The inspiratory work of breathing during assisted mechanical ventilation

We quantified the mechanical work of breathing in 6 normal subjects during assisted mechanical ventilation. Using two volume-cycled ventilators of different design, we investigated the influence of minute ventilation (V̇E) and machine settings of trigger sensitivity and flow during CO₂-driven hyperventilation to moderate and high levels (12-24 L/min). Work estimates were derived from plots of esophageal and airway pressure against inflation volume. Peak flow and trigger sensitivity were important determinants of the energy expended, and for each combination of machine settings the work done by the subject per liter of ventilation increased with V̇E. During assisted ventilation the subject expended energy equivalent to 33-50% of the work of passive inflation, even under the most favorable conditions of V̇E, sensitivity and flow. Under the least favorable conditions of V̇E, sensitivity and flow, the subject's inspiratory work of breathing substantially exceeded the energy needed by the ventilator to inflate the passive thorax. These observations imply that exertion of the respiratory muscles continues throughout inflation during assisted mechanical ventilation and call attention to the possibility that inappropriate selection of ventilatory mode or machine settings may contribute to respiratory muscle fatigue and dyspnea.