Artificial Ventilation by Means of Electrical Activation of the Intercostal/Accessory Muscles Alone in Anesthetized Dogs

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Abstract
We have previously shown that large inspired volumes can be achieved in phrenicotomized animals by intercostal/accessory muscle activation via spinal cord stimulation. In the present study, we evaluated the utility of this technique to provide complete ventilatory support for prolonged time periods (6 to 8 h, selected arbitrarily). In 10 deeply anesthetized dogs, a single electrode was introduced onto the epidural surface of the spinal cord and positioned at the T2-T3 spinal level. Bilateral phrenicotomy was performed in all animals to prevent possible diaphragm activation. The spinal cord was rhythmically stimulated approximately 13 times/min with trains of 15- to 20-Hz impulses of sufficient amplitude to achieve inspired volumes of 13 to 15 ml/kg and pressure-time index (product of duty cycle and .DELTA.P/Pmax) of < 0.15 with each contraction. Level of alveolar ventilation was monitored by end-tidal PCO2 and intermittent arterial blood gas measurements. Mean inspired volume and minute ventilation were 236 .+-. 7.84 (SE) ml and 3.12 .+-. 0.13 (SE) L/min, respectively, and not significantly different between the first and sixth hours of continuous stimulation. Mean duty cycle (Ti/Ttot) was 0.26 .+-. 0.01. Mean airway pressure (.DELTA.P) during prolonged electrical stimulation under conditions of airway occlusion was 8.05 .+-. 0.61 (SE) cm H2O. Mean ratio of .DELTA.P/Pmax was 0.47 .+-. 0.03 (SE) cm H2O; mean pressure-time index was 0.12 .+-. 0.01 (SE). There was no evidence of system fatigue, as evidenced by the lack of any significant shift in the pressure frequency curve over a 6-h time period. Mean PaCO2 was 39 mm Hg at the initiation of prolonged intercostal muscle activation and remained stable throughout the stimulation period. End-tidal PCO2 could be reduced to 25 to 30 mm Hg for 15- to 30-min periods by increasing stimulus parameters. We conclude that artificial ventilation can be maintained by electrical activation of the intercostal/accessory muscles alone and that this technique may provide a means of respiratory muscle pacing in selected quadriplegic patients.