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 (Ptp) above 15 cmH2O in the nonedematous lung and above 25 cmH2O Ptp 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 cmH2O Ptp to 342 +/- 41 cm/sec at 26 cmH2O Ptp, consistent with shear waves propagating in the lung having a shear modulus of 0.9 Ptp and lung density of 0.20 g/cm3. Stress wave velocities at 25 cmH2O Ptp 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.