A new ventilation inhomogeneity index from multiple breath indicator gas washout tests in mechanically ventilated patients

OBJECTIVESa) To determine the validity of a new method to analyze indicator gas washout tests on mechanically ventilated patients. This method takes into account the difference between the end-expiratory gas fraction and the mean gas fraction in the lung and provides the end-expiratory lung volume and a new index of ventilation inhomogeneity called volumes regression index. b) To determine the validity of this index as a predictor of chronic obstructive pulmonary disease. c) To compare this index with the moment ratio index and Becklake index. DESIGNProspective study of diagnostic test. Criterium standardsClosed-circuit indicator gas dilution technique and Tiffeneau index. SETTINGSurgical intensive care unit of a university hospital. PATIENTSA total of 38 mechanically ventilated postoperative patients, divided into two groupsthe obstructive group (n = 21) and the nonobstructive group (n = 17), based on their preoperative lung function. INTERVENTIONSNone. MEASUREMENTS AND MAIN RESULTSa) The mean coefficient of variation of all lung volume measurements in a group of nine healthy volunteers was 5%, and the difference between this technique and the closed-circuit helium dilution measurements was −2 ± 5%. In patients, the mean coefficient of variation of the lung volume measurements was 3.5%. The volumes regression index was measured as 0.02 ± 0.04 in a dummy lung, 0.37 ± 0.08 in the healthy volunteers, 0.64 ± 0.23 in the nonobstructive patients, and 1.1 ± 0.3 in the obstructive patients. The volumes regression index provided a better correlation (r2 = .46) with preoperatively determined Tiffeneau index than the Becklake index (r = .11) or the moment ratio index (r = .18). CONCLUSIONThe proposed technique provides a means for accurate measurement of the end-expiratory lung volume and the amount of ventilation inhomogeneity in mechanically ventilated intensive care unit patients. (Crit Care Med 1993; 21:1149–1158)