1. Acute Respiratory Distress Syndrome

2. Outline
Definition
Low tidal volume ventilation (6 mL/kg of predicted body weight)
Plateau pressure
Permissive hypercapnia
Hypoxemia (PEEP/FiO2)
Prone ventilation
Neuromuscular blockade
Conservative fluid management

3. At it’s core, ARDS is non-cardiogenic pulmonary edema.

4. ARDS

5. ARDS Definition: ARDS is, in essence, non-cardiogenic pulmonary edema
Clinically characterized by bilateral pulmonary infiltrates on imaging with hypoxemia that is not due to congestive heart failure
Hypoxemia in terms of PaO2/FiO2 ratio defines severity of ARDS

6. ARDS Etiologies Sepsis Aspiration Pneumonia Severe trauma
Because sepsis is the most common cause of ARDS, the management of ARDS is part of the Surviving Sepsis Guidelines
Aspiration
Pneumonia
Severe trauma
Massive transfusions
Pancreatitis

7. ARDS: Berlin Definition
Severity
PaO2/FiO2
Mild ARDS
200 mmHg < PaO2/FiO2 < 300 mmHg
Moderate ARDS
100 mmHg < PaO2/FiO2 < 200 mmHg
Severe ARDS
PaO2/FiO2 < 100 mmHg

8. ARDS Highlights Best evidence with mortality benefit
Low tidal volume ventilation (6 mL/predicted body weight in kg)
Need to have a height on every patient to calculate predicted body weight
In severe ARDS (PaO2/FiO2 < 150)
Prone ventilation
Neuromuscular blockade with cis-atracurium for 48 hours
Other things that can be tried
Inhaled nitric oxide (no mortality benefit)
Recruitment maneuvers (no mortality benefit)

9. ARDS Highlights
Once septic shock is over, keeping the patient dry is beneficial
Can help liberate from mechanical ventilation faster without increased renal failure

10. Acute Respiratory Distress Syndrome
Low tidal volume ventilation has been shown to be provide mortality benefit
Need to measure height for every patient in the ICU
Low tidal volume ventilation is based on predicted body weight
Dependent on height of patient
Neuromuscular blockade with cis-atracurium for 48 hours is beneficial in severe ARDS (PaO2/FiO2 < 150 mmHg)
Prone ventilation is beneficial in severe ARDS (PaO2/FiO2 < 150 mmHg)

11. Surviving Sepsis Guidelines 2012
Young NEJM 2013 showed no benefit to high frequency oscillation ventilation (HFOV)
Ferguson NEJM 2012 trial stopped early because of increased mortality in the group receiving (HFOV)

12. Low Tidal Volume Ventilation
NEJM 2000 trial suggested decreased mortality in patients ventilated at a tidal volume of 6 mL/kg of predicted body weight compared to 12 mL/kg of predicted body weight
Based on ideal body weight (kg)
Male: (centimeters of height – 152.4)
Female: (centimeters of height – 152.4)
All the patients were ventilated on volume assist-control mechanical ventilation

13. Low Tidal Volume Ventilation
Ventilation with Lower Tidal Volumes as Compared with Traditional Tidal Volumes for Acute Lung Injury and the Acute Respiratory Distress Syndrome NEJM May 4, 2000

14. ARDS Net Protocol
Given the decrease in mortality with low tidal volume ventilation, the ARDS Net protocol is the standard of care for patients with ARDS
The initial tidal volume is set at 8 mL/kg of predicted body weight and titrated down to 6 mL/kg of predicted body weight
It is important to know the height of the patient or have it measured, as the predicted body weight (ideal body weight) is calculated from the height of the patient
The ARDS Net protocol is given on the next two slides

17. Plateau Pressure
Mentally separate the lungs into the airways and the lung parenchyma
The plateau pressure is the pressure required to inflate the lung parenchyma (without the pressure required to push the air through the airways)
Airways
Lung Parenchyma

18. Plateau Pressure
In volume ventilation, when the ventilator pushes air into the patient, the pressure required is a sum of the pressure needed to overcome the resistance of the airways and the pressure required to inflate the lung parenchyma
Total pressure = Pressure to overcome resistance of airways + Plateau pressure (Pressure to inflate lung parenchyma)
Airways
Lung Parenchyma

19. Plateau Pressure Goal
Plateau pressure is obtained by performing an end- inspiratory pause when the patient is vented in a volume mode of ventilation
This is a measure of the pressure seen by the lung parenchyma after it is inflated by the ventilator
Plateau pressure can be thought of as a surrogate for how “stiff” the lungs
For a given tidal volume of ventilation, the higher the plateau pressure, the “stiffer” the lungs are
If you think of the lung parenchyma as a balloon, a stiffer balloon (lung) will take more pressure to inflate while inflating a less stiff balloon (lung) to the same volume will take less pressure

20. Plateau Pressure Goal
In ARDS, the goal plateau pressure is < 30 cm H2O as this was part of the ARDS Net trial protocol
If the plateau pressure is > 30 cm H2O, the ARDS Net protocol recommends decreased the tidal volume by 1 mL/kg of predicted body weight
To a minimum of 4 mL/kg of predicted body weight
To understand why lowering the tidal volume lowers the plateau pressure, again think of the lung parenchyma as a balloon.
If inflating the balloon to 500 mL requires a pressure of 35 cm H2O (this would be the plateau pressure), then inflating the same balloon to 400 mL will lower the pressure required as you are not inflating the balloon as much

21. Permissive Hypercapnia
Because of the mortality benefit with low tidal volume ventilation, the pCO2 of the patient is allowed to rise.
For patients in ARDS, the pH is allowed to drift down to 7.30
If the pCO2 rises with low tidal volume ventilation but the pH is > 7.30, there is no need to change the tidal volume as the body tolerates acidemia fairly well

22. Hypoxemia
The hypoxemia associated with ARDS is secondary to alveoli that are “flooded” because of increased pulmonary vascular permeability

23. Hypoxemia
The hypoxemia associated with ARDS can be corrected by increasing PEEP and/or FiO2
The increased PEEP (peak end expiratory pressure) allows recruitment of alveoli that are “flooded” secondary to the increased pulmonary vascular permeability due to ARDS
This recruitment of alveoli helps to improve oxygenation
Increasing FiO2 (inspired oxygen concentration) can also improve oxygenation by increasing the oxygen gradient from the inspired air to the patient’s blood

24. PEEP in ARDS
There is no mortality benefit between low PEEP versus high PEEP
NEJM 2004
Cochrane Review 2013
Using higher PEEP versus lower PEEP will increase oxygenation

25. Prone Positioning in Severe Acute Distress Syndrome
Guerin et al Prone Positioning in Severe Acute Distress Syndrome NEJM 2013
In patients with ARDS where Pa/FiO2 < 150 mmHg, there was decreased mortality in patients that received prone positioning while being mechanically ventilated compared to supine positioning
Results
28 day mortality: 16% prone vs. 32.8% supine (p < 0.001)
90 day mortality: 23.6% prone vs. 41% supine (p < 0.001)

26. Prone Positioning in Severe Acute Distress Syndrome
Because edema becomes gravitationally dependent in supine ARDS patients, prone positioning improves oxygenation by increasing alveoli recruitment of those alveoli that have been “flooded”

27. Neuromuscular Blockade in ARDS
Papazian et al Neuromuscular Blockers in Early Acute Respiratory Distress Syndrome NEJM September 16, 2010
Decreased mortality using the neuromuscular blocker (cis- atracurium) in ARDS patients with PaO2/FiO2 < 150
Cis-atracurium was used only for the first 48 hours after enrollment into the trial
There was no difference in ICU-acquired paresis between groups (placebo versus cis-atracurium)
This is important because previous studies have suggested that the use of neuromuscular blockers (paralytics) have been associated with ICU-acquired paresis

28. Conservative Fluid Management in ARDS
After the initial septic shock is over, a conservative fluid management in ARDS patients is associated with increased ventilator free days and ICU free days, without increasing mortality or renal failure requiring initiation of hemodialysis
Comparison of Two Fluid-Management Strategies in Acute Lung Injury NEJM June 2006
In patients without hypoperfusion, the conservative fluid management group had a CVP goal of < 4 mmHg
In comparison, the liberal fluid management group had a CVP goal of 4-8 mmHg
This was accomplished with the use of diuretics in patients that were not on vasopressors and did not show signs of hypoperfusion