Influence of posture on mechanical parameters derived from respiratory impedance

The influence of posture on respiratory mechanics was investigated in 10 healthy volunteers, using the forced pseudorandom noise technique. Subjects were studied in four randomly selected positions: sitting; sitting with the head turned sideways; supine; and prone with the head turned sideways. Respiratory compliance (Crs), inertance (Irs) and resistance estimated at 4 Hz (R4), were calculated by fitting respiratory impedance by a 4 parameter model with a frequency dependent resistance. When subjects changed from sitting to lying, whilst maintaining the head in the same position in relation to the body, Irs increased and Crs decreased, probably due to gravitational effects. R4 was significantly higher in the supine position than in either of the sitting or the prone positions. These results demonstrate that changes in lung volume cannot completely explain the influence of posture on respiratory resistance, and indicate upper airway geometry as a determinant factor of respiratory resistance.