In 1999 AS Slutsky described the original four ventilator-induced lung injuries (VILI), volutrauma, barotrauma, biotrauma, and atelectrauma (Table 1). Barotrauma was the first to be described in 1973 (1).
Table 1:
The argument for some is that this looks at the end-result damage to the lung instead of the cause of the damage by the ventilator. The ventilator components that can be manipulated are pressure, volume, flow, and respiratory rate.
The definitions of the different airway pressures are often confusing and vary from paper to paper (2). There are good arguments for the different definitions and I am not going to try to address them. I am using the definitions below:
Definitions:
Looking at the original four ventilator-induced lung injuries compared to the ventilator components, they are able to fit into each of these categories:
Table 2:
*Biotrauma is due to cytokine release from damage caused by the other VILIs, as well as oxygen-free radicals. It could be considered extra-ventilatory as cytokines from the disease process itself additionally cause damage.
With the shift to focus on the ventilator causes; respiratory rate, flow, and other components began to be explored.
2000:
It was discovered that the respiratory rate made a difference in ventilator-induced lung injury. In this study, increased rates with the same pressure led to increased pulmonary edema and increased perivascular hemorrhage (3). The Greek root for time is chrono- and this type of trauma can be defined as chronotrauma.
2016:
Amato et al. published a paper on driving pressure. This looked at the pressure needed to expand the alveoli, which is the transthoracic pressure. This is defined as Pplat – PEEP. It was found that a driving pressure ≤ 15 cmH20 led to decreased mortality in ARDS patients (4). Driving pressure causing VILI is called energytrauma. Energytrauma is the dissipation of energy across alveoli (5).
Also, in 2016 it was described that the flow was associated with ventilator-induced lung injury. By looking at lung strain, which is the ratio between tidal volume and functional residual capacity, the strain rate can be calculated. Strain rate is lung strain divided inspiratory time. The simplification of this is looking at volume over time, which is flow. It was found that a high rate of strain led to ventilator-induced pulmonary edema and damage (6). The Greek root for flow is rheo-, and this type of damage is called rheotrauma. Also, when there is an inadequate flow to the lungs, the body responds by increasing the work of breathing, which also leads to damage.
The new ventilator-induced lung injuries:
Chronotrauma: Trauma due to excess rate of breaths delivered to the patient
Energytrauma: Trauma due to excess transthoracic pressure
Rheotrauma: Trauma due to excess or inadequate flow being delivered
2016:
It shows that it makes more sense to focus on the ventilator for causes of ventilator-induced lung injuries. Gattinoni showed that all VILIs could be connected into one equation, mechanical power. Starting with the equation of motion he was able to create the equation for mechanical power. Table 3 includes definitions and formulas to help show how this equation was created.
Table 3 (7,8):
The equation of motion is the pressure required to deliver gas/air to the lungs.
Equation of motion:
Pmusc + Pvent = Resistance x Flow + Volume/Compliance + PEEP
The two main components of the equation define resistive and elastic work, as shown below, but the equation, as shown, is measured as a resulting pressure.
When the equation of motion is multiplied by the tidal volume (△V) it equals work and defines the energy need or energy per breath (Ebr).
Ebr = △V · △V / C · ½ + △V · R · F + △V · PEEP
△V / C: the pressure-volume curve is assumed to be linear and so the integral is triangular and therefore multiplied by ½.
Energy per breath (Ebr) is then multiplied by respiratory rate, which will give you work over time, which is power. When this is multiplied by 0.098 it changes the units to J/min and the equation for mechanical power is created.
Flow is broken down into △V/Tinsp (Inspiratory time)
Tinsp is expanded out to 1 minute = 60/RR · (I:E/(1 + I:E)), where I:E = Inspiratory to expiratory ratio
Simplified, mechanical power is the product of the transairway pressure, transthoracic pressure, and PEEP multiplied by tidal volume and respiratory rate.
Power = RR · TV · (Transairway pressure + Transthoracic Pressure + PEEP)
This equation contains all the VILIs within it. This all-encompassing VILI was described by Gattinoni as ergotrauma. Ergo is Greek for work and looks at the power across the respiratory system.
This equation is complex and can be intimidating or difficult to follow. Hopefully, it will help to see it broken down into its components and help in looking at the complete picture for preventing ventilator-induced lung injury.
*Biotrauma
References:
1. Slutsky AS. Lung injury caused by mechanical ventilation. Chest. 1999;116(1 Suppl):9S-15S. doi:10.1378/chest.116.suppl_1.9s-a
2. Wolfe DF, Sorbello JG. Comparison of published pressure gradient symbols and equations in mechanics of breathing. Respir Care. 2006;51(12):1450-1457.
3. Hotchkiss JR Jr, Blanch L, Murias G, et al. Effects of decreased respiratory frequency on ventilator-induced lung injury. Am J Respir Crit Care Med. 2000;161(2 Pt 1):463-468. doi:10.1164/ajrccm.161.2.9811008
4. Amato MB, 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/NEJMsa141063
5. Serpa Neto A, Amato MBP, Schultz MJ: Dissipated energy is a key mediator of VILI: Rationale for using low driving pressures. In Vincent JL (ed), Annual Update in Intensive Care and Emergency Medicine 2016. Cham, Springer, 2016, pp. 311–321.
6. Protti A, Maraffi T, Milesi M, et al. Role of Strain Rate in the Pathogenesis of Ventilator-Induced Lung Edema. Crit Care Med. 2016;44(9):e838-e845. doi:10.1097/CCM.0000000000001718
7. https://byjus.com/physics/work-energy-power/ Accessed 4/22/21
8. Marini JJ. Dissipation of energy during the respiratory cycle: conditional importance of ergotrauma to structural lung damage. Curr Opin Crit Care. 2018;24(1):16-22. doi:10.1097/MCC.0000000000000470
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