[Polarity of crackle waveforms: a new index for crackle differentiation]

There are three kinds of crackling sounds on chest auscultation, that is, fine crackle, coarse crackle and pleural rub. The trouble in clinical practice is that pleural rub is sometimes indistinguishable from pulmonary crackles. I have found definite rules for the polarity of the initial deflection of the waveforms (crackle polarity: CP) of these sounds. I have investigated CP distribution in 12 patients with fine crackle, 17 with coarse crackle and 8 with pleural rub. Electrical polarity was so adjusted that a positive pressure change results in a positive deflection on the recording paper. CP was positive in 93.3 +/- 6.4% of fine crackle (all on inspiration). In coarse crackle, 91.8 +/- 5.8% of CP was positive on inspiration and 93.8 +/- 10.3% was negative on expiration. In contrast to the homogeneous CP distribution of these crackles, that of pleural rub showed a remarkable heterogeneity. In six of pleural rub cases, predominant CP was positive on inspiration and negative on expiration, but this was in the reverse in the rest of two cases. A few centimeter shift of recording sites resulted in moderate to remarkable change in CP distribution in pleural rub cases, but no positional variety was observed in that of pulmonary crackles. These results show that CP distribution of pulmonary crackles may be explained with dipole stress distribution models and that of pleural rub with quadrupole one. These results suggest that the heterogeneous CP distribution with positional variety is a distinct phonopneumographic feature of pleural rub and that CP analysis is a useful method for basic and clinical study on crackling lung sounds because CP distribution reflects the generation and transmission characteristics of these sounds.