11. Respiratory Failure Flashcards
What is the definition of respiratory failure?
Impairment in gas exchange causing hypoxaemia with or without hypercapnia
What is type 1 respiratory failure?
Low paO2<8kPa or O2 sat <90% breathing room air at sea level
pCO2 normal or low
G exchange impaired at level of alveolar-capillary membrane
Can progress to type 2
What is type 2 respiratory failure?
Low paO2 and high PaCO2>6.5kPa breathing room air at sea level
Reduced ventilators effort or inability to overcome increased resistance to ventilation of entire lung
What is hypoxaemia?
Low pO2 in arterial blood
what are the effects of hypoxaemia?
Impaired CNS function, confusion, irritability, agitation
Tachypnaea
Tachycardia
Cardiac arrhythmias and cardiac ischaemia
Hypoxic vasoconstriction of pulmonary vessels
Cyanosis
What are some causes of hypoxaemia?
Low inspired O2 - high altitude
Ventilation:perfusion mismatch
Diffusion defect - problems of the alveolar capillary membrane
Intra-lung shunt - acute respiratory distress syndrome
Hypoventilation - respiratory pump failure
What are the compensatory mechanisms that decrease hypoxia in chronic hypoxaemia?
Increase EPO secreted by kidney, raised Hb
Increased 2,3 DPG - shifts Hb sat curve so oxygen released more freely
Increased capillary density
What can chronic hypoxic vasoconstriction of pulmonary vessels result in?
Pulmonary hypertension
Right heart failure
Cor pulmonale
What is the most common cause of hypoxaemia?
Ventilation:perfusion mismatch
What happens when V:Q <1?
PaO2 is low
Initially PaCO2 rises until/unless there is compensatory hyperventilation, then PaCO2 will be either normal or low
Hyperventilation induced peripheral chemoreceptors firing secondary to hypoxaemia
What happens when V:Q>1?
PaO2 rises and PaCO2 falls
How can PE lead to V:Q<1?
Embolus results in redistribution of pulmonary blood flow
Blood is diverted to unaffected areas
Leads to V:Q ratio<1 if hyperventilation cannot match increased perfusion - causes hypoxaemia
What is a typical ABG in PE?
Low PaO2 - may be normal if hyperventilation can compensate
Low PaCO2
High pH
What can cause V:Q mismatch?
Asthma Pneumonia RDS in newborn Pulmonary oedema Pulmonary embolisms
What would ABG show in hypoxaemia caused by diffusion defect?
pO2 low
pCO2 normal or low
Type 1 respiratory failure initially
What can cause diffusion impairment?
Fibrotic lung disease - thickened alveolar membrane slows gas exchange
Pulmonary oedema - fluid in interstitial space increases diffusion distance
What are the causes of diffuse lung fibrosis?
Idiopathic pulmonary fibrosis
Asbestosis
Extrinsic allergic alveolitis
Pneumoconiosis
What is a shunt in the respiratory system?
When alveolus have no ventilation at all but it is still perfused
What is the most common cause of intra pulmonary shunt?
Acute respiratory distress syndrome
What happens to the lungs in acute respiratory distress syndrome?
Increased vascular permeability, oedema, fibrin-exudation
Heavy, red lungs showing congestion and oedema, alveoli contain fluid and lined by hyaline membranes
Loss of surfactant resulting in alveolar atelectasis, lung becomes stiff and less compliant
What does hypoventilation of the lung always cause?
Hypercapnia
Therefore causes type 2 respiratory failure
What are some common causes of acute hypoventilation?
Opiate overdose
Head injury
Very severe acute asthma
What are the common causes of chronic hypoventilation?
Severe COPD
What can cause disorders of ventilation?
Problems with: - central control - motor neurons - peripheral neuropathy - neuromuscular junction Muscle weakness Muscle fatigue Chest wall disorders End stage COPD End stage restrictive lung disease Severe asthma exacerbation
Give examples of central disorders that can lead to hypoventilation
Central sleep apnea Narcotic overdose Sedatives Medullary disorders Hypothyroidism CNS trauma/brainstem herniation
Give some examples of disorders of neuromuscular junction that can cause hypoventilation
Myasthenia gravis
Organophosphate toxicity
Botulism
Which motor disorders can cause hypoventilation?
Tetanus
ALS (amyotrophic lateral sclerosis)
Spinal cord injury at C3 level
Which diseases/conditions of muscle weakness or fatigue can cause hypoventilation?
COPD Asthma Malnutrition Diaphragmatic dysfunction Muscular dystrophy Severe restrictive lung disease
Name some chest wall disorders that can cause hypoventilation
Scoliosis
Kyphosis
Kyphoscoliosis
What are the effects of acute hypercapnia?
Respiratory acidosis
Impaired CNS function - drowsiness, confusion, coma, flapping tremors
Peripheral vasodilation
Cerebral vasodilation - headache
What are the effects of chronic hypercapnia?
Respiratory acidosis compensated by retention of HCO3- by kidney
Acclimation to CNS effects - CSF pH normalised
Vasodilation mild but may still be present
