INTERRUPTER MECHANICS OF PATIENTS ADMITTED TO A CHRONIC VENTILATOR DEPENDENCY UNIT

We analyzed the results of interrupter mechanics tests of 73 consecutive patients who were admitted to a chronic ventilator dependency unit. The purposes of this study were (1) to establish guidelines for grading the severity of airway obstruction based on interrupter mechanics measurements; (2) to estimate the prevalence of airway obstruction in this population; (3) to compare interrupter test results with the clinical assessment of airway function; and (4) to characterize the distribution of bronchodilator-induced changes in isorecoil flow. Measurements from 65 of 73 patients (89%) were considered technically adequate. Of 65 patients, 29 (45%) showed signs of flow limitation during passive expiration. All patients with severe obstruction documented with spirometry during clinical stability were flow-limited and achieved maximal flows less-than-or-equal-to 0.45 L/s at recoil pressures of 10 cm H2O. In contrast to measures of expiratory dynamics, the inspiratory resistance of the respiratory system was a poor index of severity of obstruction. In seven of 12 patients, interrupter testing failed to substantiate a clinical diagnosis of severe obstruction while revealing unsuspected obstruction in six of 25 (24%) patients. The administration of 270 mug of albuterol increased flows at comparable recoil pressures by greater-than-or-equal-to 0.1 L/s in 29% of 41 patients. Changes in flow were unimodally distributed and were not correlated with severity of obstruction. In summary, the routine use of the interrupter technique in ventilator-dependent patients is helpful in determining the severity and prevalence of airway obstruction, in identifying patients with otherwise unsuspected expiratory airflow limitation or those in whom this diagnosis is clinically suspected but cannot be substantiated, and in assessing the individual responses to bronchodilators.