The effect of level of contraction on the electromyographic power spectrum of the diaphragm in pigs.

We investigated the relationship between the frequency components of myoelectric power spectra of the diaphragm and the level of diaphragmatic contraction in seven anaesthetized spontaneously breathing pigs. Electromyographic activity of the costal and crural portions of the diaphragm were recorded with fish-hook electrodes and the frequency-power spectra during inspiration were computed and expressed in terms of centroid frequency (fc). Diaphragmatic force was indirectly assessed as transdiaphragmatic pressure (Pdi) which was measured with balloon-catheter systems placed in the abdomen and oesophagus. The relationships between Pdi and costal and crural fc were assessed during brief (2 min) and incremental increases in diaphragmatic contraction, achieved by gradual occlusion of the inspiratory line of the breathing circuit. When Pdi was increased to 128, 191, 287 and 421% of the value measured during unobstructed breathing, costal and crural fc rose significantly in all animals because of an increase in the power of high-frequency components and a decline in the power of low-frequency components. Both costal and crural fc returned to control values within 5 min of the release of inspiratory occlusion. Our results indicate that the level of contraction is an important determinant of the diaphragmatic myoelectric power spectrum and should be taken into consideration when using power spectral analysis to diagnose diaphragmatic mechanical failure.