As introduced earlier, PPEAK is the maximum PAW achieved during an MV inspiration, and when displayed is principally used to measure total resistance in the proximal larger airways plus the ventilator circuit, although its absolute value is also affected by the elastic recoil force of the chest wall and lungs. Therefore, when the inspiratory VT is constant, a rising PPEAK is most often indicative of increasing airway resistance, for example, acute bronchospasm with airway narrowing from any cause, presuming no change in compliance of the lungs plus thoracic cage. Conversely, PPLAT represents the required distending pressure to balance elastic recoil at the level of terminal bronchioles and alveoli when airflow is zero (Chap. 6). This PPLAT is best measured by creating an inspiratory pause at the end of an MV inspiration, commonly 0.5-second, to allow equilibration between inspiratory and alveolar pressures, after which it is measured and/or displayed (Fig. 30.8). Conceptually, PPEAK always exceeds PPLAT since airway resistance is never zero, while PPLAT may be nearly equal to PPEAK but can never exceed it. Thus, an absolute increase in PPLAT when a mechanically delivered VT is constant indicates decreased compliance of the lungs plus chest wall, as in pneumonia, ARDS, pneumothorax, and some other restrictive lung diseases (Chap. 23 and 28).
CLINICAL CORRELATION 30.2
Common causes of increased PPEAK include acute bronchoconstriction and secretion buildup within the ventilator circuit. Therefore, an isolated elevation in PPEAK without a concomitant elevation of PPLAT is often treated with bronchodilators and endotracheal or tracheostomy tube suctioning. Additionally, kinks in the ventilator tubing or an excessively narrow endotracheal tube or tracheostomy tube may also increase PPEAK by virtue of their effects on total system resistance to air flow. Elevations of both PPEAK and PPLAT usually reflect alveolar disease or dysfunction from various etiologies. In this context, an increased PPLAT is causally associated with VALI and pneumothorax. A central goal of the ARDS Network recommendation to treat ARDS patients using VT = 6 mL/kg of PBW is to maintain PPLAT <30 cm H2O, even at the expense of low VT and permissive hypercapnia, that is, a controlled elevation of PaCO2.