EFFECTS OF PROPORTIONAL ASSIST VENTILATION ON INSPIRATORY MUSCLE EFFORT IN PATIENTS WITH CHRONIC OBSTRUCTIVE PULMONARY-DISEASE AND ACUTE RESPIRATORY-FAILURE

Background: Acute respiratory failure may develop in patients with chr onic obstructive pulmonary disease because of intrinsic positive end-e xpiratory pressure (PEEPi) and increased resistive and elastic loads. Proportional assist ventilation is an experimental mode of partial ven tilatory support in which the ventilator generates flow to unload the resistive burden (flow assistance: FA) and volume to unload the elasti c burden (volume assistance: VA) proportionally to inspiratory muscle effort, and PEEPi can be counterbalanced by application of external PE EP. The authors assessed effects of propertional assist ventilation an d optimal ventilatory settings in patients with chronic obstructive pu lmonary disease and acute respiratory failure. Methods: Inspiratory mu scles and diaphragmatic efforts were evaluated by measurements of esop hageal, gastric, and transdiaphragmatic pressures. Minute ventilation and breathing patterns were evaluated by measuring airway pressure and flow. Measurements were performed during spontaneous breathing, conti nuous positive airway pressure, FA, FA+PEEP, VA, VA+PEEP, FA+VA, and E A+VA+PEEP. Results: FA+PEEP provided the greatest improvement in minut e ventilation (89 +/- 3%) and dyspnea (62 +/- 2%). The largest reducti on in pressure time product per breath of the respiratory muscles and diaphragm (44 +/- 3% and 33 +/- 2%, respectively) also was observed du ring FA+PEEP condition. When VA was added to this setting, a reduction in respiratory rate (50 +/- 3%), an increase in inspiratory time (102 +/- 6%), and a further reduction in pressure time product per minute (65 +/- 2% and 64% for the respiratory muscles and diaphragm, respecti vely) was observed. However, values of pressure time product per liter of minute ventilation during FA+VA+PEEP did not differ with those obs erved during FA+PEEP condition. Worsening of patient-ventilator intera ction and breathing asynchrony occurred when VA was implemented. Concl usions: Application of PEEP to counterbalance PEEPi and FA to unload t he resistive burden provided the optimal conditions in such patients. Ventilator over-assistance and patient-ventilator asynchrony was obser ved when VA was added to this setting. The clinical use of proportiona l assist ventilation should be based on continuous measurements of res piratory mechanics.