Acute Respiratory Distress Syndrome

Dr. Eric Mugambi

Historical perspective

1821 – Laennec in ‘A treatise on

diseases of the chest’

Idiopathic anasarca of the lungs

Trauma associated

World war 1 & 2

Vietnam war

Historical perspective

1950s - Advances in critical care

Respirators

Airway access

Hence the term ‘respirator lung’

Other terms related to inciting agent

Post-traumatic, shock lung, wet lung,

DaNang lung)

Historical perspective

1967 – Case series of 12 patients

acute onset

tachypnœa

hypoxæmia

loss of compliance

Ashbaugh D, Boyd Bigelow D, Petty T, Levine B. ACUTE RESPIRATORY DISTRESS IN ADULTS. The Lancet. 1967;290(7511):319-23.

Defining features

Hypoxemia

Low Pa02

Stiff lungs

Reduced compliance

Hyaline membranes

Early fibrosis in patients who died

Epidemiology

Not well characterized due to lack of uniform diagnosis

Before ICUs – most patients died No opportunity for organized

investigations US data

50,000 to 200,000 cases per year (2003) 79/100,000 – ALI 59/100,000 – ARDS 4-9% in ICU settings

Epidemiology

Substantial recovery in lung function occurs within 6-12 months In few cases muscle weakness and neuropsychiatric

problems may persist

Mortality has improved significantly from 54% in 1983 to as low as 25% in 2004 but has plateaued since then Better prognosis with younger age

Leading cause of death is multiple organ failure

Definitions

1967 – Ashbaugh et al

1988 – Murray and colleagues

1992 – American European Consensus Conference (AECC)

2012 – Berlin definition

AECC (1994)

1. Acute onset of respiratory failure

2. Bilateral infiltrates resembling pulmonary

edema

3. No evidence of left atrial pressure elevation -

PCWP <18mmhg

4. Ratio of PaO2 to FiO2 is <200mmhg –

ALI – 200<PaO2/FiO2 < 300mmHg (Desaturation)

“Acute lung injury” no longer exists

Onset of ARDS (diagnosis) must be acute, as defined as within 7 days of some defined event, which may be sepsis, pneumonia, or simply a patient’s recognition of worsening respiratory symptoms.

The Berlin definition

The ARDS Definition Task Force: JAMA. 2012;307(23):2526-2533. doi:10.1001/jama.2012.5669.

Berlin definition

Bilateral opacities consistent with pulmonary edema must be present but may be detected on CT or chest X-ray.

There is no need to exclude heart failure in the new ARDS definition An “objective assessment“– meaning

an echocardiogram in most cases — should be performed if there is no clear risk factor present like trauma or sepsis.

Berlin definition

The ARDS Definition Task Force: JAMA. 2012;307(23):2526-2533. doi:10.1001/jama.2012.5669.

Hypoxemia

Cause of hypoxemia

Hypoventilation

Ventilation perfusion mismatch

Right to left shunt

Diffusion impairment

Reduced inspired oxygen tension

(Pio2)

Very low cardiac output

Etiology

Common causes (mnemonic PAST) Pneumonia Aspiration Shock Sepsis Trauma *Transfusion (multiple)

Other causes

Lung Contusion Near drowning Inhalational injury Reperfusion pulmonary edema

Other Cardio-pulmonary bypass Drug overdose Acute pancreatitis (alcohol increases risk!) Transfusion of blood products

Pathophysiology

Endothelial injury Increased vascular permeability – hallmark

Epithelial disruption Alveolar flooding Disorganized repair leads to fibrosis Septic shock in patients with bacterial

pneumonia Loss of type 2 cells

Impaired fluid transit – no fluid removal Loss of surfactant Loss of type 1 progenitor cells

Consequences

Impaired gas exchange leading to severe hypoxemia - 2/2 ventilation-perfusion mismatch, increase in physiologic dead space

Decreased lung compliance – due to the stiffness of poorly or non-aerated lung

Pulm HTN – 25% of pts, due to hypoxic vasoconstriction, Vascular compression by positive airway compression, airway collapse and lung parenchymal destruction

Normal BGB

Small bronchiole and alveolae

Alveoli and capillaries

Clinical Presentation

Acute onset within 12-36 hrs of inciting event, upto 5-7days

Dyspnoea, tachypnoea, hypoxemia, dry cough chest pain

O/E: tachycardia, cyanosis, tachypnoea, diffuse rales,

BGAs: resp alkalosis, hypoxemia

CXR: bilateral alveolar infiltrates over 75% of lung fields

No pulm venous congestion, no kerley b lines, no cardiomegaly, pleural effusions

Differential diagnoses

1. Pneumonia

2. Diffuse alveolar hemorrhage

3. Idiopathic acute eosinophilic pneumonia

4. Cryptogenic organising pneumonia

5. Acute interstitial pneumonia

6. Rapidly progressive cancer

7. Cardiogenic pulmonary edema

Management

Investigations Guided by clinical suspicion of

underlying illness (ddx) For monitoring progress in patients with

critical illness▪ Routine e.g. ABGs▪ Tests of Organ function

Management

Treat underlying illness

Minimize procedures

Prevent complications

Barotrauma

DVT

Stress ulcers

Promptly recognize nosocomial infections

Provide adequate nutrition

Therapies in ARDS

THERAPY RECOMMENDATION

Low tidal volume A (strong evidence from RCTs)

Minimize left atrial filling pressures

B (limited clinical data)

High PEEP C (recommended only as alternative)

Prone positioning C

Recruitment maneuvers C

ECMO C

High frequency ventilation D (Not recommended)

Glucocorticoids D

Surfactant, Inhaled NO, other anti-inflammatory

D

The future

Gene therapy Mesenchymal stem cells

Consequences

Impaired gas exhange leading to severe hypoxemia - 2/2 ventilation-perfusion mismatch, increase in physiologic deadspace

Decreased lung compliance – due to the stiffness of poorly or nonaerated lung

Pulm HTN – 25% of pts, due to hypoxic vasoconstriction, Vascular compression by positive airway compression, airway collapse and lung parenchymal destruction