The time constant (KT) of the respiratory system is a function of the elastic properties of the lung (compliance, CL) and its inherent capacity to resist airflow (resistance, R). It describes the time that is required for alveolar and proximal airway pressure to equilibrate. The longer the duration for equilibration, the greater the percentage change in pressure will occur, and a proportionally larger tidal volume (VT) will be delivered. For a mechanically-ventilated infant with decreased CL, the KT will be short. If the pattern of assisted ventilation does not allow for sufficient expiratory time, the patient is at risk for incomplete lung emptying and GT. To demonstrate the effects of varying ventilator rate (RR) on gas flow while holding all other ventilator parameters constant, a neonatal test lung (IngMar Medical, Pittsburgh, PA) and a standard neonatal ventilator (VIP BIRD infant/pediatric ventilator, Bird Products Corp., Palm Springs, CA) were used. Three different compliance levels were tested, 0.6, 1.3, and 1.6 mL/cm H2O. At each level, the RR was varied from 40-90 bpm. Pulmonary R was measured and flow waveforms were recorded. GT was defined as failure of the expiratory limb of the flow waveform to return to baseline before the initiation of the next breath. KT was calculated as the product of R and CL. At each level of CL, the KT and VT decreased. As CL increased, KT lengthened and GT occurred at lower RR. GT was demonstrated when the expiratory time was less than approximately six times the KT. (Table)
This model demonstrates the relationship between KT, R, and CL; when CL is low, KT is short, and a faster RR can be used. However, as CL improves and KT lengthens, rapid RR may result in GT.