BACKGROUND: To evaluate the end tidal carbon dioxide estimation in nonintubated, spontaneously breathing patients using either conventional sidestream or microstream capnometers. METHODS: Patients received a regional anesthesia technique, while the end tidal carbon dioxide partial pressure (EtCO2) was sampled through a nasal cannula (Nasal FilterLine, Nellcor, Plesanton, CA, USA) and measured using either a conventional sidestream capnometer with a 200 ml.min-1 aspiration flow rate, or a microstream capnometer (NBP-75, Nellcor Puritan Bennett, Plesanton, CA, USA) with an aspiration flow rate of 30 ml.min-1. After a 20 min period with stable hemodynamic variables (systolic arterial blood pressure within +/- 20% from baseline values), the EtCO2 was randomly recorded using one of the two capnometer while arterial blood was simultaneously drawn from the radial artery and analyzed for measurement of arterial CO2 partial pressure. Afterwards the nasal cannula was connected to the other capnometer and the procedure repeated. Both the capnometer and arterial blood gas analyzer were calibrated before each studied patient according to the manufacturer instructions. The same procedure was repeated at least two times in each patient. RESULTS: A total of 120 pairs of EtCO2 and PaCO2 measurements were drawn from 30 adults (age: 69 +/- 5 years; weight: 70 +/- 10 kg; height: 160 +/- 10 cm): 60 using the conventional sidestream capnometer and 60 with the microstream one. The median arterial to end tidal CO2 tension difference was 4.4 mmHg (range: 0.28 mmHg) with the microstream capnometer and 7 mm Hg (range: 0-22 mmHg) with the conventional capnometer (p = 0.02). CONCLUSION: The microstream capnometer provides a more accurate end tidal CO2 partial pressure measurement in nonintubated, spontaneously breathing patients than conventional sidestream capnometers, allowing for adequate monitoring of the respiratory function in nonintubated patients.
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