Elastic Power

To the Editor:

Recently, mechanical power ventilator-associated lung injury has been identified as an independent risk factor associated with intensive care mortality.¹ Dynamic and static airway pressures, which are the forces of the power concept and other respiratory parameters calculated from these forces (ie, dynamic and static compliances), are the basis of the concepts that have been well-established in intensive care literature for many years.² Failure to properly define power components corresponding to dynamic and static airway forces will further increase the current confusion in this regard.^3–6

In the recently published work of Syed et al⁷ in this journal, the elastic component of power is defined as dynamic power on the pressure-time curve. In my opinion, is not correct to define the elastic power component as dynamic power. The force of the driving power component (ie, driving pressure) is the difference between plateau pressure (P_plat) and PEEP, which are static airway pressures in the pressure-time curve. Therefore, the compliance calculated using the driving pressure is defined as static compliance. If there is a need for a different definition instead of elastic + PEEP power, I think it would be better to define this field as static power (Fig. 1).

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Fig. 1.

Pressure-time curve of volume control ventilation. Dynamic mechanical power = resistive power + driving power (green + yellow area). Static power = Driving power + PEEP power (yellow area + blue area). Total power = Static power + resistive power = Dynamic power + PEEP power (green + yellow + blue area). P_peak = peak inspiratory pressure; P_plat = plateau pressure.

The concept of dynamic power was previously defined by Aşar et al⁸ as dynamic mechanical power, and it was proposed with a simple equation that easily calculates dynamic mechanical power at the bedside during volume control ventilation using the ventilator work of breathing (WOB) parameter. In their study, the concept of dynamic power is defined as the sum of resistive power and elastic power defined by Gattinoni et al⁹: MP_dyn = ventilator WOB × MVe. Ventilator WOB is the amount of energy consumed for ventilation of 1 L of gas; its unit is expressed in Joule/L and is calculated by the ratio of work in an inspiratory cycle to tidal volume.^10,11 When the contribution of PEEP is added to this and multiplied by the conversion factor, the total dynamic power is calculated: MP_dyn = MVe × [(WOB) + (0.098 × PEEP)]. Volume control was compared with the standard power formula in 40 patients with ARDS, and it was shown that the total dynamic power is equal to the total standard power.⁸

The dynamic mechanical power force (dynamic power) on the pressure-time curve is the peak inspiratory pressure (P_peak). Therefore, the compliance calculated using the difference of P_peak and PEEP is defined as dynamic compliance. Dynamic power is a concept derived from dynamic respiratory parameters. If the contribution of airway resistance (P_peak – P_plat = resistive force) is added to the elastic (static) component of power, it can be defined as dynamic power (Fig. 1). Therefore, the concepts of dynamic and static, which are clearly defined by the intensive care scientific community and known to everyone together with the relevant airway forces, are recommended to be used properly and correctly by the authors.

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