Respiratory CO2 response depends on plasma bicarbonate concentration in mechanically ventilated patients

Abstract Objective There is controversy about the effects of high plasma bicarbonate concentration ([HCO3 − ]) and the CO 2 response test. We analyzed the relationship between [HCO3 − ] and the variation in hydrogen ion concentration (pH) for a given change in PaCO 2 , and its effects upon CO 2 response. Design A retrospective study was carried out. Setting Two intensive care units. Patients Subjects with and without chronic obstructive pulmonary disease (COPD), at the beginning of weaning from mechanical ventilation. Interventions The CO 2 response was evaluated by the re-inhalation of expired air method, measuring the hypercapnic ventilatory response (ΔV E /ΔPaCO 2 ) and hypercapnic drive response (ΔP 01 /ΔPaCO 2 ), where V E is minute volume and P 0.1 is airway occlusion pressure 0.1 s after the initiation of inspiration. Main outcome measures [HCO3 − ] and CO 2 response. Results A total of 120 patients in the non-COPD group and 48 in the COPD group were studied. COPD patients had higher mean [HCO3 − ] than non-COPD patients (33.2 ± 5.4 vs. 25.7 ± 3.7 mmol/l, p − ] and pH change per mmHg of PaCO 2 ( p E /ΔPaCO 2 ( p 0.1 /ΔPaCO 2 ( p Conclusions There is an inverse linear relationship between [HCO3 − ] and the variation of pH for a given change in PaCO 2 and the CO 2 response.