AIR-FLOW EFFECTS ON AMPLITUDE AND SPECTRAL CONTENT OF NORMAL BREATH SOUNDS

Even though it is well known that breath-sound amplitude (BSA) increas es with airflow, the exact quantitative relationships and their distri bution within the relevant frequency range have not yet been determine d. To evaluate these relationships, the spectral content of tracheal a nd chest wall breath sounds was measured during breath hold, inspirati on, and expiration in six normal men. Average spectra were measured at six flow rates from 0.5 to 3.0 l/s. The areas under the spectral curv es of the breath sounds minus the corresponding areas under the breath -hold spectra (BSA) were found to have power relationships with flow ( F), best modeled as BSA = k . F-alpha, where k and a are constants. Th e overall mean +/- SD value of the power (a) was 1.66 +/- 0.35, signif icantly less than the previously reported second power. Isoflow inspir atory chest wall sound amplitudes were 1.99 +/- 0.70- to 2.43 +/- 0.65 -fold larger than the amplitudes of the corresponding expiratory sound s, whereas tracheal sound amplitudes were not dependent on respiratory phase. Isoflow breath sounds from the left posterior base:were 32% lo uder than those from the right lung base (P < 0.01). BSA-F relationshi ps were not frequency dependent during expiration but were significant ly stronger in higher than in lower frequencies during inspiration ove r both posterior bases. These data are compatible with sound generatio n by turbulent flow in a bifurcating network with 1) flow separation, 2) downstream movement of eddies, and 3) collision of fast-moving core s of the inflowing air with carinas, all occurring during inspiration but not during expiration.