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Mechanical power and ventilator-induced lung injury

Article

Author: Jean-Pierre Revelly, Giorgio Iotti

Date of first publication: 03.04.2023

This article takes a closer look at the different components of mechanical power, its relevance in a clinical setting, and its use as a monitoring parameter.

Mechanical power and ventilator-induced lung injury

The scope of this discussion is limited to mechanical power (MP) during the inspiratory phase of controlled ventilation, assuming there is no patient effort.

In physics: 

  • Mechanical work is the energy transferred to (or from) an object via the application of force along a movement from one position to another.
  • Power is the amount of energy transferred per unit of time.

In mechanical ventilation, the power transferred from the ventilator to the respiratory system during inspiration is a unifying variable that combines the elements that may cause ventilator-induced lung injury (VILI) (Gattinoni L, Tonetti T, Cressoni M, et al. Ventilator-related causes of lung injury: the mechanical power. Intensive Care Med. 2016;42(10):1567-1575. doi:10.1007/s00134-016-4505-21​).

Mechanical power during CMV

During Controlled Mandatory Ventilation (CMV) with constant flow, MP can be described as work per breath (W) times the respiratory rate (RR) (Figure 1) (Costa ELV, Slutsky AS, Brochard LJ, et al. Ventilatory Variables and Mechanical Power in Patients with Acute Respiratory Distress Syndrome. Am J Respir Crit Care Med. 2021;204(3):303-311. doi:10.1164/rccm.202009-3467OC2​):

Where:

  • Wel,PEEP represents the Elastic Static component, i.e., the component of MP related to PEEP. 
  • Wel,DP represents the (tidal) Elastic Dynamic component, i.e., the component of MP related to tidal inflation. It has the shape of a right-angled triangle, whereby the vertical side corresponds to the tidal volume (Vt) and the horizontal side corresponds to the driving pressure (DP). The slope of the third side corresponds to compliance. 
  • Wres is the Resistive component, i.e., the energy dissipated during each inspiration to overcome the resistive and viscous properties of the respiratory system. Wres is represented by a parallelogram whose base corresponds to the difference between peak pressure (Ppeak) and plateau pressure (Pplat), while the height corresponds to Vt.
Diagram showing three different components of mechanical power
Figure 1: Pressure-volume diagram depicting the various components of mechanical power (see text for explanation)
Diagram showing three different components of mechanical power
Figure 1: Pressure-volume diagram depicting the various components of mechanical power (see text for explanation)

Mechanical power during PCV

A similar approach can be applied during Pressure Controlled Ventilation (PCV) for the calculation of Wel,PEEP and Wel,DP from Vt, PEEP and DP. Wres can be approximated by calculating the area of the rectangle with Ppeak minus PEEP as base and Vt as height, and then subtracting the triangle corresponding to Wel,DP. This calculation for Wres can be applied in the same way where Ppeak is equal to Pplat (Figure 2) or higher than it (Figure 3), i.e., where the end-inspiratory flow is zero or still positive, respectively. In both cases the approximation results in a slight overestimate of Wres, and hence of the true total ventilator work (Becher T, van der Staay M, Schädler D, Frerichs I, Weiler N. Calculation of mechanical power for pressure-controlled ventilation. Intensive Care Med. 2019;45(9):1321-1323. doi:10.1007/s00134-019-05636-83​).

Diagram showing approximation where Ppeak equals Pplat
Figure 2: Approximation of Wres in PCV where Ppeak is equal to Pplat
Diagram showing approximation where Ppeak equals Pplat
Figure 2: Approximation of Wres in PCV where Ppeak is equal to Pplat
Diagram showing approximation where Ppeak is higher than Pplat
Figure 3: Approximation of Wres in PCV where Ppeak is higher than Pplat
Diagram showing approximation where Ppeak is higher than Pplat
Figure 3: Approximation of Wres in PCV where Ppeak is higher than Pplat

Is mechanical power clinically relevant?

Quite a few investigators have computed the MP from the data of ventilatory studies in ICU patients with (Costa ELV, Slutsky AS, Brochard LJ, et al. Ventilatory Variables and Mechanical Power in Patients with Acute Respiratory Distress Syndrome. Am J Respir Crit Care Med. 2021;204(3):303-311. doi:10.1164/rccm.202009-3467OC2​, Tonna JE, Peltan I, Brown SM, Herrick JS, Keenan HT; University of Utah Mechanical Power Study Group. Mechanical power and driving pressure as predictors of mortality among patients with ARDS. Intensive Care Med. 2020;46(10):1941-1943. doi:10.1007/s00134-020-06130-24​, Robba C, Badenes R, Battaglini D, et al. Ventilatory settings in the initial 72 h and their association with outcome in out-of-hospital cardiac arrest patients: a preplanned secondary analysis of the targeted hypothermia versus targeted normothermia after out-of-hospital cardiac arrest (TTM2) trial. Intensive Care Med. 2022;48(8):1024-1038. doi:10.1007/s00134-022-06756-45​) or without ARDS (Serpa Neto A, Deliberato RO, Johnson AEW, et al. Mechanical power of ventilation is associated with mortality in critically ill patients: an analysis of patients in two observational cohorts. Intensive Care Med. 2018;44(11):1914-1922. doi:10.1007/s00134-018-5375-66​​, Robba C, Badenes R, Battaglini D, et al. Ventilatory settings in the initial 72 h and their association with outcome in out-of-hospital cardiac arrest patients: a preplanned secondary analysis of the targeted hypothermia versus targeted normothermia after out-of-hospital cardiac arrest (TTM2) trial. Intensive Care Med. 2022;48(8):1024-1038. doi:10.1007/s00134-022-06756-47​, van Meenen DMP, Algera AG, Schuijt MTU, et al. Effect of mechanical power on mortality in invasively ventilated ICU patients without the acute respiratory distress syndrome: An analysis of three randomised clinical trials. Eur J Anaesthesiol. 2023;40(1):21-28. doi:10.1097/EJA.00000000000017788​). 
In these analyses: 

  • Non-survivors received significantly larger MP than survivors
  • Increasing MP was statistically correlated with ICU and hospital mortality, fewer ventilator-free days, and increased ICU and hospital length of stay 

Overall, these retrospective studies suggest that excessive MP should preferably be avoided, assuming that a worse clinical outcome was related in part to VILI.

Is there a safe MP value for all patients?

The methods used to calculate MP in published studies must be read carefully to interpret them properly. Depending on the data available, the authors may have included or excluded some of the components of MP. There is also an ongoing debate regarding the most appropriate procedure for comparing different patients. Normalization for the patient’s size (predicted weight), compliance or end-expiratory lung volume have been proposed. 

Generally speaking, however, there is not yet a standardized approach for the calculation of MP, nor is there any widely accepted safe value for the estimated MP.

Is mechanical power ready for continuous monitoring?

Individual changes of ventilator settings have complex effects on other variables of the ventilation mechanics. The MP concept relies on the implicit assumption that all ventilatory variables have a linear relationship and the same contribution to VILI. However, this is obviously not the case, as PEEP for instance has a curvilinear (J-shape) relationship to VILI (Costa ELV, Slutsky AS, Brochard LJ, et al. Ventilatory Variables and Mechanical Power in Patients with Acute Respiratory Distress Syndrome. Am J Respir Crit Care Med. 2021;204(3):303-311. doi:10.1164/rccm.202009-3467OC2​). During inspiration, there is probably a safe volume, a threshold value and an injurious zone that should not be reached (Marini JJ, Rocco PRM. Which component of mechanical power is most important in causing VILI?. Crit Care. 2020;24(1):39. Published 2020 Feb 5. doi:10.1186/s13054-020-2747-49​). The inspiratory flow pattern, which is not accounted for in MP, may play a significant role in VILI (Marini JJ, Crooke PS, Gattinoni L. Intra-cycle power: is the flow profile a neglected component of lung protection?. Intensive Care Med. 2021;47(5):609-611. doi:10.1007/s00134-021-06375-510​). 

Although a series of open issues remains, monitoring total MP and its components may prove useful for assessing the individual patient’s evolution or their response to ventilatory setting changes. MP may become a new consideration along with several others in clinical judgement and decision making. Moreover, MP monitoring would greatly help the collection of high-quality data for any prospective study on the relationship between MP and VILI. 

So how do we reduce the risk for VILI with ventilatory settings?

Different researchers have tried to identify the most detrimental components of ventilation. A retrospective study compiling the ventilatory data of 4500 ARDS patients enrolled in controlled studies assessed the relationships of MP, Vt, RR and DP to 28-day mortality using multivariable models (Costa ELV, Slutsky AS, Brochard LJ, et al. Ventilatory Variables and Mechanical Power in Patients with Acute Respiratory Distress Syndrome. Am J Respir Crit Care Med. 2021;204(3):303-311. doi:10.1164/rccm.202009-3467OC2​). DP represents Vt normalized for compliance and is considered by many as a key component of VILI (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/NEJMsa141063911​).

Not surprisingly, the authors found that global MP was correlated with mortality. When assessing the various components of MP, only the elastic dynamic component (MPel,DP, i.e, the MP that depends on Wel,DP) was statistically significant, while the components that depend on PEEP or resistance were not. MPel,DP is particularly simple to calculate at the bedside, in both CMV and PCV.

  • MPel,DP = Vt x DP x RR / 2

Moreover, the authors found a similar predictivity of mortality just by combining DP and RR in the following index:

  • 4DP+RR Index = (4 x DP) + RR

The authors concluded that “although mechanical power was associated with mortality in patients with ARDS, the ∆P and RR were as informative and easier to assess at the beside [sic]. Whether a ventilatory strategy based on these variables improves outcomes needs to be tested in randomized controlled studies” (Costa ELV, Slutsky AS, Brochard LJ, et al. Ventilatory Variables and Mechanical Power in Patients with Acute Respiratory Distress Syndrome. Am J Respir Crit Care Med. 2021;204(3):303-311. doi:10.1164/rccm.202009-3467OC2​).

Footnotes

References

  1. 1. Gattinoni L, Tonetti T, Cressoni M, et al. Ventilator-related causes of lung injury: the mechanical power. Intensive Care Med. 2016;42(10):1567-1575. doi:10.1007/s00134-016-4505-2
  2. 2. Costa ELV, Slutsky AS, Brochard LJ, et al. Ventilatory Variables and Mechanical Power in Patients with Acute Respiratory Distress Syndrome. Am J Respir Crit Care Med. 2021;204(3):303-311. doi:10.1164/rccm.202009-3467OC
  3. 3. Becher T, van der Staay M, Schädler D, Frerichs I, Weiler N. Calculation of mechanical power for pressure-controlled ventilation. Intensive Care Med. 2019;45(9):1321-1323. doi:10.1007/s00134-019-05636-8
  4. 4. Tonna JE, Peltan I, Brown SM, Herrick JS, Keenan HT; University of Utah Mechanical Power Study Group. Mechanical power and driving pressure as predictors of mortality among patients with ARDS. Intensive Care Med. 2020;46(10):1941-1943. doi:10.1007/s00134-020-06130-2
  5. 5. Coppola S, Caccioppola A, Froio S, et al. Effect of mechanical power on intensive care mortality in ARDS patients. Crit Care. 2020;24(1):246. Published 2020 May 24. doi:10.1186/s13054-020-02963-x
  6. 6. Serpa Neto A, Deliberato RO, Johnson AEW, et al. Mechanical power of ventilation is associated with mortality in critically ill patients: an analysis of patients in two observational cohorts. Intensive Care Med. 2018;44(11):1914-1922. doi:10.1007/s00134-018-5375-6
  7. 7. Robba C, Badenes R, Battaglini D, et al. Ventilatory settings in the initial 72 h and their association with outcome in out-of-hospital cardiac arrest patients: a preplanned secondary analysis of the targeted hypothermia versus targeted normothermia after out-of-hospital cardiac arrest (TTM2) trial. Intensive Care Med. 2022;48(8):1024-1038. doi:10.1007/s00134-022-06756-4
  8. 8. van Meenen DMP, Algera AG, Schuijt MTU, et al. Effect of mechanical power on mortality in invasively ventilated ICU patients without the acute respiratory distress syndrome: An analysis of three randomised clinical trials. Eur J Anaesthesiol. 2023;40(1):21-28. doi:10.1097/EJA.0000000000001778
  9. 9. Marini JJ, Rocco PRM. Which component of mechanical power is most important in causing VILI?. Crit Care. 2020;24(1):39. Published 2020 Feb 5. doi:10.1186/s13054-020-2747-4
  10. 10. Marini JJ, Crooke PS, Gattinoni L. Intra-cycle power: is the flow profile a neglected component of lung protection?. Intensive Care Med. 2021;47(5):609-611. doi:10.1007/s00134-021-06375-5
  11. 11. 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/NEJMsa1410639

Ventilator-related causes of lung injury: the mechanical power.

Gattinoni L, Tonetti T, Cressoni M, et al. Ventilator-related causes of lung injury: the mechanical power. Intensive Care Med. 2016;42(10):1567-1575. doi:10.1007/s00134-016-4505-2



PURPOSE

We hypothesized that the ventilator-related causes of lung injury may be unified in a single variable: the mechanical power. We assessed whether the mechanical power measured by the pressure-volume loops can be computed from its components: tidal volume (TV)/driving pressure (∆P aw), flow, positive end-expiratory pressure (PEEP), and respiratory rate (RR). If so, the relative contributions of each variable to the mechanical power can be estimated.

METHODS

We computed the mechanical power by multiplying each component of the equation of motion by the variation of volume and RR: [Formula: see text]where ∆V is the tidal volume, ELrs is the elastance of the respiratory system, I:E is the inspiratory-to-expiratory time ratio, and R aw is the airway resistance. In 30 patients with normal lungs and in 50 ARDS patients, mechanical power was computed via the power equation and measured from the dynamic pressure-volume curve at 5 and 15 cmH2O PEEP and 6, 8, 10, and 12 ml/kg TV. We then computed the effects of the individual component variables on the mechanical power.

RESULTS

Computed and measured mechanical powers were similar at 5 and 15 cmH2O PEEP both in normal subjects and in ARDS patients (slopes = 0.96, 1.06, 1.01, 1.12 respectively, R (2) > 0.96 and p < 0.0001 for all). The mechanical power increases exponentially with TV, ∆P aw, and flow (exponent = 2) as well as with RR (exponent = 1.4) and linearly with PEEP.

CONCLUSIONS

The mechanical power equation may help estimate the contribution of the different ventilator-related causes of lung injury and of their variations. The equation can be easily implemented in every ventilator's software.

Ventilatory Variables and Mechanical Power in Patients with Acute Respiratory Distress Syndrome.

Costa ELV, Slutsky AS, Brochard LJ, et al. Ventilatory Variables and Mechanical Power in Patients with Acute Respiratory Distress Syndrome. Am J Respir Crit Care Med. 2021;204(3):303-311. doi:10.1164/rccm.202009-3467OC

Rationale: Mortality in acute respiratory distress syndrome (ARDS) has decreased after the adoption of lung-protective strategies. Lower Vt, lower driving pressure (ΔP), lower respiratory rates (RR), and higher end-expiratory pressure have all been suggested as key components of lung protection strategies. A unifying theoretical explanation has been proposed that attributes lung injury to the energy transfer rate (mechanical power) from the ventilator to the patient, calculated from a combination of several ventilator variables.Objectives: To assess the impact of mechanical power on mortality in patients with ARDS as compared with that of primary ventilator variables such as the ΔP, Vt, and RR.Methods: We obtained data on ventilatory variables and mechanical power from a pooled database of patients with ARDS who had participated in six randomized clinical trials of protective mechanical ventilation and one large observational cohort of patients with ARDS. The primary outcome was mortality at 28 days or 60 days.Measurements and Main Results: We included 4,549 patients (38% women; mean age, 55 ± 23 yr). The average mechanical power was 0.32 ± 0.14 J · min-1 · kg-1 of predicted body weight, the ΔP was 15.0 ± 5.8 cm H2O, and the RR was 25.7 ± 7.4 breaths/min. The driving pressure, RR, and mechanical power were significant predictors of mortality in adjusted analyses. The impact of the ΔP on mortality was four times as large as that of the RR.Conclusions: Mechanical power was associated with mortality during controlled mechanical ventilation in ARDS, but a simpler model using only the ΔP and RR was equivalent.

Calculation of mechanical power for pressure-controlled ventilation.

Becher T, van der Staay M, Schädler D, Frerichs I, Weiler N. Calculation of mechanical power for pressure-controlled ventilation. Intensive Care Med. 2019;45(9):1321-1323. doi:10.1007/s00134-019-05636-8

Mechanical power and driving pressure as predictors of mortality among patients with ARDS.

Tonna JE, Peltan I, Brown SM, Herrick JS, Keenan HT; University of Utah Mechanical Power Study Group. Mechanical power and driving pressure as predictors of mortality among patients with ARDS. Intensive Care Med. 2020;46(10):1941-1943. doi:10.1007/s00134-020-06130-2

Effect of mechanical power on intensive care mortality in ARDS patients.

Coppola S, Caccioppola A, Froio S, et al. Effect of mechanical power on intensive care mortality in ARDS patients. Crit Care. 2020;24(1):246. Published 2020 May 24. doi:10.1186/s13054-020-02963-x



BACKGROUND

In ARDS patients, mechanical ventilation should minimize ventilator-induced lung injury. The mechanical power which is the energy per unit time released to the respiratory system according to the applied tidal volume, PEEP, respiratory rate, and flow should reflect the ventilator-induced lung injury. However, similar levels of mechanical power applied in different lung sizes could be associated to different effects. The aim of this study was to assess the role both of the mechanical power and of the transpulmonary mechanical power, normalized to predicted body weight, respiratory system compliance, lung volume, and amount of aerated tissue on intensive care mortality.

METHODS

Retrospective analysis of ARDS patients previously enrolled in seven published studies. All patients were sedated, paralyzed, and mechanically ventilated. After 20 min from a recruitment maneuver, partitioned respiratory mechanics measurements and blood gas analyses were performed with a PEEP of 5 cmH2O while the remaining setting was maintained unchanged from the baseline. A whole lung CT scan at 5 cmH2O of PEEP was performed to estimate the lung gas volume and the amount of well-inflated tissue. Univariate and multivariable Poisson regression models with robust standard error were used to calculate risk ratios and 95% confidence intervals of ICU mortality.

RESULTS

Two hundred twenty-two ARDS patients were included; 88 (40%) died in ICU. Mechanical power was not different between survivors and non-survivors 14.97 [11.51-18.44] vs. 15.46 [12.33-21.45] J/min and did not affect intensive care mortality. The multivariable robust regression models showed that the mechanical power normalized to well-inflated tissue (RR 2.69 [95% CI 1.10-6.56], p = 0.029) and the mechanical power normalized to respiratory system compliance (RR 1.79 [95% CI 1.16-2.76], p = 0.008) were independently associated with intensive care mortality after adjusting for age, SAPS II, and ARDS severity. Also, transpulmonary mechanical power normalized to respiratory system compliance and to well-inflated tissue significantly increased intensive care mortality (RR 1.74 [1.11-2.70], p = 0.015; RR 3.01 [1.15-7.91], p = 0.025).

CONCLUSIONS

In our ARDS population, there is not a causal relationship between the mechanical power itself and mortality, while mechanical power normalized to the compliance or to the amount of well-aerated tissue is independently associated to the intensive care mortality. Further studies are needed to confirm this data.

Mechanical power of ventilation is associated with mortality in critically ill patients: an analysis of patients in two observational cohorts.

Serpa Neto A, Deliberato RO, Johnson AEW, et al. Mechanical power of ventilation is associated with mortality in critically ill patients: an analysis of patients in two observational cohorts. Intensive Care Med. 2018;44(11):1914-1922. doi:10.1007/s00134-018-5375-6



PURPOSE

Mechanical power (MP) may unify variables known to be related to development of ventilator-induced lung injury. The aim of this study is to examine the association between MP and mortality in critically ill patients receiving invasive ventilation for at least 48 h.

METHODS

This is an analysis of data stored in the databases of the MIMIC-III and eICU. Critically ill patients receiving invasive ventilation for at least 48 h were included. The exposure of interest was MP. The primary outcome was in-hospital mortality.

RESULTS

Data from 8207 patients were analyzed. Median MP during the second 24 h was 21.4 (16.2-28.1) J/min in MIMIC-III and 16.0 (11.7-22.1) J/min in eICU. MP was independently associated with in-hospital mortality [odds ratio per 5 J/min increase (OR) 1.06 (95% confidence interval (CI) 1.01-1.11); p = 0.021 in MIMIC-III, and 1.10 (1.02-1.18); p = 0.010 in eICU]. MP was also associated with ICU mortality, 30-day mortality, and with ventilator-free days, ICU and hospital length of stay. Even at low tidal volume, high MP was associated with in-hospital mortality [OR 1.70 (1.32-2.18); p < 0.001] and other secondary outcomes. Finally, there is a consistent increase in the risk of death with MP higher than 17.0 J/min.

CONCLUSION

High MP of ventilation is independently associated with higher in-hospital mortality and several other outcomes in ICU patients receiving invasive ventilation for at least 48 h.

Ventilatory settings in the initial 72 h and their association with outcome in out-of-hospital cardiac arrest patients: a preplanned secondary analysis of the targeted hypothermia versus targeted normothermia after out-of-hospital cardiac arrest (TTM2) trial.

Robba C, Badenes R, Battaglini D, et al. Ventilatory settings in the initial 72 h and their association with outcome in out-of-hospital cardiac arrest patients: a preplanned secondary analysis of the targeted hypothermia versus targeted normothermia after out-of-hospital cardiac arrest (TTM2) trial. Intensive Care Med. 2022;48(8):1024-1038. doi:10.1007/s00134-022-06756-4



PURPOSE

The optimal ventilatory settings in patients after cardiac arrest and their association with outcome remain unclear. The aim of this study was to describe the ventilatory settings applied in the first 72 h of mechanical ventilation in patients after out-of-hospital cardiac arrest and their association with 6-month outcomes.

METHODS

Preplanned sub-analysis of the Target Temperature Management-2 trial. Clinical outcomes were mortality and functional status (assessed by the Modified Rankin Scale) 6 months after randomization.

RESULTS

A total of 1848 patients were included (mean age 64 [Standard Deviation, SD = 14] years). At 6 months, 950 (51%) patients were alive and 898 (49%) were dead. Median tidal volume (VT) was 7 (Interquartile range, IQR = 6.2-8.5) mL per Predicted Body Weight (PBW), positive end expiratory pressure (PEEP) was 7 (IQR = 5-9) cmH20, plateau pressure was 20 cmH20 (IQR = 17-23), driving pressure was 12 cmH20 (IQR = 10-15), mechanical power 16.2 J/min (IQR = 12.1-21.8), ventilatory ratio was 1.27 (IQR = 1.04-1.6), and respiratory rate was 17 breaths/minute (IQR = 14-20). Median partial pressure of oxygen was 87 mmHg (IQR = 75-105), and partial pressure of carbon dioxide was 40.5 mmHg (IQR = 36-45.7). Respiratory rate, driving pressure, and mechanical power were independently associated with 6-month mortality (omnibus p-values for their non-linear trajectories: p < 0.0001, p = 0.026, and p = 0.029, respectively). Respiratory rate and driving pressure were also independently associated with poor neurological outcome (odds ratio, OR = 1.035, 95% confidence interval, CI = 1.003-1.068, p = 0.030, and OR = 1.005, 95% CI = 1.001-1.036, p = 0.048). A composite formula calculated as [(4*driving pressure) + respiratory rate] was independently associated with mortality and poor neurological outcome.

CONCLUSIONS

Protective ventilation strategies are commonly applied in patients after cardiac arrest. Ventilator settings in the first 72 h after hospital admission, in particular driving pressure and respiratory rate, may influence 6-month outcomes.

Effect of mechanical power on mortality in invasively ventilated ICU patients without the acute respiratory distress syndrome: An analysis of three randomised clinical trials.

van Meenen DMP, Algera AG, Schuijt MTU, et al. Effect of mechanical power on mortality in invasively ventilated ICU patients without the acute respiratory distress syndrome: An analysis of three randomised clinical trials. Eur J Anaesthesiol. 2023;40(1):21-28. doi:10.1097/EJA.0000000000001778



BACKGROUND

The mechanical power of ventilation (MP) has an association with outcome in invasively ventilated patients with the acute respiratory distress syndrome (ARDS). Whether a similar association exists in invasively ventilated patients without ARDS is less certain.

OBJECTIVE

To investigate the association of mechanical power with mortality in ICU patients without ARDS.

DESIGN

This was an individual patient data analysis that uses the data of three multicentre randomised trials.

SETTING

This study was performed in academic and nonacademic ICUs in the Netherlands.

PATIENTS

One thousand nine hundred and sixty-two invasively ventilated patients without ARDS were included in this analysis. The median [IQR] age was 67 [57 to 75] years, 706 (36%) were women.

MAIN OUTCOME MEASURES

The primary outcome was the all-cause mortality at day 28. Secondary outcomes were the all-cause mortality at day 90, and length of stay in ICU and hospital.

RESULTS

At day 28, 644 patients (33%) had died. Hazard ratios for mortality at day 28 were higher with an increasing MP, even when stratified for its individual components (driving pressure ( P  < 0.001), tidal volume ( P  < 0.001), respiratory rate ( P  < 0.001) and maximum airway pressure ( P  = 0.001). Similar associations of mechanical power (MP) were found with mortality at day 90, lengths of stay in ICU and hospital. Hazard ratios for mortality at day 28 were not significantly different if patients were stratified for MP, with increasing levels of each individual component.

CONCLUSION

In ICU patients receiving invasive ventilation for reasons other than ARDS, MP had an independent association with mortality. This finding suggests that MP holds an added predictive value over its individual components, making MP an attractive measure to monitor and possibly target in these patients.

TRIAL REGISTRATION

ClinicalTrials.gov Identifier: NCT02159196, ClinicalTrials.gov Identifier: NCT02153294, ClinicalTrials.gov Identifier: NCT03167580.

Which component of mechanical power is most important in causing VILI?

Marini JJ, Rocco PRM. Which component of mechanical power is most important in causing VILI?. Crit Care. 2020;24(1):39. Published 2020 Feb 5. doi:10.1186/s13054-020-2747-4

Intra-cycle power: is the flow profile a neglected component of lung protection?

Marini JJ, Crooke PS, Gattinoni L. Intra-cycle power: is the flow profile a neglected component of lung protection?. Intensive Care Med. 2021;47(5):609-611. doi:10.1007/s00134-021-06375-5

Driving pressure and survival in the acute respiratory distress syndrome.

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/NEJMsa1410639



BACKGROUND

Mechanical-ventilation strategies that use lower end-inspiratory (plateau) airway pressures, lower tidal volumes (VT), and higher positive end-expiratory pressures (PEEPs) can improve survival in patients with the acute respiratory distress syndrome (ARDS), but the relative importance of each of these components is uncertain. Because respiratory-system compliance (CRS) is strongly related to the volume of aerated remaining functional lung during disease (termed functional lung size), we hypothesized that driving pressure (ΔP=VT/CRS), in which VT is intrinsically normalized to functional lung size (instead of predicted lung size in healthy persons), would be an index more strongly associated with survival than VT or PEEP in patients who are not actively breathing.

METHODS

Using a statistical tool known as multilevel mediation analysis to analyze individual data from 3562 patients with ARDS enrolled in nine previously reported randomized trials, we examined ΔP as an independent variable associated with survival. In the mediation analysis, we estimated the isolated effects of changes in ΔP resulting from randomized ventilator settings while minimizing confounding due to the baseline severity of lung disease.

RESULTS

Among ventilation variables, ΔP was most strongly associated with survival. A 1-SD increment in ΔP (approximately 7 cm of water) was associated with increased mortality (relative risk, 1.41; 95% confidence interval [CI], 1.31 to 1.51; P<0.001), even in patients receiving "protective" plateau pressures and VT (relative risk, 1.36; 95% CI, 1.17 to 1.58; P<0.001). Individual changes in VT or PEEP after randomization were not independently associated with survival; they were associated only if they were among the changes that led to reductions in ΔP (mediation effects of ΔP, P=0.004 and P=0.001, respectively).

CONCLUSIONS

We found that ΔP was the ventilation variable that best stratified risk. Decreases in ΔP owing to changes in ventilator settings were strongly associated with increased survival. (Funded by Fundação de Amparo e Pesquisa do Estado de São Paulo and others.).