Expanded Explanation - Acute Respiratory Distress Syndrome

08 Apr 2022
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Here is an example of an Expanded Explanation for Acute Respiratory Distress Syndrome:

Overview

Key Facts

Severe inflammation of the lungs results in an inefficient supply of oxygen to the body that can be life threatening.
Acute respiratory distress syndrome (ARDS) is severe inflammation of the lungs resulting in a life-threateningly inefficient supply of oxygen to the body.
ARDS is usually caused by a serious underlying health condition and most people who develop the condition are already hospitalised due to this.

ARDS comprises the following:
- acute onset (within one week)
- arterial hypoxaemia
- bilateral, fluffy pulmonary infiltrates on chest X-ray, not explained by effusion/nodules or collapse
- non-cardiogenic pulmonary oedema (pulmonary capillary wedge pressure <18cmH20)
- reduced lung compliance

Diagnosis is based on a combination of examination and imagining findings.
The severity of ARDS is classified by the degree of hypoxaemia occurring.
Management is usually based in an Intensive Care setting with ventilation and supportive measurements as well as treating the underlying cause.
The mortality rate is around 1/3 of all cases; this is often due to the underlying cause.
Those that do survive often have no lasting lung damage.

Epidemiology

Most cases of ARDS will occur in patients already admitted to hospital.
It can develop quickly in the community, such as patients with influenza/pneumonia/aspiration.

Aetiology

Conditions directly affecting the respiratory system include:
- cardiopulmonary bypass
- pulmonary contusion
- smoke inhalation
- oxygen toxicity
- pneumonia
- near-drowing
- aspiration of gastric contents

Other conditions include:
- sepsis
- burns
- disseminated intravascular coagulation (DIC)
- pancreatitis, uraemia
- drug overdoses (e.g. diamorphine, methadone, barbiturates, paraquat)
- trauma

Pathophysiology

Damage to capillary endothelium and alveolar epithelium and increased capillary permeability result in an accumulation of protein-rich fluid in the alveoli, causing diffuse alveolar damage.
Activation of neutrophils causes inflammation and further lung damage.

Clinical Presentation

Patients with ARDS can present with the following:
- severe shortness of breath
- raised respiratory rate
- shallow breathing
- drowsiness
- confusion
- feeling lightheaded

Differential Diagnosis

Some differential diagnosis of ARDS include:
- panic attack
- diabetic ketoacidosis (in early stages, the breathing in shallow and can progress to Kussmaul breathing, indicating severe metabolic acidosis)
- asthma attack
- carbon monoxide poisoning

Diagnosis/Investigation

Diagnosis and investigations for ARDS are as follows:
- blood including septic screen, cultures
- blood gases
- pulse oximetry
- chest X-ray - pulmonary oedema, bilateral alveolar shadowing, air bronchograms

Other investigations may include echocardiogram and urine culture. This depends on the clinical scenario and suspected underlying cause.

Management

No specific treatment is available for ARDS, and management is essentially supportive.

Non-pharmacological

Management is guided by the severity of hypoxaemia:
- mild (200 mm Hg < PaO₂/FiO = 300 mmHg)
- moderate (100 mm Hg < PaO₂/FiO = 200 mmHg)
- severe (PaO₂/FiO = 100 mmHg)
Patients usually require management within the Intensive Care Unit.

Supplemental oxygen is given and patients frequently require mechanical ventilation:
- pressure-controlled, inverse-ratio ventilation is used because this lowers peak airway pressure, reduces barotrauma and creates better distribution of gas in the lungs
- with the addition of positive end-expiratory pressure (PEEP), there is greater alveolar recruitment, increased functional residual capacity, better lung compliance and reduced shunt
- turning the patient into the prone position intermittently allows those dependent parts of the lung that are susceptible to atelectasis to re-expand and improves blood flow to the ventilated parts of the lung

Guidelines from the American Thoracic Society/European Society of Intensive Care Medicine/Society of Critical Care Medicine Clinical Practice:
- for all patients with ARDS, the recommendation is strong for mechanical ventilation using lower tidal volumes and lower inspiratory pressures
- for patients with severe ARDS, the recommendation is strong for prone positioning for more than 12 h/d
- for patients with moderate or severe ARDS, the recommendation is strong against routine use of high-frequency oscillatory ventilation and conditional for higher positive end-expiratory pressure and recruitment manoeuvres

Additional evidence is necessary to make a definitive recommendation for or against the use of extracorporeal membrane oxygenation in patients with severe ARDS.
Other supportive measures include IV hydration and nutritional support, and venous thromboembolism prophylaxis.

Pharmacological

Sepsis should be treated empirically until causative organisms are identified.
Inhaled nitric oxide (NO) causes selective vasodilatation of the ventilated areas of the lung when inhaled at low concentrations.
Corticosteroids have been shown to be beneficial in the latter stages of ARDS (ie progressive pulmonary interstitial fibroproliferation is present).

Prognosis

The prognosis of ARDS is:
- associated with substantial morbidity
- fatal in 1/3 of cases, though usually due to the underlying illness and not directly as a result of ARDS

The lungs make a full recovery in most cases of those who survive.

Other

The use of exogenous surfactant in adult patients has no proven value.