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Objectives
Apply the concept of time constant to the physiology of mechanical ventilation.
Compare constant flow and descending ramp flow patterns during volume-controlled ventilation.
Describe the effect of respiratory mechanics on the airway pressure waveform during volume-controlled ventilation.
Describe the effect of resistance and compliance on flow during pressure-controlled ventilation.
Describe the effect of rise time adjustment during pressure-controlled and pressure support ventilation.
Describe the effect of termination flow during pressure support ventilation.
Discuss the role of sigh breaths during mechanical ventilation.
Discuss the physiologic effects of I:E ratio manipulations.
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Microprocessor-controlled ventilators allow the clinician to choose among various inspiratory flow waveforms. This chapter describes the technical and physiologic aspects of various inspiratory waveforms during mechanical ventilation.
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An important principle for understanding pulmonary mechanics during mechanical ventilation is that of the time constant. The time constant determines the rate of change in the volume of a lung unit that is passively inflated or deflated. It is expressed by the relationship:
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where Vt is the volume of a lung unit at time t, Vi is the initial volume of the lung unit, e is the base of the natural logarithm, and τ is the time constant. The relationship between Vt and τ is illustrated in Figure 9-1. Note that the volume change is nearly complete in five time constants.
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For respiratory physiology, τ is the product of resistance and compliance. Lung units with a higher resistance and/or a higher compliance will have a longer time constant and require more time to fill and to empty. Conversely, lung units with a lower resistance and/or compliance will have a shorter time constant and thus require less time to fill and to empty. A simple method to measure the expiratory time constant is to divide the expired tidal volume by the peak expiratory flow during passive positive pressure ventilation:
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where VT is the expired tidal volume and V̇e(peak) is the peak expiratory flow. Although this is a useful index of the global expiratory time constant, it treats the lung as a single compartment and thus does not account for time constant heterogeneity in the lungs.
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Volume-Controlled Ventilation
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The flow, pressure, and volume waveforms produced with a constant flow pattern are shown in Figure 9-2. This is often called square-wave or rectangular-wave ventilation due to the shape of the flow waveform. With the constant flow pattern, the volume (per ...