Noninvasive monitoring of the arterial partial pressures of CO2 (PaCO2) of critically ill patients by measuring their end-tidal partial pressures of CO2 (PetCO(2)) would be of great clinical value. However, the gradient between PetCO(2) and PaCO2 (Pet-aCO(2)) in such patients typically varies over a wide range. A reduction of the Pet-aCO(2) gradient can be achieved in spontaneously breathing healthy humans using an end-inspiratory rebreathing technique. We investigated whether this method would be effective in reducing the Pet-aCO(2) gradient in a ventilated animal model.
Six anesthetized pigs were ventilated mechanically. End-tidal gases were systematically adjusted over a wide range of PetCO(2) (30-55 mmHg) and PetO(2) (35-500 mmHg) while employing the end-inspiratory rebreathing technique and measuring the Pet-aCO(2) gradient. Duplicate arterial blood samples were taken for blood gas analysis at each set of gas tensions.
PetCO(2) and PaCO2 remained equal within the error of measurement at all gas tension combinations. The mean +/- A SD Pet-aCO(2) gradient (0.13 +/- A 0.12 mmHg, 95% CI -0.36, 0.10) was the same (p = 0.66) as that between duplicate PaCO2 measurements at all PetCO(2) and PetO(2) combinations (0.19 +/- A 0.06, 95% CI -0.32, -0.06).
The end-inspiratory rebreathing technique is capable of reducing the Pet-aCO(2) gradient sufficiently to make the noninvasive measurement of PetCO(2) a useful clinical surrogate for PaCO2 over a wide range of PetCO(2) and PetO(2) combinations in mechanically ventilated pigs. Further studies in the presence of severe ventilation-perfusion (V/Q) mismatching will be required to identify the limitations of the method.
- End-inspiratory rebreathing
- CONGENITAL HEART-DISEASE
- CARBON-DIOXIDE GRADIENT