Mean airway pressure – Minute ventilation product (mM): A simple and universal surrogate equation to calculate mechanical power in both volume and pressure controlled ventilation

Ehab G. Daoud, Philip Lee, Shane Toma, Claudio L. Franck

Abstract

Introduction

Mechanical power represents the energy delivered by a mechanical ventilator onto the lungs. It incorporates all the variables participating in ventilator-induced lung injury, including driving pressure, tidal volume, positive end expiratory pressure, and respiratory rate. The pitfall of mechanical power is its mathematical complexity, as the gold standard method of calculation involves deriving the inspiratory area under the pressure-volume curve of each breath. Prior studies attempted to create simplified equations, they lack clinical utility as calculations cannot be done by solely looking at ventilator settings or they require manipulation of variables. There are also different formulas depending on the type of the mode of ventilation used. This study offers a simplified, universal equation called the mean airway pressure – Minute ventilation product (mM equation) which renders mechanical power clinical application more feasible at the bedside.

Methods and Statistics

Data collection used the online SIVA simulator, which simulate mechanical ventilation and calculate the geometrical area of the inspiratory limb of the pressure-volume curve. Different combinations of passive scenarios with varying compliances (10-80 ml/cmH₂O) and resistances (5-30 cmH₂O/L/S) in each the VCV and PCV modes were accomplished by adjusting ventilator settings with respiratory rate (5-40 BPM), tidal volume (150-700 mL), DP (5-30 cmH₂O), and PEEP (0-15 cmH₂O), with different inspiratory times in PCV and different flows rates in the VCV.

A total of 2,000 values were collected in each mode. Range of Mechanical power measured by the simulator: 0.1 – 105 J/min and range of mM equation (mean airway pressure x Minute ventilation): 0.37 – 820 cmH₂O/L/min. Pearson correlation coefficients were calculated to compare the relationship of the mM equation to the measured MP, and linear regressions were used for predicting the MP derived from the mM equation in each mode separately and when combining all data from both modes. T-test for equal variance and Bland Altmann plot were used to compare the reference MP measured (MPR) from the simulator to the one derived from the Mm formula (MPD).

Results

There was a statistically significant linear relationship (P < 0.001) and strong correlation of determination (R² = 0.931), CI (0.961, 0.967) between the mM formula and the gold-standard method of calculating mechanical power for the combined two modes. For the VCV: there was a statistically significant linear relationship (P < 0.001) and strong correlation of determination (R² = 0.936), CI (-0.963, 0.971). For the PCV: there was a statistically significant linear relationship (P < 0.001) and strong correlation of determination (R² = 0.936), CI (-0.964, 0.970).

A linear regression model predicted the MP from the mM as follows: for both modes MP = 0.13 (mM) + 3.41, for PCV MP = 0.15 (mM) + 3.79, for VCV MP = 0.13 (mM) + 2.48.

The derived mechanical power from the mM was not statistically different (P 0.498) from the calculated  reference MP using two sample T-tests assuming equal variance.

The Bland-Altman plot for VCV mode showed a mean of 0.78 with 95% CI (0.34, 1.22), SD (-13.27, 14.83). In PCV, a mean of – 0.53 with 95% CI (-0.68, -0.38), SD (-6.28, 5.22). For both modes, a mean of 0, with 95% CI (-0.2, 0.2), SD (-10.06, 10.05).

Conclusion

The mM equation and its MP derived formula is a reliable method of calculating mechanical power. The simplicity and universal nature of its calculation can provide significant clinical utility at the bedside. More studies are needed to validate this method of calculation.

Keywords

Mechanical power, mean airway pressure, minute ventilation

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