Assessment of diaphragm strength and activation by transvenous phrenic nerve stimulation

Summary The purpose of the present work is to report the feasibility, humaneness and reliability of a method for measurement of diaphragm performance in intact, awake horses. Phrenic nerves were activated through the vena cava by repeated transvenous stimulations (40 to 80 V, 100 to 200 μs square-wave pulses), using an electrode-catheter advanced from the jugular vein. Twitches were produced that caused changes in gastric and oesophageal pressures, abdominal and rib cage motion and airflow tracings, suggesting that the diaphragm was activated. Four sites were selected over each hemithorax for placement of surface electrodes able to detect and to ensure constancy of evoked compound diaphragmatic action potentials (CDAP). Trains of repeated pulses applied to nerves (10 to 100 Hz) gave repeatable diaphragmatic force-frequency relationships. Nerve stimulation produced no visible signs of discomfort and increases in Cortisol and β-endorphins similar to those produced by nasogastric intubation. Transvenous single and repeated activations of the phrenic nerves in ponies is feasible, free of iatrogenic risks and ethically acceptable, recording reliable evoked potentials via surface electrodes is possible, and force-frequency characteristics of the diaphragm can be conveniently obtained from simultaneous oesophageal and gastric pressures measurements.