ARDS Flashcards
define
Acute respiratory distress syndrome (ARDS) is caused by the increased permeability of alveolar capillaries leading to fluid accumulation in the alveoli, i.e. non-cardiogenic pulmonary oedema.
It is a serious condition that has a mortality of around 40% and is associated with significant morbidity in those who survive.
causes: direct lung injury
direct lung injury
- diffuse pulmonary infection - mostly viral
- aspiration pneumonitis
- inhalation injury - smoke and other noxious gases such as nitrogen dioxide
- near drowning
- oxygen therapy and mechanical ventilation
causes: indirect lung injury
These include:
- shock - from:
- sepsis
- multiple trauma
- acute pancreatitis
- burns
- complicated abdominal surgery
- fat embolism
- disseminated intravascular coagulation
- multiple blood transfusions
- uraemia
- eclampsia
- raised intracranial pressure
- radiation
- high altitude
- prolonged hypotension
cardiopulmonary bypass
drugs and toxins - salicylates, heroin, opiates, paraquat
hypersensitivity or anaphylactic reaction
multisystem disease - vasculitis, thrombotic thrombocytopaenic purpura, acute hepatic failure
s/s
Clinical features are typically of an acute onset and severe:
- dyspnoea
- elevated respiratory rate
- bilateral lung crackles
- low oxygen saturations
Patients with ARDS show a gradually worsening picture of respiratory disturbance which may rapidly become life threatening.
Usually, there is a latent period of hours or days after the insult with the patient hospitalised for one of the known aetiologic conditions.
This is then followed by stages of:
- respiratory distress - dyspnoea, tachypnoea - but with a normal chest radiograph
- increasing cyanosis, arterial hypoxaemia, and respiratory failure; the chest x-ray now shows diffuse bilateral shadowing which may be asymmetric depending on cause and recent posture
- hypoxaemia becomes refractory to high inspired oxygen and respiratory acidosis develops
- there may be death from hypoxic cardiac arrest
ix
A chest x-ray [bilateral infiltrates] and arterial blood gases are the key investigations.
bloods: fbc, u+e, lfts, clotting, amylase, crp, cultures, abg
criteria for ARDS
Criteria (American-European Consensus Conference)
- acute onset (within 1 week of a known risk factor)
- pulmonary oedema: bilateral infiltrates on chest x-ray (‘not fully explained by effusions, lobar/lung collapse or nodules)
- non-cardiogenic (pulmonary artery wedge pressure needed if doubt)
- pO2/FiO2 < 40kPa (200 mmHg)
mx
Consult expert advice.
- Once the cause has been identified and treated, the aim of treatment is to resuscitate the patient and achieve adequate gas exchange without further exacerbating injury.
- Mechanical ventilation is often required using high inspired oxygen, usually for a long time. A positive end-expiratory pressure (PEEP) of 5-15 cm H2O is generally considered helpful to prevent premature alveolar closure. Although PEEP helps with oxygenation, it does so at the expense of cardiac output.
- Blood gases should be checked regularly. Inspired oxygen concentrations should be kept to a minimum that will prevent severe hypoxaemia - oxygen toxicity is not a problem if FiO2 is below 50%.
- Fluids should be replaced, but with care not to overload. Assessment may be made by measuring blood pressure and and urine output. Finer assessment may be necessary using a Swan Ganz catheter to measure the pulmonary capillary wedge pressure - this reflects the left atrial pressure. Monitoring of central venous pressure is not generally considered appropriate in ARDS.
- If there is evidence of circulatory failure despite adequate hydration - such as fall in cardiac or urine output - then consider low dose dopamine as a renal arterial dilator, and dobutamine for its positive inotropic action. If there is fluid overload, consider frusemide 40-120 mg per 24 hours i.v.
- Antibiotics should be given if there has been an infective aetiology.
define transudate [which the fluid might be]
= extravascular fluid
low protein content
low specific gravity
define exudate [which the fluid might be]
fluid that leaks out of blood vessels into nearby tissues due to inflammation
high protein content
ddx of pulm oedema [transudates]
high cap hydrostatic pressure
- ccf
- iatrogenic fluid overload
- renal failure
- relative increase in neg pressure pulmonary
- oedema
low cap oncotic pressure
- liver failure
- nephrotic syndrome
- malnutrition, malabsorption, protein losing enteropathy
high interstitial pressure
- low lymphatic drainage eg. ca
ddx of pulm oedema [exudates]
ARDS
