Resistance to respiratory airflow of the nasal passages: comparisons between different common methods of calculation.

Computer assisted active posterior rhinomanometric determinations of resistance were made with four adult subjects. A face mask and pneumotach were used to measure respiratory airflow. The magnitude and variation of six different instantaneous and time averaged methods of calculation of resistance resulting from simultaneous measurements were compared. Over a resistance range of 1-6 cm H2O/l/sec (0.1-0.6 Pa/cm3/sec) time averaged results approximated those computed at 75 Pa and were 20-25% less than those at 150 Pa. Over the same range of nasal patencies, the coefficients of variation averaged 6-8% in 144 series of 10 measurements which were obtained from six modes of resistance computation in four subjects (total 1440). Time averaged results showed the least variation. A frequency range of 10-26 breaths/min increased the coefficient of variation only to 9% and a ventilation range of 7-24 l/min increased it to 11%, quantitative relationships between resistances and pattern of breathing were not evident. Mask positioning was critical, small maladjustments resulted in large resistive changes.