ASSESSMENT OF INSPIRATORY FLOW LIMITATION INVASIVELY AND NONINVASIVELY DURING SLEEP

To define the standard of airway flow limitation, pharyngeal pressure and flow rate were measured during wakefulness and sleep in seven habi tual snorers with widely varying degrees of sleep-induced increases in upper airway resistance. Inspiratory pressure:flow relationships were used to group breaths into four categories of flow limitation, includ ing linear (Level 1), mildly alinear (Level 2), constant flow rate wit h no pressure dependence (Level 3), and decreasing flow rate throughou t significant portions of inspiration, i.e., negative pressure depende nce (Level 4). These pressure:flow rate gold standards of flow limitat ion were used to evaluate a flow limitation index derived from the tim e profile (or ''shape'') of three noninvasive estimates of flow rate: (1) pneumotach flow rate, (2) differentiated sum respiratory inductanc e plethysmography (RIP), and (3) nasal pressure. A nonflow limited tem plate for each of these noninvasive measurements was taken from awake breaths and the difference in area determined between the template bre ath and each of the noninvasive signals measured during nonrapid eye m ovement (NREM) sleep. The noninvasive flow limitation indices were fou nd to be effective in differentiating severe types of inspiratory flow limitation, i.e., Level 1 versus Level 3 or Level 4 (sensitivity/spec ificity > 80%). On the other hand, these indirect indices were not abl e to consistently detect mild levels of flow limitation (Level 1 versu s Level 2; sensitivity/specificity = 62 to 72%); nor were these noninv asive estimates of flow rate ''shape'' sensitive to breaths with a hig h but fixed resistance throughout inspiration. The area index derived from measurements of pressure at the nares (Pn) was the most sensitive , nonperturbing, noninvasive measure of flow rate and flow limitation, and we recommend its use for recognizing most of the common types of moderate to severe levels of airway flow limitation in sleeping subjec ts.