Patients with severe air-flow obstruction receiving mechanical ventilation are at risk of inadvertent pulmonary hyperinflation with morbidity and mortality caused by pneumothorax and circulatory depression. Nine patients with severe air-flow obstruction (5 asthma, 4 chronic air-flow obstruction) requiring mechanical ventilation were studied while sedated and therapeutically paralyzed. Pulmonary hyperinflation during steady-state ventilation was quantified by measuring total exhaled volume during 20- to 40-s apnea (end-inspiratory lung volume, VEI). Patients were studied at 3 levels of minute ventilation (E) (10, 16, and 26 L/min) and at each E, 3 levels of tidal volume (VT) (0.6, 1.0, and 1.6 L) and 3 levels of inspiratory flow (I) (40, 70, and 100 L/min for VT = 1.0 L). There were progressive increases in VEI when VT was increased or when expiratory time (TE) was decreased either by an increase in rate (and hence E) or by a decrease in I (at a constant E) reaching lung volumes as high as 3.6 ± 0.4 L above FRC. Alveolar, central venous, and esophageal pressure rose in parallel with lung volumes, and hypotension was seen in most patients at highest lung volumes. Peak airway pressure (Ppk) was predominantly related to inspiratory flow and did not reflect changes in lung volume. Levels of ventilation required for normocapnia prior to paralysis (15.7 ± 2.3 L/min) were associated with hypotension in 7 patients and probable hyperinflation in excess of 1.96 ± 0.17 L above FRC.
VEI is a simple, reproducible measurement of pulmonary hyperinflation and may be more important than Ppk in the causation of barotrauma. Small VT and long TE (i.e., low E and high I) were critical factors in reducing pulmonary hyperinflation, circulatory depression, and the potential for barotrauma.