Respiratory Failure Flashcards
What values constitute respiratory failure?
A paO2 7kPa
When does respiratory failure occur?
Respiratory failure occurs when either or both of pulmonary gas exchange or respiratory muscles fail.
What are the two types of respiratory failure? What differentiates them?
Type one : Low oxygen, normal/ low carbon dioxide
Type two : low oxygen, high carbon dioxide
Type one and type two are on a physiological continuum. Mild to moderate shunts –> type I failure- paCO2 is maintained by hyperventilation but as the disease progresses and the shunting becomes more severe, the ability of the central respiratory drive to compensate is overcome –> hypercapnia and type II failure.
How does respiratory failure occur?
Disturbances in ventilation and/ or perfusion in pulmonary diseases leads to VA/ Q imbalance —> acid base imbalance and abnormal changes in arterial blood gases.
What type of diseases most often lead to type I respiratory failure?
Restrictive lung diseases such as pulmonary oedema and pneumonia which cause a reduction in long volume –> hypoxaemia
What is respiratory failure?
Respiratory failure is when alveolar ventilation is not enough to maintain normal arterial blood gases.
What type of diseases most often lead to type II respiratory failure?
Obstructive diseases such as COPD and asthma. Lung volume is unchanged but airflow is reduced = reduction in alveolar ventilation = failure of gas exchange ( blood is not oxygenated and co2 is not expired ). There is some reactive hyperventilation but the respiratory centre becomes less sensitive.
What happens in asthma?
Brocnchoconstriction, mucus hypersecretion and mucosal inflammation –> narrowing of airways = increased airway resistance and voluntary respiratory effort is needed to expel air and small airways close before tidal volume is expelled. The functional residual capacity increases leaving over expanded lungs which makes inspiration more and more difficult —> airway obstruction and uneven ventilation. Continued blood flow to some hypoventilated areas causes ventilation perfusion imbalance —> arterial hypoxaemia. Early in the attack, the patient compensates by hyperventilating the unobstructed areas = type one respiratory failure but later on this capacity is impaired by increased FRC, more extensive airway narrowing and muscle fatigue. Hypoxaemia worsens and paco2 rises leading to respiratory acidosis and type two respiratory failure.
What happens in COPD?
COPD is characterised by Chronic persistent irreversible airway obstruction, increased inflammation and mucus hypersecretion. Small airways also collapse easily due to destruction of pulmonary elastic tissue. This leads to a rise in FRC —> barrel chest. The effort required to ventilate the alveoli against increased resistance gives rise to dyspnoea. Inefficient gas exchange = type chronic two respiratory failure.
What happens in emphysema?
There is enlargement of alveolar sacs by the destruction of elastin in the alveolar walls by excess activity of neutrophil elastase. Large spaces called bullae form, which reduces the surface area for gas exchange —> in efficient gas exchange = type two respiratory failure.
What are the clinical effects of respiratory failure?
Hypoxia –> inability to carry out aerobic metabolism = cellular dysfunction; effects on the brain may give rise to confusion and drowsiness –> coma and death. Renal tubular cells increase erythropoietin production in response to reduced pao2. Polycethaemia increases O2 carrying capacity of blood. Pulmonary vasoconstriction diverts blood from poorly ventilated areas to ventilated parts of the lung —> pulmonary hypertension.
Hypercapnia –> respiratory acidosis.
What is hypoxaemia?
Low partial pressure of oxygen in the peripheral blood.
What can cause hypoxaemia?
- hypoventilation
- gas diffusion impairment
- shunt
- ventilation perfusion mismatch
What can cause hypoventilation?
Decreased respiratory drive : Central alveolar hypoventilation, COPD, CNS sedation
Peripheral neuromuscular disease: muscular dystrophy, myasthenia graves, MND, myotonic dystrophy, polio
Thoracic cage abnormalities: kyphoscoliosis, ankylosing spondylitis
Diaphragmatic paralysis
What can cause diffusion impairment?
- Emphysema
- interstitial lung diseases
- left ventricular failure
