Tuesday, July 1, 2014

ARDS: Early Ventilator Settings Matter

Author: Sean Weaver, DO MPH
Emergency Medicine Resident
University of Nevada, School of Medicine

The following blog post appeared initially at www.lasvegasemr.com/foam-blog and is reproduced with the permission of the author.

A 40y/o male with no known significant PMH presents with non-productive cough, SOB, DOE, diarrhea, and possibly fever though he is not sure. He has not had a similar illness in the past, denies any recent travel, no sick contacts and denies ever smoking.

Vitals: HR 102, RR 20, O2 sats: 90% on RA, BP: 116/56, Temp: 98ºF

Physical exam reveals an ill appearing male in mild distress. Mucous membranes are dry with a thrush like appearance isolated to the soft palate. Pulmonary exam reveals equal breath sounds bilaterally with some scattered rhonchi, dry crackles and occasional wheeze. ECG was unremarkable. Two view chest x ray was positive for diffuse prominence of the interstitium with interstitial infiltrates vs interstitial edema.

While awaiting admission patient becomes tachypneic and requires NRB mask. Patient stated that he was having difficulty breathing, felt tired and agreed to intubation.

Vitals: HR 103, RR 40, O2sats: 94 % on NRB, BP: 124/86

The patient is successfully intubated, placed on AC/VC w/ PEEP of 10, TV 500, RR 14 and FiO2 of 100% with orders to titrate to O2 sats >90%. An ABG was ordered for two hours post intubation.

Representative post intubation chest X-ray:

Chest x ray is representative of the patient’s portable chest X-ray after intubation.
This image is freely available on-line, courtesy the University of Washington (2004) copyright policy at the following link: http://courses.washington.edu/med620/mechanicalventilation/case3answers.html

Post intubation ABG: pH: 7.39; PCO2: 29; PO2: 112; FiO2: 100; SO2: 98

What is your diagnosis?

What interventions, specifically regarding the ventilator settings, could dramatically reduce the mortality risk in this patient?

Acute Respiratory Distress Syndrome (ARDS)
  • PaO2/FiO2 = 112/1.0 = 112
  • Patient was admitted to the ICU with sepsis and pneumonia of unknown etiology. In the ICU the patient was diagnosed with HIV/AIDS and PCP was confirmed after BAL. This was the most likely the underlying factor that contributed to his rapid decompensation in the emergency department and progression to ARDS.
Prior to 2000 and the publication of the Acute Respiratory Distress Networks (ARDSNet) ventilated patients were routinely placed on 10 to 15ml/kg tidal volume. This landmark multicenter randomized control trial found that lower tidal volumes led to increased probability of survival and discharge at 180 days as well as lower overall mortality rates in patients with ARDS (1).

Figure 1. Probability of survival and of being discharged home and breathing without assistance during the first 180 days after randomization in patients with acute lung injury and acute respiratory distress syndrome.

This data challenged the status quo and pushed clinicians to rethink their approach to ventilator patients. Unfortunately, 14 years later, emergency department physicians are still struggling to adopt these lung-protective ventilation strategies (2). While patients with ARDS have an improved chance of survival relative to twenty years ago, the mortality rate for severe ARDS is very high at approximately 45% (4). Our role as emergency physicians is to give these patients the best chance at making a full recovery. The following outlines the basics about ARDS and the specifics interventions that can be utilized in the ED.

Definition of ARDS (3)
  • Hypoxia
  • PaO2/FiO2 ratio of ≤300 
  • Bilateral pulmonary infiltrates (patchy, diffuse or homogenous) on chest x ray 
  • No clinical evidence of left atrial hypertension
Classifications and Associated Mortality (4)
In 2011 a panel of experts established a new criteria for defining ARDS. At issue was the lack of reliability and validity of the previous definition established in 1994. This new effort was an initiative of the European Society of Intensive Care Medicine and was endorsed by the American Thoracic Society and the Society of Critical Care Medicine. After examining the degree of hypoxemia as well as four other variables, the panel determined that ARDS was best classified as mild, moderate and severe based on the PaO2/FIO2 ratio. They arrived at the Berlin Criteria.

The Berlin Criteria

The panel also decided to remove the term acute lung injury (ALI). The initial intent of the term was to create an inclusive term for all patients with ARDS. However, the panel felt that clinicians were using it incorrectly to describe a less severe form of hypoxia. They felt removing it from the nomenclature used to describe ARDS was appropriate (4).

Pathogenesis (5)
  • Inflammatory process that involves both lungs.
  • Circulating neutrophils adhere to the endothelium of therapy pulmonary capillaries and release cytoplasmic granules resulting in damage to the endothelium. This produces leaky capillaries and an exudation of fluid into the lung parenchyma resulting in pulmonary edema.
  • Neutrophils pass through the leaky capillaries and produce and inflammatory response in the lung parenchyma, resulting in further damage and respiratory decompensation.

Lung Protective Strategies (3)

Low volume is key to preventing additional injury (6) and improves mortality (1, 7)
  • Goal tidal volume: ≤6ml/kg ideal body weight (minimum 4ml/kg)
  • Original ARDSNet study identified 6ml/kg ideal body weight as the goal but a minimum of 4ml/kg was acceptable (1)
  • Ideal body weight (IBW): Females = 50 + 2.3(ht -60); Males = 45.5 + 2.3(ht -60) 
    • Weight: kg 
    • Height: in
  • ARDSNet Ventilator Protocol card outlines initial ventilator set up.
    • Based on ideal body weight start on any ventilator mode with tidal volume of ≤ 8ml/kg
    • Reduce tidal volume by 1ml/kg at intervals ≤2 hours until 6ml/kg is achieved 
    • Set RR to match minute ventilation. Do not exceed 35 bpm.
  • Given the realities of practicing in a busy emergency department, titrating tidal volume may not be a practical approach. One possible option would be to initiate a standardized protocol for each patient post-intubation and empower respiratory therapists and/or nurses to titrate the tidal volume as appropriate.
Positive End Expiratory Pressure (PEEP)
  • Start with a PEEP of 5cm/H2O
  • Oxygenation goal: PaO2 of 55-88mmHg or SpO2 of 88-95%
  • Increase PEEP by 1-2cm/H2O to meet oxygenation goals
  • Recruitment maneuvers are not necessary in the ED and do not definitely reduce mortality or decrease time on ventilation (8)
  • Peak plateau pressures should remain ≤30cmH2O

Prone Position Reduces Mortality (9)
  • Placing the patient in a prone position while on mechanical ventilation improves ventilation, oxygenation and reduces mortality.
  • Prone positioning should be reserved for the ICU where patients are closely monitored and respiratory therapy is close.

Implications for Emergency Medicine
  • ARDS has an incredibly high mortality. Through early recognition and proper initial ventilator management we can help maximize the patient's chance at a positive outcome.
  • Low tidal volumes are key. Start at ≤8ml/kg IBW and aggressively titrate down to achieve goal tidal volume of 6ml/kg
  • PEEP should be utilized. Start at a PEEP of 5cmH20 but if oxygenation goals are not met increase the PEEP in increments of 1-2
  • While prone positioning improves mortality it is a complicated maneuver that should only be performed in the ICU setting.

1. Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome. The Acute Respiratory Distress Syndrome Network. N Engl J Med. 2000;342(18):1301-8.

2. Fuller BM, Mohr NM, Dettmer M, et al. Mechanical ventilation and acute lung injury in emergency department patients with severe sepsis and septic shock: an observational study. Acad Emerg Med. 2013;20(7):659-69.

3. Available at: http://www.ardsnet.org/system/files/Ventilator Protocol Card. Accessed April 7, 2014.

4. Ranieri VM, Rubenfeld GD, Thompson BT, et al. Acute respiratory distress syndrome: the Berlin Definition. JAMA. 2012;307(23):2526-33.

5. Marino PL. The Little ICU Book of Facts and Formulas. Lippincott Williams & Wilkins; 2008.

6. Lipes J, Bojmehrani A, Lellouche F. Low Tidal Volume Ventilation in Patients without Acute Respiratory Distress Syndrome: A Paradigm Shift in Mechanical Ventilation. Crit Care Res Pract. 2012;2012:416862.

7. Petrucci N, De feo C. Lung protective ventilation strategy for the acute respiratory distress syndrome. Cochrane Database Syst Rev. 2013;2:CD003844.

8. Hodgson C, Keating JL, Holland AE, et al. Recruitment manoeuvres for adults with acute lung injury receiving mechanical ventilation. Cochrane Database Syst Rev. 2009;(2):CD006667.

9. Sud S, Friedrich JO, Adhikari NK, et al. Effect of prone positioning during mechanical ventilation on mortality among patients with acute respiratory distress syndrome: a systematic review and meta-analysis. CMAJ. 2014.

1 comment:

  1. Thanks for helping me better understand this disease. I think this disease affects more Americans than I realized. I think giving good senior care to these people is the most important thing we can do for them.