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Respiratory Failure & Mechanical Ventilation

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The adequacy of flow during volume-controlled ventilation can be evaluated with the pressure-time waveform. Since the total work performed during the breath is the sum of the patient work and the ventilator work, we can evaluate the relative contributions of both by comparing the shapes of the pressure-time waveform during 2 different conditions: completely passive breathing, during which the waveform has a defined pattern based on the type of flow (constant-flow, descending-ramp, or sinusoidal) and patient-triggered breathing, during which the additional patient effort “dishes out” (ie, makes concave) the pressure waveform, relative to the amount of patient work performed. The hatched area illustrates the pressure-time product and represents the effort the patient contributed to the delivery of the breath.

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