Phrenic Nerve Stimulation
Magnetic stimulation can be used for bilateral or unilateral phrenic nerve stimulation. Its advantages over electrical stimulation are ease of positioning, repeatability and much improved patient tolerance. It is used reliably to assess the function of respiratory muscles by measuring the diaphragmatic response and strength. The ease of application and patient comfort makes it possible to also carry out longitudinal monitoring studies. Magnetic stimulation can be used to obtain responses even when the phrenic nerve cannot be located using conventional electrical stimulation.
Mills et al (1995) looked at the ability of four different magnetic coils to produce unilateral phrenic stimulation in five normal subjects, assessed from twitch transdiaphragmatic pressure (TwPDI) measurements and diaphragmatic electromyogram (EMG) recordings. They found that unilateral magnetic phrenic nerve stimulation is easy to apply and is a reproducible technique in the assessment of hemidiaphragm contractility. It is well tolerated and allows hemidiaphragm contractility to be rapidly and reliably assessed because precise positioning of the coils is not necessary. This may be particularly useful in patients. In addition, the anterolateral positioning of the coil allows the use of the magnet in the supine patient such as in the operating theatre or intensive care unit (Mills et al. 1995).
Twitch Pdi measurement may be used to confirm or refute the diagnosis of bilateral or unilateral diaphragm weakness. In patients with neuromuscular disease Twitch Pdi may permit accurate clinical assessment of disease progress. Ongoing developments include the adaptation of magnetic stimulation for use with neonates or children. Additionally, by measuring pressure changes in the mouth (Twitch Pmo) it is now possible to obtain a non-volitional measure of diaphragm strength.
- Mills et al., Thorax, 1995.