Generation and detection of lung stress waves from the chest surface

In anesthetized pigs, we generated stress waves by imposing a distortion on the intercostal muscle between the 5th and 6th ribs. Stress waves were detected by two accelerometers, 5-7 cm apart, oriented in either the ventral-dorsal or cranial-caudal direction. Cross-spectral analysis was used to calculate transit time. Waves of velocities similar to those of lung shear waves were detected at transpulmonary pressures (P-tp) above 15 cmH(2)O in the nonedematous lung and above 25 cmH(2)O P-tp in the edematous lung. Waves were detected in the frequency range 9-40 Hz. Stress wave velocity increased from 287 +/- 24 (SD) cm/sec at 18 cmH(2)O P-tp to 342 +/- 41 cm/sec at 26 cmH(2)O P-tp, consistent with shear waves propagating in the lung having a shear modulus of 0.9 P-tp and lung density of 0.20 g/cm(3). Stress wave velocities at 25 cmH(2)O P-tp decreased with the increases in lung density induced by alveolar edema, consistent with elasticity theory. An elasticity analysis showed the existence of lung-rib cage interfacial waves with properties similar to the measured stress waves. (C) 1997 Elsevier Science B.V.