Effects of diaphragm shortening on the mean action potential conduction velocity in canines.

1. The present study was designed to test if the mean muscle fibre action potential conduction velocity (VAPC) in the costal diaphragm changes with muscle length, in spontaneously breathing mongrel dogs. 2. VAPC was determined by the electromyogram (EMG) power spectrum 'dip' method, which is based on the bipolar electrode transfer function. A bipolar EMG electrode with a 20 mm fixed interelectrode distance was sutured to the costal diaphragm in the fibre direction, and in a region with a low density of motor endplates. Diaphragm length was measured with piezoelectric crystals positioned next to the EMG electrode. Seven dogs were vagotomized and spinally anaesthetized in order to increase diaphragmatic shortening, reduce velocity of shortening and abolish possible cross-talk signals from adjacent muscles. 3. Our results showed that VAPC in the canine costal diaphragm was 3.4 m s-1 and was not significantly related to diaphragmatic shortening.