MOUTH PRESSURE IN RESPONSE TO MAGNETIC STIMULATION OF THE PHRENIC NERVES

Background - Diaphragm strength can be assessed by the measurement of gastric (TW PGA), oesophageal (TW POES), and transdiaphragmatic (TW PD I) pressure in response to phrenic nerve stimulation. However, this re quires the passage of two balloon catheters. A less invasive method of assessing diaphragm contractility during stimulation of the phrenic n erves would be of clinical value. A study was undertaken to determine whether pressure measured at the mouth (TW PM) during magnetic stimula tion of the phrenic nerves accurately reflects TW POES, and to investi gate the relations between TW PM and TW PDI; and also to see whether g lottic closure and twitch potentiation can be avoided during these mea surements. Methods - Eight normal subjects and eight patients with sus pected respiratory muscle weakness without lung disease were studied. To prevent glottic closure magnetic stimulation of the phrenic nerves was performed at functional residual capacity during a gentle expirato ry effort against an occluded airway incorporating a small leak. TW PD I, TW POES, and TW PM were recorded. Care was taken to avoid potentiat ion of the diaphragm. Results - In normal subjects mean TW PM was 13.7 cm H2O (range 11.3-16.1) and TW POES was 13.3 cm H2O (range 10.4-15.9 ) with a mean (SD) difference of 0.4 (0.81) cm H2O. In patients mean T W PM was 9.1 cm H2O (range 0.5-18.2) and TW POES was 9.3 (range 0.7-18 .7) with a mean (SD) difference of -0.2 (0.84) cm H2O. The relation be tween TW PM and TW PDI was less close but was well described by a line ar function. In patients with diaphragm weakness (low sniff PDI) TW PM was <10 cm H2O. Conclusions - TW PM reliably reflects TW POES and can be used to predict TW PDI in normal subjects and patients without lun g disease. TW PM may therefore be a promising non-invasive, non-voliti onal technique for the assessment of diaphragm strength.