Simple, accurate calculation of mechanical power in pressure controlled ventilation (PCV)

Abstract

Background: Mechanical power is a promising new metric to assess energy transfer from a mechanical ventilator to a patient, which combines the contributions of multiple parameters into a single comprehensive value. However, at present, most ventilators are not capable of calculating mechanical power automatically, so there is a need for a simple equation that can be used to estimate this parameter at the bedside. For volume-controlled ventilation (VCV), excellent equations exist for calculating power from basic ventilator parameters, but for pressure-controlled ventilation (PCV), an accurate, easy-to-use equation has been elusive.

Results: Here, we present a new power equation and evaluate its accuracy compared to the three published PCV power equations. When applied to a sample of 50 patients on PCV with a non-zero rise time, we found that our equation estimated power within an average of 8.4% ± 5.9% (mean ± standard deviation) of the value obtained by numerical integration of the P-V loop. The other three equations estimated power with an error of 19.4% ± 12.9% (simplified Becher equation), 10.0% ± 6.8% (comprehensive Becher equation), and 16.5% ± 14.6% (van der Meijden equation).

Conclusions: Our equation calculates power more accurately than the other three published equations, and is much easier to use than the only previously published equation with similar accuracy. The proposed new mechanical power equation is accurate and simple to use, making it an attractive option to estimate power in PCV cases at the bedside.

Keywords: Mechanical power equation; Mechanical ventilation; Pressure-controlled ventilation.

Fig. 1

Four equations for estimating mechanical power in PCV patients (bottom), and Bland–Altman plots for each of these equations [4, 5] (top). Bland–Altman plots were generated by plotting the mean of the calculated value (MP) and the numerically integrated value (MP_ref) for each equation (x-axis) against the difference between the calculated value and numerically integrated value (y-axis)

Fig. 2

Agreement between each of the four mechanical power equations (y-axis) and the numerically integrated value (MP_ref) (x-axis). Dashed line represents 1:1 slope

Fig. 3

Percent error (MP_ref − MP)/MP_ref for each of the four equations [2, 3] as a function of the parameter t_insp/(R·C). Calculations that resulted in greater than 15% error are highlighted in red

Fig. 4

Comparison of original function, f(x), and linear approximation, g(x). Functions show excellent agreement over a realistic range of x values

Fig. 5

Comparison of mechanical power predicted by the comprehensive Becher, et al. equation and the linear model derived here. The two equations show excellent agreement over a realistic range of t_slope/(RC) values