Flow volume loop pattern in sitting and supine positions in obstructive sleep apnea

ABSTRACT

Episodes of obstructive sleep apnea depend on posture assumed during sleep, being more frequent in the supine position. Findings on supine flow volume loop [FVL] may therefore correlate better with obstructive apneic episodes than sitting FVL. In this study we investigated the FVL pattern in 27 obstructive sleep apnea [OSA] patients having upper airway obstruction in both sitting and supine positions compared to 20 control subjects. Diagnosis of OSA was based on full night polysonographic study. Spirometric measures were done in either group in sitting and supine positions in a random fashion. Sixteen patients with oropharyngeal airway obstruction underwent uvulopalatopharyngo plasty. Polysomnography and FVL were repeated 6 weeks later and compared to preoperative pattern. Apnea-hypopnea index showed more than 50% improvement in 13 patients postoperatively. Spirometric features in OSA group were Vital capacity [VC] was less in supine [2.4 +/- 0.5] than sitting position [2.7 +/- 0.6]. Both were less than normal group [4.2 +/- 0.9] p<0.05. Postoperatively, there was insignificantly increase in both supine [2.5 +/- 0.4] and sitting [2.8 +/- 0.5] VC in responders. The expiratory and inspiratory flow rates were less in supine position than sitting position throughout VC especially at higher lung volumes, both were less than normal group. The Expiratory flow curve showed an expiratory flow plateau which could be identified between 85-61% of VC in 83% of OSA in supine position and in 66% in sitting positions. The mid portion of expiratory curve was convex away from volume axis in 89% of cases of OSA compared to slightly concave one in normal group. On the other hand, the inspiratory flow curve showed an inspiratory plateau occurred in 66% of sitting OSA patients. It extended over 55% VC. Expiratory flow ratio MEF[50]/ M1F[50] ratio was higher in OSA[0.71 in sitting and 0.78 in supine] compared to control group [0.3 in sitting and 0.6 in supine]. In the postoperatively group the pattern of FVL was still retaining the preoperative features specially in the bad responders group. FVL reflects the dynamic upper airway narrowing in OSA patients. Two features are added to previously reported FVL characteristics in OSA patients. The pattern is more evident in supine position. It is persistent after surgical treatment. FVL is useful physiological test for studying behavior of upper airways in individual OSA patients rather than diagnostic tool for screening these patients. Supine FVL may be used as a helping tool to predict the postoperative success when sitting FVL is not fully informative in a Suspicious case