Chest
Volume 87, Issue 5, May 1985, Pages 612-618
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The Inspiratory Work of Breathing during Assisted Mechanical Ventilation

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We quantified the mechanical work of breathing in six normal subjects during assisted mechanical ventilation. Using two volume-cycled ventilators of different design, we investigated the influence of minute ventilation (VE) 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 VE. During assisted ventilation the subject expended energy equivalent to 33-50 percent of the work of passive inflation, even under the most favorable conditions of VE, sensitivity and flow. Under the least favorable conditions of VE, sensitivity and flow, the subjects 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.

Section snippets

Rationale of Method

The mechanical work performed in inflating the chest by a specified volume increment comprises two components: that done expanding the lung, and that done moving the chest wall through the same volume change. When the relaxed chest is inflated passively by a mechanical ventilator, each of these components can be calculated by integrating the instantaneous product of the rate of volume change (V) and the relevant trans-structural pressure:3 WTHX=t1t2(PAWPATM)V˙dt=WL+WCWWL=t1t2(PAWPES)V˙

RESULTS

Airway resistance and dynamic compliance of the thorax, lungs, and chest (Table 1) were within the ranges expected for supine normal subjects.8, 11, 12, 13 Ventilation during moderate and high levels of ventilation averaged 12.3 ±0.2 L/min and 23.5 ± 0.3 L/min respectively, across all conditions. Tidal volume (VT) corrected for system compliance ranged among individuals from 650-1020 ml. At 12 L/min, VT averaged 827.5 ± 122.1 ml (assist) and 826.7 ± 127.4 ml (control, NS, p >.05). At 24 L/min

DISCUSSION

These results demonstrate that significant work was performed by the subject during assisted mechanical ventilation and that energy expenditure increased when gas delivery from the machine was insufficient to match the subjects ventilatory demand. Minute ventilation, assist sensitivity level, and gas flow rate were important interactive determinants of the subjects workload. Reducing the flow setting or trigger sensitivity increased the active component of respiratory work, particularly at high

ACKNOWLEDGMENT

The authors thank Jack Hildebrandt, Ph.D. for helpful suggestions and thoughtful review of this work; Douglas Williams for expert technical assistance; and Brenda Plunkett for preparation of the typescript.

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    Manuscript received and accepted January 14.

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