Pathophysiology of Respiratory Failure Fern White & Annabel Fothergill

Definition? Acute respiratory failure occurs when the pulmonary system is no longer able to meet the metabolic demands of the body due to inadequate gas exchange. Hypoxaemic respiratory failure: drop in blood oxygenation. Hypercapnic respiratory failure: rise in arterial CO2. PaO2 <8 kPa PaCO2 >6.7 kPa

What does the oxygenation of the blood in the lungs depend on? ● P A O 2 (Partial pressure of oxygen in alveolus) ● Diffusing capacity ● Perfusion ● Ventilation-perfusion matching What layers make up the diffusion barrier for oxygen within the lungs?

What does the removal of CO2 from the blood in the lungs depend on? ● Largely dependent on alveolar ventilation o RR = respiratory rate o V T = Tidal volume o V D = Dead space ● Respiratory rate ● Tidal volume ● Ventilation-perfusion matching

Ventilation and perfusion If alveoli are ventilated without being perfused: Increased dead space as these alveoli do not take part in gas exchange. Shunting: Non-ventilated alveoli remain perfused. Blood leaving the lungs is poorly oxygenated. In time, hypoxic vasoconstriction will result in a reduction in perfusion to non-ventilated alveoli and a relative increase in perfusion to ventilated alveoli, thus reducing the magnitude of the shunt and increasing the arterial saturation

Name 4 mechanisms of respiratory failure ● Low inspired Po 2 ● Hypoventilation: O2 in the alveolus is not replenished, CO2 is not removed. Alveolar partial pressure of oxygen falls with a corresponding fall in the arterial partial pressure and hence saturation. The fall in alveolar PO2 is small and is easily compensated for by small increase in inspired oxygen concentration ● Ventilation-perfusion mismatch ● Diffusion abnormality: Abnormality of the alveolar membrane or a reduction in the number of capillaries resulting in a reduction in alveolar surface area. Desaturation on exercise.

Type 1 or Type 2? Type 2: Inadequate alveolar ventilation; both oxygen and carbon dioxide are affected. Defined as the build up of carbon dioxide levels (PaCO2) that has been generated by the body but cannot be eliminated. Causes: drug overdose (reduced breathing effort), neuromuscular disease, chest wall abnormalities, and severe airway disorders with increased resistance or reduced lung surface area (eg, asthma and COPD). Type 1: Hypoxemia without hypercapnia - PaCO2 may be normal or low. It is typically caused by a ventilation/perfusion (V/Q) mismatch. Causes: anything that creates a mismatch (acute lung diseases such as pneumonia, PE, ARDS) or hypoventilation (low ambient oxygen, neuromuscular disease).

What does this x-ray show? What type of respiratory failure can it lead to?

What are the clinical signs? ● Respiratory compensation o Tachypnoea o Accessory muscles o Recession o Nasal flaring ●Sympathetic stimulation o Increased HR o Increased BP (early) o Sweating ●Tissue hypoxia o Altered mental state o Lactic acidosis o Decreased HR and BP (late) ●Haemoglobin desaturation o Cyanosis ●Hypercapnia o Flapping tremor o If severe (>10kPa) - unconsciousness + respiratory

Treatment Type 1 Respiratory Failure Pneumonia – Antibiotics – Physiotherapy Pulmonary Oedema – Vasodilators – Diuretics Pneumothorax – Chest drain or aspiration Type 2 Respiratory Failure Asthma – Bronchodilators (salbutamol) – Corticosteroids Drug Overdose – Antagonist Guillain–Barré Syndrome – (autoimmune demyelination) – Immunoglobulin or Plasmapheresis

Thanks! Any questions?