A control system for automatic electrical stimulation of abdominal muscles to assist respiratory function in tetraplegia

People with tetraplegia have poor respiratory function leading to limited tidal volume (VT) and reduced cough peak flow (CPF). These problems may cause respiratory failure during the initial admission or subsequent intercurrent illness. Electrical stimulation of the abdominal muscles during expiration can improve respiratory function by increasing VT and CPF. We developed a novel control system to automatically trigger muscle stimulation, synchronised with the subject's voluntary respiratory activity. The system was tested in four subjects with a functionally complete lesion at level C4 to C6, aged between 16 and 46 years, 3 months to 5 years post injury, who were breathing spontaneously. The algorithm delivered automatic stimulation patterns, detecting cough and quiet breathing while suppressing stimulation during other activities such as speaking. Marked increases in VT (between 9% and 71% of baseline) and CPF (between 31% and 54% of baseline) were observed, suggesting that the technique may have potential use in both acute and established tetraplegia to increase minute ventilation and to improve cough clearance of secretions.