Usefulness of pulmonary function tests and blood gases in acute neuromuscular respiratory failure

Background and purpose: Define the usefulness of pulmonary function tests (PFT) and arterial blood gases (ABG) in patients admitted to the ICU with acute neuromuscular respiratory failure (NMRF). Methods: We reviewed 76 patients admitted to an ICU at Mayo Clinic (Rochester) between 2003 and 2009 with acute NMRF defined as need for mechanical ventilation (MV) because of primary impairment of the peripheral nervous system. Poor functional outcome was defined as a modified Rankin score >3. Results: Median age was 65years. The most frequent diagnosis was myasthenia gravis (25 patients); 54% of patients had no known neuromuscular diagnosis before admission, and 11% had no specific diagnosis at discharge. Median MV time was 16days; 14% of patients died during hospitalization, and 63% were severely disabled at discharge. Maximal expiratory pressure ≤30cm H ₂O and maximal inspiratory pressure (MIP) worse than -28cm H ₂O before MV were associated with need for invasive MV for longer than 7days (P=0.02). Indicators of chronic respiratory acidosis (low pH, high pCO ₂, and high HCO ₃) before MV were associated with in-hospital death and poor functional outcome, but mostly in patients with progressive, untreatable neuromuscular diagnoses. Conclusions: In patients with primary acute NMRF, bedside PFT and ABG before MV can be used to predict evolution and outcome. Lower MIP and MEP portend prolonged MV and are more useful than forced vital capacity. Presentation with chronic respiratory acidosis is associated with high risk of in-hospital mortality and severe disability, especially in patients without treatable diagnoses.