Measurements during mechanical ventilation

Constant-flow ventilation is provided by most intensive care unit ventilators when the machine breath is volume controlled. A constant volume is delivered for every time increment, and so the horizontal time axis becomes a linear analog for volume. It is important that there is no inspiratory pause and that the inspiratory waveform is a square wave. The shape of the inflation pressure-volume curve can be determined from a machine breath when the patient is relaxed. With care it is possible to make this measurement when the patient is not completely relaxed (Fig 1).

Fig. 1 An example of a sequence of pressure-time curves. Many are irregular, particularly when the patient is interacting with the ventilated breath. Only when the curve is close to the idealized shape (example 5) is it possible to measure the pulmonary mechanics.

As inspiration begins, pressure is required to overcome the flow resistance of the branching airways and the ventilatory apparatus. This can be seen as a sudden upstroke of pressure (Fig, 2(a)). At this point little gas has entered the chest. If the inspiratory flow is known or set by the ventilator, resistance at this point of the respiratory cycle can be estimated. As flow occurs, the volume of the chest increases (,Fj.9s..M2(b]...iand...i2(c)). Further pressure is required to overcome the expansion of the elastic components of the lung and chest wall. This portion of the waveform is the pressure-volume curve of the lung and chest wall. The gradient of the line is equal to the effective dynamic compliance, which is calculated as peat airway pressure — PEEP


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Fig. 2 Constituent parts of an individual pressure-time diagram: (a) initially little gas has entered the chest, no volume expansion of the lung has occurred, and the rapid pressure upstroke overcomes airway resistance; (b) as time progresses expansion of the lungs and chest wall occurs and the pressure curve increases; (c) at the end of the breath the pressure curve is linear, and the pressure-volume curve can be assessed from this line; (d) a completed breath.

This method of assessing the pressure-volume relationship has its limitations. Firstly, although pressure may be linearly related to volume over the volume range, this may not be the case for larger volumes where pressure may be markedly non-linear. In addition, the opening pressure of the system may change with disease, and it may not be accurately assessed as the initial upstroke of this waveform is related to the resistance of the system rather than the tissue elasticity which needs to be overcome to prevent airway closure.

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