Bedside estimation of the inspiratory work of breathing during mechanical ventilation

The work of chest inflation, WI, is a primary determinant of the need for ventilatory support and an integrative index of elastic and resistive impedance. Although the mechanical work performed by a ventilator in moving gas into the passive chest (WI = integral of PV dt) can be determined by measuring the area enclosed by a display of airway pressure (P) against delivered volume (V), the instrumentation required is not routinely available at the bedside. Under conditions of constant flow, however, inspiratory time represents an analog of delivered volume, and airway pressure can be recorded easily by equipment normally employed to monitor pulmonary vascular pressures. We reasoned that the area beneath the airway pressure vs time tracing should accurately reflect WI for unassisted breaths delivered by the ventilator at constant flow. We computed estimates of WI from simultaneous pressure-volume (PV) and pressure-time (PT) plots during square-wave inflation in 20 acutely ill patients. Ventilator settings were varied over the usual clinical range for tidal volume (10 to 15 ml/kg) and inspiratory flow (40 to 80 L/min). PV and PT estimates agreed closely; across the four setting combinations tested, the difference between PV and PT estimates averaged 2.4 +/- 5.6 percent (means +/- SD, r = 0.99). Furthermore, the reproducible geometric configuration of the curves generated allowed accurate estimation of WI from routine beside observations of tidal volume and peak dynamic and static inflation pressures, without the need for specialized equipment or area measurement. Such simplified estimates could serve in clinical practice to gauge the ventilatory workload and to monitor changes in respiratory impedance.