Skip to main content
Figure 1 | Critical Care

Figure 1

From: Expiratory time constant for determinations of plateau pressure, respiratory system compliance, and total resistance

Figure 1

Pressure, flow, and volume waveforms for determination of expiratory time constant. In the first breath a patient is ventilated with pressure support ventilation (PSV) 20 cm H2O and positive end expiratory pressure (PEEP) 5 cm H2O. The expiratory time constant (ƬE) was estimated during passive exhalation between 0.10 and 0.50 seconds using exhaled flow rate and tidal volume waveform data (see Methods). In the second breath the same patient is temporarily changed to volume controlled synchronized intermittent mandatory ventilation (VC-SIMV) with an end inspiratory pause (EIP) for 0.5 second at PEEP 5 cm H2O, while applying similar peak inspiratory flow rate and tidal volume. This is done to determine peak airway pressure during inhalation (Paw) and inspiratory plateau pressure (Pplt) or static elastic recoil pressure of the respiratory system, needed for calculations of respiratory system compliance and total resistance (see equations 1 and 2). The third breath is an example of an EIP that is not tolerated, that is, the patient attempts to spontaneously inhale during the pause, precluding accurate measurement of Pplt.

Back to article page