Mechanical Power – Paradigm Shift in Mechanical Ventilation in ARDS

Mechanical Power

Blog Written by – Dr Jose Chacko

Although a life-supporting intervention, mechanical ventilation is associated with its own

perils. Improved gas exchange and amelioration of the work of breathing occurs at the

expense of potential damage to the lung, described as ventilator-induced lung injury (VILI).

Over the past few decades, we have become increasingly familiar with the propagators of

VILI; the benchmark ARDSnet trial conclusively established the safety of low tidal volume

ventilation with limitation of plateau pressure (Pplat).1 Subsequently, the driving pressure

(Δp) was identified as a potential trigger for VILI.2 However, the wide heterogeneity among

critically ill patients with respiratory failure may be an important consideration in planning an

appropriate ventilation strategy. For instance, a tidal volume based on the predicted body

weight does not consider the wide variation in the extent of lung involvement in patients with

acute respiratory distress syndrome (ARDS).

What is mechanical power of ventilation?

In mechanistic terms, lung injury may be induced by the strain inflicted on the lung

represented by the tidal volume, and the applied stress, represented by the transpulmonary

pressure (airway pressure – pleural pressure). Besides, there are other important arbiters of

lung injury, including the respiratory rate and the flow (the rate of strain). Clearly, an

increase in the rate of ventilation would also exacerbate the potential for lung injury. The

concept of mechanical power has evolved to combine all the potential propagators of VILI to

represent the total energy expense incurred over the time during mechanical ventilation.

Components of mechanical power – the equation of motion

According to the basic equation of motion, the pressure applied to the respiratory system

includes the pressure required to overcome airway and tissue resistance (kinetic energy) and

the pressure required to inflate the lung and the chest wall (potential energy).

The individual components of the applied pressure include:

1. The pressure required to overcome the elastance of the lung and the chest wall

elastance:

(ELrs) x tidal volume (Δv) = driving pressure (Δp). This constitutes potential

energy and is related to elastic work

2. The pressure required to overcome airway and lung tissue resistance:

Raw x flow. This constitutes kinetic energy, related to resistive work

3. The positive-end expiratory pressure (PEEP), which is the static baseline

pressure of the lung at when the flow and Δv are zero

The components of mechanical power from the pressure-volume graph

The energy required to inflate the lung from its resting volume to the applied tidal volume

may be represented by the pressure-volume curve (Figure 1). The elastic work is represented

by the area of the yellow triangle, the resistive work by the area of the green parallelogram,

and the PEEP-related work by the pink rectangle.

Figure 1. Derivation of the components of mechanical power from the pressure-volume

graph. The pink rectangle represents PEEP-related work; the yellow triangle, elastic work;

the green parallelogram, resistive work. Vt, tidal volume; Pplat, plateau pressure; Ppeak,

peak pressure

 

Derivation of the individual components of power from the pressure-volume graph

Elastic work

The elastic work is represented by the area of the yellow triangle, calculated as follows:

Area of a triangle =   base x height. The base of the yellow triangle is constituted by the tidal

volume (Δv), and the height by Pplat – PEEP (Δp).

Elastance (ELrs) = Δp/Δv

Hence (Δp) = ELrs x Δv

Thus, elastic work = area of the yellow triangle =   Δv x ELrs x Δv =   (Δv2 x ELrs) ……

(1)

Resistive work

The resistive work, represented by the area of the green parallelogram.

This equation is further simplified for bedside calculation as:

Power RS = RR . 0.098 . tidal volume . (Ppeak –   Δp).

The evidence so far –

Mechanical power was calculated using the above equation by Gattinoni et al. in 80 patients

– 30 with normal lungs and 50 with ARDS.3 Calculations were based on the pressure-volume

curve at PEEP levels of 5 and 15 cm H2O at different tidal volumes – 6, 8, 10, and 12 ml/kg.

The authors observed that the mechanical power increased exponentially with tidal volume,

driving pressure, flow, and respiratory rate; it increased linearly with increasing levels of

PEEP.

In a study on healthy piglets, Cressoni et al. used a tidal volume of 38 ml/kg, known to be

lethal, at rates of 12, 9, 6, and 3 breaths/min.4 Widespread lung edema was noted only when

the mechanical power was higher than 12 J/min, regardless of the respiratory rate. In another

phase of the same study, the respiratory rate was maintained constant at 35 breaths/min, at

tidal volumes of 11 and 22 ml/kg. The mechanical power varied, depending on the tidal

volume used. VILI occurred when the mechanical power exceeded the threshold level of 12

J/min, regardless of the tidal volume. This experimental study suggested that neither tidal

volume nor the respiratory rate independently predicted the development of VILI; instead, the

mechanical power, determined by a combination of both variables, induced VILI above a

threshold of 12 J/min.4

Gurin et al. performed a secondary analysis of patients with ARDS included in ACURASYS

and the PROSEVA trials.5 A total of 787 patients were enrolled in these two randomized

controlled trials with a ventilator strategy similar to the low tidal volume arm of the ARDSnet

trial.1 The tidal volume, PEEP, compliance, respiratory rate, driving pressure, and mechanical

power were compared between survivors and non-survivors at 90 days. The 90-day survival

was significantly higher among patients with a driving pressure ≤13 cm H2O on day 1.

Notably, the 90-day survival was also significantly higher among patients in whom the

mechanical power was ≤12 J/min on day 1. The authors also noted that the numerical value

of the mechanical power expressed in J/min was similar to the driving pressure in cm H2O.

In another study, critically ill patients who underwent mechanical ventilation for 48 h or more

from two databases were analyzed.6 The authors evaluated the impact of mechanical power in

the second 24 h of mechanical ventilation on clinical outcomes. Among the 8207 patients

enrolled, mechanical power was significantly associated with in-hospital mortality.

Furthermore, higher levels of mechanical power resulted in a significantly higher ICU and

30-d mortality. The ICU and hospital length of stay were lower in patients who were

subjected to lower mechanical power. A consistent increase in the risk of death was observed

above a threshold power of 17 J/min. Higher mechanical power correlated with worse clinical

outcomes even with lower tidal volumes and driving pressures.

Unanswered questions and future research

Mechanical power combines ventilation parameters known to trigger ventilator-induced lung

injury. The evidence available so far suggests that a higher mechanical power is

independently associated with adverse outcomes among patients receiving mechanical

ventilation. Apart from the already established ventilation targets, including low tidal

volume, plateau and driving pressures, mechanical power may be a more decisive and

composite target to aim for to reduce ventilation-induced damage to the lungs. Considering

its likely important role in the causation of VILI, an algorithm to calculate mechanical power

may be incorporated into ventilators for continuous monitoring. The critical level of applied

power that may induce VILI is uncertain, although a threshold of 12–17 J/min has been

suggested based on available evidence. The relative impact of the different components of

mechanical power towards the causation of VILI also needs further detailed evaluation.

Summary

• Tidal volume based on predicted body weight is widely employed as a lung-protective

strategy in mechanically ventilated patients. However, such a strategy may not be

ideal, considering the wide heterogeneity of the extent of lung involvement in ARDS

• VILI is more likely to be related to the energy transfer from the ventilator to the

respiratory system over time, represented by the mechanical power

• The components of transferred energy include tidal volume, respiratory rate, airway

pressure, and flow rate

• The mechanical power of ventilation includes three basic components – elastic work,

represented by the pressure required to overcome the elastance of the lung and the

chest wall elastance, resistive work, related to the pressure required to overcome

airway and tissue resistance, and the PEEP-related work

• Early studies suggest that mechanical power, easily calculated by the bedside, may

more closely correlate with the extent of VILI compared to individual ventilation

parameters including tidal volume and driving pressure

• Although a threshold level of 12–17 Joules/min has been suggested, further detailed

studies are required to evaluate an appropriate target for mechanical power during

ventilation

• The incorporation of an algorithm into ventilators for the calculation of mechanical

power will facilitate continuous monitoring of mechanical power and optimization of

ventilation parameters

References

1. Acute Respiratory Distress Syndrome Network: Brower RG, Matthay MA, Morris A, et

al. Ventilation with lower tidal volumes as compared with traditional tidal volumes for

acute lung injury and the acute respiratory distress syndrome. N Engl J Med. 2000 May

4;342(18):1301-8. doi: 10.1056/NEJM200005043421801. PMID: 10793162.

2. Amato MBP, Meade MO, Slutsky AS, et al. Driving pressure and survival in the acute

respiratory distress syndrome. N Engl J Med. 2015;372(8):747-755.

doi:10.1056/NEJMsa1410639

3. Gattinoni L, Tonetti T, Cressoni M, et al. Ventilator-related causes of lung injury: the

mechanical power. Intensive Care Med. 2016 Oct;42(10):1567-1575. doi:

10.1007/s00134-016-4505-2. Epub 2016 Sep 12. PMID: 27620287.

4. Cressoni M, Algieri I, Montaruli C, Dondossola D, Pugni P. Mechanical Power and

Development of Ventilator-induced Lung Injury. CRITICAL CARE MEDICINE.:9.

5. Gu rin C. Effect of driving pressure on mortality in ARDS patients during lung

protective mechanical ventilation in two randomized controlled trials. Published online

2016:9.

6. Neto A, Deliberato RO, Johnson AEW, et al. PROVE Network Investigators.

Mechanical power of ventilation is associated with mortality in critically ill patients: an

analysis of patients in two observational cohorts. Intensive Care Med. 2018

Nov;44(11):1914-1922. doi: 10.1007/s00134-018-5375-6. Epub 2018 Oct 5. PMID:

30291378.