The Science Journal of the American Association for Respiratory Care

2010 OPEN FORUM Abstracts


Lara Brewer, Joseph Orr; Bioengineering, University of Utah, Salt Lake City, UT

Background: The ARDSnet ventilation protocol recommends a tidal volume (Vt) setting of 6-8 mL/kg predicted body weight. Respiratory rate (RR) is adjusted to achieve a target pH of (7.3-7.45) while avoiding PaCO2 lower than 25 mmHg. It should be possible to estimate an initial RR that is close to the ventilation requirements of the patient without needing to wait for several arterial blood gas (ABG) panels to be run. Such an estimate would be based on inputs of set Vt, directly measured CO2 excretion (VCO2), modeled estimate of metabolic VCO2, pulmonary dead space fraction(Vd/Vt), airway dead space, current PaCO2 from ABG, and desired PaCO2. The inputs would be analyzed to calculate the RR required for delivery of sufficient alveolar minute volume to support excretion of the metabolically produced CO2 and to reach the target PaCO2. Methods: A simple set of equations was developed to take inputs related to a patient’s metabolism, PaCO2, Vd/Vt, and recommended Vt and from those parameters, calculate a suggested RR. In an assessment of the equations in mechanically ventilated swine, the subject-specific, measured input parameters were used to calculate the RR needed to eliminate the measured VCO2 at the given tidal volume during a period of stabilization. The calculated RR was compared to the monitored RR. After a change in Vt was made and 60 minutes had been allowed for stabilization, a second ABG was sampled and the RR was again calculated. Calculated and monitored RR were compared for each measurement pair. Results: The RR calculation was in good agreement with the monitored rate. The calculated RR differed from the monitored rate by mean SD of -0.02 ± 0.06 br/min (-0.12 ± 0.3%) and r2 was 0.99 (n=8). Conclusions: Accurate selection of initial RR based on patient-specific ventilation, metabolism and pulmonary dead space parameters appears possible. When small tidal volumes are selected, a large percentage of the Vt is lost to airway deadspace, such that minute ventilation must be increased compared to large Vt ventilation in order to maintain the same effective alveolar ventilation. Future work should be done in vivo to compare the calculated RR with the RR empirically found from multiple ABG samples during ARDSnet ventilation adjustment. The model could also be expanded for cases where large pulmonary shunt is present. An interesting approach for future research would be to target a specific pH in addition to or instead of the target PaCO2. Sponsored Research - Philips-Respironics