Venous blood gases in the assessment of respiratory failure in patients undergoing sleep studies: a randomized study.

STUDY OBJECTIVES: Venous blood gases (VBGs) are not consistently considered suitable surrogates for arterial blood gases (ABGs) in assessing acute respiratory failure due to variable measurement error. The physiological stability of patients with chronic ventilatory failure may lead to improved agreement in this setting. METHODS: Adults requiring ABGs for sleep or ventilation titration studies had VBGs drawn before or after each ABG, in a randomized order. Veno-arterial correlation and agreement were examined for carbon dioxide tension (PCO2), pH, oxygen tension (PO2) and oxygen saturation (SO2). RESULTS: We analyzed 115 VBG-ABG pairs from 61 patients. Arterial and venous measures were correlated (with p<0.05) for PCO2 (r=0.84) and pH (r=0.72), but not for PO2 or SO2. Adjusted mean veno-arterial differences (95% limits of agreement) were +5.0mmHg (-4.4 to +14.4) for PCO2; -0.02 (-0.09 to +0.04) for pH; -34.3mmHg (-78.5 to +10.0) for PO2; and -23.9% (-61.3 to +13.5) for SO2. VBGs obtained from the dorsal hand demonstrated a lower mean PCO2 veno-arterial difference (p<0.01). A venous PCO2 threshold of ≥45.8mmHg was >95% sensitive for arterial hypercapnia, so measurements below this can exclude the diagnosis without an ABG. A venous PCO2 threshold of ≥53.7mmHg was >95% specific for arterial hypercapnia, so such readings can be assumed diagnostic. The area under the receiver operating characteristic curve of 0.91 indicated high discriminatory capacity. CONCLUSIONS: A venous PCO2 <45.8mmHg or ≥53.7mmHg would exclude or diagnose hypercapnia, respectively, in patients referred for sleep studies, but VBGs are poor surrogates for ABGs where precision is important. CLINICAL TRIAL REGISTRATION: Registry: Australian New Zealand Clinical Trials Register; Name: A comparison of arterial and blood gas analyses in sleep studies; Identifier: ACTRN12617000562370; URL https://www.anzctr.org.au/Trial/Registration/TrialReview.aspx?id=372717.

A randomized controlled trial of a multi-dimensional intervention to improve CPAP use and self-efficacy.

OBJECTIVES: To assess the utility of a tailored intervention program to improve continuous positive airway pressure (CPAP) use and self-efficacy in individuals with obstructive sleep apnea (OSA). METHODS: 81 participants (mean age 52.1 ± 11.6 years; 35 females) with OSA were randomized to either a multi-dimensional intervention (PSY CPAP, n = 38) or treatment as usual (TAU CPAP, n = 43). The intervention included a psychoeducation session prior to CPAP initiation, a booster psychoeducation session in the first weeks of commencing CPAP, follow-up phone calls on days 1 and 7, and a review appointment on day 14. CPAP use was compared between the PSY CPAP and TAU CPAP groups at 1 week, 1 month, and 4 months. Self-efficacy scores (risk perception, outcome expectancies, and CPAP self-efficacy) were compared between groups following the initial psychoeducation session and again at 1 month and 4 months. RESULTS: CPAP use was higher in the PSY CPAP group compared to the TAU CPAP group for all time points (p = .02). Outcome expectancies improved significantly over time in PSY CPAP participants (p = .007). Change in risk perception was associated with CPAP use at 1 week (p = .02) for PSY CPAP participants. However, risk perception did not mediate the effect between group and CPAP use at 1 week. CONCLUSIONS: Interventions designed to increase self-efficacy and administered prior to CPAP initiation, repeated in the early stages of CPAP therapy, and combined with a comprehensive follow-up regime are likely to improve CPAP use. Sustained improvement in CPAP use is the ultimate goal but remains to be investigated.

A Randomized Controlled Clinical Trial of Lung Volume Recruitment in Adults with Neuromuscular Disease.

Rationale: Clinical care guidelines advise that lung volume recruitment (LVR) be performed routinely by people with neuromuscular disease (NMD) to maintain lung and chest wall flexibility and slow lung function decline. However, the evidence base is limited, and no randomized controlled trials of regular LVR in adults have been published. Objectives: To evaluate the effect of regular LVR on respiratory function and quality of life in adults with NMD. Methods: A randomized controlled trial with assessor blinding was conducted between September 2015 and May 2019. People (>14 years old) with NMD and vital capacity <80% predicted were eligible, stratified by disease subgroup (amyotrophic lateral sclerosis/motor neuron disease or other NMDs), and randomized to 3 months of twice-daily LVR or breathing exercises. The primary outcome was change in maximum insufflation capacity (MIC) from baseline to 3 months, analyzed using a linear mixed model approach. Results: Seventy-six participants (47% woman; median age, 57 [31-68] years; mean baseline vital capacity, 40 ± 18% predicted) were randomized (LVR, n = 37). Seventy-three participants completed the study. There was a statistically significant difference in MIC between groups (linear model interaction effect P = 0.002, observed mean difference, 0.19 [0.00-0.39] L). MIC increased by 0.13 (0.01-0.25) L in the LVR group, predominantly within the first month. No interaction or treatment effects were observed in secondary outcomes of lung volumes, respiratory system compliance, and quality of life. No adverse events were reported. Conclusions: Regular LVR increased MIC in a sample of LVR-naive participants with NMD. We found no direct evidence that regular LVR modifies respiratory mechanics or slows the rate of lung volume decline. The implications of increasing MIC are unclear, and the change in MIC may represent practice. Prospective long-term clinical cohorts with comprehensive follow-up, objective LVR use, and clinically meaningful outcome data are needed. Clinical trial registered with anzctr.org.au (ACTRN12615000565549).

External proficiency testing improves inter-scorer reliability of polysomnography scoring.

PURPOSE: This study evaluated whether or not polysomnography (PSG) inter-scorer reliability (ISR) across sleep centres could be improved by external proficiency testing (EPT), or by EPT combined with method alignment training. METHODS: Experienced scorers form 15 sleep centres were randomised to the following: (1) a control group, (2) a group that received a self-directed intervention of EPT reports (EPTPassive) or (3) a group that received an active intervention of method alignment training and EPT reports (EPTActive). Respiratory, arousal and sleep scoring ISR from sixteen PSG fragments were compared between groups across time. RESULTS: Among 30 scorers, there were no ISR changes in controls between baseline (BL) and 6 months (6 m). Both EPT groups showed ISR improvement from BL to 6 m for respiratory, arousal and sleep scoring (p < 0.05). Respiratory scoring back-transformed mean (95CI) proportion of specific agreement (PSA) for the EPTPassive group improved from 0.78 (0.72-0.84) to 0.80 (0.74-0.86) and for the EPTActive group from 0.80 (0.74-0.85) to 0.82 (0.76-0.88). Arousal scoring PSA for the EPTPassive group improved from 0.72 (0.66-0.77) to 0.74 (0.69-0.79) and for the EPTActive group from 0.71 (0.65-0.76) to 0.77 (0.72-0.82). Sleep scoring kappa for the EPTPassive group improved from 0.64 (0.58-0.69) to 0.73 (0.68-0.77) and for the EPTActive group from = 0.75 (0.71-0.80) to 0.80 (0.76-0.85). Overall, poorer performers achieved greater improvement. CONCLUSION: External proficiency testing produced modest, statistically significant PSG inter-scorer reliability improvements among experienced scorers across sleep centres, with potential to improve clinical management of individual patients and increase research study statistical power.

Fetal heart rate events during sleep, and the impact of sleep disordered breathing, in pregnancies complicated by preterm fetal growth restriction: An exploratory observational case-control study.

OBJECTIVES: To evaluate fetal heart rate (FHR) patterns during sleep in pregnancies complicated by preterm fetal growth restriction (FGR). To determine whether co-existing sleep-disordered breathing (SDB) impacts on acute FHR events or perinatal outcome. DESIGN: Observational case control study. SETTING AND POPULATION: Women with preterm FGR and gestation-matched well grown controls (estimated fetal weight above the 10th percentile with normal Doppler studies); tertiary maternity hospital, Australia. METHODS: A polysomnogram, a test used to measure sleep patterns and diagnose sleep disorders, and concurrent cardiotocography (CTG), were analysed for respiratory events and FHR changes. MAIN OUTCOME MEASURES: Frequency of FHR events overnight in FGR cases versus controls and in those with or without SDB. RESULTS: Twenty-nine patients with preterm FGR and 29 controls (median estimated fetal weight 1st versus 60th percentile, P < 0.001) underwent polysomnography with concurrent CTG at a mean gestation of 30.2 weeks. The median number of FHR events per night was higher among FGR cases than among controls (3.0 events, interquartile range [IQR] 1.0-4.0, versus 1.0 [IQR 0-1.0]; P < 0.001). Women with pregnancies complicated by preterm FGR were more likely than controls to be nulliparous, receive antihypertensive medications, be supine at sleep onset, and to sleep supine (32.9% of total sleep time versus 18.3%, P = 0.03). SDB was common in both FGR and control pregnancies (48% versus 38%, respectively, P = 0.55) but was generally mild and not associated with an increase in overnight FHR events or adverse perinatal outcome. CONCLUSIONS: Acute FHR events overnight are more common in pregnancies complicated by preterm FGR than in pregnancies with normal fetal growth. Mild SDB was common in late pregnancy and well tolerated, even by fetuses with preterm FGR. TWEETABLE ABSTRACT: Mild sleep-disordered breathing seems well tolerated even by highly vulnerable fetuses.

Rapidly and slowly progressive neuromuscular disease: differences in pulmonary function, respiratory tract infections and response to lung volume recruitment therapy (LVR).

INTRODUCTION: Reduced lung volumes are a hallmark of respiratory muscle weakness in neuromuscular disease (NMD). Low respiratory system compliance (Crs) may contribute to restriction and be amenable to lung volume recruitment (LVR) therapy. This study evaluated respiratory function and the immediate impact of LVR in rapidly progressive compared to slowly progressive NMD. METHODS: We compared vital capacity (VC), static lung volumes, maximal inspiratory and expiratory pressures (MIP, MEP), Crs and peak cough flow (PCF) in 80 adult participants with motor neuron disease ('MND'=27) and more slowly progressive NMDs ('other NMD'=53), pre and post a single session of LVR. Relationships between respiratory markers and a history of respiratory tract infections (RTI) were examined. RESULTS: Participants with other NMD had lower lung volumes and Crs but similar reduction in respiratory muscle strength compared with participants with MND (VC=1.30±0.77 vs 2.12±0.75 L, p<0.001; Crs=0.0331±0.0245 vs 0.0473±0.0241 L/cmH2O, p=0.024; MIP=39.8±21.3 vs 37.8±19.5 cmH2O). More participants with other NMD reported an RTI in the previous year (53% vs 22%, p=0.01). The likelihood of having a prior RTI was associated with baseline VC (%predicted) (OR=1.03 (95% CI 1.00 to 1.06), p=0.029). Published thresholds (VC<1.1 L or PCF<270 L/min) were, however, not associated with prior RTI.A single session of LVR improved Crs (mean (95% CI) increase = 0.0038 (0.0001 to 0.0075) L/cmH2O, p=0.047) but not VC. CONCLUSION: These findings corroborate the hypothesis that ventilatory restriction in NMD is related to weakness initially with respiratory system stiffness potentiating lung volume loss in slowly progressive disease. A single session of LVR can improve Crs. A randomised controlled trial of regular LVR is needed to assess longer-term effects.

Genioglossus muscle responses to resistive loads in severe OSA patients and healthy control subjects.

This study aimed to determine whether there is impairment of genioglossus neuromuscular responses to small negative pressure respiratory stimuli, close to the conscious detection threshold, in obstructive sleep apnea (OSA). We compared genioglossus electromyogram (EMGgg) responses to midinspiratory resistive loads of varying intensity (≈1.2-6.2 cmH2O·L-1·s), delivered via a nasal mask, between 16 severe OSA and 17 control participants while the subjects were awake and in a seated upright position. We examined the relationship between stimulus intensity and peak EMGgg amplitude in a 200-ms poststimulus window and hypothesized that OSA patients would have an increased activation threshold and reduced sensitivity in the relationship between EMGgg activation and stimulus intensity. There was no significant difference between control and OSA participants in the threshold (P = 0.545) or the sensitivity (P = 0.482) of the EMGgg amplitude vs. stimulus intensity relationship, where change in epiglottic pressure relative to background epiglottic pressure represented stimulus intensity. These results do not support the hypothesis that deficits in neuromuscular response to negative upper airway pressure exist in OSA during wakefulness; however, the results are likely influenced by a counterintuitive and novel genioglossus muscle suppression response observed in a significant proportion of both OSA and healthy control participants. This suppression response may relate to the inhibition seen in inspiratory muscles such as the diaphragm in response to sudden-onset negative pressure, and its presence provides new insight into the upper airway neuromuscular response to the collapsing force of negative pressure.NEW & NOTEWORTHY Our study used a novel midinspiratory resistive load stimulus to study upper airway neuromuscular responses to negative pressure during wakefulness in obstructive sleep apnea (OSA). Although no differences were found between OSA and healthy groups, the study uncovered a novel and unexpected suppression of neuromuscular activity in a large proportion of both OSA and healthy participants. The unusual response provides new insight into the upper airway neuromuscular response to the collapsing force of negative pressure.

The effect of sex and body weight on lung volumes during sleep.

STUDY OBJECTIVES: Low lung volumes are thought to contribute to obstructive sleep apnea (OSA). OSA is worse in the supine versus lateral body position, men versus women, obese versus normal-weight (NW) individuals and REM versus NREM sleep. All of these conditions may be associated with low lung volumes. The aim was to measure FRC during wake, NREM, and REM in NW and overweight (OW) men and women while in the supine and lateral body positions. METHODS: Eighty-one healthy adults were instrumented for polysomnography, but with nasal pressure replaced with a sealed, non-vented mask connected to an N2 washout system. During wakefulness and sleep, repeated measurements of FRC were made in both supine and right lateral positions. RESULTS: Two hundred eighty-five FRC measures were obtained during sleep in 29 NW (body mass index [BMI] = 22 ± 0.3 kg/m2) and 29 OW (BMI = 29 ± 0.7 kg/m2) individuals. During wakefulness, FRC differed between BMI groups and positions (supine: OW = 58 ± 3 and NW = 68 ± 3% predicted; lateral OW = 71 ± 3, NW = 81 ± 3% predicted). FRC fell from wake to NREM sleep in all participants and in both positions by a similar amount. As a result, during NREM sleep FRC was lower in OW than NW individuals (supine 46 ± 3 and 56 ± 3% predicted, respectively). FRC during REM was similar to NREM and no sex differences were observed in any position or sleep stage. CONCLUSIONS: Reductions in FRC while supine and with increased body weight may contribute to worsened OSA in these conditions, but low lung volumes appear unlikely to explain the worsening of OSA in REM and in men versus women.

Apnoea and hypopnoea scoring for people with spinal cord injury: new thresholds for sleep disordered breathing diagnosis and severity classification.

STUDY DESIGN: Descriptive study. OBJECTIVES: To determine the effect of respiratory event rule-set changes on the apnoea hypopnoea index, and diagnostic and severity thresholds in people with acute and chronic spinal cord injury. SETTING: Eleven acute spinal cord injury inpatient hospitals across Australia, New Zealand, Canada and England; community dwelling chronic spinal cord injury patients in their own homes. METHODS: Polysomnography of people with acute (n = 24) and chronic (n = 78) tetraplegia were reanalysed from 1999 American Academy of Sleep Medicine (AASM) respiratory scoring, to 2007 AASM 'alternative' and 2012 AASM respectively. Equivalent cut points for published 1999 AASM sleep disordered breathing severity ranges were calculated using receiver operator curves, and results presented alongside analyses from the able-bodied. RESULTS: In people with tetraplegia, shift from 1999 AASM to 2007 AASM 'alternative' resulted in a 22% lower apnoea hypopnoea index, and to 2012 AASM a 17% lower index. In people with tetraplegia, equivalent cut-points for 1999 AASM severities of 5,15 and 30 were calculated at 2.4, 8.1 and 16.3 for 2007 AASM 'alternative' and 3.2, 10.0 and 21.2 for 2012 AASM. CONCLUSION: Interpreting research, prevalence and clinical polysomnography results conducted over different periods requires knowledge of the relationship between different rule-sets, and appropriate thresholds for diagnosis of disease. SPONSORSHIP: This project was proudly supported by the Traffic Accident Commission (Program grant) and the National Health and Medical Research Council (PhD stipend 616605).

Evidence against a subcortical gate preventing conscious detection of respiratory load stimuli.

Respiratory related evoked potentials (RREP) were used to examine respiratory stimulus gating. RREPs produced by consciously detected vs. undetected loads, near the detection threshold, were compared. Participants (n = 17) were instrumented with EEG and a nasal mask connected to a loading manifold, which presented a range of mid-inspiratory resistive loads, plus a control, in a random block design. Participants were cued prior to the stimulus and signalled detection by a button press. There were statistically significant differences in peak-to-peak amplitude of the P1 RREP peak for detected (mean ± SD; 3.86 ± 1.45 µV; P = 0.020) and undetected loads (3.67 ± 1.27 µV; P = 0.002) vs. control (2.36 ± 0.81 µV), although baseline-to-peak differences were not significantly different. In contrast peak-to-peak P3 amplitude was significantly greater for detected (5.91 ± 1.54 µV; P < 0.001) but not undetected loads (3.33 ± 0.98 µV; P = 0.189) vs. control (3.69 ± 1.46 µV), with the same pattern observed for baseline-to-peak measurements. The P1 peak, thought to reflect arrival of somatosensory information, appeared to be present in response to both detected and undetected loads, but the later P3 peak, was present for detected loads only. This suggests that for sub-threshold loads sensory information may reach the cortex, arguing against a sub-cortical gating process.

Diagnostic accuracy of a two-stage model for detecting obstructive sleep apnoea in chronic tetraplegia.

BACKGROUND: Obstructive sleep apnoea (OSA) is highly prevalent in people with spinal cord injury (SCI). Polysomnography (PSG) is the gold-standard diagnostic test for OSA, however PSG is expensive and frequently inaccessible, especially in SCI. A two-stage model, incorporating a questionnaire followed by oximetry, has been found to accurately detect moderate to severe OSA (MS-OSA) in a non-disabled primary care population. This study investigated the accuracy of the two-stage model in chronic tetraplegia using both the original model and a modified version for tetraplegia. METHODS: An existing data set of 78 people with tetraplegia was used to modify the original two-stage model. Multivariable analysis identified significant risk factors for inclusion in a new tetraplegia-specific questionnaire. Receiver operating characteristic (ROC) curve analyses of the questionnaires and oximetry established thresholds for diagnosing MS-OSA. The accuracy of both models in diagnosing MS-OSA was prospectively evaluated in 100 participants with chronic tetraplegia across four international SCI units. RESULTS: Injury completeness, sleepiness, self-reported snoring and apnoeas were included in the modified questionnaire, which was highly predictive of MS-OSA (ROC area under the curve 0.87 (95% CI 0.79 to 0.95)). The 3% oxygen desaturation index was also highly predictive (0.93 (0.87-0.98)). The two-stage model with modified questionnaire had a sensitivity and specificity of 83% (66-93) and 88% (75-94) in the development group, and 77% (65-87) and 81% (68-90) in the validation group. Similar results were demonstrated with the original model. CONCLUSION: Implementation of this simple alternative to full PSG could substantially increase the detection of OSA in patients with tetraplegia and improve access to treatments. TRIAL REGISTRATION NUMBER: Results, ACTRN12615000896572 (The Australian and New Zealand Clinical Trials Registry) and pre-results, NCT02176928 (clinicaltrials.gov).

Genioglossus reflex responses to negative upper airway pressure are altered in people with tetraplegia and obstructive sleep apnoea.

KEY POINTS: Protective reflexes in the throat area (upper airway) are crucial for breathing. Impairment of these reflexes can cause breathing problems during sleep such as obstructive sleep apnoea (OSA). OSA is very common in people with spinal cord injury for unknown reasons. This study shows major changes in protective reflexes that serve to keep the upper airway open in response to suction pressures in people with tetraplegia and OSA. These results help us understand why OSA is so common in people with tetraplegia and provide new insight into how protective upper airway reflexes work more broadly. ABSTRACT: More than 60% of people with tetraplegia have obstructive sleep apnoea (OSA). However, the specific causes are unknown. Genioglossus, the largest upper-airway dilator muscle, is important in maintaining upper-airway patency. Impaired genioglossus muscle function following spinal cord injury may contribute to OSA. This study aimed to determine if genioglossus reflex responses to negative upper-airway pressure are altered in people with OSA and tetraplegia compared to non-neurologically impaired able-bodied individuals with OSA. Genioglossus reflex responses measured via intramuscular electrodes to ∼60 brief (250 ms) pulses of negative upper-airway pressure (∼-15 cmH2 O at the mask) were compared between 13 participants (2 females) with tetraplegia plus OSA and 9 able-bodied controls (2 females) matched for age and OSA severity. The initial short-latency excitatory reflex response was absent in 6/13 people with tetraplegia and 1/9 controls. Genioglossus reflex inhibition in the absence of excitation was observed in three people with tetraplegia and none of the controls. When the excitatory response was present, it was significantly delayed in the tetraplegia group compared to able-bodied controls: excitation onset latency (mean ± SD) was 32 ± 16 vs. 18 ± 9 ms, P = 0.045; peak excitation latency was 48 ± 17 vs. 33 ± 8 ms, P = 0.038. However, when present, amplitude of the excitation response was not different between groups, 195 ± 26 vs. 219 ± 98% at baseline, P = 0.55. There are major differences in genioglossus reflex morphology and timing in response to rapid changes in airway pressure in people with tetraplegia and OSA. Altered genioglossus function may contribute to the increased risk of OSA in people with tetraplegia. The precise mechanisms mediating these differences are unknown.

Light sensors for objective light measurement in ambulatory polysomnography.

Ambulatory polysomnography (PSG) does not commonly include an objective measure of light to determine the time of lights off (Loff), and thus cannot be used to calculate important indices such as sleep onset latency and sleep efficiency. This study examined the technical specifications and appropriateness of a prototype light sensor (LS) for use in ambulatory Compumedics Somte PSG.Two studies were conducted. The first examined the light measurement characteristics of the LS when used with a portable PSG device, specifically recording trace range, linearity, sensitivity, and stability. This involved the LS being exposed to varying incandescent and fluorescent light levels in a light controlled room. Secondly, the LS was trialled in 24 home and 12 hospital ambulatory PSGs to investigate whether light levels in home and hospital settings were within the recording range of the LS, and to quantify the typical light intensity reduction at the time of Loff. A preliminary exploration of clinical utility was also conducted. Linearity between LS voltage and lux was demonstrated, and the LS trace was stable over 14 hours of recording. The observed maximum voltage output of the LS/PSG device was 250 mV, corresponding to a maximum recording range of 350 lux and 523 lux for incandescent and fluorescent light respectively. At the time of Loff, light levels were within the recording range of the LS, and on average dropped by 72 lux (9-245) in the home and 76 lux (4-348) in the hospital setting. Results suggest that clinical utility was greatest in hospital settings where patients are less mobile. The LS was a simple and effective objective marker of light level in portable PSG, which can be used to identify Loff in ambulatory PSG. This allows measurement of additional sleep indices and support with clinical decisions.

The Influence of CO2 on Genioglossus Muscle After-Discharge Following Arousal From Sleep.

Objectives: Ventilatory after-discharge (sustained elevation of ventilation following stimulus removal) occurs during sleep but not when hypocapnia is present. Genioglossus after-discharge also occurs during sleep, but CO2 effects have not been assessed. The relevance is that postarousal after-discharge may protect against upper airway collapse. This study aimed to determine whether arousal elicits genioglossus after-discharge that persists into sleep, and whether it is influenced by CO2. Methods: Twenty-four healthy individuals (6 female) slept with a nasal mask and ventilator. Sleep (EEG, EOG, EMG), ventilation (pneumotachograph), end-tidal CO2 (PETCO2), and intramuscular genioglossus EMG were monitored. NREM eucapnia was determined during 5 minutes on continuous positive airway pressure (4 cmH2O). Inspiratory pressure support was increased until PETCO2 was ≥2 mm Hg below NREM eucapnia. Supplemental CO2 was added to reproduce normocapnia, without changing ventilator settings. Arousals were induced by auditory tones and genioglossus EMG compared during steady-state hypocapnia and normocapnia. Results: Eleven participants (4 female) provided data. Prearousal PETCO2 was less (p < .05) during hypocapnia (40.74 ± 2.37) than normocapnia (43.82 ± 2.89), with differences maintained postarousal. After-discharge, defined as an increase in genioglossus activity above prearousal levels, occurred following the return to sleep. For tonic activity, after-discharge lasted four breaths irrespective of CO2 condition. For peak activity, after-discharge lasted one breath during hypocapnia and 6 breaths during normocapnia. However, when peak activity following the return to sleep was compared between CO2 conditions no individual breath differences were observed. Conclusions: Postarousal genioglossal after-discharge may protect against upper airway collapse during sleep. Steady-state CO2 levels minimally influence postarousal genioglossus after-discharge.

Arousal-Induced Hypocapnia Does Not Reduce Genioglossus Activity in Obstructive Sleep Apnea.

Study Objectives: To determine whether arousals that terminate obstructive events in obstructive sleep apnea (OSA) (1) induce hypocapnia and (2) subsequently reduce genioglossus muscle activity following the return to sleep. Methods: Thirty-one untreated patients with OSA slept instrumented with sleep staging electrodes, nasal mask and pneumotachograph, end-tidal CO2 monitoring, and intramuscular genioglossus electrodes. End-tidal CO2 was monitored, and respiratory arousals were assigned an end-arousal CO2 change value (PETCO2 on the last arousal breath minus each individual's wakefulness PETCO2). This change value, in conjunction with the normal sleep related increase in PETCO2, was used to determine whether arousals induced hypocapnia and whether the end-arousal CO2 change was associated with genioglossus muscle activity on the breaths following the return to sleep. Results: Twenty-four participants provided 1137 usable arousals. Mean ± SD end-arousal CO2 change was -0.2 ± 2.4 mm Hg (below wakefulness) indicating hypocapnia typically developed during arousal. Following the return to sleep, genioglossus muscle activity did not fall below prearousal levels and was elevated for the first two breaths. End-arousal CO2 change and genioglossus muscle activity were negatively associated such that a 1 mm Hg decrease in end-arousal CO2 was associated with an ~2% increase in peak and tonic genioglossus muscle activity on the breaths following the return to sleep. Conclusions: Arousal-induced hypocapnia did not result in reduced dilator muscle activity following return to sleep, and thus hypocapnia may not contribute to further obstructions via this mechanism. Elevated dilator muscle activity postarousal is likely driven by non-CO2-related stimuli.

Sensory detection of threshold intensity resistive loads in severe obstructive sleep apnoea.

Respiratory related evoked potentials (RREPs) were used to investigate whether sensory detection of small mid-inspiratory resistive loads (≈1.2-6.2 cmH2OL-1s), delivered during wakefulness, was impaired in obstructive sleep apnoea (OSA). It was reasoned that impaired detection of minor airway patency challenge may lead to difficult-to-remedy further collapse. There was a significant reduction in OSA (n=16) vs. control (n=17) participants in the slope of the relationship between the P1 RREP component amplitude, which reflects arrival of somatosensory information at the cortex, and stimulus intensity, expressed as change in epiglottic pressure (mean [95% confidence intervals]: -0.50 [-0.97, -0.03] vs. -1.78 [-2.54, -1.02]; P=0.004), suggesting a reduction in sensitivity to small respiratory loads. However there was no significant difference in sensitivity after background Pepi was taken into account (P=0.268). Additionally, there were no significant group differences in the threshold of the P1 amplitude/stimulus intensity relationship, or in the P1 latency. These results indicate a reduced sensitivity to detection of small upper airway negative pressure stimuli in OSA related to a reduction in mechanoreceptor activation (likely related to increased airway resistance in OSA vs. controls; P=0.002) rather than defective mechanosensory function.

The effect of body mass and sex on the accuracy of respiratory magnetometers for measurement of end-expiratory lung volumes.

Respiratory magnetometers are increasingly being used in sleep studies to measure changes in end-expiratory lung volume (EELV), including in obese obstructive sleep apnea patients. Despite this, the accuracy of magnetometers has not been confirmed in obese patients nor compared between sexes. Thus we compared spirometer-measured and magnetometer-estimated lung volume and tidal volume changes during voluntary end-expiratory lung volume changes of 1.5, 1, and 0.5 l above and 0.5 l below functional respiratory capacity in supine normal-weight [body mass index (BMI) < 25 kg/m] and healthy obese (BMI > 30 kg/m) men and women. Two different magnetometer calibration techniques proposed by Banzett et al. [Banzett RB, Mahan ST, Garner DM, Brughera A, Loring SH. J Appl Physiol (1985) 79: 2169-2176, 1995] and Sackner et al. [Sackner MA, Watson H, Belsito AS, Feinerman D, Suarez M, Gonzalez G, Bizousky F, Krieger B. J Appl Physiol (1985) 66: 410-420, 1989] were assessed. Across all groups and target volumes, magnetometers overestimated spirometer-measured EELV by ~65 ml (<0.001) with no difference between techniques (0.07). The Banzett method overestimated the spirometer EELV change in normal-weight women for all target volumes except +0.5 l, whereas no differences between mass or sex groups were observed for the Sackner technique. The variability of breath-to-breath measures of EELV was significantly higher for obese compared with nonobese subjects and was higher for the Sackner than Banzett technique. On the other hand, for tidal volume, both calibration techniques underestimated spirometer measurements (<0.001), with the underestimation being more marked for the Banzett than Sackner technique (0.03), in obese than normal weight (<0.001) and in men than in women (0.003). These results indicate that both body mass and sex affect the accuracy of respiratory magnetometers in measuring EELV and tidal volume.

Nasal Resistance Is Elevated in People with Tetraplegia and Is Reduced by Topical Sympathomimetic Administration.

STUDY OBJECTIVES: Obstructive sleep apnea (OSA) is common in individuals with tetraplegia and associated with adverse health outcomes. The causes of the high prevalence of OSA in this population are unknown, but it is important to understand as standard treatments are poorly tolerated in tetraplegia. Nasal congestion is common in tetraplegia, possibly because of unopposed parasympathetic activity. Further, nasal obstruction can induce OSA in healthy individuals. We therefore aimed to compare nasal resistance before and after topical administration of a sympathomimetic between 10 individuals with tetraplegia (T) and 9 able-bodied (AB) controls matched for OSA severity, gender, and age. METHODS: Nasal, pharyngeal, and total upper airway resistance were calculated before and every 2 minutes following delivery of ≈0.05 mL of 0.5% atomized phenylephrine to the nostrils and pharyngeal airway. The surface tension of the upper airway lining liquid was also assessed. RESULTS: At baseline, individuals with tetraplegia had elevated nasal resistance (T = 7.0 ± 1.9, AB = 3.0 ± 0.6 cm H2O/L/s), that rapidly fell after phenylephrine (T = 2.3 ± 0.4, p = 0.03 at 2 min) whereas the able-bodied did not change (AB = 2.5 ± 0.5 cm H2O/L/s, p = 0.06 at 2 min). Pharyngeal resistance was non-significantly higher in individuals with tetraplegia than controls at baseline (T = 2.6 ± 0.9, AB = 1.2 ± 0.4 cm H2O/L/s) and was not altered by phenylephrine in either group. The surface tension of the upper airway lining liquid did not differ between groups (T = 64.3 ± 1.0, AB = 62.7 ± 0.6 mN/m). CONCLUSIONS: These data suggest that the unopposed parasympathetic activity in tetraplegia increases nasal resistance, potentially contributing to the high occurrence of OSA in this population.

A prospective cohort study of pulmonary function during pregnancy in women with and without asthma.

BACKGROUND AND OBJECTIVE: Pregnancy alters the severity of asthma unpredictably. Uncertainty still exists about longitudinal changes in pulmonary function during pregnancy in both healthy and asthmatic women. This study aimed to compare pulmonary function changes during pregnancy in healthy and asthmatic women and to determine the relationship between pulmonary function and asthma-related quality of life during pregnancy. A secondary aim was to investigate the application of forced expiratory volume in 6 s (FEV6) for monitoring asthma during pregnancy. METHODS: Pregnant women with (n = 20) and without asthma (n = 20) had pulmonary function tests at 8-20, 21-28 and 29-40 weeks gestation. Those with asthma also completed the Asthma Control Questionnaire (ACQ) and mini Asthma Quality of Life Questionnaire (mAQLQ) at each visit. RESULTS: Pulmonary function declined in both groups at follow-up #1 (more markedly in those with asthma) but then improved at follow-up #2 (more markedly in those with asthma). In those with asthma, ACQ scores increased, while mAQLQ scores declined at follow-up #1; whilst at follow-up #2 these changes were in the opposite direction. FEV6 and forced vital capacity (FVC) were highly correlated (r = 0.88, p < 0.01) in asthmatics. CONCLUSIONS: Pulmonary function changes during second and third trimesters were more pronounced in asthmatics than in healthy women. FEV6 monitoring may assist pregnant women and their health professionals in optimizing asthma management. The changes in pulmonary function in women with asthma were not significantly associated with changes in asthma control or asthma-related quality of life.