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 (plusmn10 L/min) and is characterized by a low dead space (2 mL). The calibration curve is quadratic and coefficient values for Rohrer equation have been obtained by fitting experimental data (R²=0.99, MSE=1 Pa²). Sensitivity varies from about 25 PamiddotL^-1middotmin for flow rates lower than 4 L/min to about 58 PamiddotL^-1middotmin 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 19-37degC 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 1 Pa² to 0.3 Pa² 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%/degC. 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.