Answer: Calculating pulmonary function score ratio (PFS ratio) in patients with Guillain-Barré syndrome may guide into the time of tracheostomy as it predicts the length of intubation.
PFS = VC (in mL/kg) + MIP (in cm H2O) + MEP (in cm H2O)
VC = vital capacity
MIP = maximal inspiratory pressure
MEP = maximal expiratory pressure.
The PFS ratio: is obtained by dividing PFS immediately prior to intubation and PFS on the 12th day of mechanical ventilation.
A PFS ratio < 1 predicts the need for mechanical ventilation for more than 3 weeks with the specificity of 100 percent.
Lawn ND, Wijdicks EF. Post-intubation pulmonary function test in Guillain-Barré syndrome. Muscle Nerve 2000; 23:613.