Mechanical Power in Pressure-Controlled Ventilation: A Simple and Reliable Bedside Method

Abstract

Background: Mechanical power (MP) represents the amount of energy applied by the ventilator to the respiratory system over time. There are two main methods to calculate MP in mechanical ventilation. The first is the geometric method, which directly measures the dynamic inspiratory area of the pressure-volume loop during the respiratory cycle. The second involves using various algebraic equations to estimate MP. However, almost all calculations are either complex or not reliable compared with the geometric method, considered the gold standard. This study aimed to develop an easy to use, reliable equation for bedside calculation of MP and to compare its accuracy with other existing equations for calculating MP.

Methods: In a preliminary study, we measured MP in 56 cases who were mechanically ventilated and without spontaneous breathing efforts. The measurements were done at the ICU of a single university medical center in the Netherlands.

Results: We found that the MP can be accurately calculated using an equation that incorporates the plateau pressure in 56 cases in 42 patients. The MP estimated with our new proposed equation (MP calculated using plateau pressure) correlated well with the reference value of MP with a bias of 0.2 J/min. The 95% limits of agreement (LoAs) were -3.1 to + 3.4 J/min. Other equations give the following bias and LoAs; bias of -0.8, LoA -3.8 to 1.9 J/min (van der Meijden equation), bias of -1.9, LoA -3.7 to -0.0 J/min (comprehensive Becher equation), bias of -2.4, LoA -4.5 to -0.3 J/min (simplified Becher equation), and a bias of -1.9, LoA -3.7 to 0.1 J/min (linear model equation).

Conclusions: The equation we propose to calculate MP in pressure-controlled ventilation is a reliable, simple, and accurate alternative for the previously published equations. Consequently, this method is highly suitable for routine use in clinical practice.

Figure 1.

Graphic representation of the dynamic pressure-volume loop during pressure-controlled ventilation. A, A dynamic pressure-volume (PV) loop showing the three components of mechanical power (MP) in pressure-controlled ventilation. In this case, peak airway pressure (P_peak) is 12 cm H₂O and plateau pressure (P_plat) is 11 cm H₂O. The red dotted line is indicative, not exact. B, The blue and yellow zones represent the real PV loop, which is the MP of a single breath. The blue and orange zones are the MP as calculated with our new equation, using P_plat instead of P_peak (an equal pressure line is at P_plat). The yellow zone is subtracted from the real PV loop, while adding the orange zone. Simplified equation MP = 0.098 · RR · Vt · P_plat. Paw = airway pressure, PEEP = positive end-expiratory pressure, RR = respiratory rate, Vt = tidal volume.

Figure 2.

Bland-Altman plots showing the differences between mechanical power (MP) measured and MP calculated using the equations for estimating MP in pressure-controlled ventilation. For Bland-Altman plot of the comprehensive Becher equation, see e-supplement (http://links.lww.com/CCX/B477). MP_Pplat = mechanical power calculated using plateau pressure.