A NEW VENTILATION INHOMOGENEITY INDEX FROM MULTIPLE BREATH INDICATOR GAS WASHOUT TESTS IN MECHANICALLY VENTILATED PATIENTS

Objectives: a) To determine the validity of a new method to analyze in dicator gas washout tests on mechanically ventilated patients. This me thod takes into account the difference between the end-expiratory gas fraction and the mean gas fraction in the lung and provides the end-ex piratory lung volume and a new index of ventilation inhomogeneity call ed volumes regression index. b) To determine the validity of this inde x as a predictor of chronic obstructive pulmonary disease. c) To compa re this index with the moment ratio index and Becklake index. Design: Prospective study of diagnostic test. Criterium standards: Closed-circ uit indicator gas dilution technique and Tiffeneau index. Setting. Sur gical intensive care unit of a university hospital. Patients: A total of 38 mechanically ventilated postoperative patients, divided into two groups: the obstructive group (n = 21) and the nonobstructive group ( n = 17), based on their preoperative lung function. Interventions. Non e. Measurements and Main Results: a) 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-circ uit 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 lun g, 0.37 +/- 0.08 in the healthy volunteers, 0.64 +/- 0.23 in the nonob structive patients, and 1.1 +/- 0.3 in the obstructive patients. The v olumes regression index provided a better correlation (r2 = .46) with preoperatively determined Tiffeneau index than the Becklake index (r2 = .11) or the moment ratio index (r2 = .18). Conclusion: The proposed technique provides a means for accurate measurement of the end-expirat ory lung volume and the amount of ventilation inhomogeneity in mechani cally ventilated intensive care unit patients.