Mechanical power of ventilation during invasive mechanical ventilation in critically ill patients – A prospective observational study

Introduction

 Invasive Mechanical ventilation is a life-saving treatment to improve oxygenation, ventilation, lung recruitment and decrease work of breathing. However, over time it was realized that in addition to these beneficial effects, it also poses risk of damage to lungs called as ventilator induced lung injury or VILI. The risk of VILI depends on variables related to both ventilator and the lungs. Through experimental and observational studies it has been found that variables related to ventilator contributing to VILI in differing measures include tidal volume, driving pressure, PEEP, inspiratory flow rate and respiratory rate. These variables mostly have been studied independently as factors causing VILI. Gattinoni et al suggested that all the above mentioned variables causing VILI may be unified into a unique summary or composite variable called the Mechanical Power (MP). Mechanical power is the energy transferred from the ventilator to the respiratory system of the patient with each breath multiplied by respiratory rate, that is the energy transferred per minute. The association of trans-pulmonary MP with VILI was first established by Cressoni et al in a porcine study in 2016 (1)and this was later followed by multiple animal and human studies verifying the same.

 The most precise way to measure mechanical power would be to measure the area under the inspiratory Pressure-volume curve. But this is not possible at the bedside since the area under the P-V curve isn’t generally bound by straight lines and will require special equipment or software addition to the ventilator to measure and solve for integral of airway pressure with respect to the change in volume. Secondly and more importantly, measurement of MP like this will not provide the composition and relative contribution of the individual variables that affect MP and hence may be altered at the bedside to change MP to prevent VILI. To solve this problem, Gattinoni et al invented an algebraic equation from the equation of motion for the respiratory system and validated it by establishing its correlation with measured MP for volume controlled ventilation with constant flow(2). Later, a simpler and easier surrogate equation was proposed and validated by Giosa et al(3). Similar Equations have also been invented for pressure controlled mode of ventilation(4)(5).

One of the limitations of the idea of mechanical power include the lack of normalization. Mechanical power threshold that may induce lung injury in a child may not be same as mechanical power threshold for an adult and among adults too, there may be variability in the sizes of ventilable lungs. Mechanical power thus needs normalization to predicted body weight or functional residual capacity to refine its prognostic ability.

Another problem with this index is that it includes the energy or power required to inflate the lung including the chest wall. As such, it may not truly represent the characteristics of lung in patient with higher contribution from the latter. Use of trans-pulmonary mechanical power may offset this problem. That would require us to use an esophageal probe to measure esophageal pressure as a surrogate for pleural pressure to calculate trans-pulmonary pressures and trans-pulmonary mechanical power.

 Multiple human studies, mostly retrospective and secondary analysis of data from previous randomized controlled trials, have demonstrated that higher MP is associated with worse VILI and clinical outcomes including death in both ARDS and non-ARDS patients.

In our study we intend to look at the Mechanical power delivered to critically ill patients on mechanical ventilation and its association with clinical outcomes including mortality and analyze other associated clinical, laboratory and epidemiological variables.

Objectives

Primary Objective –

Descriptive analysis of Mechanical power being delivered to mechanically ventilated patients.

                               Secondary Objectives –

1.      To assess the association between mechanical power and mortality within 30 days from the start of invasive mechanical ventilation.

2.      To assess association between mechanical power and ventilator free days till day 30.

To arrive at a threshold for mechanical power for predicting mortality.

Methodology

After screening for inclusion and exclusion criteria, data will be recorded as per the preformed case record form. Variables needed for Mechanical power calculation will be recorded soon after the patient is intubated and will be recorded every 6 hours until 96 hours or until the patient starts breathing spontaneously, whichever is earlier.

In our study, we’d limit ourselves to measuring the total mechanical power transferred to the respiratory system including the lungs as well as the chest wall assuming the contribution from the chest wall will be not so significant.

In addition to ventilatory variables and Mechanical power, other patient data including the Clinical and demographic details, APACHE and SOFA scores, hospital diagnosis, comorbidities and ICU diagnosis will also be collected. Patient would be followed up for 30 days from the day of start of invasive mechanical ventilation and outcomes including ventilator free days and/or mortality will be recorded.

Mechanical power will be calculated using Gattinoni’s simplified equation as well as Giosa’s surrogate equations.

Study Design – Prospective observational study

Study Place – Medical and surgical Intensive care units of Tata Memorial Centre, Parel, Mumbai

Sampling method – Sample of convenience

Statistical analysis –

Mean mechanical power delivered to each patient will be calculated.Patient characteristics and outcomes will be described as frequency with percentages or mean (SD) / Median (IQR) as appropriate. Quantitative data will be summarized using Mean (SD) if normally distributed. Median (IQR) will be reported if the data is non-normally distributed. The normality of quantitative data will be accessed using Kolmogorov-Smirnov’s test for normality.The association between Mechanical power and mortality and ventilator free days will be assessedusing multivariable logistic regression analysis. The threshold for Mechanical power will be determined through receiver operating characteristics analysis and Youden J index. A two tailed p value of <0.05 will be considered statistically significant.All statistical analysis will be performed using IBM SPSS v25.