Continuous carbon dioxide monitoring in the exhaled breath of mechanically ventilated rats.

In small animals, carbon dioxide monitoring is either limited by the need to take blood samples for gas analysis, or it interferes with respiratory efficiency and lung mechanics analysis. We introduced a novel approach for continuous monitoring of CO₂ in the expiratory limb of the breathing circuit. The relevance of the method is assessed by CO₂ measurements at different respiratory settings. Rats were ventilated with a tidal volume (V_T) of 8 mL kg^-1. Respiratory rates were adjusted to achieve arterial CO₂ partial pressure ( $P_{\mathrm{aC}{O}_2}$ ) between 35 and 45 mmHg. We measured partial pressure of CO₂ in the expiratory limb of the breathing circuit (exCO₂). exCO₂ values were compared to $P_{\mathrm{aC}{O}_2}$ from blood gas analysis. The agreement between the two measurements was assessed by correlation and Bland-Altman analysis. The validity of the novel approach was established through additional experimental runs with V_T of 7 or 6 mL kg^-1, where the respiratory rate was set in accordance to exCO₂. Measurements of exCO₂ reflected $P_{\mathrm{aC}{O}_2}$ with high correlation (R² = 0.8658). Bland-Altman analysis showed high agreement between the two measurements. Respiratory rate setting guided by exCO₂ (39 ± 2 or 40 ± 2 mmHg during ventilation with V_T of 7 or 6 mL kg^-1, respectively) was appropriate to maintain $P_{\mathrm{aC}{O}_2}$ (41 ± 2 mmHg for both V_T). Measurement of exCO₂ provides a robust estimate of $P_{\mathrm{aC}{O}_2}$ in anaesthetized small animals during mechanical ventilation. Continuous monitoring of CO₂ in the exhaled breath could be used to guide mechanical ventilation settings.