Apply the concept of time constant to the physiology of mechanical ventilation.
Compare constant flow and descending ramp flow patterns during volume-controlled ventilation (VCV).
Describe the effect of respiratory mechanics on the airway pressure waveform during VCV.
Describe the effect of resistance and compliance on flow during pressure-controlled ventilation (PCV).
Describe the effect of rise time adjustment during PCV and pressure support ventilation (PSV).
Describe the effect of termination flow during PSV.
Discuss the role of sigh breaths during mechanical ventilation.
Discuss the physiologic effects of I:E manipulations.
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.
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:
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.
The time constant function for lung emptying. After one time constant, 37% of the volume remains in the lungs, 13% remains after two time constants, 5% remains after three time constants, 2% remains after four time constants, and < 1% remains after five time constants.
For respiratory physiology, τ is the product of resistance and compliance. Lung units with a high resistance and/or a high compliance will have a longer time constant and require more time to fill and to empty. Conversely, lung units with a low resistance and/or a low compliance will have a shorter time constant and thus require less time to fill and to empty. A method to measure the expiratory time constant during mechanical ventilation is to divide the expired tidal volume by the peak expiratory flow during passive exhalation:
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.