Optimum stimulus frequency for contracting the inspiratory muscles with chest-surface electrodes to produce artificial respiration.

Electroventilation is the term used to describe the production of inspiration by applying a train of short-duration pulses to chest-surface electrodes. Studies were conducted in the dog to determine the optimum frequency to produce a smooth air flow in the trachea. It was found that a stimulus frequency of 25/sec or slightly higher meets this criterion. To illustrate that electroventilation can be carried out for a prolonged period using these parameters, electroventilation was applied continuously for 5 hours in a dog without a noticeable change in blood pressure.

The safety factor for electroventilation measured by production of cardiac ectopy in the anesthetized dog.

The safety factor of electroventilation (ie, the ratio of the current required to produce an ectopic beat to the current required to produce an inspired volume of 225 ml, which is approximately twice tidal volume) was determined in 12 pentobarbital-anesthetized dogs using transthoracic electrodes positioned at the optimal electroventilation site. The optimal stimulation site for electroventilation was first determined using hand-held, stimulating electrodes. Then electrodes, 4.1 cm in diameter, were sutured bilaterally to the optimal stimulation site. The relationship between inspired volume and stimulus intensity was determined using a 0.8-s burst of stimuli (60/s) with a pulse duration of 0.1 ms. Using the same electrodes, the threshold current for producing ectopic beats was determined for single pulses ranging from 0.1 to 10 ms duration. In all dogs, the current required to produce an ectopic beat increased greatly as the pulse duration decreased. At 0.1 ms, the safety factor for electroventilation was calculated to be 25.8.

Electroventilation in the baboon.

The optimum sites for chest-surface electrodes and the relationship between the inspired volume of air and stimulus intensity were determined in six pentobarbital-anesthetized baboons. The optimum stimulation sites were identified using hand-held, trans-chest stimulating electrodes. Thereafter, conducting adhesive electrodes 4 cm in diameter were placed over each of the optimal sites. The relationship between inspired volume and stimulus intensity was determined with the forelimbs restrained and unrestrained using an 800-msec burst of 60/sec stimuli with a pulse duration of 20 microseconds. Two optimum stimulation sites were identified, one in the anterior axillary region, and another just lateral to the xiphoid process. The maximum volume of inspired air was obtained when the stimulating electrodes were placed at the anterior axillary sites. With the forelimbs restrained or unrestrained, an increased stimulus intensity resulted in an inspired volume well in excess of the spontaneous tidal volume of the animal. These studies indicate that, with chest-surface electrodes at the optimal locations, electroventilation can produce inspired volumes greater than spontaneous tidal volume in anesthetized baboons. With electrodes in the xiphoid region a smaller maximum inspired volume was obtained because of the stimulation of expiratory (abdominal) muscles.