The Science Journal of the American Association for Respiratory Care

2011 OPEN FORUM Abstracts


John S. Emberger, Francis Gott III, Joel M. Brown II; Respiratory Care, Christiana Care Health System, Newark, DE

BACKGROUND: Time Constants (TC) can be measured in volume control ventilation by measuring static plateau pressure (PLAT), lung compliance (Cst) and airway resistance (Raw). TC calculation is as follows: TC = Cst X Raw. TC relates to the the time it takes to empty lung volume. (1TC=63%, 2TC=84.5% and 3TC=95%) Time low (Tlow) for Airway pressure release ventilation (APRV) is set to avoid alveolar collapse. Tlow is typically set based on exhalation flow graphic, but it has been suggested that Tlow could be optimally set with TC1. We wanted to determine if TC value of the Tlow setting correlates to the volume exhaled during the APRV release breath. METHODS: An IRB approved retrospective review of patients switched from volume controlled ventilation to APRV was performed. Patients who had Cst and Raw measurement in volume control within 2 hours on being placed on APRV were examined. TC was calculated from Cst and Raw. Maximum exhalation volume in APRV was calculated: Pressure High (Phigh) setting X Cst. Percentage of Maximal exhalation was calculated by Actual exhaled tidal volume/Maximum exhalation volume. TC value was calculated for each Tlow setting: TC of Tlow Setting = Tlow/TC. Correlation (r2)for % of maximal exhalation volume and TC of Tlow settings. RESULTS: 26 patients with 46 sets of data were identified that had CST and Raw measured within 2 hours of being placed on APRV in the last year. Average TC measured in volume control was 0.61 +/- 0.27 seconds. Average Phigh was 25 +/- 4 (range 18-32)cmH2O. Average Plow was 0.79 +/- 0.16 (range 0.5-1.2)seconds. Average TC value of the Tlow was 1.49 +/- 0.53. % of exhalation correlated (r2=0.53) to TC of Tlow. 1TC correlated to ~60% exhalation and 2TC correlated to ~88% exhalation volume in APRV (see chart). CONCLUSIONS: Based on TC calculated in volume controlled ventilation for patients moving to APRV: 1) % of exhaled lung volume correlates to time constant value of the Tlow setting, 2) One to two TC for Tlow setting appears to give 60% to 88% of maximal exhalation volume. Using calculated time constants while in volume controlled ventilation could help guide setting of APRV Tlow to optimally maintan lung volume and avoid alveolar collapse. Reference: 1) A Modrykamien, R Chatburn, APRV: Alternative mode of mechanical ventilation in acute respiratory distress syndrome, Cleveland Clinic Medical Journal, Feb 2011
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