Mechanical Power during ventilation
Only recently an interesting concept has arisen that sees mechanical, positive-pressure ventilation as a means of applying energy / power to the lung. The individual ventilation parameters as well as the lung characteristics affect the total amount of delivered power. This may be a way of measuring the contribution of individual parameters to Ventilator-Induced-Lung Injury (VILI)
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Gattinoni et al. have described this concept in detail. Please find below an overview of the key messages:
The Mechanical Power Formula
Values are expressed in Joules/min.
From the above formula one can calculate how the mechanical power is affected by incremental changes in the individual parameters. From this the following relationships derive:
From the above one can derive that
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- Mechanical power applied to the lung increases with all variables
- Mechanical power increases most with high TV, Flow, driving pressure and RR
- Mechanical power increases with PEEP. While PEEP is static during ventilation this effect is caused by distension of the lung
parenchyma
- Mechanical power increases more significantly with an increase in elastane / decrease in compliance than wit high airway
resistance
power increases
power DEcreases
- Longer breath duration
- Lower TV
- Lower PEEP
- Lower Flow
- Lower R
- Lower Compliance
- Higher Elastane
- Shorter breath duration
- Higher TV
- Higher PEEP
- Higher Flow
- Higher R
- Lower Compliance
- Higher Elastane
Clinical application
There is currently insufficient data on how to incorporate the above concept into choosing ventilation parameters at the bedside. Studies suggest that driving pressure more than TV or PEEP contribute to lung injury and mortality. Given the fact that driving pressure is part of mechanical power one can argue that the mechanical power idea should be part of a clinicians thought process during individualisation of ventilation for any given patient. Research needs to focus on how to integrate the above calculations in a meaningful way to help guide clinicians' decision making.
Graphical Representation
- The blue triangle represents the baseline stretch of the lung tissue and therefore the energy to be overcome at each tidal volume
- The orange triangle represents the energy needed to overcome the elasticity of the respiratory system
- The green parallelogram represents the energy needed to overcome the resistance to gas flow
- The pink triangle represents the static component not taking part in the equation of power as it is only delivered once
