Intermittent hypoxia suppresses adiponectin secretion by adipocytes

Obstructive sleep apnea (OSA), characterized by cyclic intermittent hypoxia (IH) during sleep, is an independent risk factor for cardiovascular disease. Adiponectin (APN), an adipocytokine secreted exclusively by adipocytes, possesses antiatherogenic properties. Low levels of APN, particularly the high-molecular-weight (HMW) form, are associated with an increased risk of cardiovascular disease. Here, we hypothesized that IH would result in the dysregulation of APN expression and secretion. 3T3-L1 adipocytes were exposed to IH at 12 cycles/h for 6 h/d to simulate the IH condition similar to that encountered in OSA. Control adipocytes were exposed to 21% O(2) under identical conditions. After 48 h of incubation, IH caused a decrease in the secretion of total and HMW APN in spite of a significant upregulation of APN mRNA expression by adipocytes. This study suggested a novel mechanism of how the cyclic hypoxemia in OSA predisposes OSA patients to cardiovascular disease through the dysregulation of secretion of APN by adipocytes. Further studies are needed to determine the exact molecular mechanism how IH reduces the release of APN by adipocytes.

Intermittent hypoxia reduces upper airway stability in lean but not obese Zucker rats

Obstructive sleep apnea involves intermittent periods of airway occlusions that lead to repetitive oxygen desaturations. Exposure to chronic intermittent hypoxia (IH) in rats increases diurnal blood pressure and alters skeletal muscle physiology. The impact of IH on upper airway muscle function is unknown. We hypothesize that IH exposure increases upper airway collapsibility in rats due to alterations of the muscles surrounding the upper airway. Lean and obese rats were exposed to cyclic alterations in O(2) levels (20.6%-5%) every 90 s, 8 h/day for 6 days/wk for 12 wk. Following the exposure period, arterial pressure was recorded via the tail artery in conscious unrestrained rats. Mean arterial pressure was increased in lean IH but not in obese IH-exposed Zucker rats (P < 0.05). The pharyngeal pressure associated with airway collapse (P(crit)) was measured under anesthesia during baseline conditions and then during supramaximal stimulation of the hypoglossal nerve (cnXII). Baseline P(crit) was more positive (more collapsible) in lean but not obese rats following 12 wk of IH (P < 0.05), while supramaximal stimulation of cnXII increased airway stability (decreased P(crit)) in both lean and obese Zucker rats following IH to levels that were similar to their respective room air controls. The in vitro peak tension and the expression of the individual myosin heavy chain isoforms from the upper airway muscles were unaltered following IH. We conclude that IH leads to increases in baseline collapsibility in lean Zucker rats exposed to IH by nonmyogenic mechanisms.

Medical therapy for obstructive sleep apnea: a review by the Medical Therapy for Obstructive Sleep Apnea Task Force of the Standards of Practice Committee of the American Academy of Sleep Medicine

A significant number of patients with obstructive sleep apnea neither tolerate positive airway pressure (PAP) therapy nor achieve successful outcomes from either upper airway surgeries or use of an oral appliance. The purpose of this paper, therefore, was to systematically evaluate available peer-reviewed data on the effectiveness of adjunctive medical therapies and summarize findings from these studies. A review from 1985 to 2005 of the English literature reveals several practical findings. Weight loss has additional health benefits and should be routinely recommended to most overweight patients. Presently, there are no widely effective pharmacotherapies for individuals with sleep apnea, with the important exceptions of individuals with hypothyroidism or with acromegaly. Treating the underlying medical condition can have pronounced effects on the apnea/hypopnea index. Stimulant therapy leads to a small but statistically significant improvement in objective sleepiness. Nonetheless, residual sleepiness remains a significant health concern. Supplemental oxygen and positional therapy may benefit subsets of patients, but whether these therapies reduce morbidities as PAP therapy does will require rigorous randomized trials. PAP therapy has set the bar high for successful treatment of sleep apnea and its associated morbidities. Nonetheless, we should strive towards the development of universally effective pharmacotherapies for sleep apnea. To accomplish this, we require a greater knowledge of the neurochemical mechanisms underlying sleep apnea, and we must use this infrastructure of knowledge to design well-controlled, adequately powered studies that examine, not only effects on the apnea/hypopnea index, but also the effects of pharmacotherapies on all health related outcomes shown beneficial with PAP therapy.

Adiponectin inhibits superoxide generation by human neutrophils

Adiponectin (Ad), a member of the adipocytokine family, has been reported to possess antiinflammatory properties. We investigated the effects of full-length human Ad (hAd) on phorbol 12-myristate 13-acetate (PMA)-induced O2-* generation by human neutrophils. hAd, even at the lowest tested concentration of 0.001 microg/ml, after 30-min pretreatment of cells, significantly inhibited O2-* generation by neutrophils stimulated with PMA (100 nM). However, no relation between the dose of hAd and extent of inhibition of PMA-induced O2-* generation was observed with increasing the concentration of hAd up to 1 microg/ml. hAd also significantly inhibited neutrophil O2-* generation stimulated by N-formyl-methionyl-leucyl-phenylalanine (100 microM) and diacylglycerol (500 nM), as well as the PMA-induced neutrophil nitroblue tetrazolium reduction and H2O2 formation. Pretreatment of neutrophils with pronase-digested hAd failed to inhibit the PMA-induced O2-* generation. For the first time, this study revealed that Ad inhibited O2-* generation by neutrophils, possibly through regulation of NADPH oxidase.

Prevalence of Positional Sleep Apnea in Patients Undergoing Polysomnography

STUDY OBJECTIVES

The primary aim of this study was to determine the prevalence of positional obstructive sleep apnea using a functional definition. Positional sleep apnea was defined as a total apnea-hypopnea index (AHI) > or = 5 with a > 50% reduction in the AHI between the supine and nonsupine postures, and an AHI that normalizes (AHI < 5) in the nonsupine posture. A secondary aim was to determine if positional sleep apnea can be diagnosed accurately during a split-night study.

DESIGN

Retrospective chart review.

SETTING

Two sleep centers in Buffalo, NY, one a Veterans Affairs Western New York Healthcare System Sleep Center (VAWNY) and the other a freestanding ambulatory center (Associated Sleep Center [ASC]).

PATIENTS

Three hundred twenty-six patients from the VAWNY, including 57 patients who underwent a split-night study and 242 patients from the ASC who underwent polysomnography.

INTERVENTIONS

None.

MEASUREMENTS

Patient characteristics and sleep study results.

RESULTS

Positional sleep apnea was seen in 49 of 99 patients (49.5%) with mild sleep apnea (AHI, 5 to 15/h), 14 of 72 patients (19.4%) with moderate sleep apnea (AHI, 15 to 30/h), and 5 of 77 patients (6.5%) with severe sleep apnea (AHI > 30/h). Sufficient sleep (> 15 min) in both postures was not seen in 104 of 269 patients (38.7%) and 80 of 242 overnight studies (33.1%) at the VAWNY and ASC, respectively, and was not seen in 47 of 57 split-night studies (82.5%). The percentage of studies with insufficient sleep in both postures was significantly greater for split-night studies (p < 0.0001).

CONCLUSIONS

Positional sleep apnea is common particularly in patients with mild disease. Positional sleep apnea cannot usually be assessed during a split-night study.

Systemic Administration of Serotonin 2A/2C Agonist Improves Upper Airway Stability in Zucker Rats

The effects of [+/-]-2,5-dimethoxy-4-iodoaminophentamine, a serotonin(2A/2C) receptor agonist, on pharyngeal airflow mechanics were examined in isoflurane-anesthetized lean and obese Zucker rats. The pharyngeal pressure associated with flow limitation, maximum inspiratory flow, oronasal resistance, genioglossus muscle activity, and arterial blood pressure (BP) were measured before and after the intravenous administration of the agonist. A robust activation of the genioglossus muscle in all lean and obese rats was associated with decreased upper airway (UA) collapsibility (p < 0.05), unchanged maximum flow, and increased oronasal resistance (p < 0.05) in both groups. The changes in UA mechanics and BP after the drug were similar in lean and obese rats. The serotonin agonist had no effect on UA mechanics in a group of paralyzed (pancuronium bromide) rats, despite similar elevations in BP. There was a smaller decrease (p < 0.05) in UA collapsibility that was also associated with increased upstream resistance when the drug was administered after bilateral hypoglossal nerve transection. We conclude that systemic administration of a serotonin(2A/2C) receptor agonist improves UA collapsibility predominantly, but not exclusively, via stimulation of the hypoglossal nerves and also increases upstream resistance, at least in part, through activation of nonhypoglossal motoneuronal pools innervating the UA muscles.

Prediction of the apnea-hypopnea index from overnight pulse oximetry

STUDY OBJECTIVE

s: To compare the relative usefulness of the different indexes derived from pulse oximetry in the diagnosis of obstructive sleep apnea (OSA), and to determine if a combination of these indexes improves the prediction of the apnea-hypopnea index (AHI) measured by polysomnography.

DESIGN

Prediction model developed from 224 patients, validated prospectively in 101 patients from the same center (group 1) and in 191 patients from a different sleep center (group 2).

SETTING

Two independent sleep clinics run by university sleep specialists.

PARTICIPANTS

Patients who underwent polysomnography for suspicion of OSA.

INTERVENTIONS

The following indexes were calculated from pulse oximetry recordings performed simultaneously during polysomnography: (1) Delta index, the average of the absolute differences of oxygen saturation between successive 12-s intervals; (2) desaturation events per hour to 2%, 3%, and 4% levels; and (3) cumulative time spent below 90%, 88%, 86%, 84%, 82%, and 80% saturation.

MEASUREMENTS AND RESULTS

The best predictor was the Delta index, although desaturation events provided similar levels of diagnostic accuracy. An aggregation of multivariate models using combination of indexes reduced the prediction error (r(2) = 0.70) significantly (p < 0.05) compared to using the Delta index alone (r(2) = 0.60). The proportion of subjects from the validation groups within 95% confidence interval (CI) of the derivation group was 90% (95% CI, 83 to 95%) and 91% (95% CI, 86 to 95%) for groups 1 and 2, respectively. The overall likelihood ratios for the aggregated model in all patient groups were 4.2 (95% CI, 3.3 to 15.3), 3.4 (95% CI, 2.7 to 4.3), 3.0 (95% CI, 2.2 to 4.1), and 6.7 (95% CI, 4.9 to 9.2) for normal (AHI < 5/h), mild (AHI 5 to < 15/h), moderate (AHI 15 to < 30/h), and severe (AHI > or = 30/h) disease, respectively.

CONCLUSIONS

The Delta index and oxygen desaturation indexes provided similar levels of diagnostic accuracy. The combination of indexes improved the precision of the predicted AHI and may offer a potentially simpler alternative to polysomnography.

Behavioral and pharmacologic therapy of obstructive sleep apnea

Previous attempts at using pharmacologic agents in the treatment of OSA have been disappointing. Medroxyprogesterone has not been found to be useful in the treatment of OSA. Use of protriptyline is limited by frequent side effects, but its role in mild and REM-related OSA must be clarified. SSRIs seem to be ineffective in treatment of severe OSA. Further studies are needed to determine their effect in persons with mild disease. This is important because patients with mild OSA (AHI < 15 hours) are most likely to be noncompliant with CPAP therapy [91]. A recent systematic review of drug treatments for OSA concluded that the current data do not support the use of any drug as an alternative to CPAP [92]. Of 56 studies identified, only 9 studies met methodologic criteria. Clearly, basic research and adequately powered clinical trials are needed to identify an effective medication for OSA.

The utility of neural network in the diagnosis of Cheyne-Stokes respiration

The aim of this study was to design a diagnostic model to identify patients with Cheyne-Stokes respiration (CSR-CSA) based on indices of oximetric spectral analysis. A retrospective analysis of oximetric recordings of 213 sleep studies conducted over a one-year period at a Veterans Affairs medical facility was performed. A probabilistic neural network (PNN) was developed from salient features of the oximetric spectral analysis, desaturation events and the delta index. A fivefold cross-validation was used to assess the accuracy of the neural network in identifying CSR-CSA. When compared to overnight polysomnography, the PNN achieved a sensitivity of 100% (95% confidence interval [CI] 85%-100%) and a specificity of 99% (95% 97%-100%) with a corresponding area under the curve of 99% (95% CI 99%-100%). When combined with overnight pulse oximetry, PNN offers an accurate and easily applicable tool to detect CSR-CSA.

Opioidergic modulation of ventilatory response to sustained hypoxia in obese Zucker rats

OBJECTIVE

To determine whether altered central and/or peripheral opioidergic mechanisms contribute to the altered ventilatory response to sustained hypoxia in obese Zucker rats.

RESEARCH METHODS AND PROCEDURES

Eight lean (176 +/- 8 [SEM] g) and eight obese (225 +/- 12 g) Zucker rats were studied at 6 weeks of age. Pulmonary ventilation ((E)), tidal volume (V(T)), and breathing frequency (f) at rest and in response to sustained (30 minutes) hypoxic (10% O(2)) challenges were measured on three separate occasions by the barometric method after the randomized, blinded administration of equal volumes of saline (control), naloxone methiodide (N(M); 5 mg/kg, peripheral opioid antagonist), or naloxone hydrochloride (N(HCl); 5 mg/kg, peripheral and central opioid antagonist).

RESULTS

Administration of N(M) and N(HCl) in lean animals had no effect on (E) either at rest or during 30 minutes of sustained exposure to hypoxia. Similarly, N(M) failed to alter (E) in obese rats. In contrast, N(HCl) significantly (p < 0.05) increased (E) and V(T) both at rest and during 2 to 10 minutes of hypoxic exposure in obese rats. After 20 to 30 minutes of hypoxic exposure, V(T) remained elevated with N(HCl), but the earlier elevation of (E) seemed to be attenuated due to a decrease in f at 20 minutes of exposure to hypoxia.

DISCUSSION

Thus, endogenous opioids modulate both resting (E) and the ventilatory response to sustained hypoxia in obese, but not in lean, Zucker rats by acting specifically on opioid receptors located within the central nervous system.

Serotonergic modulation of ventilation and upper airway stability in obese Zucker rats

To elucidate the role of serotonin in the maintenance of normal breathing and upper airway (UA) patency in obesity, we studied the effects of systemic administration of ritanserin, a serotonin (5-HT) 2A and 2C receptor antagonist, on ventilation (V E) during room air breathing and during hypoxic (10% O2) and hypercapnic (4% CO2) ventilatory challenges in awake young (6-8 wk) and older (7-8 mo) obese and lean Zucker (Z) rats. Older obese Z rats adopted a more rapid shallow breathing pattern compared with older lean rats. The administration of ritanserin (1 mg/kg intraperitoneally) to older obese rats resulted in a reduction in V E (439 +/- 35 [SD] to 386 +/- 41 ml/kg/min, p < 0.01), a decrease in respiratory rate, a prolongation of inspiratory time, and an increase in V O2 (16.4 +/- 1.7 to 18.2 +/- 1.9 ml/kg(0.75)/min, p < 0.05) during room air breathing. By comparison, it had little effect on ventilation in young lean and obese Z or older lean Z rats. Ritanserin also had no effect on ventilatory responses to either hypoxia or hypercapnia in young or older lean and obese Z rats. The collapsibility of the isolated UA was examined in older Z rats. The pharyngeal critical pressure (Pcrit) of older obese rats was significantly greater than that of lean rats (p < 0.05), indicating that obese rats have more collapsible UA than lean rats. The administration of ritanserin significantly increased Pcrit in older obese rats (-1.6 +/- 0.3 to -0.8 +/- 0.2 cm H2O, p < 0.01) and in lean rats (-3.1 +/- 1.0 to -2.4 +/- 0.6 cm H2O, p < 0.05). We suggest that the 5-HT(2A/2C) receptor subtype plays an important role in the maintenance of UA stability and normal breathing in obesity, and we speculate that older obese Z rats may have augmented serotonergic control of UA dilator muscles as a mechanism to prevent pharyngeal collapse.

Comparison of cervical magnetic and transcutaneous phrenic nerve stimulation before and after threshold loading

Brief supramaximal stimulation of the phrenic nerves (twitch) is considered a promising technique to detect diaphragmatic fatigue in humans. However, the most commonly employed methodology (transcutaneous stimulation) is technically difficult. Cervical magnetic stimulation is a recently described technique that is potentially simpler and may obviate some of the problems inherent with transcutaneous stimulation. The purpose of this study was to determine the ability of cervical magnetic stimulation to evaluate diaphragmatic function. Accordingly, we measured transdiaphragmatic pressure (Pdi) during transcutaneous and cervical magnetic stimulation of the phrenic nerves before and after a potentially fatiguing task; inspiratory threshold loading to task failure. During threshold loading, subjects generated approximately 60% of their maximal esophageal pressure with each breath until they could no longer reach the target pressure. At least 10 twitches were obtained during both transcutaneous and magnetic stimulation before and 10, 30, 60, and 120 min after threshold loading. Control twitch Pdi was significantly larger during magnetic stimulation compared with transcutaneous stimulation: 39.3 +/- 3.0 (mean +/- SE) versus 27.4 +/- 2.3 cm H2O, p < 0.0005. This increase in twitch Pdi was solely due to the esophageal component. Following threshold loading, a significant reduction in transcutaneous twitch Pdi was seen in only three of the 10 subjects. Mean transcutaneous twitch Pdi fell only slightly from 27.4 +/- 2.3 during control to 25.1 +/- 2.2 cm H2O at 10 min after loading (p < 0.004). In contrast, magnetic twitch Pdi was significantly reduced in nine of the 10 subjects following threshold loading. Mean magnetic twitch Pdi fell from 39.3 +/- 3.0 during control to 31.1 +/- 3.0 cm H2O at 10 min after loading (p < 0.0001). The average fall in twitch Pdi post-loading (expressed as a percentage of the control value) was significantly greater for magnetic stimulation compared with transcutaneous stimulation: 21.0 +/- 3.1 versus 7.8 +/- 2.9%, p < 0.0001. In summary: (1) in the fresh state, twitch Pdi is larger with magnetic stimulation compared with transcutaneous stimulation, and (2) transcutaneous and cervical magnetic twitch Pdi are affected differently by threshold loading to task failure.

Reproducibility of Borg scale measurements of dyspnea during exercise in patients with COPD

The purpose of this study was to evaluate the moderate term (5 weeks) reproducibility of Borg scale ratings of the effort to breathe (Borge) and the degree of discomfort evoked by breathing (Borgd) in patients with COPD during exercise. Six subjects with moderately severe COPD (FEV1, 1.42 +/- 0.50 L) underwent progressive incremental exercise (15 W/min) on a cycle ergometer to a symptom-limited maximum every week for 6 weeks (first week used as practice session). Minute ventilation (VE), oxygen consumption (VO2), and Borg ratings were obtained every minute during exercise. Borge and Borgd were highly correlated in each subject (r = 0.99 +/- 0.01). Borg scores were not significantly different across study days during both maximal and submaximal exercise. The within-subject coefficient of variation (CV) for Borge during maximal exercise was 13.9 +/- 9.0% (range, 6 to 31%) which was not significantly different from that observed for the physiological indices: 8.2 +/- 4.1% (range, 4 to 15%) for VE and 5.2 +/- 3.4% (range, 1 to 10%) for VO2. In contrast, at 66% of the maximum workload, the within-subject CV for Borge was 25.0 +/- 13.6% (range, 12 to 50%) which was significantly greater than that observed for the physiologic indices: 5.8 +/- 2.0% (range, 3 to 9%) for VE and 4.6 +/- 1.1% (range, 3 to 6%) for VO2. In every subject, Borge was linearly correlated with VE, VO2, and workload. However, within an individual subject, the slope of these relationships varied between trials; within-subject CV for the slope of the Borge/VE relationship was 20.2 +/- 8.0% (range, 12 to 32%). In conclusion, during incremental exercise Borg ratings of dyspnea are not as reproducible as physiologic indices in patients with COPD.

Determination of gas exchange threshold by nonparametric regression

The gas exchange threshold (GET) has been used an an index of anaerobic threshold because it can be measured noninvasively. GET is estimated from a breakpoint in breath by breath values of carbon dioxide uptake (Vco2) and oxygen uptake (Vo2) obtained during a progressive exercise test. Three methods of estimating GET were evaluated: (1) the original V slope method (OVS) using two adjoining standard linear regressions, (2) the modified V slope method (MVS) where the breakpoint is detected by visual inspection, and (3) a new method that we developed with nonparametric regression (NPM) using cubic splines. Simulated data were used because the existence of a breakpoint is known with certainty. Detection accuracy for OVS and MVS never exceeded 63% because of a low specificity. The detection accuracy of NPM ranged between 50 and 89% depending on the amount of noise and abruptness of the threshold, and exceeded that of OVS and MVS at low levels of noise. NPM was significantly more accurate (p < 0.05) than OVS and MVS for detecting GET except with high levels of noise. Both NPM and OVS have similar degrees of numerical accuracy and are superior to the currently used MVS method in this respect. All three methods gave similar results on 20 exercise tests. We conclude from the simulated data that NPM is more accurate than OVS and MVS at detecting GET. NPM can be applied to human data and it provides results that are consistent with OVS and MVS.

Twitch potentiation following voluntary diaphragmatic contraction

The purpose of this study was to determine whether twitch potentiation (a transient augmentation of twitch tension following vigorous muscular contraction) occurs in the human diaphragm in vivo. Six healthy subjects were studied. To determine whether twitch potentiation occurs, the subjects attempted to maximally activate their diaphragm by performing the combined Mueller-expulsive maneuver with visual feedback (Pdi max maneuver). Twitches were obtained before, at 10 s after the transdiaphragmatic pressure (Pdi) maneuver, and at intervals over the ensuing 10 min. We also determined whether twitch potentiation would occur following submaximal voluntary diaphragmatic contractions (33 and 66% of Pdi max). In addition, we examined whether repeated voluntary contractions would result in greater twitch potentiation compared with that observed after a single voluntary contraction. Twitch potentiation was observed in every subject. The number of maximal voluntary contractions (MVC) (one, two, or four) had no significant effect on the degree of twitch potentiation. The increase in twitch amplitude (expressed as a percentage of the control value) averaged 63 +/- 35% (SD)(pooled data from one, two, and four contraction trials). Twitch potentiation decayed in a monoexponential fashion (r = 0.99) with a time constant of 125 s (95% Cl = 100 to 160 s). Twitch potentiation was also observed after submaximal voluntary diaphragmatic contractions. Again, the number of voluntary contractions (one or four) had no significant effect on the degree of twitch potentiation. After submaximal diaphragmatic contractions of 66% of Pdi max, the degree of twitch potentiation was not significantly different, 61 +/- 36% (pooled data from one and four contraction trials) from that observed following the maximal voluntary contraction maneuvers.(ABSTRACT TRUNCATED AT 250 WORDS)

Diaphragmatic fatigue after exercise in healthy human subjects

The purpose of this study was to determine whether diaphragmatic fatigue occurs after high-intensity constant-load whole-body exercise to volitional exhaustion. Ten sedentary subjects with a maximal oxygen uptake of 2.52 +/- 0.47 L/min were studied. Subjects exercised on a bicycle ergometer at 80% of their maximal working capacity until volitional exhaustion. Minute ventilation during the last minute of exercise was 89.9 +/- 13.6 L/min, which represented 50 +/- 6% of the subjects' 12-s maximal voluntary ventilation. During the last minute of exercise, mean inspiratory esophageal pressure was 18.1 +/- 5.3 cm H2O, which represented only 15 +/- 4% of the subjects' maximal static inspiratory pressure. Bilateral transcutaneous supramaximal phrenic nerve stimulation was performed before and 10, 30, 45 and 60 min after exercise. Twitch diaphragmatic pressure (twitch Pdi) was significantly decreased after exercise in seven of the 10 subjects. For the group as a whole, twitch Pdi fell from 28.9 +/- 3.7 cm H2O during control to 23.9 +/- 5.1 cm H2O at 10 min after exercise (p < 0.005). The fall in twitch Pdi was due to a significant decrease in twitch esophageal pressure from 19.6 +/- 4.3 cm H2O during control to 15.5 +/- 4.9 cm H2O (p < 0.001). Twitch gastric pressure was not significantly different: 8.7 +/- 4.0 cm H2O, compared with 9.2 +/- 3.8 cm H2O during control. Twitch Pdi recovered to 93 +/- 7% of control values at 60 min after exercise. The fall in twitch Pdi after exercise indicates that diaphragmatic fatigue can occur following heavy endurance exercise in sedentary healthy persons.