Know how to Measure Plateau Pressure when Using Pressure-Regulated Volume Control Ventilation Mode and Know what to do with the Value Once it is Obtained
David N. Hager MD
Mechanical ventilation has been shown to cause ventilation-induced lung injury by at least two mechanisms. First, using a low lung volume allows the repeated opening and closing of small airways and alveoli. This is associated with high shear forces on the lung interstitium and alveolar surfactant depletion, both of which result in ventilation-induced lung injury. To reduce this low-volume injury, the use of positive end-expiratory pressure (PEEP) is advocated, though the ideal amount of PEEP continues to be the topic of intense study. Second, the use of too much PEEP and/or the use of supraphysiologic tidal volumes have been shown to cause ventilation-induced lung injury by overdistention of alveoli. Historically, alveolar overdistention has been a concern when plateau pressure (Pplat) is >30 cm H2O. Although much attention is often given to high peak airway pressure, it is Pplat that reflects alveolar pressure, value most associated with ventilation-induced lung injury. Two approaches to limiting alveolar overdistention are (a) to reduce tidal volume until plateau pressure ≤30 cm H2O during assist volume control (AC) ventilation or (b) to limit inspiratory airway pressure to ≤30 cm H2O during pressure-regulated volume control (PRVC) ventilation. The astute clinician will be able to understand (a) the differences between AC and PRVC ventilation modes, (b) that alveolar pressures ≤30cm H2O might not be uniformly safe, and (c) the appropriate strategy that limits both low volume and overdistention lung injury.
Controlled Modes of Ventilation
AC and PRVC are both “controlled” modes of mechanical ventilation. For each, the clinician sets a minimum respiratory rate, which ensures a set number of breaths even if the patient is apneic or paralyzed. A nonparalyzed/nonapneic patient who triggers the ventilator in excess of this set rate will receive full ventilator support for each additional breath. However, the modes differ in how the magnitude of tidal volumes are determined. During assist control ventilation, the
clinician sets a tidal volume, inspiratory flow rate, inspiratory time, and inspiratory flow waveform. Irrespective of lung compliance, airway resistance, or simultaneous attempts at active exhalation by the patient, the set tidal volume will be delivered. For this reason, even in the apneic patient, a minimum minute ventilation (respiratory rate times tidal volume) is guaranteed, though there is the potential for high peak airway pressures. Plateau pressure is measured at end inspiration by occluding the expiratory valve of the ventilator circuit. This allows airway pressure to equilibrate throughout the lungs.
clinician sets a tidal volume, inspiratory flow rate, inspiratory time, and inspiratory flow waveform. Irrespective of lung compliance, airway resistance, or simultaneous attempts at active exhalation by the patient, the set tidal volume will be delivered. For this reason, even in the apneic patient, a minimum minute ventilation (respiratory rate times tidal volume) is guaranteed, though there is the potential for high peak airway pressures. Plateau pressure is measured at end inspiration by occluding the expiratory valve of the ventilator circuit. This allows airway pressure to equilibrate throughout the lungs.
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