Peak Inspiratory Pressure to Estimate Plateau Pressure in Pressure Controlled Modes: Be Aware of Age and Disease*

Abstract

Driving pressure (ΔP = plateau pressure [Pplat] minus positive endexpiratory pressure [PEEP]) has generated increasing interest in recent years since it is considered the measure that best stratifies mortality risk in critically ill adults with acute respiratory distress syndrome (ARDS) (1). Consequently, ΔP is a potential predictor of ventilator-induced lung injury and a possible target to define the safety limits of mechanical ventilation (2, 3). In children, similar results have recently been published: Díaz et al (4) reported that ΔP is the formula that best discriminated patients with pediatric ARDS (PARDS) from those without, and van Schelven et al (5) showed that ΔP was independently associated with increased time to extubation.Pplat measurement is performed in adults in the static condition when gasflow reaches zero using the volume-controlled mode (1). Contrasting with adult critical care, pressure controlled modes with decelerating flow are predominantly used in the PICU (6), and peak inspiratory pressure (PIP) is used in some studies to estimate Pplat in PARDS for ΔP calculation (7-9). In this issue of Pediatric Critical Care Medicine, Patel et al (10) address a knowledge gap in the reliability of Pplat estimation using PIP in pressure controlled mode. The authors show that in a cohort of 52 patients with PARDS (median age of 8.6 yr old [3.2-16.0 yr old]), PIP was close to Pplat measured in the static condition, that is, PIP was 1.0 ± 0.6 cm H 2 O above Pplat (95% limits of agreement of -0.3 to 2.2). Therefore, Patel et al (10) concluded that PIP measured during decelerating flow ventilation may be an adequate surrogate of Pplat in PARDS when inspiratory flow approaches zero.This new study by Patel et al ( 10) is well designed, including a cohort of PARDS children with an oxygenation index of 15.7 (9.0-21.0) and bilateral infiltrates on chest radiograph. In such a cohort, inspiratory airway resistance (Raw) is most likely low, which is the ideal condition for a zero flow plateau at the end of inspiratory time. Indeed, the inspiratory time constant (τ) in such circumstances is estimated to be 0.04 seconds. That is, since τ = respiratory system compliance × Raw, which is the same as this equation: (Vt [mL/kg] × weight [kg])/(Plat-PEEP) (cm H 2 O) × R (cm H 2 O/L / s) (11). τ is arrived at by using median tidal volume = 6.4 mL/kg, mean body weight = 27 kg at 8.6 years old (12), Pplat = 29.5 cm H 2 O, PEEP = 12 cm H 2 O (see article Supplemental Table in reference [10]), normal inspiratory resistance = 4 cm H 2 O/L / s (13). As zero flow is obtained at five time constants (≈ 0.2 s) (11), an inspiratory time set at more than 0.8 seconds (see the Supplemental Table in reference [10])

Pediatric Acute Respiratory Distress Syndrome: Approaches in Mechanical Ventilation

“…In addition, VCV delivers lower Qi for the same Vt and Ti, which theoretically reduces strain rate. Pplat can also be accurately measured in PC-modes (58, 59). That said, there are four principles that need to be considered, whichever mode is selected.…”

Plateau Pressure and Driving Pressure in Volume- and Pressure-Controlled Ventilation: Comparison of Frictional and Viscoelastic Resistive Components in Pediatric Acute Respiratory Distress Syndrome

“…The role of the resistive component is relevant in pediatric MV because airway diameter is inversely related to airway resistance (9)(10)(11). The resistive component has two subcomponents (3): frictional resistance corresponds to changes in the resistive airway pressure due to inspiratory flow variations; viscoelastic resistance is the lung stress relaxation and gas redistribution, which dissipates energy through the lung parenchyma.…”