[Application of capnography and SpO2 measurement in the evaluation of respiratory failure in patients with chronic obstructive pulmonary disease].

OBJECTIVE: To investigate the accuracy and potential error range of noninvasive estimation of CO2 pressure (PCO2), arterial O2 pressure (PaO2), and oxygenation index (OI) by measuring the end-tidal CO2 pressure (PETCO2) and pulse oxygen saturation (SpO2) in patients with chronic obstructive pulmonary disease (COPD) and respiratory failure, and assess the feasibility of this method for dynamic monitoring of arterial CO2 pressure (PaCO2) and PaO2 in the primary care facilities where arterial blood gases analysis is not available.

METHODS: All the 30 patients with COPD and respiratory failure received routine clinical treatment including bronchodilators, mucolytics, glucocorticosteroid, antibiotics and oxygen therapy (titrated to keep SpO2 above 90%) for 5-7 days. A subgroup of the patients also received NIPPV treatment. All the patients were tested with both the eupnea method and prolonged expiratory method before and after the treatment to obtain the data of PCO2 and SpO2 were respectively performed before and after treatment.

RESULTS: The PETCO2 with eupnea (PETCO2(Q)) was 50.72-/+8.93 mmHg, significantly lower than PaCO2 (71.25-/+9.08 mmHg, Plt;0.01), but the PETCO2(P) (70.35-/+8.91 mmHg) was comparable with PaCO2 (P>0.05). Similar results were obtained after the treatment. The PETCO2(P) before treatment and after treatment was positively correlated to PaCO2 (r=0.96 and 0.97, respectively, P<0.01). The PaO2(Y) before the treatment derived from the oxygen dissociation curve based on SpO2 measurement was close to SpO2 (59.96-/+1.42 mmHg vs 59.07-/+2.22 mmHg, P>0.05). The OI derived from PaO2 and OI(Y) from PaO2 (Y) was also similar (215.70-/+22.77 vs 219.15-/+24.63, P>0.05). Linear regression analysis showed positive correlations between PaO2(Y) to PaO2 (r=0.81, P<0.01) and between OI(Y) and OI (r=0.95, P<0.01).

CONCLUSIONS: In patients with COPD (especially those with also type II respiratory failure), the modified monitoring method of PCO2 and maintenance of SpO2 above 90% can precisely estimate PaCO2 and PaO2. This method is feasible for clinical noninvasive and dynamic evaluation of respiratory failure in COPD patients, especially in primary care facilities where arterial blood gases analysis is not available.