Influence of gas temperature on the performances of a low dead space capillary type pneumotachograph for neonatal ventilation

The design and calibration of a pneumotachograph with capillary type resistance is here described. The pneumotacograph has been designed aimed to the measurement of flow rate in the neonatal ventilation range (±10 L/min) and is characterized by a low dead space (2mL). The calibration curve is quadratic and coefficient values for Rohrer equation have been obtained by fitting experimental data (R2=0.99, MSE=1Pa2). Sensitivity varies from about 25 Pa·L-1·min for flow rates lower than 4 L/min to about 58 Pa•L -1•min for flow rates higher than 7 L/min. The influence of airflow temperature on Rohrer equation coefficients has then been analyzed. A gas temperature variation in the range 19-37°C corresponds to a 10% average output percent variation, being the discrepancy higher at higher flow rates. A linear dependence of Rohrer equation second order term coefficient from temperature has been hypothesized. By fitting experimental data with the proposed equation MSE decreases from 1Pa2 to 0.3Pa2 thus, increasing repeatability (<2%) in the overall flow rate and temperature range considered. The second order term coefficient in Rohrer equation increases with temperature of about 0.6%/°C. Rohrer equation, corrected for gas temperature, allows then to increase the repeatability of the here proposed capillary type pneumotacograph, while maintaining a good sensitivity with low dead space.