Respiratory tissue properties derived from flow transfer function in healthy humans.

Assuming homogeneity of alveolar pressure, the relationship between airway flow and flow at the chest during forced oscillation at the airway opening [flow transfer function (FTF)] is related to lung and chest wall tissue impedance (Zti): FTF = 1 + Zti/Zg, where Zg is alveolar gas impedance, which is inversely proportional to thoracic gas volume. By using a flow-type body plethysmograph to obtain flow rate at body surface, FTF has been measured at oscillation frequencies (f(os)) of 10, 20, 30 and 40 Hz in eight healthy subjects during both quiet and deep breathing. The data were corrected for the flow shunted through upper airway walls and analyzed in terms of tissue resistance (Rti) and effective elastance (Eti,eff) by using plethysmographically measured thoracic gas volume values. In most subjects, Rti was seen to decrease with increasing f(os) and Eti,eff to vary curvilinearly with f(os)2, which is suggestive of mechanical inhomogeneity. Rti presented a weak volume dependence during breathing, variable in sign according to f(os) and among subjects. In contrast, Eti,eff usually exhibited a U-shaped pattern with a minimum located a little above or below functional residual capacity and a steep increase with decreasing or increasing volume (30-80 hPa/l2) on either side. These variations are in excess of those expected from the sigmoid shape of the static pressure-volume curve and may reflect the effect of respiratory muscle activity. We conclude that FTF measurement is an interesting tool to study Rti and Eti,eff and that these parameters have probably different physiological determinants.