The Science Journal of the American Association for Respiratory Care

2012 OPEN FORUM Abstracts


Aanchal Kapoor, Carla Wollens, Robert L. Chatburn; Respiratory Institute, Cleveland Clinic, Cleveland, OH

BACKGROUND The assessment of pulmonary dead space to tidal volume fraction (Vd/Vt) has been used as a predictor of extubation success. It is a useful tool in titrating PEEP during mechanical ventilation and has been used as a predictor of mortality in acute lung injury (ALI) patients. Siddiki et el (Critical Care 2010;14(4):R141) have modified the Harris Benedict equation to estimate CO2 production as a basis for calculation of Vd/Vt. These authors noted that there has been no comparison of estimated with measured CO2 production (V’eCO2). The purpose of this pilot study was to compare Vd/Vt values calculated from estimated and measured values for V’eCO2. METHODS After obtaining informed consent, the following data were collected at the bedside; age (years), height (cm) weight (kg) sex, diagnosis, and body temperature (degrees C). A NICO monitor (Philips Healthcare) was used to measure V’eCO2 and calculate Vd/Vt. Minute ventilation (V’e) was obtained from the ventilator (PB 840, Covidien) as an average of 10 breaths. An Arterial Blood Gas was drawn at the same time and results entered into NICO. The following equations were used for manual calculation: Vd/Vt = 1-[(0.86 x estV’eCO2)/(V’e x PaCO2)] where V’e is the expired minute ventilation (from the ventilator) estV’eCO2 is the estimated CO2 production est V’eCO2 = (predHB x hf x 0.8)/6.8644 where predHB is the predicted resting energy expenditure (Harris Benedict): predHB females = 655.1 + (6.56 x weight) + height – (4.56 x age) predHB males = 66.45 + (13.75 x weight) + (5 x height) – (6.76 x age), hf is a metabolic correction factor; 1.13 per degree C over 37 degrees, 1.2 for minor surgery, 1.35 for major trauma and 1.6 for severe infection. RESULTS Data for Vd/Vt values are illustrated in the figure (shows inter-quartile range and median). There was no significant difference between Vd/Vt values calculated from measured and estimated values for V’eCO2 (0.44 vs 0.52; P = 0.327). However, the small sample size resulted in a low power (0.054). CONCLUSIONS Based on these preliminary data, a larger sample size might reveal a significant and clinically important difference in calculated Vd/Vt values. Furthermore, estimating V’eCO2 based on relatively constant parameters (eg, weight and height) may not be appropriate when using Vd/Vt for dynamic medical decisions like ventilator management. Further research is warranted. Sponsored Research - None