Determination of Pharyngeal Size
The PCSA was measured during wakefulness by CT as previously described.” All measurements were obtained in the supine position during a 5-s period of breathing cessation at FRC. Subjects were very carefully instructed and observed to avoid glottic closure, swallowing, and respiratory motion during this period. Images were taken at 0.8-cm intervals between the hard palate and a level at or near the tip of the epiglottis. The software of the scanner (Siemens Somatom-2 Body Scanner), automatically calculated PCSA at each interval. One patient was not evaluated for PCSA because he exceeded the 136-kg (300-lb) weight limit of the scanners table.
We defined the nasopharynx as extending from the lower edge of the hard palate to the lower edge of the soft palate or the uvula and defined the oropharynx as extending from the lower edge of the soft palate to the tip of the epiglottis. Images at the edge of the hard palate and at the tip of the epiglottis were eliminated in the data analysis because of disproportionately large mean PCSA values at these two levels. Elimination of these PCSA values did not signifi¬cantly affect mean differences in nasopharyngeal and oropharyngeal cross-sectional area between patients and controls. Four measures of PCSA, all expressed in square millimeters, were determined: (1) minimal area (the smallest area observed); (2) mean nasopharyngeal area; (3) mean oropharyngeal area; and (4) mean pharyngeal area (including both nasopharynx and oropharynx).
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Determination of Pharyngeal Airflow Resistance
Pharyngeal resistance was defined as inspiratory airflow resis-tance between the choanae and epiglottis at 0.3 L/s. Our technique of measuring Rp, previously described in detail, requires simul-taneous recordings of nasal airflow and pressures at the choanae and the tip of the epiglottis. Choanal pressure was determined by anterior rhinometry, and supraglottic pressure was determined by a balloon-tipped catheter. Airflow was measured with a tight- fitting cushioned face mask connected to a pneumotachograph (Fleisch No. 2) having an internal resistance of 0.6 to 0.7 cm H20/L/s. Pharyngeal resistance was calculated by the equation, Rp = (P supraglottic—P nasal)/0.3, and was reported as the mean of ten measurements. All measurements were obtained with subjects supine and awake, with the head and neck in a neutral position. Measurements of Rp could not be obtained in one patient with OS A and two control subjects because of intolerance of the supraglottic catheter or technical problems. Measurements of Rp were not obtained in five patients with OSA who were studied in a protocol that did not require Rp measurements but otherwise was identical.
Body mass index (BMI) (body weight in kilograms divided by the square of height in meters) was selected as the index of obesity.” We employed nonparametric statistical methods because most of the recorded variables were not evenly distributed. Group differ-ences were assessed with the Mann-Whitney two-sample test and correlations between variables with the Spearman rank sum corre-lation. Partial correlation analysis examined the relationship be-tween two variables while controlling for a third variable. The 0.05 level of confidence was selected to indicate statistical significance. All tests are two-sided unless specified otherwise.
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