Intramuscular Electrical Activation of the Phrenic Nerve

In acute studies of 18 supine anesthetized dogs, the diaphragm was activated using intramuscular monopolar stimulating electrodes placed bilaterally within 2 cm of the entry point of the phrenic nerve. Inspiratory contraction was evoked using 10 to 40 Hz bursts of balanced charge biphasic stimulus pulses. The pulse width during a burst was ramped exponentially from 0 to 100 ¿s with a time constant between 50 and 250 ims. Evoked tidal volume and end inspiration transdiaphragmatic pressure saturated at depolarizing current amplitudes less than 16 mA for all stimulus frequencies. Inward collapse of the upper rib cage accounted for a loss in evoked tidal volume of 14 to 33 percent of the tidal volume possible with the rib cage held fixed. The mean maximal evoked tidal volume was 350 percent of the volume required for basal metabolic needs at 20 breaths per min. The projected maximum tidal volume without fatigue was at least 160 percent of the basal volume for rates of breathing between 10 and 30 breaths per min. A respiratory mechanical model was developed which suggests that the upper rib cage is decoupled from the lower rib cage and costal margin in the supine apneic dog.