Mechanical power and ventilator variables in patients with acute respiratory distress syndrome

METHODOLOGY

 Patients presenting in Department of Critical Care Medicine with ARDS requiring mechanical ventilation will be recruited for the study. A written informed consent will be obtained from the patients’ caretaker. Participants will be enrolled into the study based on eligibility criteria.

ARDS will be defined as per Berlin definition and patients will be classified as mild (200 mmHg

All the selected patients will undergo evaluation and general physical examination and appropriate routine baseline laboratory investigations. Patients who require mechanical ventilation will be included in the study further. Retrospectively patients will be grouped under survivors and non survivors for further analysis.

ICU admission date will be considered as Day 0. The next date of ICU admission will be defined as Day 1. The following patient data will be recorded within 24 h after admission: Age, gender, Body mass index, medical history, SOFA and APACHE II score.

On arrival in ICU arterial blood gas will be takenand PaO2/FiO2 ratio will be noted.

Mechanical Ventilator settings:

At baseline, patients will be maintained deeply sedated and paralyzed, ventilated in volume control with a square wave form without any inspiratory pause, applying a tidal volume between 4 and 8 mL/kg of ideal body weight with a PEEP value set by the attending physician to ensure an arterial saturation between 88 and 95%. Ventilator settings will be readjusted on the basis of arterial blood gas values after 2 h of the first setting. Ventilator setup, adjustment andweaning will be performed at regular intervals of 12hrs and as necessary, based on the ARDS Clinical Network Mechanical Ventilation Protocol.(15)

Baseline Endotracheal aspirate for gram staining and culture sensitivity will be sent on intubation.

Monitoring:

Blood values of arterial pH, partial pressure of arterial oxygen (PaO2), partial pressure of arterial carbon dioxide (PaCO2), ratio of PaO2 and fraction of inspired oxygen (FiO2) i.e., PaO2/FIO2, will be recorded on arrival in ICU and at an interval of 12hours subsequently. Mechanical ventilator setting which includes VT (scaled to the predicted body weight [PBW]), RR, PEEP, respiratory system compliance, elastance, Plateau pressure and Driving Pressure (DP) will be recorded on intubation and then at an interval of 12 hours subsequently. Serial data will be recorded 12th hrly for a maximum of 14 days. The removal of the neuromuscular blocker or death will determine the end of the collection.

 Computed Tomography (CT) scan acquisition and analysis: Patients will be moved to the radiology department and a Multidetector row computed tomography (Philips Ingenuity 128 slice CT Scanner, Netherlands) from the thoracic inlet to the diaphragm will be performed. CT scan will be taken at full inspiration with PEEP of 5cm H2O sustained by a portable ventilator. All images will be reconstructed using standard reconstruction algorithms with a slice thickness of 1.0 mm and a reconstruction interval of 0.8 mm. The lung gas volume and amount of well-inflated tissue will be computed with an automated algorithm software (Philips Intellispace portal 11 solutions). Lung regions will be classified into 4 categories by CT attenuation densities: 1) hyperinflated, density between -1000 and -901 Hounsfield units (HU); 2) normally aerated, density between -900 and -501 HU; 3) poorly aerated, density between -500 and -101 HU; and 4)

non-aerated, density between +100 and -100 HU. The volume of each lung region will be divided by predicted total lung capacity to correct for differences in physique among patients.

 Calculation of mechanical power:

Mechanical power will be obtained as the algebraic sum of three components as described earlier by Gattinoni et al. (5):

1. Elastic–static power (J/min) =0.098 *VT*RR*PEEP.

2. Elastic–dynamic power (J/min)=0.098*VT*RR*0.5*DP.

3. Resistive power (J/min) = 0.098 * VT * RR * (Ppeak-Pplat).