Effect of gas density on respiratory sounds

The generation of normal lung sounds by turbulent air flow has been questioned because gas density appears to have only a minor effect. We studied whether gas density has a greater influence on lung sounds at higher frequencies than traditionally measured. Six healthy adult men breathed air followed by a mixture of 80% helium and 20% oxygen (He-O2) at a target flow of 1.5 L/s. Flow and sounds at the trachea and posterior right lower lobe were simultaneously acquired by computer. Fourier analysis was applied to sounds at target flow +/- 0.2 L/s. Average power spectra were computed for each recording site, respiratory phase, and respired gas. He-O2 reduced the power of inspiratory lung sounds below 300 Hz by only 1.7 +/- 1.5 dB whereas power between 300 and 600 Hz was reduced by 4.6 +/- 1.4 dB (p<0.05). Tracheal sound power was reduced less consistently but all subjects showed an upward frequency shift in power maxima on He-O2, similar to formant shifts observed in helium speech. Our findings suggest that flow turbulence is the major determinant of lung sounds at higher frequencies. Current instruments for auscultation and recording of respiratory sounds may have to be modified to optimize their response in this higher frequency range.