Twitch pressures in the assessment of diaphragm weakness

To assess the value of phrenic nerve stimulation in the investigation of diaphragm function, transdiaphragmatic pressures were measured in 20 healthy subjects and in 15 patients with diaphragm weakness, during unilateral and bilateral transcutaneous phrenic nerve stimulation at 1 Hz at functional residual capacity (twitch Pdi). Diaphragm function was initially assessed by measuring transdiaphragmatic pressure during a voluntary manoeuvre, the maximal sniff (sniff Pdi); normal readings were confirmed in the control subjects (102-157 (normal greater than 98) cm H2O in the 10 men, 79-102 (normal greater than 70) cm H2O in the 10 women) and reduced values were found in the 15 patients with diaphragm weakness (7.5-90 cm H2O in the 13 men, 23 and 53 cm H2O in the two women). Twitch Pdi during bilateral phrenic nerve stimulation ranged from 8.8 to 33 cm H2O in the control subjects and from 3.1 to 27 cm H2O in the 10 patients in whom a measurement could be obtained. Bilateral twitch Pdi correlated with sniff Pdi both in the control subjects and in the patients with diaphragm weakness (r = 0.75). Only four patients had a bilateral twitch Pdi below the lowest value seen in the control group, including the three with the lowest sniff Pdi (3.1-8.5 cm H2O). These results indicate that transdiaphragmatic pressure recorded during bilateral phrenic nerve stimulation discriminated between control subjects and patients with known weakness of the diaphragm only when this was severe.