PURPOSE:An alternative to clinician selected ventilator settings is to use mathematical models. Adaptive support ventilation (ASV) is one model that determines optimum tidal volume and frequency by minimizing work. We developed an alternative model called Mid Frequency Ventilation (MFV) that determines optimum tidal volume and frequency by maximizing alveolar ventilation. We studied: 1) How different are clinician chosen ventilator settings from these mathematical models, and 2) Can MFV improve lung protective ventilation strategies?.
METHODS:A survey of critical care physicians, fellows and respiratory therapist provided reference ventilator settings for a 70 kg paralyzed patient in 5 scenarios (normal lungs/ normal acid base, ARDS/mixed acidosis, morbidly obese/respiratory acidosis, COPD / respiratory alkalosis and status asthmaticus / severe respiratory acidosis).The survey derived values for minute ventilation and minute alveolar ventilation were used as goals for ASV and MFV respectively. A lung simulator (Ingmar ASL 5000) programmed with each scenario respiratory system characteristics was ventilated with the clinician settings, ASV and MFV.
RESULTS:Table 1 shows the results of the survey and mathematical models applied on the lung simulator. Tidal volumes ranged from 6.1 to 8.2 mL/kg for the clinician, 6.7 to 11.6 mL/kg for ASV and 3.7 to 9.9 mL/kg for MFV. Mean airway pressures were similar for all strategies; however, set inspiratory pressures were lower for the mathematical models. Clinician selected tidal volumes were similar (<1 ml/kg difference) to the ASV settings for all scenarios except asthma, where the tidal volumes were larger for ASV and MFV. MFV delivered the same alveolar minute ventilation using less volume and pressure.
CONCLUSION:1. ASV performed close to clinician chosen ventilator settings. 2. MFV used less volume and pressure for a given alveolar ventilation.
CLINICAL IMPLICATIONS:Ventilator settings selected by mathematical models may improve delivery of mechanical ventilation.
DISCLOSURE:Eduardo Mireles-Cabodevila, None.