The effect of gender on compensatory neuromuscular response to upper airway obstruction in normal subjects under midazolam general anesthesia

Critical to Know Pcrit: A Review on Pharyngeal Critical Closing Pressure in Obstructive Sleep Apnea

It is crucial to understand the underlying pathophysiology of obstructive sleep apnea (OSA). Upper airway collapsibility is an important pathophysiological factor that affects the upper airway in OSA. The aim of the current study was to review the existing body of knowledge on the pharyngeal collapsibility in OSA. After a thorough search through Medline, PubMed, Scopus, and Web of science, the relevant articles were found and used in this study. Critical closing pressure (Pcrit) is the gold standard measure for the degree of collapsibility of the pharyngeal airway. Various physiological factors and treatments affect upper airway collapsibility. Recently, it has been shown that the baseline value of Pcrit is helpful in the upfront selection of therapy options. The standard techniques to measure Pcrit are labor-intensive and time-consuming. Therefore, despite the importance of Pcrit, it is not routinely measured in clinical practice. New emerging surrogates, such as finite element (FE) modeling or the use of peak inspiratory flow measurements during a routine overnight polysomnography, may enable clinicians to have an estimate of the pharyngeal collapsibility. However, validation of these techniques is needed.

Critical closing pressure (Pcrit) of the pharyngeal airway during routine drug-induced sleep endoscopy: feasibility and protocol

In obstructive sleep apnea (OSA), there are various pathophysiological factors affecting the upper airway during sleep. Two prominent factors contributing to OSA are site and pattern of upper airway collapse and degree of pharyngeal collapsibility. In a clinical setting, drug-induced sleep endoscopy (DISE) is used to visualize the structures of the upper airway. Critical closing pressure (Pcrit) is the gold standard measure of pharyngeal collapsibility. This prospective clinical study aimed to investigate the feasibility and protocol of Pcrit measurements during DISE. Thirteen patients with OSA were included. Pcrit was calculated using peak inspiratory airflow and inspiratory ventilation. The proposed protocol was successful in Pcrit measurement during DISE in all subjects [median[Q1;Q3] Pcrit for “peak inspiratory method” (n = 12): −0.84[−2.07;0.69] cmH₂O, “ventilation method” (n = 13): −1.32[2.32;0.47] cmH₂O], highlighting the feasibility of the approach. There was no significant difference (P = 0.67) between calculated Pcrit with either of the calculation methods, indicating high reliability. Correlation analysis showed Pcrit as an independent parameter of any of the anthropometric or polysomnographic parameters. The ventilation method proved to be more successful in assessment of Pcrit in subjects with epiglottic collapse (e.g., with high negative effort dependence). Subjects with palatal complete concentric collapse during DISE had a wide Pcrit range ([−2.86;2.51]cmH₂O), suggesting no close correlation between Pcrit and this DISE pattern (P = 0.38). Incorporation of Pcrit measurements into DISE assessments is feasible and may yield valuable additional information for OSA management. Combining Pcrit and DISE provides information on both the site and degree of upper airway collapse and the degree of pharyngeal collapsibility.

NEW & NOTEWORTHY The protocol of this study was successful in concomitant measurement of Pcrit during routine clinical endoscopy. Comparison of two calculation methods for Pcrit showed that the inspiratory ventilation method was more successful in assessment of Pcrit in subjects with epiglottic collapse who have high negative effort dependence. Subjects with palatal complete concentric collapse during DISE had a wide Pcrit range and did not have a greater Pcrit than patients in other site of collapse categories.

Upper-airway collapsibility and compensatory responses under moderate sedation with ketamine, dexmedetomidine, and propofol in healthy volunteers

BACKGROUND

Ketamine is a potent sedative drug that helps to maintain upper-airway patency, due to its higher upper-airway dilator muscular activity and higher level of duty cycle, as seen in rats. However, no clinical trials have tested passive upper-airway collapsibility and changes in the inspiratory duty cycle against partial upper-airway obstruction in humans. The present study evaluated both the passive mechanical upper-airway collapsibility and compensatory response against acute partial upper-airway obstruction using three different sedative drugs in a crossover trial.

METHODS

Eight male volunteers entered this nonblinded, randomized crossover study. Upper-airway collapsibility (passive critical closing pressure) and inspiratory duty cycle were measured under moderate sedation with ketamine, propofol, and dexmedetomidine. Propofol, dexmedetomidine, and ketamine anesthesia were induced to obtain adequate, same-level sedation, with a BIS value of 50-70 and the OAA/S score of 2-3 and RASS score of -3.

RESULTS

The median passive critical closing pressure of 0.08 [-5.51 to 1.20] cm H2 O was not significantly different compared to that of propofol sedation (-0.32 [-1.41 to -0.19] cm H2 O) and of dexmedetomidine sedation (-0.28 [-0.95 to -0.03] cm H2 O) (p = .045). The median passive RUS for ketamine 54.35 [32.00 to 117.50] cm H2 O/L/s was significantly higher than that for propofol 5.50 [2.475 to 19.60] cm H2 O/L/s; (mean difference, 27.50; 95% CI 9.17 to 45.83) (p = .009) and for dexmedetomidine 19.25 [4.125 to 22.05] cm H2 O/L/s; (mean difference, 22.88; 95% CI 4.67 to 41.09) (p = .021). The inspiratory duty cycle increased significantly as the inspiratory airflow decreased in passive conditions for each sedative drug, but behavior differed among the three sedative drugs.

CONCLUSION

Our findings demonstrate that ketamine sedation may have an advantage of both maintained passive upper-airway collapsibility and a compensatory respiratory response, due to both increase in neuromuscular activity and the increased duty cycle, to acute partial upper-airway obstruction.

A review on drug-induced sedation endoscopy - Technique, grading systems and controversies

Sleep disordered breathing (SDB) comprises a spectrum of disorders, ranging from simple snoring to severe obstructive sleep apnoea (OSA), with a significant burden to health care systems in high income countries. If left untreated, OSA has significant cumulative, long-term health consequences. In the 1990s drug induced sedation endoscopy (DISE) has been developed to become a primary tool in the diagnosis and management of OSA. It allows meticulous endoscopic evaluation of the airway and identifies areas of collapse, thereby informing both on the selection of surgical techniques, where efficacy depends entirely on success at relieving obstruction at a certain level and on the usefulness of conservative measures, such as mandibular advancement splints. This article provides a review of the literature on DISE, covering different grading systems and techniques, explaining different rationales and discussing controversies.

Pharmacokinetics and pharmacodynamics of a new highly concentrated intranasal midazolam formulation for conscious sedation

AIM

To evaluate the pharmacokinetics, pharmacodynamics, nasal tolerance and effects on sedation of a highly concentrated aqueous intranasal midazolam formulation (Nazolam) and to compare these to intravenous midazolam.

METHODS

In this four-way crossover, double-blind, double-dummy, randomized, placebo-controlled study, 16 subjects received 2.5 mg Nazolam, 5.0 mg Nazolam, 2.5 mg intravenous midazolam or placebo on different occasions. Pharmacokinetics of midazolam and α-hydroxy-midazolam were characterized and related to outcome variables for sedation (saccadic peak velocity, the Bond and Lader visual analogue scale for sedation, the simple reaction time task and the observer's assessment of alertness/sedation). Nasal tolerance was evaluated through subject reporting, and ear, nose and throat examination.

RESULTS

Nazolam bioavailability was 75%. Maximal plasma concentrations of 31 ng ml^-1 (CV, 42.3%) were reached after 11 min (2.5 mg Nazolam), and of 66 ng ml^-1 (coefficient of variability, 31.5%) after 14 min (5.0 mg Nazolam). Nazolam displayed a significant effect on OAA/S scores. Sedation onset (based on SPV change) occurred 1 ± 0.7 min after administration of 2.5 mg intravenous midazolam, 7 ± 4.4 min after 2.5 mg Nazolam, and 4 ± 1.8 min after 5 mg Nazolam. Sedation duration was 118 ± 95.6 min for 2.5 mg intravenous midazolam, 76 ± 80.4 min for 2.5 mg Nazolam, and 145 ± 104.9 min for 5.0 mg Nazolam. Nazolam did not lead to nasal mucosa damage.

CONCLUSIONS

This study demonstrates the nasal tolerance, safety and efficacy of Nazolam. When considering the preparation time needed for obtaining venous access, conscious sedation can be achieved in the same time span as needed for intravenous midazolam. Nazolam may offer important advantages in conscious sedation.

A pilot study of upper airway management using a remote-controlled artificial muscle device during propofol anesthesia

STUDY OBJECTIVE

To test the hypothesis that the jaw closure using a pneumatic actuator device affect airway collapsibility and resistance during propofol anesthesia.

DESIGN

Prospective, randomized study.

SETTING

University-affiliated hospital.

PATIENTS

Six male subjects were included in the present study.

INTERVENTION

We used pressure-flow relationships to evaluate critical closing pressure (PCRIT) and upper airway resistance in different conditions of body and head position. Anesthesia was induced and maintained with a propofol infusion, targeting a constant blood concentration of 1.5 to 2.0μg/mL to establish an adequate depth of anesthesia, with patients breathing spontaneously through a nasal mask. An air-inflatable pneumatic actuator was used to achieve jaw closure. Nasal mask pressure was intermittently reduced to evaluate upper airway collapsibility (passive PCRIT) and upstream resistance under 4 different conditions: (1) neutral occlusion at 0-cm head elevation (baseline), (2) jaw closure at 0-cm head elevation, (3) neutral occlusion at 6-cm head elevation, and (4) jaw closure at 6-cm head elevation. PCRIT and upstream resistance under each condition were compared using 1-way analysis of variance. P<.05 was considered significant.

MEASUREMENTS

The pressure and inspiratory flow at the subjects' nose mask were recorded. Polysomonographic parameters (electroencephalograms, electrooculograms, submental electromyograms, and plethysmogram) were also recorded.

MAIN RESULTS

The combination of 6-cm head elevation with jaw closure using the pneumatic actuator decreased upper airway collapsibility (PCRIT≈-3.0 cm H2O) compared to the baseline position (PCRIT≈-1.2 cm H2O; P=.0003).

CONCLUSION

We demonstrated that jaw closure using an air-inflatable pneumatic actuator device can produce substantial decreases in upper airway collapsibility and maintain upper airway patency during propofol anesthesia.

A Remote-Controlled Airbag Device Can Improve Upper Airway Collapsibility by Producing Head Elevation With Jaw Closure in Normal Subjects Under Propofol Anesthesia

Continuous maintenance of an appropriate position of the mandible and head purely by manual manipulation is difficult, although the maneuver can restore airway patency during sleep and anesthesia. The aim of this paper was to examine the effect of head elevation with jaw closure using a remote-controlled airbag device, such as the airbag system, on passive upper airway collapsibility during propofol anesthesia. Seven male subjects were studied. Propofol infusion was used for anesthesia induction and maintenance, with a target blood propofol concentration of 1.5–2 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} }{}\(\mu \) \end{document}g/ml. Nasal mask pressure (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} }{}\({P}_{\text {N}}\) \end{document}) was intermittently reduced to evaluate upper airway collapsibility (passive \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} }{}\({P}_{_{\text {CRIT}}}\) \end{document}) and upstream resistance (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} }{}\({R}_{_{\text {US}}}\) \end{document}) at three different head and jaw positions, jaw opening position in the supine position, jaw opening position in the sniffing position with 6-cm head elevation, and jaw closure at a 6-cm height sniffing position. The 6-cm height sniffing position with jaw closure was achieved by an airbag device that was attached to the subject’s head-like headgear. Patient demographics, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} }{}\({P}_{_{\text {CRIT}}}\) \end{document} and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} }{}\({R}_{_{\text {US}}}\) \end{document} in each condition were compared using one-way ANOVA with a post hoc Tukey test. \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} }{}\({\rm P}<0.05\) \end{document} was considered significant. We also confirmed the effects of our airbag device on improvement of upper airway collapsibility in three obstructive sleep apnea patients in a clinical study. The combination of 6-cm head elevation with jaw closure using the air-inflatable robotic airbag system decreased upper airway collapsibility (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} }{}\({P}_{_{\text {CRIT}}}\sim -3.4\) \end{document}-cm H\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} }{}\(_{2}\) \end{document}O) compared with the baseline position (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} }{}\({P}_{_{\text {CRIT}}}\sim -0.8\) \end{document}-cm H\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} }{}\(_{2}\) \end{document}O, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} }{}\({P} = 0.0001\) \end{document}). In the clinical study, there was improvement of upper airway obstruction in sleep apnea patients, including decreased apnea and hypopnea duration and increased the lowest level of oxygen saturation. We demonstrated that establishment of head elevation with jaw closure achieved by a remote-controlled airbag device using an inflatable airbag system can produce substantial decreases in upper airway collapsibility and maintain upper airway patency during propofol anesthesia and sleep.

[Drug-induced sleep videoendoscopy: clinical usefulness and literature review]

Fiberoptic examination of the pharynx under drug-induced sleep is a test that helps to detect the areas of vibration and collapse in patients with sleep-disordered breathing. This article is a review of the available literature on the subject, aimed at helping otolaryngologists to understand the procedure and to resolve some controversies surrounding it.

Postoperative Respiratory Muscle Dysfunction

Postoperative pulmonary complications are responsible for significant increases in hospital cost as well as patient morbidity and mortality; respiratory muscle dysfunction represents a contributing factor. Upper airway dilator muscles functionally resist the upper airway collapsing forces created by the respiratory pump muscles. Standard perioperative medications (anesthetics, sedatives, opioids, and neuromuscular blocking agents), interventions (patient positioning, mechanical ventilation, and surgical trauma), and diseases (lung hyperinflation, obesity, and obstructive sleep apnea) have differential effects on the respiratory muscle subgroups. These effects on the upper airway dilators and respiratory pump muscles impair their coordination and function and can result in respiratory failure. Perioperative management strategies can help decrease the incidence of postoperative respiratory muscle dysfunction. Such strategies include minimally invasive procedures rather than open surgery, early and optimal mobilizing of respiratory muscles while on mechanical ventilation, judicious use of respiratory depressant anesthetics and neuromuscular blocking agents, and noninvasive ventilation when possible.

The effects of hormonal status on upper airway patency in normal female subjects during propofol anesthesia

STUDY OBJECTIVE

To determine the mechanical upper airway properties and compensatory neuromuscular responses to obstruction during propofol anesthesia in the follicular and luteal phases of the menstrual cycle.

DESIGN

Prospective, randomized study.

SETTING

University-affiliated hospital.

SUBJECTS

12 premenopausal female volunteers for studies of upper airway collapse throughout their menstrual cycle during the follicular phase (6 -10 days) and mid-late luteal phase (20 - 24 days).

MEASUREMENTS

The level of propofol anesthesia (1.5 - 2.0 μg/mL) required to suppress arousal responses was determined by Observer's Assessment of Alertness/Sedation scoring (level 2) and confirmed by bispectral index monitoring. Pressure-flow relationships were constructed to evaluate collapsibility (P(CRIT)) and up-stream resistance (R(US)) during acute [Passive; hypotonic electromyography (EMG)] and sustained (Active; elevated EMG) changes in nasal mask pressure. The difference between passive P(CRIT) and active P(CRIT) (ΔP(CRIT A-P)) represented the magnitude of the compensatory response to obstruction.

MAIN RESULTS

Passive P(CRIT) was significantly higher in the mid-late luteal phase (-4.7 cm H(2)O) than in the follicular phase (-6.2 cmH(2)O; P < 0.05). Active P(CRIT) significantly decreased compared with passive P(CRIT) in the follicular phase (-10.1 cm H(2)O) and in the mid-late luteal phase (-7.7 cm H(2)O) and (P < 0.05). No significant difference was noted in ΔP(CRIT) between the follicular (3.9 ± 2.9 cm H(2)O) and mid-late luteal phases (3.0 ± 2.6 cm H(2)O). No differences were seen in R(US) between the menstrual phases for either the passive (P = 0.8) or active (P = 0.75) states.

CONCLUSIONS

Menstrual phase has an effect on anatomical alterations (mechanical properties) in the hypotonic upper airway during propofol anesthesia.

Effect of head elevation on passive upper airway collapsibility in normal subjects during propofol anesthesia

BACKGROUND

Head elevation can restore airway patency during anesthesia, although its effect may be offset by concomitant bite opening or accidental neck flexion. The aim of this study is to examine the effect of head elevation on the passive upper airway collapsibility during propofol anesthesia.

METHOD

Twenty male subjects were studied, randomized to one of two experimental groups: fixed-jaw or free-jaw. Propofol infusion was used for induction and to maintain blood at a constant target concentration between 1.5 and 2.0 μg/ml. Nasal mask pressure (PN) was intermittently reduced to evaluate the upper airway collapsibility (passive PCRIT) and upstream resistance (RUS) at each level of head elevation (0, 3, 6, and 9 cm). The authors measured the Frankfort plane (head flexion) and the mandible plane (jaw opening) angles at each level of head elevation. Analysis of variance was used to determine the effect of head elevation on PCRIT, head flexion, and jaw opening within each group.

RESULTS

In both groups the Frankfort plane and mandible plane angles increased with head elevation (P < 0.05), although the mandible plane angle was smaller in the free-jaw group (i.e., increased jaw opening). In the fixed-jaw group, head elevation decreased upper airway collapsibility (PCRIT ~ -7 cm H₂O at greater than 6 cm elevation) compared with the baseline position (PCRIT ~ -3 cm H₂O at 0 cm elevation; P < 0.05).

CONCLUSION

: Elevating the head position by 6 cm while ensuring mouth closure (centric occlusion) produces substantial decreases in upper airway collapsibility and maintains upper airway patency during anesthesia.

Critical closing pressure during midazolam-induced sleep

The critical closing pressure (Pcrit) is the airway pressure at which the airway collapses and reflects the anatomical contribution to the genesis of obstructive sleep apnea. Pcrit is usually determined during non-rapid eye movement sleep at night, but has been determined under midazolam sedation during the day in the absence of sleep stage monitoring. Indeed, little is known about the effects of midazolam on sleep architecture. Moreover, deeper sedation with midazolam can decrease upper airway muscle activity and increase collapsibility compared with natural sleep. Pcrit under sedation has not been systematically compared with the usual method performed during natural sleep. Therefore, this study aimed to test the hypothesis that Pcrit following low doses of midazolam during the day would be comparable to Pcrit measured during natural sleep in the same patient. Fifteen men (age 54 ± 10 yr, body mass index 30 ± 4 kg/m(2)) with obstructive sleep apnea underwent a baseline standard overnight polysomnogram (apnea-hypopnea index 38 ± 22 events/h, range: 8-66 events/h), and Pcrit was determined during natural sleep and following midazolam. Sleep induction was obtained with low doses of midazolam (2.4 mg, range 2.0-4.4 mg), and sleep architecture was comparable to natural sleep. Natural sleep and induced sleep Pcrit were similar (-0.82 ± -3.44 and -0.97 ± 3.21 cmH(2)O, P = 0.663) and closely associated (intraclass correlation coefficient = 0.92; 95% confidence interval, 0.78-0.97, P < 0.001). Natural and midazolam-induced Pcrit correlated with obstructive sleep apnea severity, indicating that both Pcrit measures provided meaningful physiological information. Pcrit determined during the day with sleep induction is similar to natural overnight sleep and is a valid alternative approach in which to determine Pcrit.

Effect of end-expiratory lung volume on upper airway collapsibility in sleeping men and women

The relationship between changes in absolute end-expiratory lung volume (EELV) and collapsibility has not been rigorously quantified. We hypothesized that pharyngeal collapsibility varies inversely with absolute lung volume in sleeping humans during 1) conventional and 2) isovolume measurements of passive critical pressure (Pcrit). Eighteen healthy subjects (11 male, 7 female) slept in a negative pressure ventilator for measurements of pharyngeal collapsibility (Pcrit) during non-rapid eye movement sleep. EELV was 1) allowed to vary with changes in nasal pressure for conventional Pcrit measurements and 2) controlled by maintaining a fixed pressure difference across the respiratory system (P(RS)) from the nose to the body surface for isovolume Pcrit measurements at elevated EELV (P(RS) = +10 cmH(2)O), reduced EELV (P(RS) = -5 cmH(2)O), and functional residual capacity (FRC; P(RS) = 0 cmH(2)O). In each condition, the absolute EELV was determined and the corresponding Pcrit was derived from upper airway pressure-flow relationships. In the entire group, Pcrit varied inversely with EELV (P < 0.001). Pcrit decreased as EELV increased from the conventional to the FRC isovolume condition by -3.5 ± 1.0 cmH(2)O/l (P < 0.003). Subjects with a conventional Pcrit below -2 cmH(2)O exhibited greater reductions in EELV and correspondingly greater decreases in the FRC isovolume compared with the conventional Pcrit (P < 0.001). The overall response, ΔPcrit/ΔEELV, was -2.0 ± 0.2 cmH(2)O/l (P < 0.001) and did not differ between men and women (P = 0.16). Nevertheless, men and women differed significantly in FRC (2.63 ± 0.16 vs. 1.88 ± 0.13 liters, P <0.05) and FRC isovolume Pcrit (-2.3 ± 0.8 vs. -7.2 ± 1.2 cmH(2)O, P < 0.05), implying that the men had larger lungs and more collapsible airways than the women. The ΔPcrit/ΔEELV response was independent of sex, conventional Pcrit, body mass index, and neck, waist, and hip circumferences. We conclude that Pcrit varies inversely with absolute EELV, which may lead to 1) an underestimation of the magnitude of quantitative differences in Pcrit across the spectrum from health (negative Pcrit) to disease (positive Pcrit) and 2) increases in sleep apnea susceptibility in obesity.