Phrenic nerve stimulation is possible only with normal nerves and a sufficient amount of normal muscle.
1. Normal nerve
Nerve function is determined electroneurophysiologically (ENMG study) by measuring the conduction time or latency, i.e. the time the pulse travels along the nerve from the location it is launched to the muscle. When studying the phrenic nerve, the electric pulse to the nerve is applied in the neck to the nerve at about the level of the larynx and the response, a muscle twitch, is picked up on the side of the thorax below the 8th rib by electromyogram electrodes glued to the skin. Conduction time is longer in hurt nerves than in normal nerves. No conduction means no nerve function.
2. Sufficient amount of normal muscle
One can imagine that, by chance, the stimulating and the pick-up electrode touch the same and only axon; the result would be a normal latency. Therefore, the compound muscle action potential (CMAP) is registered; in the case of one axon it would be zero. However, the CMAP can be misleading large without sufficient amount of muscle mass in the diaphragm. Therefore, it is strongly recommended to check the contraction of the diaphragm when the phrenic nerve is stimulated supramaximally. There should be a brisk ipsilateral hemidiaphragmatic contraction present, which is visible and grossly palpable on physical examination.