PURPOSE: Noninvasive monitoring of the arterial partial pressures of CO(2) (PaCO(2)) of critically ill patients by measuring their end-tidal partial pressures of CO(2) (PETCO(2)) would be of great clinical value. However, the gradient between PETCO(2) and PaCO(2) (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. METHODS: 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. RESULTS: PETCO(2) and PaCO(2) remained equal within the error of measurement at all gas tension combinations. The mean ± SD PET-aCO(2) gradient (0.13 ± 0.12 mmHg, 95% CI -0.36, 0.10) was the same (p = 0.66) as that between duplicate PaCO(2) measurements at all PETCO(2) and PETO(2) combinations (0.19 ± 0.06, 95% CI -0.32, -0.06). CONCLUSIONS: 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 PaCO(2) 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.
PURPOSE: Noninvasive monitoring of the arterial partial pressures of CO(2) (PaCO(2)) of critically illpatients by measuring their end-tidal partial pressures of CO(2) (PETCO(2)) would be of great clinical value. However, the gradient between PETCO(2) and PaCO(2) (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. METHODS: 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. RESULTS: PETCO(2) and PaCO(2) remained equal within the error of measurement at all gas tension combinations. The mean ± SD PET-aCO(2) gradient (0.13 ± 0.12 mmHg, 95% CI -0.36, 0.10) was the same (p = 0.66) as that between duplicate PaCO(2) measurements at all PETCO(2) and PETO(2) combinations (0.19 ± 0.06, 95% CI -0.32, -0.06). CONCLUSIONS: 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 PaCO(2) 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.
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