The Science Journal of the American Association for Respiratory Care

2010 OPEN FORUM Abstracts

COMPARISON OF THREE METHODS TO SET TLOW ON AIRWAY PRESSURE RELEASE VENTILATION - A MODEL STUDY.

Mohammad F. Siddiqui1, Eduardo Mireles-Cabodevila1, Robert L. Chatburn2; 1Division of Pulmonary and Critical Care Medicine, UAMS, Little Rock, AR; 2Respiratory Institute, Cleveland Clinic, Cleveland, OH

BACKGROUND: There is no consensus on how to set the release phase duration (Tlow) in APRV. Reviewing the literature, we were able to identify 3 methods to set APRV, each with distinct goals. No theoretical, animal or clinical studies have been used to demonstrate their effect. The purpose of this study was to evaluate the ventilation outcomes of the 3 techniques using a simulation. METHODS: We implemented a mathematical model of pressure control ventilation (J Appl Physiol 1989; 67(3):1081-92) in a spreadsheet. Three different methods of APRV settings were evaluated: 1) Target peak expiratory flow rate (PEFR) between 50-75% by Tlow (Crit Care Med 2005; 33[Suppl.]:S228–40); 2) Titrate Tlow to target VT 4-6 cc/kg (http://ccmtutorials.com); and 3) Set Tlow to 4 time constants to achieve complete exhalation and titrate the release phase pressure (Plow) to achieve VT 4-6 cc/kg (Fundamentals of Mechanical Ventilation, 2nd Ed, 2006). For all methods, we kept the high pressure (Phigh) = 25 cm H2O and the time on Phigh (Thigh) = 4 s. We used an expiratory resistance (RE) of 15 cmH2O/L/s, and three sets of static compliance (C) 15, 30 and 60 mL/cm H2O, values which have been previously published for ARDS patients. For methods 1 and 2 we set the Plow at 0 cm H20, changed the Tlow, and calculated VT and aPEEP. For method 3 we varied the Plow to target the VT 4 – 6 cc/kg. RESULTS: For method 1, a PEFR between 50-75% was only achieved with a C of 30 and 60 mL/cmH20 with a Tlow of 0.2-0.3 s and 0.3-0.6 s respectively. These settings generated significant aPEEP and a broad range of VT (Table 1). For method 2, Tlow ranged between 0.4-6 s for a C of 15 mL/cmH2O with minimal aPEEP, but only single Tlow values of 0.3 s and 0.2 s were possible for C of 30 and 60 mL/cmH2O respectively; however, this was associated with aPEEP of 13 and 20 cmH2O. For method 3, with C of 15, 30 and 60 mL/cm H2O, Plow values ranged between 0-4, 12-14, 18-20 cmH2O respectively. For all methods, mean airway pressure was similar. CONCLUSION: Method 1 and 2 to set APRV have limited combinations of settings for the respiratory characteristics of patients with ARDS. These methods may result in injurious tidal volumes and significant aPEEP. Furthermore, there is dependence between aPEEP and VT making it difficult to achieve the goals of lung protective ventilation. Method 3 allows the best titration of settings to achieve ventilation goals while maintaining the same mean airway pressure. Sponsored Research - None