ICU Fellowship Vivas – MH, Failure to Wean and Pancreatitis Dr Swapnil Pawar
Inguinal Hernia Dr Swapnil Pawar
Acute Pericarditis Dr Swapnil Pawar
Stopping Nasogastric feeds in ICU – Time for a CHANGE Dr Swapnil Pawar
Mechanical Power – Paradigm Shift in Mechanical Ventilation in ARDS
Dr Swapnil Pawar
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
(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,
Derivation of the individual components of power from the pressure-volume graph
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) ……
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
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.
• 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
• The incorporation of an algorithm into ventilators for the calculation of mechanical
power will facilitate continuous monitoring of mechanical power and optimization of
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.
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
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: