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Ventilator-induced lung injury (VILI) carries significant attributable mortality in acute respiratory distress syndrome (ARDS). Even though all the ventilatory variables contribute to VILI, current guidelines focus almost exclusively on tidal volume (VT) and plateau pressure/driving pressure (ΔP).
In 2016, we formalised the concept of mechanical power (MP) with the aim to: (1) quantify the contribution of respiratory rate (RR) and the positive end-expiratory pressure (PEEP) to the total power delivered by the ventilator; (2) aggregate these variables into a single physical measure, whose value might relate to the risk of VILI [1].
In this manuscript, we aim to review the conceptual and analytical derivation of the MP, discuss the role of each component in the generation of VILI and the association between MP and outcome.
Derivation of mechanical power
To develop the MP formula, we started from the classical equation of motion and multiplied each pressure component (elastic, resistive and static) by the VT (i.e., expressing the work, or energy to the system), and then by the RR to obtain power in Joules/minute [1].
This original computation was subsequently adapted for different flow delivery (e.g., volume or pressure-controlled ventilation), and simplified for easier calculation and applicability to conditions different from passive ventilation [2, 3] Fig. 1.
Components of mechanical power and risk of VILI
The role of each MP component in the generation of VILI is still debated. The main controversy relates to whether PEEP should be excluded from MP computation given that PEEP is a static pressure (i.e., not associated with dynamic volume change) [4]. While this objection may seem plausible, it must be considered that PEEP generates a volume when first applied to the respiratory system, and the energy required to move the lung from its new energy position is equal to the sum of the internal energy of the system (i.e., VT × PEEP) and the energy needed to reach the desired inspiratory volume (i.e., VT × ΔP). In other words, it is the absolute pressure, not just the change in pressure that determines VILI [5]. This has been demonstrated in models where increasing PEEP while keeping the other components of the MP (e.g., VT, RR) constant [6], caused a proportional increase in VILI. Despite these debates, PEEP is included in all the equations proposed for the MP computation [2, 3].
Association between MP and outcome
Several studies including ARDS [7] and non-ARDS patients [8] have now demonstrated the association between MP and mortality. Its prognostic value has been compared with single variables (e.g., ΔP), and more recently with a combination of driving pressure and frequency: 4 × ΔP + RR [9] which reflects the relative effect on the odds of death. All these proposed variables have, however, predictive values comparable with MP, which is unsurprising given that they all share the same component.
Important questions are about the definition of a safe MP is, and the appropriate upper and lower safety thresholds. A “normal” MP provides acceptable PO2 and PCO2 and is included within safe boundaries. Experimentally we found in 30 kg pigs, a safe threshold was between an upper threshold of 12 J/min and a lower threshold of 4–7 J/min [10]. However, these are just average values, and in theory the distribution of MP within the respiratory cycle may play an equally important role. Indeed, MP is concentrated at the beginning of inspiration during pressure support ventilation, while in volume-controlled ventilation the MP distribution is more equally distributed throughout the inspiratory time [11]. Moreover, the MP dissipation during expiration may be more uniform if the expiratory flow is constant [12, 13]. Also, not all the elements of MP have equal weight: doubling the VT leads to a fourfold increase in MP; doubling RR leads to a 1.4-fold increase, and doubling PEEP to a twofold increase in MP [1]. Experimentally, three iso-power packages of 20 J/min, achieved adjusting VT or RR or PEEP [14] resulted in similar lung weight and wet-to-dry ratio at 48-h suggesting that iso-power, however, achieved, produces the same injury. Of note, the lung damage obtained at iso-power with the increasing of VT resulted in greater impairment in lung mechanics, while the iso-power damage induced by increasing PEEP resulted in greater systemic complications such as hemodynamic impairment and water retention. The concept of iso-power seems a convenient way to guide titration of the ventilator settings if MP goes beyond the – yet to be defined – upper thresholds of safety.
Mechanical power normalisation
The additional essential step for the clinical use of MP is its “normalisation” to the size of the lung as to move the respiratory system of an elephant or a mouse will obviously require different energy, and this will affect the safety thresholds. Scaling, however, must be applied to humans of different ages, sex and lung dimensions. Currently, this problem remains unsolved although attempts have been made normalising MP for body weight, functional residual capacity, and compliance. We are currently investigating the possibility of MP normalisation through the use of an MP ratio–analogously to what was done to derive the ventilatory ratio –between the measured MP and the MP applied to normal lungs to eliminate a normal amount of VCO2.
Importantly, using oesophageal pressure as an estimate of the changes in pleural pressure it is possible to quantify the proportion of the total MP which is delivered to the lungs – an essential information for critical patients with different chest wall elastance.
MP is a summary variable derived from solid physical and biological foundations, that can be included in any ventilator to monitor the safety of mechanical ventilation and guide lung protective strategies. In a sense, MP is to ventilation what SvO2 is to the body’s homeostasis a variable that reflects the combination of different elements (e.g. arterial oxygenation, oxygen consumption, cardiac output or haemoglobin) all potentially associated with mortality. In the same way MP indicates that one or more variables are excessive and expose patients to the risk of VILI. Protective ventilation may be achieved by the combination of variables that achieves the lowest MP compatible with adequate gas exchange and hemodynamics.
References
Gattinoni L, Tonetti T, Cressoni M, Cadringher P, Herrmann P, Moerer O, Protti A, Gotti M, Chiurazzi C, Carlesso E, Chiumello D, Quintel M (2016) Ventilator-related causes of lung injury: the mechanical power. Intensive Care Med 42(10):1567–1575. https://doi.org/10.1007/s00134-016-4505-2
Giosa L, Busana M, Pasticci I, Bonifazi M, Maria Macrì M, Romitti F, Vassalli F, Chiumello D, Quintel M, Marini JJ, Gattinoni L (2019) Mechanical power at a glance: a simple surrogate for volume-controlled ventilation. Intensive Care Med Exp 7(1):61. https://doi.org/10.1186/s40635-019-0276-8
Trinkle CA, Broaddus RN, Sturgill JL, Waters CM, Morris PE (2022) Simple, accurate calculation of mechanical power in pressure controlled ventilation (PCV). Intensive Care Med Exp 10(1):22. https://doi.org/10.1186/s40635-022-00448-5
Huhle R, Serpa-Neto A, Schultz MJ, de Abreu MG (2018) Is mechanical power the final word on ventilator-induced lung injury? No. Ann Transl Med 6(19):394. https://doi.org/10.21037/atm.2018.09.65
Vasques F, Duscio E, Pasticci I, Romitti F, Vassalli F, Quintel M, Gattinoni L (2018) Is the mechanical power the final word on ventilator-induced lung injury? We are not sure. Ann Transl Med 6(19):395. https://doi.org/10.21037/atm.2018.08.17
Collino F, Rapetti F, Vasques F et al (2019) Positive end-expiratory pressure and mechanical power. Anesthesiology 130(1):119–130. https://doi.org/10.1097/ALN.0000000000002458
Serpa Neto A, Deliberato RO, Johnson AEW et al (2018) Mechanical power of ventilation is associated with mortality in critically ill patients: an analysis of patients in two observational cohorts. Intensive Care Med 44(11):1914–1922. https://doi.org/10.1007/s00134-018-5375-6
Wu H-P, Chu C-M, Chuang L-I et al (2021) The association between mechanical power and mortality in patients with pneumonia using pressure-targeted ventilation. Diagnostics (Basel) 11(10):1862. https://doi.org/10.3390/diagnostics11101862
Costa E, Slutsky AS, Brochard LJ et al (2021) Ventilatory variables and mechanical power in patients with acute respiratory distress syndrome. Am J Respir Crit Care Med 204(3):303–311. https://doi.org/10.1164/rccm.202009-3467OC
Romitti F, Busana M, Palumbo MM et al (2022) Mechanical power thresholds during mechanical ventilation: an experimental study. Physiol Rep 10(6):225. https://doi.org/10.14814/phy2.15225
Crooke PS, Gattinoni L, Michalik M, Marini JJ (2022) Intracycle power distribution in a heterogeneous multi-compartmental mathematical model: possible links to strain and VILI. Intensive Care Med Exp 10(1):21. https://doi.org/10.1186/s40635-022-00447-6
Goebel U, Haberstroh J, Foerster K, Dassow C, Priebe H-J, Guttmann J, Schumann S (2014) Flow-controlled expiration: a novel ventilation mode to attenuate experimental porcine lung injury. Br J Anaesth 113(3):474–483. https://doi.org/10.1093/bja/aeu058
Busana M, Zinnato C, Romitti F et al (2022) Energy dissipation during expiration and ventilator-induced lung injury: an experimental animal study. J Appl Physiol 133(5):1212–1219. https://doi.org/10.1152/japplphysiol.00426.2022
Vassalli F, Pasticci F, Romitti F et al (2020) Does iso-mechanical power lead to iso-lung damage? An experimental study in a porcine model. Anesthesiology 132(5):1126–1137. https://doi.org/10.1097/ALN.0000000000003189
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Gattinoni, L., Collino, F. & Camporota, L. Mechanical power: meaning, uses and limitations. Intensive Care Med 49, 465–467 (2023). https://doi.org/10.1007/s00134-023-06991-3
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DOI: https://doi.org/10.1007/s00134-023-06991-3