Abstract 212 Poster Session III, Monday, 5/3 (poster 99)

Purpose: To evaluate the stability of respiratory inductive plethysmography (RIP) during SomnoStar PT (SensorMedics, Loma Linda, CA), a test lung (Siemens Test Lung 190, 1 liter, Schamburg, IL), and a high frequency oscillatory ventilator (SensorMedics 3100A).

Methods: Following a one half-hour stabilization period to allow machine warm-up, the plethysmograph was calibrated according to manufacturer specifications using HFOV at 3 Hz, 0.5 inspiratory time, Paw 30 cm HO, P 50 cm HO, flow 20 liters/minute. Both abdominal and thoracic bands were used for end-expiratory lung volume (EELV) measurements. After calibration, the frequency of HFOV was increased to 7 Hz to simulate clinical conditions and the EELV data was collected for 4 hours. Using RespiEvents off-line Event Data Processing (EDP) software (Non-Invasive Monitoring Systems, Miami Beach, FL) and a laptop computer (Toshiba 110 CS), the EELV drift was analyzed over the four hour period. Initial EELV measurements were recorded off-line. At ten minute sampling intervals over the four hours, the EELV counts for ten 20-second periods were averaged. The resulting twenty-four average EELV counts were compared to the initial EELV. The % difference (average measured count initial count/total EELV count) was defined as the drift. (Figure)

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Results: The overall drift over four hours was 3.15% of initial EELV and the rate of drift was 0.8%/hour. Most of the drift (2.18% or 70% of the total) occurred within the first hour of testing.

Discussion: During spontaneous breathing and conventional ventilation, RIP has been shown to have a drift of 1-5% in calibrated studies (JAP 55: 598-603, 1983; JAP 55: 263-6, 1983; Chest 102:234-8, 1992). This drift is even higher if a 30 minute thermal equilibration period is not allowed (JAP 65:306-8, 1988; JAP 64:42-9, 1988). After allowing for this warm-up period, we found a 3% drift at a rate of less than 1%/hour with most occurring in the first hour. We found this system stable during high frequency ventilation.