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A-1557
2004
Partial Pressure End-Tidal CO2 (PetCO2) Accurately Predicts PaCO2 for a Large Majority of Patients with Acute Respiratory Distress Syndrome (ARDS)
Michael J. Banner, Ph.D., Vic Brennan, Ph.D., Andrea Gabrielli, M.D.
Department of Anesthesiology, University of Florida College of Medicine, Gainesville, Florida.
Introduction: Increased physiologic deadspace volume, a component of which is alveolar deadspace volume (VDalv), has been identified as an independent predictor of mortality in patients with ARDS (1). Substantial increases in the physiologic deadspace volume-to-tidal volume ratio (VD/VT) predisposes to compromised arterial blood gas exchange and a widening of the arterial minus end-tidal carbon dioxide (Pa-etCO2) value. The purpose of this descriptive study is to evaluate the accuracy of PetCO2 for predicting PaCO2 in patients with ARDS.

Methods:
IRB consent was obtained on 48 intubated, mechanically ventilated adults with ARDS (67 ± 10 yrs, 76 ± 16 kg, 37 males, 11 females). All received various combinations of intermittent mandatory ventilation (2-8 per min), positive end expiratory pressure (5-20 cm H2O), and pressure support ventilation (5-30 cm H2O). Signals from a combined pressure / flow / carbon dioxide sensor, positioned between the endotracheal tube and ventilator Y-piece, were directed to a respiratory monitoring system (NICO, Novametrix- Respironics and NeuroDimension) for calculation of VD/VT and VDalv using the single breath CO2 elimination method (2). An arterial blood gas was obtained. Patients were divided in two groups; Group A: Pa-etCO2 < 10 mm Hg, Group B: Pa-etCO2 > 10 mm Hg. Data were analyzed using a Mann-Whitney U test; alpha was set at 0.05 for statistical significance.

Results: Table. Group A consisted of 85 % (n = 41) and Group B 15 % (n = 7) of the patients.

Discussion:
Decreased lung volume and the acute formation of Zone 4 lung units in ARDS (3,4) results in increased regional pulmonary vascular resistance predisposing to areas of increased alveolar ventilation-to-perfusion matching, i. e., increased VDalv. Inappropriately large increases in physiologic deadspace volume (VD/VT > 0.60) as in severe forms of ARDS, contribute to large differences in Pa-etCO2, limiting the accuracy of PetCO2 to predict PaCO2 (i. e., Group B patients). In the large majority of patients (greater than 8 in 10) with moderate forms of ARDS, although VD /VT is increased, PetCO2 accurately predicts PaCO2. A clinical implication of these findings is cost savings. Fewer arterial blood gases may be required for determining PaCO2 by using PETCO2 monitoring for patients with ARDS.

References

(1) Nuckton et al, NEJM, 2002; 346: 1281; (2) Arnold et al, Crit Care Med, 1996; 24: 96; (3) Benumof J. L., in Anesthesia, R. D. Miller (ed), 2000, pp. 579, 580, 585, (4) West J B, Respiratory Physiology, 1976, pp. 40, 69

Anesthesiology 2004; 101: A1557
PaCO2PetCO2Pa-etCO2PaO2/FIO2VD/VTVDalv
Group A37±5.532.2±5.04.5±0.5293±1050.46±0.08104±59
Group B44.3±5.0*30±1.214.3±3.8*194±45*0.61±0.06*196±57*
Data are mean ± SD. p < 0.05 compared to patients in Group A (*)