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Biomedical subjects

L A Engel

Publications and source records attributed to L A Engel.

At least 19 recordsLinked to original sources
PubMed1995

Effects of increased +Gz on chest wall mechanics in humans.

We studied the effects of head-to-foot acceleration (+Gz) on chest wall mechanics in five normal subjects seated in a human centrifuge. Results were compared with those previously obtained in the same subjects in microgravity during parabolic flights. In all subjects, end-expiratory abdominal pressure (Pga) and volume (Vab) increased with Gz. On average, end-expiratory Pga increased from 7.4 +/- 1.7 cmH2O at + 1 Gz to 14.9 +/- 2.8 cmH2O at + 3 Gz and end-expiratory Vab increased by 0.32 +/- 0.06 liter between + 1 and + 3 Gz. On the other hand, the abdominal contribution to tidal volume (Vab/VT) and abdominal compliance decreased from 34.7 +/- 5.9% and 52 +/- 6 ml/cmH2O at + 1 Gz to 29.3 +/- 5.1% and 26 +/- 4 ml/cmH2O at + 3 Gz, respectively. Changes in end-expiratory Pga were linear between 0 and + 3 Gz, but changes in end-expiratory Vab, Vab/VT, and abdominal compliance were greater in microgravity than in hypergravity. In contrast to weightlessness, which did not alter minute ventilation and tidal changes in Pga and transdiaphragmatic pressure, these variables increased with increasing Gz. These results indicate that, although changes in Gz have a linear effect on abdominal transmural pressure, hypergravity and weightlessness do not have symmetrical effects on chest wall mechanics.

Abdomen↗
PubMed1995

Pulmonary vascular dilatation and diffusion-dependent impairment of gas exchange in liver cirrhosis.

To test the hypothesis that diffusion-limitation for oxygen is due to abnormal vascular dilatation and significantly contributes to the arterial hypoxaemia of liver cirrhosis requires an experimental approach that detects both diffusion-limitation for oxygen and the presence of abnormal dilatation of pulmonary vessels exposed to alveolar gas. We therefore studied the gas exchange of a 64 year old man with alcoholic liver cirrhosis and severe resting arterial hypoxaemia (arterial oxygen tension (Pa,O2) 7.5 kPa) whilst breathing air and 100% O2 using conventional blood gas (CBG) analysis, the multiple inert gas elimination technique (MIGET) and whole body scintigraphy (WBS) following the i.v. administration of radiolabelled boli of macroaggregates with a minimum diameter of 15 microM. During air breathing, there was a consistently positive difference between the arterial oxygen tension predicted by MIGET and that actually measured (P-M Pa,O2, average 0.9 kPa). During O2 breathing, P-M Pa,O2 became negative, (average -12.2 kPa), and shunt estimated by the O2 method (% of Q') was consistently less than that measured by MIGET. Whereas both O2 method and MIGET estimates of shunt never exceeded 25%, the WBS shunt was 40%, indicating that a substantial fraction of cardiac output flowed through abnormally dilated pulmonary vessels, some of which were exposed to alveolar gas and, hence, participated in gas exchange. Although our observations pertain to one subject, we believe they provide the most convincing in vivo evidence to date that abnormal dilatation of interalveolar vessels may, per se, result in a significant diffusion impairment for O2. Furthermore, in view of the consistently negative P-M Pa,O2 observed during oxygen breathing, we speculate that such abnormal vascular dilatation may also have produced a significant diffusive impairment of one or more of the less soluble inert gases used in the MIGET analysis.

Dilatation, Pathologic↗
PubMed1994

Respiratory-related activity of soft palate muscles: augmentation by negative upper airway pressure.

We studied respiratory-related activity of the soft palate muscles in 10 anesthetized tracheostomized supine dogs. Moving time average (MTA) electromyographic (EMG) activity was measured in the palatinus (PAL), levator veli palatini (LP), and tensor veli palatini (TP) with bipolar fine-wire electrodes and in the diaphragm with bipolar hook electrodes. Measurements were made during tracheostomy breathing and nasal breathing with the mouth sealed (NB). During tracheostomy breathing, all soft palate muscles displayed respiratory-related phasic inspiratory and expiratory as well as tonic EMG activity. During NB, peak inspiratory EMG activity increased in PAL, LP, and TP because of an increase in both phasic inspiratory and tonic MTA activity. In contrast, phasic expiratory activity did not change. A constant negative pressure equal to peak inspiratory tracheal pressure during NB was applied to the caudal end of the isolated upper airway with the nose occluded. This was associated with soft palate muscle responses qualitatively similar to the responses during NB but accounted for only 39, 25, and 32% of the magnitude of the peak inspiratory MTA EMG responses to NB in PAL, LP, and TP, respectively. Our results demonstrate that the soft palate muscles exhibit respiratory-related activity in common with other upper airway muscles. Furthermore, such activity is augmented in each soft palate muscle during NB, and negative upper airway pressure makes a substantial contribution to the recruitment of soft palate muscle activity.

Anesthesia↗
PubMed1993

Cardiorespiratory consequences of expiratory chest wall compression during mechanical ventilation and severe hyperinflation.

OBJECTIVES: To measure and compare the effects of manual expiratory compression of either the rib cage or abdomen on cardiac output, end-expiratory lung volume, and other cardiorespiratory variables in an animal model that mimics the severe pulmonary hyperinflation and hemodynamic impairment occurring in patients with severe acute asthma during mechanical ventilation. DESIGN: Prospective, randomized, crossover trial. SETTING: Research laboratory. SUBJECTS: Seven cross-bred, anesthetized, supine dogs. INTERVENTIONS: The following sequence was employed: a) spontaneous breathing without pulmonary hyperinflation; b) positive-pressure ventilation with severe pulmonary hyperinflation (produced by an external variable expiratory flow resistor); c) approximately 7 mins of manual expiratory compression of either the rib cage or abdomen during positive-pressure ventilation-hyperinflation. This sequence was then repeated, incorporating the alternative type of expiratory compression. MEASUREMENTS AND MAIN RESULTS: Cardiac output (measured by thermodilution), aortic pressure, pleural (esophageal) pressure, and changes in end-expiratory lung volume were measured. The decrease in cardiac output due to mechanical ventilation with pulmonary hyperinflation was exacerbated by rib cage compression (p < .001; spontaneous breathing 2.9 +/- 0.2 L/min, hyperinflation 1.5 +/- 0.1 L/min, and rib cage compression 1.0 +/- 0.1 [SEM] L/min). However, the positive-pressure ventilation-hyperinflation-induced decrease in cardiac output was attenuated by abdominal compression (p < .001; spontaneous breathing 3.3 +/- 0.2 L/min, hyperinflation 1.4 +/- 0.1 L/min, and abdominal compression 2.1 +/- 0.1 L/min). Mean aortic pressure returned to prehyperinflation levels during abdominal compression (p < .001; spontaneous breathing 126 +/- 2 mm Hg, hyperinflation 75 +/- 5 mm Hg, and abdominal compression 120 +/- 3 mm Hg). Both types of compression were similarly effective (p > .75) in increasing mean expiratory pleural pressure, so that end-expiratory lung volume was similarly (p > .25) reduced (0.45 +/- 0.05 and 0.40 +/- 0.05 L for rib cage and abdominal compressions, respectively) in this non-air flow, limiting animal model. CONCLUSIONS: The cardiorespiratory effects of manually compressing the rib cage or abdomen during expiration in this animal study suggest that these techniques should be carefully evaluated in mechanically ventilated patients with severe acute asthma.

Abdomen↗
PubMed1993

Lung volume and effectiveness of inspiratory muscles.

We related inspiratory muscle activity to inspiratory pressure generation (Pmus) at different lung volumes in five seated normal subjects. Integrated electromyograms were recorded from diaphragmatic crura (Edi), parasternals (PS), and lateral external intercostals (EI). At 20% increments in the vital capacity (VC) subjects relaxed and then made graded and maximal inspiratory efforts against an occluded airway. At any given level of pressure generation, Edi, PS, and EI increased with increasing lung volume. The Pmus generated at total lung capacity as a fraction of that at a low lung volume (between residual volume and 40% VC) was 0.39 +/- 0.15 (SD) for the diaphragm, 0.20 +/- 0.06 for PS, and 0.22 +/- 0.04 for the lateral EI muscles. Our results indicate a lesser volume dependence of the Pmus-EMG relationship for the diaphragm than for PS and EI muscles. This difference in muscle effectiveness with lung volume may reflect differences in length-tension and/or geometric mechanical advantage between the rib cage muscles and the diaphragm.

Diaphragm↗
PubMed1993

Abdominal compliance, parasternal activation, and chest wall motion.

We measured abdominal compliance (Cab) and rib cage displacement (delta Vrc) relative to abdominal displacement (delta Vab) during relaxation and tidal breathing in upright (U) and supine (S) postures in five normal subjects. In S, an abdominal binder was used to decrease Cab in two to five increments. We also measured the electrical activity of the parasternal muscle (EMGps) with the use of fine-wire intramuscular electrodes during CO2 rebreathing in U and in supine unbound (SU) and supine bound (SB) postures. During maximum binding (SB2), Cab decreased to 39 +/- 7% of the SU value (P = 0.01), matching Cab in U (P = 0.16). In the SB condition, the ratio of tidal delta Vrc/delta Vab to relaxation delta Vrc/delta Vab increased as Cab decreased, matching the data in U. For the group, this ratio decreased during SU to 47 +/- 10% (P = 0.02) but increased during SB2 to 86 +/- 7% (P = 0.18) of the value in U. During CO2 rebreathing, EMGps increased linearly with tidal volume (r > 0.727, P < 0.01). However, at any given tidal volume, the SU and SB2 EMGps were not significantly different (P = 0.12), and both were less than that in U (P < 0.02). The results suggest that the differences in chest wall motion between U and S may be due to the difference in Cab and not to different patterns of respiratory muscle recruitment. The mechanism may relate to changes in mechanical coupling between the diaphragm and the rib cage.

Abdominal Muscles↗
PubMed1993

Blood flow distribution to upper airway muscles.

Radiolabeled (15-microns) microspheres were used to measure blood flow to upper airway muscles [alae nasi (AN), intrinsic laryngeal, tongue, cervical strap, and hyoid musculature], diaphragm (DI), and parasternals (PS) during spontaneous breathing in 24 anesthetized tracheotomized supine dogs. Six dogs were also studied while -28 +/- 3 (SE) cmH2O tracheal airway pressure was generated against an inspiratory resistance (IR) (upper airway bypassed). Blood flow to posterior cricoarytenoid muscle (PCA) [24.0 +/- 2.1 (SE) ml.min-1.100 g-1] was greater than that to DI (18.0 +/- 2.3 ml.min-1.100 g-1) and comparable to that to PS (21.4 +/- 2.9 ml.min-1.100 g-1). Blood flow per unit weight did not differ between AN, tongue muscles, laryngeal adductors, cervical strap muscles, and cricothyroid (CT). Average blood flow to these muscles was only 8.0 +/- 0.8 ml.min-1.100 g-1. With the exception of CT, blood flow to these upper airway muscles was less than that to DI and PCA. Relative to blood flow during spontaneous breathing, IR loading increased blood flow to AN by a factor of 7.5, to PCA by 3.4, to DI by 3.2 and to PS by 1.9. There was no change in blood flow in the other muscles during loading. Our results show that at rest blood flow to main glottic dilator (PCA) is similar to that to main inspiratory muscles. Furthermore, in response to an IR load, blood flow to PCA and AN increased by an equivalent or greater amount than that to DI.(ABSTRACT TRUNCATED AT 250 WORDS)

Airway Resistance↗
PubMed1992

Effects of cricothyroid muscle contraction on upper airway flow dynamics in dogs.

We studied the effects of cricothyroid muscle (CT) contraction on upper airway flow dynamics in eight prone open-mouth anesthetized dogs. Animals were mechanically ventilated via a tracheostomy while a constant airflow (Vuaw) passed through the isolated upper airway. Nasal airflow (Vn) was monitored using a nasal mask and pneumotachograph. Bilateral CT contraction was induced by electrical stimulation of the external branches of the superior laryngeal nerves. During CT contraction with Vuaw of 100-443 ml/s in the inspiratory direction, total upper airway resistance (Ruaw) fell by 49.1 +/- 5.4% (SE) while supraglottic resistance fell by 63.6 +/- 3.6%; simultaneously Vn fell by 55.3 +/- 3.8% and Vuaw increased by 7.2 +/- 1.7%. Similar results were obtained when Vuaw was in the expiratory direction. In three dogs in which the attachments of the CT to either the thyroid or cricoid cartilage were severed, superior laryngeal nerve stimulation had no systematic effect on Ruaw. Because visual assessment during CT contraction consistently revealed dilation of the piriform recesses, we suggest that CT contraction is associated with pharyngeal dilation, which in open-mouth dogs (with overlapping soft palate and epiglottis) redistributes flow to the oral route with a net reduction in Ruaw. Thus the CT may have a respiratory role as a pharyngeal dilator.

Airway Resistance↗
PubMed1992

Pharyngeal dilation associated with cricothyroid muscle contraction in dogs.

The mechanical function of phasic respiratory-related activity of the cricothyroid muscle of the larynx is poorly understood. We studied five adult cross-bred dogs (weight 14-20 kg) deeply anesthetized with pentobarbitone sodium, mechanically ventilated via a tracheostomy, and placed prone with the mouth open. Bilateral cricothyroid muscle contraction was induced by supramaximal electrical stimulation of the external branches of the superior laryngeal nerve. Computerized axial tomography was used to assess effects of cricothyroid muscle contraction. During cricothyroid muscle contraction, oropharyngeal (tip of epiglottis) cross-sectional area increased by 18.0 +/- 3.0% (SE) (P = 0.008), whereas combined left and right piriform recess cross-sectional area increased by 85 +/- 25% (n = 4; P = 0.02) at the midepiglottic level and by 152 +/- 37% (P = 0.01) at the base of the epiglottis. Furthermore, at the base of the epiglottis the maximum horizontal distance between the alae of the thyroid cartilage increased by 21 +/- 8% (P = 0.05). In contrast, lateral glottic diameter decreased by 52 +/- 2% (n = 4; P = 0.01), whereas dorsoventral glottic diameter increased by 18 +/- 5% (n = 4; P less than 0.02). The cricothyroid muscle, therefore, has the capacity to act simultaneously as a pharyngeal dilator and a glottic constrictor and thus may play a role in the control of oropharyngeal as well as laryngeal patency.

Animals↗
PubMed1992

O2 cost of breathing: ventilatory vs. pressure loads.

We compared the O2 cost of breathing (VO2resp) at high levels of ventilation (HV) with that against high inspiratory pressure loads (HP) using an external elastance when end-expiratory volume, work rate (W), and pressure-time product (P) were matched at two levels of ventilation and elastic loading. Each of five normal subjects performed three pairs of loaded runs (one HV and one HP) bracketed by two resting runs. Mean O2 consumption from the pairs of resting runs was subtracted from that of each of the loaded runs to give VO2resp during loaded breathing. Matching for W and P was within 15% in all 15 pairs of runs. During HV runs, ventilation was 398 +/- 24% of corresponding values during HP runs (P < 0.01). Although there was no difference in W (P > 0.05), the VO2resp during HV runs was 237 +/- 33% of that during HP (P < 0.01) and efficiency of HV was 51 +/- 5% of that during HP (P < 0.01). When W was normalized for the decrease in maximum inspiratory pressure with increased mean lung volume, efficiency during HV and HP runs did not differ (P > 0.05). In the second series of experiments, when both HV and HP runs were matched for W but P was allowed to vary, efficiency increased by 1.42 +/- 0.42% (P < 0.05) for each percent decrease in P during HV runs but was unchanged (P > 0.05) during HP runs despite a 193 +/- 10% increase in P.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1992

Chest wall and trunk muscle activity during inspiratory loading.

We measured the electromyographic (EMG) activity in four chest wall and trunk (CWT) muscles, the erector spinae, latissimus dorsi, pectoralis major, and trapezius, together with the parasternal, in four normal subjects during graded inspiratory efforts against an occlusion in both upright and seated postures. We also measured CWT EMGs in six seated subjects during inspiratory resistive loading at high and low tidal volumes [1,280 +/- 80 (SE) and 920 +/- 60 ml, respectively]. With one exception, CWT EMG increased as a function of inspiratory pressure generated (Pmus) at all lung volumes in both postures, with no systematic difference in recruitment between CWT and parasternal muscles as a function of Pmus. At any given lung volume there was no consistent difference in CWT EMG at a given Pmus between the two postures (P > 0.09). However, at a given Pmus during both graded inspiratory efforts and inspiratory resistive loading, EMGs of all muscles increased with lung volume, with greater volume dependence in the upright posture (P < 0.02). The results suggest that during inspiratory efforts, CWT muscles contribute to the generation of inspiratory pressure. The CWT muscles may act as fixators opposing deflationary forces transmitted to the vertebral column by rib cage articulations, a function that may be less effective at high lung volumes if the direction of the muscular insertions is altered disadvantageously.

Adult↗
PubMed1992

Effects of inspiratory resistance, inhaled beta-agonists and histamine on canine tracheal blood flow.

Tracheobronchial blood flow is potentially important in asthma as it could either influence the clearance of mediators from the airways, thus affecting the duration and severity of bronchospasm, or enhance oedema formation with a resultant increase in airflow obstruction. In anaesthetized dogs, spontaneously breathing via a tracheostomy, we investigated the effects of three interventions which are relevant to acute asthma attacks and could potentially influence blood flow and its distribution to the mucosa and remaining tissues of the trachea: 1) increased negative intrathoracic pressure swings (-25 +/- 1 cmH2O) induced by an inspiratory resistance; 2) variable inhaled doses of a beta-adrenoceptor-agonist (terbutaline); and 3) aerosolized histamine sufficient to produce a threefold increase in pulmonary resistance. Microspheres labelled with different radioisotopes were used to measure blood flow. Resistive breathing did not influence tracheobronchial blood flow. Following a large dose of terbutaline, mucosal blood flow (Qmb) increased by 50%. After inhaled histamine, Qmb reached 265% of the baseline value. We conclude that, whereas increased negative pressure swings do not influence tracheobronchial blood flow or its distribution, inhalation of aerosolized terbutaline, corresponding to a conventionally nebulized dose, increases mucosal blood flow. Our results also confirm that inhaled histamine, in a dose sufficient to produce moderate bronchoconstriction, increases tracheal mucosal blood flow in the area of deposition.

Administration, Inhalation↗
PubMed1991

Effect of microgravity on the respiratory system.

Because the pleural pressure gradient and regional distribution of pulmonary function are gravity dependent, substantial changes may be expected during weightlessness. Although very few measurements have been made during spaceflights, a number of observations during brief periods of weightlessness inside aircraft flying with parabolic trajectories confirm these predictions. Single-breath N2 washouts suggest a marked reduction in the inequality of ventilation distribution seen at 1 G. Similarly, inferences made from cardiogenic oscillations during single-breath washouts suggest a greater uniformity of perfusion during weightlessness. This is supported by changes seen on chest radiographs as well as by more direct measurements of regional blood flow distribution using radioactive iodine-labeled macroaggregates. Vital capacity is only slightly reduced, but functional residual capacity decreases by approximately 10% and maximum expiratory flow rates are slightly decreased, especially at low lung volumes. Weightlessness decreases abdominal girth, increases abdominal compliance, and substantially increases the abdominal contribution to tidal volume during resting breathing. Despite these changes, there does not appear to be any alteration in the temporal pattern of breathing. However, the deposition of inhaled medium-sized aerosol particles is substantially reduced, as predicted by model analyses of gravitational sedimentation. Virtually all these observations describe effects at the very onset of weightlessness. Practically nothing is known of slower functional changes and adaptations to prolonged weightlessness. Systematic repeated measurements during manned spaceflights will hopefully begin to provide some information on this subject in the near future.

Blood Volume↗
PubMed1991

Oxygen cost of inspiratory loading: resistive vs. elastic.

We measured the O2 cost of breathing (VO2resp) against external inspiratory elastic (E) and resistive loads (R) when end-expiratory lung volume, tidal volume, breathing frequency, work rate, and pressure-time product were matched in each of six pairs of runs in six subjects. During E, peak inspiratory mouth pressure was 65.7 +/- 1.8% (SD) of the maximum at functional residual capacity. However, during resistive runs, peak inspiratory mouth pressure was 41.1 +/- 2.8% of the maximum at functional residual capacity. In 36 paired runs, where both work rate and pressure-time product were within 10%, VO2resp for E was less than for R (81 and 96 ml/min, respectively; P less than 0.01). During loaded and unloaded breathing with the same tidal volume, we measured the changes in anteroposterior diameter of the lower rib cage in five subjects. In four subjects we also recorded the electromyograms of several fixator and stabilizing muscles. During E and R, the change in anteroposterior diameter of the lower rib cage was -116 +/- 5 and -45 +/- 4% (SE), respectively, of the unloaded value (P less than 0.01), indicating greater deformation during E. Although the peak electromyographic activity was 72 +/- 16% greater during E (P less than 0.01), there was no difference between the loads for area under the electromyogram time curve (P greater than 0.05). However, the time to 50% peak activity was less during R (P less than 0.02). We conclude that, even when work rate and pressure-time product are matched, VO2resp during R is greater than that during E. This difference may be due to preferential recruitment of faster and less efficient muscle fibers.

Adult↗
PubMed1991

Respiratory-related activity of cricothyroid muscle in awake normal humans.

The role of the cricothyroid muscle (CT) in respiration is unclear. To examine the respiratory-related electrical activity of the CT, we measured its electromyogram (EMG) and compared it with that of the alae nasi (AN) in eight healthy subjects. During quiet breathing the CT EMG phasing was inspiratory in seven subjects. This pattern was similar to the AN with respect to phasing and shape of the integrated EMG. The onset of phasic CT and AN activity related to inspiration preceded flow by 173 +/- 39 and 570 +/- 76 (SE) ms, respectively (P less than 0.01). We measured the duration from onset of phasic activity to peak of the EMG (TA) and the total cycle duration (TT). TA/TT of the CT was 0.29 +/- 0.02, similar to that of the AN (0.28 +/- 0.03). Inspiratory resistive loading, panting, and voluntary hyperventilation increased CT activity above the peak level seen during tidal breathing. Voluntary glottic closure increased CT activity to a level above tonic but below peak tidal activity. The findings suggest that the phasic electrical activity of the CT simulates predominantly that of an upper airway dilator.

Adult↗
PubMed1991

Cricothyroid muscle responses to increased chemical drive in awake normal humans.

To examine the response of the cricothyroid muscle (CT) to increased chemical drive, we measured its electromyogram simultaneously with that of the alae nasi (AN) in seven normal awake subjects. During both progressive hyperoxic hypercapnia and hypoxia, peak integrated inspiratory activity (moving time average, MTA) of the CT and AN increased as a power function of mean inspiratory flow (ratio of tidal volume to inspiratory time, VT/TI), given by MTA = a(VT/TI)b + c (where a, b, and c are constants). The exponent b varied from 0.009 to 3.4 among subjects but was correlated between CT and AN both during hypercapnia (r = 0.86) and hypoxia (r = 0.81). The onset of inspiratory activity of the CT and AN preceded that of inspiratory flow. Expressed as a percentage of expiratory time, the CT lead time rose from 7% at rest to 20% during hyperpnea. The corresponding values for the AN were from 22 to 52% (both P less than 0.03). Thus the pattern of response of the CT and AN is similar and related to that of the inspiratory muscles in a curvilinear manner. The findings suggest that during chemical stimulation the electrical activity of the CT is analogous to that of the AN, an upper airway dilator.

Adult↗
PubMed1991

Pressure-diameter relationships of the upper airway in awake supine subjects.

In awake supine normal subjects, dimensional changes of the oropharyngeal airway were measured during exposure to negative intraluminal pressures. The pressure was generated 1) "actively" by subjects inspiring against an externally occluded airway or 2) "passively" by external suction at the mouth during voluntary glottic closure with no inspiratory effort. Airway dimensions were imaged with X-ray fluoroscopy and anteroposterior diameters measured at levels corresponding to cervical vertebra 3 and 4 (C3 and C4). Cephalad axial displacement of the hyoid bone (CDHY) was also measured. During the "active" maneuver, airway diameters and position were maintained at resting levels despite airway pressure up to -15 cmH2O. In contrast, during the passive maneuver at -15 cmH2O, C3 was only 15 +/- 9% and C4 only 47 +/- 8% of control; CDHY was 5.6 +/- 1.8 mm. In three subjects airway wall apposition occurred and persisted until an active inspiratory effort. We conclude that, in the absence of inspiratory effort, negative oropharyngeal airway pressures result in marked narrowing and cephalad displacement of the upper airway, even during wakefulness. Therefore, our data suggest that the complex interaction of upper airway and thoracic muscle activity is critical in determining the effective compliance and patency of the upper airway, which is readily collapsible even in normal subjects.

Adult↗