The Science Journal of the American Association for Respiratory Care

2006 OPEN FORUM Abstracts

MAXIMAL INSPIRATORY PRESSURE AS AN INDICATOR FOR EARLY INITIATION OF NPPV IN ALS PATIENTS

Mario Gonzalez MD, Erika Simpson MD, Stanley Appel MD, Michael Bocci RRT,
Mary Barnes CRT, Peggy Ingels Allred PT, Brian Gaden RRT, Brian Timon RRT, Mickey Wells RRT, Ken D. Hargett, RRT,
The Methodist Hospital, The Methodist Neurological Institute-Weill Cornell Medical College, Houston, Texas 

Background: The early use of noninvasive positive pressure ventilation (NPPV) in Amyotrophic Lateral Sclerosis (ALS) patients is associated with improved quality of life and survival. Current American Association of Neurology and American College of Chest Physicians guidelines recommend initiation of NPPV when upright forced vital capacity (FVC) decreases below 50% of predicted. Medicare eligibility criteria for NPPV coverage includes documented sleep-associated hypoventilation and one of the following: FVC less than 50% or Maximal Inspiratory Pressure (MIP) less than 60 cmH2O. Measurement of MIP is thought to be useful and a more sensitive indicator of respiratory dysfunction and need for NPPV even in patients with FVC greater than 50% predicted. Our objective was to determine the prevalence of diaphragmatic dysfunction as measured by upright MIP in ALS patients with FVC's above 50% and to determine the relationship of MIP and FVC.

Methods: 152 patients with a diagnosis of definite or probable ALS were studied in our ALS clinic. Forced vital capacity and maximal inspiratory pressures were measured with standardized technique and the studies were interpreted by the clinic board certified pulmonologist. Disease disability was assessed with the Appel ALS rating scale at the time of testing. EMG studies of the diaphragm were performed for a sub-set of patients. Correlation analysis of study variables were performed with linear regression and Chi Square analysis.

Results: 126 patients had both FVC and MIP measured during the study period (39% females, 31% bulbar). The average AALS score was 81 with a range of 109 to 53 at the time of study and disease duration of 35 months. The mean FVC was 71% with a range of 97% to 45%. The mean MIP was 46cmH2O with a range of 72 to 20 cmH2O. FVC and MIP significantly correlated with the lower MIP associated with lower FVC (p<0.0001; r=0.63).  However, 61% of patients with FVC > 70% had MIP < 60 cmH2O. 98% of patients with FVC < 70% had MIP < 60 cmH20. Over all 26% of patients with a FVC of > 50% had a MIP < 60cmH2O.  93% of patients with a FVC of < 50% had a MIP < 60 cmH2O.

Conclusion: Respiratory dysfunction as measured by MIP is present in 25% of ALS patients who would not otherwise meet current "standard of care" guidelines for NPPV. Our data indicates that greater than 50% of the patients with a FVC > 70% had evidence of diaphragmatic dysfunction as measured by MIP < 60 cmH2O. MIP appears to be a more sensitive indicator of diaphragm weakness in ALS patients and provides the basis for earlier initiation of NPPV. Polysomnographic sleep studies should be performed to determine accurate pressure settings for NPPV. Additional studies should be performed to determine if earlier intervention based on these criteria result in improved quality of life and extended survival.

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