Respiratory related evoked responses to graduated pressure pulses using wavelet transform methods

In this study, the respiratory related evoked responses (RREPs) from ten normal subjects in response to brief varying pressure pulses at -6, - 10 and - 17 cm H2O with a duration of 200 ms were recorded to investigate how midlatency cortical evoked potentials measured on the scalp are affected in response to pressure pulses of varying magnitude at the mouth. Wavelet decomposition was performed for eight frequency scales in time for the RREPs. The RREPs at each wavelet scale were enhanced by eliminating the wavelet coefficients due to the artifacts and noise. After denoising, the third (125-250 Hz), fourth (62.5-125 Hz), fifth (31.25-62.5 Hz) and sixth (15.62-31.25 Hz) wavelet scales were quantified using the global field power estimates which serve to reduce the contamination by facial electromyogram responses evoked by the pressure stimulus. Our results show that the estimates of the global field power (GFP) at the third, fourth and fifth wavelet scales between 25 and 100 ms poststimulus were significantly increased when the pressure pulse was increased from -6 to -17cm H2O. On average, the total GFP from all scales, summed over the period 30-90 ms poststimulus, doubled from baseline with the -6 cm H2O stimulus, and increased linearly by 40% between -6 and -17 H2O. This supports the use of the GFP as an index of respiratory mechanoreceptor input to the central nervous system.