Chest
The Inspiratory Work of Breathing during Assisted Mechanical Ventilation
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
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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2017, Journal of Critical CareCitation Excerpt :Flow dyssynchrony refers to a mismatch between the inspiratory flow delivered by the ventilator and the flow demanded by the effort generated by the patient's respiratory muscles [1,2]. In most cases of flow dyssynchrony the delivered flow is lower than that demanded, which can result in increased breathing discomfort [3] and inspiratory work of breathing [4], particularly when high minute ventilation is demanded [5]. This type of dyssynchrony develops mainly during volume control ventilation [6], when the delivered flow is fixed, and tends to be more frequent when lower tidal volumes are set [7,8].
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2016, Clinics in Chest MedicineCitation Excerpt :Graphically, this is appreciated by the airway pressure-time waveform being sucked down during breath delivery.1 An estimate of this imposed loading can be calculated by comparing the difference between the area under the curve of the pressure-time tracings of the assisted or supported to a controlled breath.28,29 At no point is this dys-synchrony likely more apparent than in the setting of acute respiratory failure when inspiratory flow demands are high and can vary from breath to breath.
Manuscript received and accepted January 14.