1.3% vs. 96.6±1.7%, p<0.001), and lower EtCO2 (4.65±0.68% vs. 4.82±1.02%, p = 0.025). One hundred and two (18.5%) of the participants had HLA-DQB1*06:02. No association was seen between HLA-DQB1*06:02 and HVR or HCVR. Genetic analysis revealed point wise, uncorrected significant associations between two TASK2/KCNK5 variants (rs2815118 and rs150380866) and HCVR.

Conclusions

This is the largest study to date reporting the relationship between gender and HVR/ HCVR and the first study assessing the association between genetic polymorphisms in humans and HVR/HCVR. The data suggest that gender has a large effect on hypoxic breathing response.

Gender differences in patients with obesity hypoventilation syndrome

The role of gender and menopause in obstructive sleep apnoea is well known; however, no study has reported the impact of gender on the clinical presentation and the nocturnal respiratory events in patients with obesity hypoventilation syndrome. Therefore, this study prospectively evaluated differences in the clinical characteristics of women and men with obesity hypoventilation syndrome in a large cohort of patients with obstructive sleep apnoea. During the study period, a total of 1973 patients were referred to the sleep clinic with clinical suspicion of obstructive sleep apnoea. All patients underwent overnight polysomnography, during which time spirometry, arterial blood samples and thyroid tests were routinely obtained. Among 1973 consecutive patients, 1693 (617 women) were diagnosed with obstructive sleep apnoea, among whom 144 suffered from obesity hypoventilation syndrome (96 women). The prevalence of obesity hypoventilation syndrome among women and men was 15.6% and 4.5%, respectively (P < 0.001). Women with obesity hypoventilation syndrome were significantly older than men with obesity hypoventilation syndrome (61.5 ± 11.9 years versus 49.1 ± 12.5 years, P < 0.001). Although there were no significant differences between genders regarding symptoms, body mass index, spirometric data or daytime PaCO2 , women with obesity hypoventilation syndrome suffered significantly more from hypertension, diabetes and hypothyroidism. The prevalence of obesity hypoventilation syndrome was higher in post-menopausal (21%) compared with pre-menopausal (5.3%) women (P < 0001). HCO3 and duration of SpO2 <90% were the only independent predictors of obesity hypoventilation syndrome. In conclusion, this study reported that among subjects referred to the sleep disorders clinic for evaluation of obstructive sleep apnoea, obesity hypoventilation syndrome is more prevalent in women than men, and that women with obesity hypoventilation syndrome suffer from significantly more co-morbidities. Post-menopausal women with obstructive sleep apnoea have the highest prevalence of obesity hypoventilation syndrome.

Correlates of obesity-related chronic ventilatory failure

Introduction

Only a third of obese patients develop chronic ventilatory failure. This cross-sectional study assessed multiple factors potentially associated with chronic ventilatory failure.

Materials/patients and methods

Participants had a body mass index (BMI) >30 kg/m², with or without chronic ventilatory failure (awake arterial partial pressure of carbon dioxide >6 kPa or base excess (BE) ≥2 mmols/L). Factors investigated were grouped into domains: (1) obesity measures, (2) pulmonary function, (3) respiratory and non-respiratory muscle strength, (4) sleep study derivatives, (5) hypoxic and hypercapnic responses, and (6) some hormonal, nutritional and inflammatory measures.

Results

71 obese participants (52% male) were studied over 27 months, 52 (SD 9) years and BMI 47 (range 32–74) kg/m². The best univariate correlates of BE from each domain were: (1) dual-energy X-ray absorptiometry measurement of visceral fat (r=+0.50, p=0.001); (2) supine forced expiratory volume in 1 s (r=−0.40, p=0.001); (3) sniff maximum pressure (r=−0.28, p=0.02); (4) mean overnight arterial oxygen saturation (r=−0.50, p<0.001); (5) ventilatory response to 15% O₂ breathing (r=−0.28, p=0.02); and (6) vitamin D (r=−0.30, p=0.01). In multivariate analysis, only visceral fat and ventilatory response to hypoxia remained significant.

Conclusions

We have confirmed that in the obese, BMI is a poor correlate of chronic ventilatory failure, and the best independent correlates are visceral fat and hypoxic ventilatory response.

Trial registration number

NCT01380418.

Obesity hypoventilation syndrome in the critically ill

This article summarizes available data on the obesity hypoventilation syndrome and its pertinence to intensivists, outlines clinical and pathophysiologic aspects of the disease, discusses multidisciplinary treatments, and reviews the available literature on outcomes specific to the critically ill patient.

Obstructive sleep apnea and other sleep-related syndromes

Obstructive sleep apnea syndrome (OSAS) is a common disorder characterized by repetitive episodes of breathing cessation due to complete or partial collapse of the upper airway therefore affecting ventilation. It is quite common, with a prevalence of about 2-4%, has a strong genetic component, and creates a proinflammatory state with elevated TNFα and other cytokines. If untreated, OSA can lead to significant neurological problems that include stroke, cognitive decline, depression, headaches, peripheral neuropathy, and nonarteritic ischemic optic neuropathy (NAION). Treatment reverses some of these neurological problems. Treatment includes continuous positive airway pressure and its variants, oral appliances, weight loss, upper airway surgery, and rarely maxillofacial procedures. Other sleep breathing disorders such as hypoventilation, central sleep apnea, complex sleep apnea, and Cheyne-Stokes respiration are less common and are sometimes associated with neuromuscular disorders causing diaphragmatic paralysis, but can also be seen in opiate exposure and severe obesity.

Women candidates for diving with oxygen-enriched gas mixtures have a lower end tidal CO2 than men during moderate exercise

We have previously determined the thresholds for CO2 detection (conscious recognition of elevated CO2) and retention in male divers, beyond which a diving candidate should not continue his diving activity due to an increased risk of CNS oxygen toxicity. The purpose of the present study was to establish whether there is a difference in end tidal PCO2 between male and female divers who use oxygen-enriched gas mixtures. Ventilatory and perceptual responses to variations in inspired CO2 (range 0-42 mm Hg) were assessed during moderate exercise in 18 males and 18 females. End tidal PCO2 was lower in the female divers when breathing oxygen with 42 mm Hg CO2 (58.2±3.0 mm Hg vs. 61.5±4.5 mm Hg, P<0.03). These results suggest that female divers have a lower end tidal CO2 than males when breathing a hyperoxic gas mixture during exercise, which might imply that women are less susceptible to CNS oxygen toxicity than men.

Respiratory mechanics and ventilatory control in overlap syndrome and obesity hypoventilation

The overlap syndrome of obstructive sleep apnoea (OSA) and chronic obstructive pulmonary disease (COPD), in addition to obesity hypoventilation syndrome, represents growing health concerns, owing to the worldwide COPD and obesity epidemics and related co-morbidities. These disorders constitute the end points of a spectrum with distinct yet interrelated mechanisms that lead to a considerable health burden. The coexistence OSA and COPD seems to occur by chance, but the combination can contribute to worsened symptoms and oxygen desaturation at night, leading to disrupted sleep architecture and decreased sleep quality. Alveolar hypoventilation, ventilation-perfusion mismatch and intermittent hypercapnic events resulting from apneas and hypopneas contribute to the final clinical picture, which is quite different from the “usual” COPD. Obesity hypoventilation has emerged as a relatively common cause of chronic hypercapnic respiratory failure. Its pathophysiology results from complex interactions, among which are respiratory mechanics, ventilatory control, sleep-disordered breathing and neurohormonal disturbances, such as leptin resistance, each of which contributes to varying degrees in individual patients to the development of obesity hypoventilation. This respiratory embarrassment takes place when compensatory mechanisms like increased drive cannot be maintained or become overwhelmed.

Although a unifying concept for the pathogenesis of both disorders is lacking, it seems that these patients are in a vicious cycle. This review outlines the major pathophysiological mechanisms believed to contribute to the development of these specific clinical entities. Knowledge of shared mechanisms in the overlap syndrome and obesity hypoventilation may help to identify these patients and guide therapy.

Sex differences in white matter alterations accompanying obstructive sleep apnea

STUDY OBJECTIVES

Females with obstructive sleep apnea (OSA) show different psychological and physiological symptoms from males, which may be associated with sex-related variations in neural injury occurring with the disorder. To determine whether male- or female-specific brain injury is present in OSA, we assessed influences of sex on white matter changes in the condition.

DESIGN

Two-group factorial.

SETTING

University medical center.

PATIENTS OR PARTICIPANTS

80 subjects total, with newly diagnosed, untreated OSA groups of 10 female (age mean ± SE: 52.6 ± 2.4 years, AHI 22.5 ± 4.1 events/h) and 20 male (age 48.9 ± 1.7, AHI 25.5 ± 2.9) patients, and 20 female (age 50.3 ± 1.7) and 30 male (age 49.2 ± 1.4) healthy control subjects.

INTERVENTIONS

None.

MEASUREMENTS AND RESULTS

Brain fiber integrity was assessed with fractional anisotropy (FA), a diffusion tensor imaging-derived measure. Sleep quality, daytime sleepiness, depression, and anxiety were assessed with questionnaires. We identified regions of differing injury in male versus female OSA patients by assessing brain regions with significant interaction effects of OSA and sex on FA. Areas of sex-specific, OSA-related FA reductions appeared in females relative to males, including in the bilateral cingulum bundle adjacent to the mid hippocampus, right stria terminalis near the amygdala, prefrontal and posterior-parietal white matter, corpus callosum, and left superior cerebellar peduncle. Females with OSA showed higher daytime sleepiness, anxiety and depression levels, and reduced sleep quality.

CONCLUSIONS

Sex differences in white matter structural integrity appeared in OSA patients, with females more affected than males. These female-specific structural changes may contribute to or derive from neuropsychological and physiological symptom differences between sexes.

Increased propensity for central apnea in patients with obstructive sleep apnea: effect of nasal continuous positive airway pressure

RATIONALE

There is increasing evidence of increased ventilatory instability in patients with obstructive sleep apnea (OSA), but previous investigations have not studied whether the hypocapnic apneic threshold is altered in this group.

OBJECTIVES

To compare the apneic threshold, CO2 reserve, and controller gain between subjects with and without OSA matched for age, sex, and body mass index.

METHODS

Hypocapnia was induced via nasal mechanical ventilation for 3 minutes. Cessation of mechanical ventilation resulted in hypocapnic central hypopnea or apnea depending upon the magnitude of the hypocapnia. The apnea threshold (Pet(CO2)-AT) was defined as the measured Pet(CO2) at which the apnea closest to the last hypopnea occurred. The CO2 reserve was defined as the change in Pet(CO2) between eupneic Pet(CO2) and Pet(CO2)-AT. Controller gain was defined as the ratio of change in Ve between control and hypopnea or apnea to the DeltaPet(CO2).

MEASUREMENTS AND MAIN RESULTS

Eleven pairs of subjects were studied. There was no difference in the Pet(CO2)-AT between the two groups. However, the CO2 reserve was smaller in the subjects with OSA (2.2 +/- 0.6 mm Hg) compared with the control subjects (4.5 +/- 1.4 mm Hg; P < 0.001). The controller gain was increased in the subjects with OSA (3.7 +/- 1.3 L/min/mm Hg) compared with the control subjects (1.6 +/- 0.5 L/min/mm Hg; P < 0.001). Controller gain decreased and CO2 reserve increased in seven subjects restudied after using continuous positive airway pressure for 1 month.

CONCLUSIONS

Ventilatory instability is increased in subjects with OSA and is reversible with the use of continuous positive airway pressure.

Intermittent hypoxia and respiratory plasticity in humans and other animals: does exposure to intermittent hypoxia promote or mitigate sleep apnoea?

This review focuses on two phenomena that are initiated during and after exposure to intermittent hypoxia. The two phenomena are referred to as long-term facilitation and progressive augmentation of respiratory motor output. Both phenomena are forms of respiratory plasticity. Long-term facilitation is characterized by a sustained elevation in respiratory activity after exposure to intermittent hypoxia. Progressive augmentation is characterized by a gradual increase in respiratory activity from the initial to the final hypoxic exposure. There is much speculation that long-term facilitation may have a significant role in individuals with sleep apnoea because this disorder is characterized by periods of upper airway collapse accompanied by intermittent hypoxia, one stimulus known to induce long-term facilitation. It has been suggested that activation of long-term facilitation may serve to mitigate apnoea by facilitating ventilation and, more importantly, upper airway muscle activity. We examine the less discussed but equally plausible situation that exposure to intermittent hypoxia might ultimately lead to the promotion of apnoea. There are at least two scenarios in which apnoea might be promoted following exposure to intermittent hypoxia. In both scenarios, long-term facilitation of upper airway muscle activity is initiated but ultimately rendered ineffective because of other physiological conditions. Thus, one of the primary goals of this review is to discuss, with support from basic and clinical studies, whether various forms of respiratory motor neuronal plasticity have a beneficial and/or a detrimental impact on breathing stability in individuals with sleep apnoea.

Effect of menopause on the chemical control of breathing and its relationship with acid-base status

This study examined the role of alterations in the chemoreflex control of breathing, acid-base balance, and their interaction in postmenopausal ventilatory adaptations. A modified iso-oxic hyperoxic and hypoxic CO(2)-rebreathing procedure was employed to evaluate central and peripheral chemoreflex drives to breathe, respectively, in 15 healthy postmenopausal and 20 premenopausal women of similar age. Arterialized venous blood samples were collected at rest for the estimation of arterial Pco(2) (Pa(CO(2))) and H(+) concentration ([H(+)]), plasma strong ion difference ([SID]) and total weak acid ([A](tot)) concentrations, and serum progesterone ([P(4)]) and 17beta-estradiol ([E(2)]) concentrations. In post- compared with premenopausal women, Pa(CO(2)), [SID], and the central chemoreflex ventilatory recruitment threshold for Pco(2) (VRTco(2)) were higher, whereas [P(4)] and [E(2)] were lower (all P < 0.05), with no significant change in central or peripheral chemoreflex sensitivity, peripheral chemoreflex VRTco(2), and [A](tot). The acidifying effect of an increased Pa(CO(2)) was offset by the alkalizing effect of an increased [SID], such that [H(+)] was preserved in post- compared with premenopausal women. Pa(CO(2)) correlated positively with the central chemoreflex VRTco(2) (r = 0.67, P < 0.01), which in turn correlated positively with [SID] (r = 0.53, P < 0.01) within the pooled data. In conclusion, the relative alveolar hypoventilation and attendant arterial hypercapnia in healthy post- compared with premenopausal women could be explained, in part, by the interaction of 1) reduced central, but not peripheral, chemoreflex VRTco(2), 2) increased [SID], and 3) reduced circulating female sex steroid hormone concentrations.

Gender differences in obstructive sleep apnea and treatment implications

Obstructive sleep apnea (OSA) is a common cause of daytime sleepiness for millions of Americans. It is also a disease associated with an increased likelihood of hypertension, cardiovascular disease, stroke, daytime sleepiness, motor vehicle accidents, and diminished quality of life. A number of population-based studies have shown that OSA is more common in men than in women and this discrepancy is often evident in the clinical setting. There are a number of pathophysiological differences to suggest why men are more prone to the disease than women. Although the exact mechanisms are unknown, differences in obesity, upper airway anatomy, breathing control, hormones, and aging are all thought to play a role. The purpose of this review was to examine the literature on gender differences in OSA and to analyze whether or not these differences in pathogenic mechanisms affect diagnosis or treatment.

Central chemoreflex sensitivity and sympathetic neural outflow in elite breath-hold divers

Repeated hypoxemia in obstructive sleep apnea patients increases sympathetic activity, thereby promoting arterial hypertension. Elite breath-holding divers are exposed to similar apneic episodes and hypoxemia. We hypothesized that trained divers would have increased resting sympathetic activity and blood pressure, as well as an excessive sympathetic nervous system response to hypercapnia. We recruited 11 experienced divers and 9 control subjects. During the diving season preceding the study, divers participated in 7.3 ± 1.2 diving fish-catching competitions and 76.4 ± 14.6 apnea training sessions with the last apnea 3–5 days before testing. We monitored beat-by-beat blood pressure, heart rate, femoral artery blood flow, respiration, end-tidal CO2, and muscle sympathetic nerve activity (MSNA). After a baseline period, subjects began to rebreathe a hyperoxic gas mixture to raise end-tidal CO2 to 60 Torr. Baseline MSNA frequency was 31 ± 11 bursts/min in divers and 33 ± 13 bursts/min in control subjects. Total MSNA activity was 1.8 ± 1.5 AU/min in divers and 1.8 ± 1.3 AU/min in control subjects. Arterial oxygen saturation did not change during rebreathing, whereas end-tidal CO2 increased continuously. The slope of the hypercapnic ventilatory and MSNA response was similar in both groups. We conclude that repeated bouts of hypoxemia in elite, healthy breath-holding divers do not lead to sustained sympathetic activation or arterial hypertension. Repeated episodes of hypoxemia may not be sufficient to drive an increase in resting sympathetic activity in the absence of additional comorbidities.

Weight loss as a treatment for obstructive sleep apnoea

Obesity is a major health problem in well-developed societies. It is frequently associated with metabolic, cardiovascular and psychological comorbid conditions. Besides, it is known that obesity constitutes a risk factor for obstructive sleep apnoea (OSA). Weight loss has been evaluated as a treatment for OSA. We reviewed the nature of the link between obesity and OSA as well as the conservative and surgical therapeutic approaches of obesity. We consider the effectiveness of weight loss alone or combined with other modalities of treatment in OSA. We conclude that although weight loss is important and can facilitate the treatment of OSA, it can rarely cure it without being associated to classical techniques, such as nasal continuous positive airway pressure (nCPAP).

Long-term facilitation of breathing is absent after episodes of hypercapnic hypoxia in awake humans

Despite the failure by many previous investigators to demonstrate a long-term facilitation of breathing following episodes of hypoxia in awake humans, we attempted to produce it using a pattern of hypercapnic hypoxic episodes similar to that experienced by obstructive sleep apnoea patients, reasoning that if long-term facilitation was relevant to these patients then it is appropriate to test the effectiveness of such episodes. Ten subjects drawn from the University student population were instrumented to measure ventilation, heart rate and end-tidal PCO2 and PO2 breath-by-breath while seated in a comfortable reclining chair. After an initial resting period breathing room air they experienced fifteen, 30-s episodes breathing 6% O2 and 5% CO2 separated by 90 s of breathing air. We examined the measured variables for an hour after the episodes but found no trends toward an increase in ventilation or decrease in end-tidal PCO2 that would indicate the presence of a long-term facilitation. We therefore concluded that long-term facilitation of ventilation was not demonstrated in awake humans using this pattern of stimuli.

Association between ventilatory response to hypercapnia and obstructive sleep apnea–hypopnea index in asymptomatic subjects

The majority of awake ventilatory control studies have shown normal or decreased ventilatory response to hypercapnia (HCVR) in obstructive sleep apnea-hypopnea syndrome (OSAHS) patients. These findings are contrary to experimental studies suggesting increased loop gain and greater breathing instability in OSAHS patients. We have investigated the relationship between central chemoreflex sensitivity tested by HCVR and obstructive sleep apnea/hypopnea index (OSAHI) in asymptomatic subjects. Twenty normal volunteers (10 men and 10 women) from the general population without physical complaints including sleep-related symptoms were included. The subjects were studied for awake ventilatory responses to hypoxia (HVR) and hypercapnia. Overnight polysomnography (PSG) was performed in two consecutive nights with the first night used as acclimatization. The subjects have an average body mass index (BMI) of 27 +/- 5 SD kg/m(2), ages of 35 +/- 9 SD years and Epworth sleepiness scale of 2.1 +/- 1.8 SD. A positive linear relationship was found between HCVR and logarithmically transformed OSAHI (r = 0.67, p = 0.001). BMI and age were not significantly correlated to HCVR or Log OSAHI. No relationship was found between HVR and Log OSAHI (r = 0.25, p = 0.29). Percentage oxygen saturation nadir during sleep was found to significantly correlate to both daytime HCVR (r = -0.60, p = 0.005) and Log OSAHI (r = -0.65, p = 0.002) and tended to correlate to HVR (r = -0.41, p = 0.07). Arousal index during sleep was not associated with either HCVR (p = 0.93) or HVR (p = 0.26). In conclusion, heightened central chemosensitivity was positively related to OSAHI in asymptomatic volunteers. We believe these findings are in keeping with the evolving theory of loop gain being a significant factor for respiratory control instability and obstructive apnea genesis. The mechanism can be applied to asymptomatic subjects with even minimal sleep-disordered breathing.

Effect of treatment with nasal continuous positive airway pressure on ventilatory response to hypoxia and hypercapnia in patients with sleep apnea syndrome

BACKGROUND

The increase in peripheral chemoreflex sensitivity in patients with obstructive sleep apnea (OSA) is associated with activation of autonomic nervous system and hemodynamic responses. Nasal CPAP (nCPAP) is an effective treatment for OSA, but little is known on its effect on chemoreflex sensitivity.

OBJECTIVES

To assess the effect of nCPAP treatment or placebo (sham nCPAP) on ventilatory control in patients with OSA.

SETTING

Sleep laboratory of Azienda Ospedaliera Garibaldi.

PATIENTS

Twenty-five patients with moderate-to-severe OSA.

DESIGN AND MEASUREMENTS

Patients were randomly assigned to either therapeutic nCPAP (use of optimal pressure, n = 15) or sham nCPAP (suboptimal pressure of 1 to 2 cm H2O, n = 10) in a double-blind fashion and treated for 1 month. A rebreathing test to assess ventilatory response to normocapnic hypoxia and normoxic hypercapnia was performed at basal condition and after 1 month of treatment.

RESULTS

The use of therapeutic nCPAP or sham nCPAP did not affect daytime percentage of arterial oxygen saturation (SaO2%) or end-tidal P(CO2). The normocapnic hypoxic ventilatory response was reduced after 1 month of treatment with nCPAP (the slope was 1.08 +/- 0.02 L/min/SaO2% at basal condition and 0.53 +/- 0.07 L/min/SaO2% after 1 month of treatment, p = 0.008) [mean +/- SD], but not in patients treated with sham nCPAP (slope, 0.83 +/- 0.09 L/min/SaO2% and 0.85 +/- 0.19 L/min/SaO2% at basal condition and after 1 month, respectively). The normoxic hypercapnic ventilatory response remained unchanged after 1 month in both groups. No changes in ventilatory response to either hypoxia or hypercapnia were observed after a single night of nCPAP treatment.

CONCLUSION

The ventilatory response to hypoxia is reduced during regular treatment, but not after short-term treatment, with nCPAP. Readjusted peripheral oxygen chemosensitivity during nCPAP treatment may be a side effect of both reduced sympathetic activity and increased baroreflex activity, or a possible continuous positive airway pressure-related mechanism leading to a reduced activation of autonomic nervous system per se.

Clinical characteristics of obesity-hypoventilation syndrome in Japan: a multi-center study

OBJECTIVE

To clarify the prevalence and clinical characteristics of obesity-hypoventilation syndrome (OHS) in a large number of patients with moderate to severe obstructive sleep apnea syndrome (OSAS).

METHODS

Subjects comprised 611 patients with OSAS registered from 7 sleep centers and clinics and analyzed according to the definitions of the Respiratory Failure Research Group of the Japanese Ministry of Health and Welfare. Baseline characteristics, polysomnographic data during sleep, laboratory blood examinations, excessive daytime sleepiness, pulmonary functions, and arterial blood gases were compared between OHS and non-OHS patients. Determinants of daytime hypercapnia were also examined in OHS patients.

RESULTS

OHS was identified in 55 of the 611 patients with OSAS (9%). OHS patients were younger, heavier, and more somnolent than non-OHS patients and displayed more severe OSAS, liver dysfunctions, higher total cholesterol, and impaired pulmonary function. However, these differences were resolved except for pulmonary function after correction for obesity. Daytime hypercapnia was associated with impaired pulmonary function. Percent vital capacity (%VC) was most closely correlated with PaCO2 in OHS.

CONCLUSION

OHS patients display numerous abnormalities due to obesity compared with non-OHS patients. Impaired pulmonary function, particularly %VC, may play an important role in the development of daytime hypercapnia independent of obesity in OHS patients.

Respiratory control stability and upper airway collapsibility in men and women with obstructive sleep apnea

Obstructive sleep apnea (OSA) is two to three times more common in men as in women. The mechanisms leading to this difference are currently unclear but could include gender differences in respiratory stability [loop gain (LG)] or upper airway collapsibility [pharyngeal critical closing pressure (Pcrit)]. The aim of this study was to compare LG and Pcrit between men and women with OSA to determine whether the factors contributing to apnea are similar between genders. The first group of 11 men and 11 women were matched for OSA severity (mean +/- SE apnea-hypopnea index = 43.8 +/- 6.1 and 44.1 +/- 6.6 events/h). The second group of 12 men and 12 women were matched for body mass index (BMI; 31.6 +/- 1.9 and 31.3 +/- 1.8 kg/m2, respectively). All measurements were made during stable supine non-rapid eye movement sleep. LG was determined using a proportional assist ventilator. Pcrit was measured by progressively dropping the continuous positive airway pressure level for three to five breaths until airway collapse. Apnea-hypopnea index-matched women had a higher BMI than men (38.0 +/- 2.4 vs. 30.0 +/- 1.9 kg/m2; P = 0.03), but LG and Pcrit were similar between men and women (LG: 0.37 +/- 0.02 and 0.37 +/- 0.02, respectively, P = 0.92; Pcrit: 0.35 +/- 0.62 and -0.18 +/- 0.87, respectively, P = 0.63). In the BMI-matched subgroup, women had less severe OSA during non-rapid eye movement sleep (30.9 +/- 7.4 vs. 52.5 +/- 8.1 events/h; P = 0.04) and lower Pcrit (-2.01 +/- 0.62 vs. 1.16 +/- 0.83 cmH2O; P = 0.005). However, LG was not significantly different between genders (0.38 +/- 0.02 vs. 0.33 +/- 0.03; P = 0.14). These results suggest that women may be protected from developing OSA by having a less collapsible upper airway for any given degree of obesity.

Daytime hypercapnia in adult patients with obstructive sleep apnea syndrome in France, before initiating nocturnal nasal continuous positive airway pressure therapy

CONTEXT

Daytime hypercapnia in patients with obstructive sleep apnea syndrome (OSAS) has a highly variable prevalence in the published studies, and is usually thought to be the consequence of an associated disease, COPD, or severe obesity.

STUDY OBJECTIVES

To assess the prevalence of daytime hypercapnia in a very large population of adult patients with OSAS, free of associated COPD, and with a wide range of body mass index (BMI), and to evaluate the relationship between daytime hypercapnia and the severity of obesity and obesity-related impairment in lung function.

DESIGN

Retrospective analysis of prospectively collected data.

METHODS

The database of the observatory of a national nonprofit network for home treatment of patients with chronic respiratory insufficiency (Association Nationale pour le Traitement a Domicile de l'Insuffisance Respiratoire Chronique) was used. Collected data at treatment initiation were age, apnea-hypopnea index, BMI, FEV(1), vital capacity (VC), and arterial blood gases. The study included 1,141 adult patients with OSAS treated in France with nocturnal nasal continuous positive airway pressure (CPAP), FEV(1) >/= 80% predicted, FEV(1)/VC >/= 70%, and absence of restrictive respiratory disease other than related to obesity.

RESULTS

The prevalence of daytime hypercapnia (Paco(2) >/= 45 mm Hg) before initiating CPAP therapy was 11% in the whole study population. The prevalence of daytime hypercapnia was 7.2% (27 of 377 patients) with BMI < 30, 9.8% (58 of 590 patients) with BMI from 30 to 40, and 23.6% (41 of 174 patients) with BMI > 40. Patients with daytime hypercapnia had significantly higher BMI values and significantly lower VC, FEV(1), and Pao(2) values than the normocapnic patients. Stepwise multiple regression showed that Pao(2), BMI, and either VC or FEV(1) were the best predictors of hypercapnia, but these variables explained only 9% of the variance in Paco(2) levels.

CONCLUSION

Daytime hypercapnia was observed in > 1 of 10 patients with OSAS needing CPAP therapy and free of COPD, and was related to the severity of obesity and obesity-related impairment in lung function. However, other mechanisms than obesity are probably involved in the pathogenesis of daytime hypercapnia in OSAS.

Overnight changes of chemoreflex control in obstructive sleep apnoea patients

We hypothesized that the numerous episodes of hypoxia, hypercapnia and arousal experienced by obstructive sleep apnoea (OSA) patients induce overnight changes in respiratory chemoreflexes. A modification of the Read rebreathing technique assessed chemoreflex characteristics in the evening and the morning of patients undergoing diagnostic assessment for OSA in a clinical sleep laboratory. Two groups were studied: those with apnoea-hypopnoea indices (AHI) greater than 30 composed the OSA group (n = 12), and those with AHI indices less than 10 composed the non-OSA group (n = 12). There was a significant (approximately 30%) overnight increase in chemoreflex sensitivities, without changes in thresholds, in the OSA group. In the non-OSA group there was a significant overnight reduction in chemoreflex thresholds (approximately 5%), without changes in sensitivities. We suggest that these changes affect the stability of the chemoreflex control system in opposite ways as the night proceeds: destabilizing breathing for patients in the OSA group, and stabilising breathing for patients in the non-OSA group.

Chemoreflex control of breathing during wakefulness in healthy men and women

This study used a modified CO(2) rebreathing procedure to examine the effect of gender on the chemoreflex control of breathing during wakefulness in healthy men (n = 14) and women (n = 14). Women were tested in the follicular phase of the menstrual cycle. During rebreathing trials, subjects hyperventilated to reduce the partial pressure of end-tidal CO(2) (Pet(CO(2))) below 25 Torr and were then switched to a rebreathing bag containing a normocapnic hypoxic or hyperoxic gas mixture. During the trial, Pet(CO(2)) increased, while O(2) was maintained at a constant level. The point at which ventilation began to rise as Pet(CO(2)) increased was identified as the ventilatory recruitment threshold (VRT). Ventilation below the VRT was measured, and the slope of the ventilatory response above the VRT was determined. Gender had no effect on the hyperoxic or hypoxic VRT for CO(2). Central chemoreflex sensitivity was significantly greater in men than women but not after correction for forced vital capacity. Measures of peripheral chemoreflex sensitivity were similar between genders. However, the slope of the tidal volume (Vt) response to hyperoxic and hypoxic CO(2) rebreathing (corrected and uncorrected) was greater in men than women, respectively. We conclude that central chemoreflex sensitivity is greater in men compared with women as reflected by differences in ventilatory (uncorrected) and Vt (corrected and uncorrected) responses to CO(2). However, gender has no significant effect on the central chemoreflex VRT for CO(2). The peripheral chemoreflex control of breathing during wakefulness is similar between men and women.

The effect of upper airway structural changes on central chemosensitivity in obstructive sleep apnea-hypopnea

We examined the efficiency of upper airway structural changes in uvulopalatopharyngoplasty and/or tonsillectomy on central chemosensitivity, and whether the outcome of such surgeries can be predicted by the central chemosensitivity in obstructive sleep apnea-hypopnea syndrome (OSAHS) patients. In 11 patients with OSAHS group, the average of the hypercapnic ventilatory response (HCVR) slope was 1.93 +/- 0.20 L/min/mm Hg preoperatively and 1.78 +/- 0.22 L/min/mm Hg postoperatively. The average of the mouth occlusion pressure at 0.1 second after the onset of inspiration (P (0.1)) slope was 0.47 +/- 0.06 cm H (2)O/mm Hg and 0.44 +/- 0.08 cm H (2)O/mm Hg, before and after surgery, respectively. There were no significant differences before and after treatment, although OSAHS was improved by these surgeries. In control group with 5 patients, the HCVR slope and P (0.1) slope also showed no significant difference before and after the procedure. When we divided the 11 OSAHS patients into 7 responders (apnea-hypopnea index < 20 events/h and > 50% reduction) and 4 poor responders, there was a significant difference between the average HCVR slope of responders (1.59 +/- 0.21 L/min/mm Hg) and that of poor responders (2.52 +/- 0.20 L/min/mm Hg). We saw no significant difference in physiologic (age, body mass index, one-piece tonsil weight), blood gas values, cephalometric, spirometric, or sleep parameters.

The influence of gender and upper airway resistance on the ventilatory response to arousal in obstructive sleep apnoea in humans

The termination of obstructive respiratory events is typically associated with arousal from sleep. The ventilatory response to arousal may be an important determinant of subsequent respiratory stability/instability and therefore may be involved in perpetuating obstructive respiratory events. In healthy subjects arousal is associated with brief hyperventilation followed by more prolonged hypoventilation on return to sleep. This study was designed to assess whether elevated sleeping upper airway resistance (R(UA)) alters the ventilatory response to arousal and subsequent breathing on return to sleep in patients with obstructive sleep apnoea (OSA). Inspired minute ventilation (V(I)), R(UA) and end-tidal CO(2) pressure (P(ET,CO(2))) were measured in 22 patients (11 men, 11 women) with OSA (mean +/-s.e.m., apnoea-hypopnoea index (AHI) 48.9 +/- 5.9 events h(-1)) during non-rapid eye movement (NREM) sleep with low R(UA) (2.8 +/- 0.3 cmH(2)O l(-1) s; optimal continuous positive airway pressure (CPAP) = 11.3 +/- 0.7 cmH(2)O) and with elevated R(UA) (17.6 +/- 2.8 cmH(2)O l(-1) s; sub-optimal CPAP = 8.4 +/- 0.8 cmH(2)O). A single observer, unaware of respiratory data, identified spontaneous and tone-induced arousals of 3-15 s duration preceded and followed by stable NREM sleep. V(I) was compared between CPAP levels before and after spontaneous arousal in 16 subjects with tone-induced arousals in both conditions. During stable NREM sleep at sub-optimal CPAP, P(ET,CO(2)) was mildly elevated (43.5 +/- 0.8 versus 42.5 +/- 0.8 Torr). However, baseline V(I) (7.8 +/- 0.3 versus 8.0 +/- 0.3 l min(-1)) was unchanged between CPAP conditions. For the first three breaths following arousal, V(I) was higher for sub-optimal than optimal CPAP (first breath: 11.2 +/- 0.9 versus 9.3 +/- 0.6 l min(-1)). The magnitude of hypoventilation on return to sleep was not affected by the level of CPAP and both obstructive and central respiratory events were rare following arousal. Similar results occurred after tone-induced arousals which led to larger responses than spontaneous arousals. V(I) for the first breath following arousal under optimal CPAP was greater in men than women (11.0 +/- 0.4 versus 7.6 +/- 0.6 l min(-1)). These results demonstrate that the ventilatory response to arousal is influenced by pre-arousal airway resistance and gender. Whether this contributes to the perpetuation of respiratory events and the pathogenesis of OSA is unclear.

Delayed chemoreceptor responses in infants with apnoea

AIMS

To test the hypothesis that apnoea of infancy (AOI) is due to a deficit in chemoreception.

METHODS

Tests were performed on 112 infants: 43 healthy control infants, 28 infants with periodic breathing or central apnoea (PBCA), and 41 infants with obstructive apnoea (OA) on overnight polysomnography. Chemoreceptor responses to hypercapnia (4% and 6% CO2 in air) for 6-8 minutes and hyperoxia (100% O2) for 60 seconds were expressed in terms of response strength and reaction time. Age at birth (gestational week 37-41) and age at test (2-34 postnatal weeks) were comparable across groups (median, min-max value). A total of 70 CO2 and 71 O2 tests were analysed.

RESULTS

The strongest and fastest CO2 responders were control infants: their median increase in ventilation was 291%/kPaCO2 and their reaction time 16 breaths. In infants with PBCA and OA, the increase in ventilation was 41% and 130%/kPaCO2, and reaction time 64 and 54 breaths, respectively. There was a significant negative correlation between CO2 response strength and response time. In response to hyperoxia there was a comparable decrease in ventilation in all infants (12-20%), but a significantly longer response time in infants with apnoea (20 v 12 breaths). There was no correlation between the response strength and response time to O2 and CO2.

CONCLUSION

An inappropriate central control of respiration is an important mechanism in the pathogenesis of apnoea of infancy.

Effects of opioid dependence and tobacco use on ventilatory response to progressive hypercapnia

Respiratory depression is a serious medical risk of opioid use. Most opioid abusers also smoke cigarettes, perhaps further compromising breathing. Differences in ventilatory response to nonhypoxic hypercapnia were studied in healthy volunteers with limited substance use (LU), tobacco smokers (SM), and opioid-dependent, methadone-maintained smokers (OD). The last two groups had similar current cigarette use and all groups were similar in gender and body mass index. Because previous data suggest that SM are sensitive to hypoxia but not hypercapnia, it was predicted that only the OD group would exhibit decreased carbon dioxide (CO(2)) sensitivity. All subjects rebreathed CO(2) during three identical sessions (four trials per session). Fractional end-tidal (Fet) CO(2) levels during repeated 4-min exposures to progressive hypercapnia (6% to 10%) were similar across groups. Ventilatory response (breathing rate, tidal volume and minute volume) linearly increased with FetCO(2) concentration and did not differ significantly across sessions. Relative to the LU and SM groups (which did not significantly differ), the CO(2)-minute volume and CO(2)-breathing rate functions were significantly shifted rightward (decrease in intercept but not slope) for OD subjects. These data are consistent with the hypothesis that chronic opioid exposure and/or short-term methadone maintenance (but not tobacco or nicotine use) produces a specific decrease in CO(2) sensitivity, primarily through an inhibitory effect on respiratory frequency.

Chemoreflex control of ventilation is altered during wakefulness in humans with OSA

We hypothesized that patients with obstructive sleep apnea (OSA) have a different awake ventilatory response to carbon dioxide above and below eupnea compared with normal. Eight male subjects with OSA and control subjects matched for gender, race, age, height and weight voluntarily hyperventilated during wakefulness to reduce the partial pressure of carbon dioxide (PET(CO2)) below 25 mmHg. Subjects were then switched into a rebreathing bag containing a normocapnic (42 mmHg) hypoxic [partial pressure of end tidal oxygen (PET(O2))=50 mmHg (H50)] or hyperoxic [PET(O2)=140 mmHg (H140)] gas mixture. During the trial PET(CO2) increased while PET(O2) was maintained at a constant level. The point at which ventilation and PET(CO2) increased linearly was considered to be the carbon dioxide ventilatory recruitment threshold (VRT(CO2)). Measurements of ventilation and its components (i.e. tidal volume and breathing frequency) were made below this threshold and the slope of the minute ventilation; tidal volume or breathing frequency response above the threshold was determined. Four trials for a given oxygen level were completed. The PET(CO2) that demarcated the VRT(CO2) was increased (H(50)=43.43+/-0.92 vs. 41.05+/-0.67; H(140)=47.65+/-0.80 vs. 45.28+/-0.75), as were measures of ventilation below the threshold (H(50)=18.50+/-2.11 vs. 13.44+/-1.43; H(140)=19.66+/-2.71 vs. 10.83+/-1.24) in the OSA subjects compared with control. In contrast the OSA and control subjects did not respond differently to changes in PET(CO2) above the threshold. We conclude that the PET(CO2) that delineates the VRT(CO2) and ventilation below this threshold is elevated in subjects with OSA.

Gender differences in sleep apnea: epidemiology, clinical presentation and pathogenic mechanisms

Sleep apnea syndromes are a common cause of sleepiness and neurocognitive impairment and have been implicated as an independent risk factor for cardiovascular disease. While both epidemiological and sleep clinic-based studies indicate that sleep apnea syndromes are more common in men than in women, the gender difference in prevalence is more marked within the sleep clinic. Reasons for the relative failure of women to attend sleep clinics and the pathophysiologic differences that give rise to the male predominance of sleep apnea syndromes are unknown. The purpose of this review was to examine the literature with regard to these aspects, to provide clinical guidance to improve the reduced attendance of women to sleep laboratories and to stimulate research interest into the causes of these differences.

Effect of obesity and/or sleep apnea on chemosensitivity: differences between men and women

The purpose of this study was to identify if obesity or obstructive sleep apnea (OSA) influence hypercapnic response (HCR) and hypoxic response (HR) taking into account differences between gender. Data are expressed in % predicted based on reference values of our laboratory ('Respir. Physiol. 113 (1998) 157'). Obese women without OSA (n=117, body mass index (BMI) 43+/-8 kg/m(2)) demonstrated an increased ventilatory (VE) and occlusion pressure (P(0.1)) HCR and HR slope: VE HCR 113 (NS), VE HCR/vital capacity (VC) 126 (P<0.05), P(0.1) HCR 130 (P<0.05), VE HR 136 (P<0.05), VE HR/VC 154 (P<0.001) and P(0.1) HR 210 (P<0.001) % predicted. Obese women with OSA (n=34, BMI 42+/-9 kg/m(2)) presented similar increased values for HCR and even more increased values for HR than obese women without OSA matched for age, height and BMI: VE HR 155 (P=NS), VE HR/VC 205 (P<0.05) and P(0.1) HR 273 (P<0.05) % predicted. In obese men (without or with OSA) HCR and HR values were similar to our reference values.

Determinants of chronic hypercapnia in Japanese men with obstructive sleep apnea syndrome

STUDY OBJECTIVE

To identify the determinants of chronic hypercapnia (ie, PaCO(2), > or = 45 mm Hg) in men with obstructive sleep apnea syndrome (OSAS) without airflow obstruction.

DESIGN

An analysis was conducted of 143 male patients with OSAS, which had been diagnosed by polysomnography (PSG), who had been referred to a university hospital. Patients were classified as hypercapnic (ie, PaCO(2), > or = 45 mm Hg) and normocapnic (ie, PaCO(2), < 45 mm Hg), and obese (ie, body mass index [BMI], > or = 30 kg/m(2)) or nonobese (ie, BMI, < 30 kg/m(2)). Patients with airflow obstruction (ie, FEV(1)/FVC ratio, < 70%) were excluded from the study. Baseline clinical characteristics, pulmonary function, PSG data, and blood gas data were compared between hypercapnic and normocapnic patients. Correlations between PaCO(2) and several anthropometric, respiratory, and polysomnographic variables were determined by stepwise multiple regression analysis.

RESULTS

Fifty-five patients (38%) were hypercapnic. Hypercapnic patients were younger and heavier, and had more abnormalities on pulmonary and PSG testing. Stepwise multiple regression analysis revealed that the PaCO(2) level was influenced significantly by the mean level of arterial oxygen saturation (SaO(2)) during sleep and by the percent of vital capacity (%VC) (R(2) = 0.430; p < 0.0001), indicating that 43% of the total variance in the PaCO(2) could be explained by the mean SaO(2) and %VC in hypercapnic patients. In contrast, only 13% of the total variance in the PaCO(2) was accounted for by the mean SaO(2) and BMI in normocapnic patients (R(2) = 0.134; p = 0.0034). The mean SaO(2), %VC, and PaO(2) were selected as independent variables for predicting the PaCO(2) in obese patients. These variables explained 41% of the total variance in the PaCO(2) (R(2) = 0.407; p < 0.0001), whereas the mean SaO(2) only accounted for 13% of the total variance in PaCO(2) levels in nonobese patients (R(2) = 0.134; p = 0.0064).

CONCLUSION

Nocturnal desaturation and restrictive pulmonary impairment play major roles in determining the PaCO(2) in hypercapnic and obese OSAS patients without airflow obstruction.

Epidemiology and pathogenesis of sleep-disordered breathing

Sleep-disordered breathing is common in the general population, but the observed prevalence depends on the criteria used to establish the diagnosis. Obesity is a strong risk factor, but other conditions such as allergic upper and lower airways disease may also be important. Differences in risk between the sexes and ethnic groups appear to be present even after established risk factors have been considered. The pathogenesis is likely mutifactorial with anatomic and physiologic factors of varying importance in different individuals. The natural history is uncertain, but without treatment or reduction in risk factors, some progression is likely. Ongoing epidemiologic investigations such as the Sleep Heart Health Study are beginning to provide important information on these questions.