Hypoxaemia and Respiratory Failure Flashcards
Define hypoxia
Oxygen deficiency at tissue level
Define type 1 respiratory failure
Low pO2 with normal or low pCO2 (hypoxia)
Define type 2 respiratory failure
Low pO2 and high pCO2 (hypoxaemia and hypercapnia)
What is normal ventilation and perfusion ratio determined by
- Alveolar ventilation per minute = 5250 ml/min = ~5 L/min
- Stroke volume x heart rate = 70ml x 70bpm = 4900 ml/min = ~5 L/min
Explain how ventilation/perfusion mismatch causes type 1 respiratory failure
- Some alveoli may be poorly ventilated, leading to V/Q < 1
- Causes pO2 to fall and pCO2 to rise - Hyperventilation occurs to compensate for this by causing unaffected alveoli to have increased ventilation - V/Q > 1
- This causes pO2 to rise and pCO2 to fall in these segments - However haemoglobin is well saturated and further increases in pO2 have no effect and do not significantly increase pO2 in the body
- pCO2 drops due to increased ventilation, thus the final result is hypoxaemia with normal or low pCO2
List some causes of V/Q mismatch
- Occurs in disorders where some alveoli are being poorly ventilated
- Asthma - variable airway narrowing
- Pneumonia - exudate in affected alveoli
- Respiratory distress in newborn - some alveoli not expanded due to lack of surfactant
- Pulmonary oedema - fluid in alveoli
- Pulmonary embolism
Outline how pulmonary embolism causes type 1 respiratory failure
- Embolus results in redistribution of pulmonary blood flow
§ Blood is diverted to unaffected areas of the pulmonary circulation- Causes V/Q < 1 in unaffected alveoli and V/Q > 1 in affected
- Leads to V/Q < 1 if hyperventilation cannot match the increased perfusion
- Hyperventilation sufficient to get rid of CO2, therefore type 1 respiratory failure
Explain how diffusion defects result in type 1 respiratory failure
- CO2 is more soluble than oxygen and therefore more readily diffuses between the lung and blood
- Diffusion defects cause low pO2 and normal (or low) pCO2
Explain some causes of diffusion defects
- Fibrotic lung disease - thickened alveolar membrane slows gas exchange
- CO2 able to cross but oxygen has trouble
- Pulmonary oedema - fluid in interstitial space increases diffusion distance
- CO2 more soluble and more readily diffuses across
Explain how hypoventilation causes type 2 respiratory failure
- When the entire lung is poorly ventilated
- Hypoxaemia - alveolar pO2 falls leading to fall in arterial pO2
- Hypercapnia - alveolar pCO2 rises leading to rise in arterial pCO2
- Hypoventilation always causes hypercapnia, therefore type 2 respiratory failure
Explain if acute or chronic hypoventilation is more severe
- Acute hypoventilation
- Need urgent treatment
- Artificial ventilation may be needed
- Chronic hypoventilation
- Chronic hypoxia and chronic hypercapnia
- Slow onset and progression therefore time for compensation
- Can be better tolerated
List some causes of acute and chronic hypoventilation
Acute - opiate overdose, head injury, very severe acute asthma
Chronic - severe COPD, lung fibrosis, kyphoscoliosis, Guillain Barre syndrome, myasthenia gravis
Explain how type 1 respiratory failure can progress to type 2
- Type 1 respiratory failure can progress to type 2 as more areas of the lungs are involved in disease
- Asthma/COPD- when CO2 unable to leave lungs and reduces CO2 diffusion, trouble sustaining hyperventilation
- Lung fibrosis - very thick barrier that CO2 has trouble diffusing across, trouble sustaining hyperventilation
- Respiratory disease of the newborn - loss of hyperventilation as baby cannot maintain so alveoli without sufficient surfactant lose ventilation
Explain what cyanosis is
Cyanosis - bluish discoloration of the skin and mucous membranes due to presence of > 50 g/L of unsaturated haemoglobin
Distinguish between central and peripheral cyanosis
- Central cyanosis - serious problem
- Seen in oral mucosa, tongue, lips
- Tongue normally warm and well perfused - severe if cyanosis seen
- Indicates hypoxaemia
- Congenital heart defect - right to left shunt allowing deoxygenated blood to leave the aorta
- Seen in oral mucosa, tongue, lips
- Peripheral cyanosis
- Seen in fingers and toes
- Poor local circulation
- Seen in fingers and toes
Explain the effects of acute hypoxia
- Impaired CNS function - confusion, irritability
- Cyanosis
- Cardiac arrhythmias
- Hypoxic vasoconstriction of pulmonary vessels - redistribution of blood to well-ventilated alveoli
Explain the chronic effects of hypoxia
- Compensatory mechanisms to increase oxygen delivery
- Increased EPO secreted by kidney - raised haemoglobin
- Increased 2,3 DPG
- Chronic hypoxic vasoconstriction of pulmonary vessels results in:
- Pulmonary hypertension
- Right heart failure
- Cor pulmonale - heart failure secondary to pulmonary hypertension
Explain the acute effects of hypercapnia
- Respiratory acidosis
- Impaired CNS function - drowsiness, confusion, coma, flapping tremors
- Peripheral vasodilation - warm hands, bounding pulse
- Cerebral vasodilation - headache
Explain the chronic effects of hypercapnia
Respiratory acidosis compensated by retention of HCO3 by kidney
Explain the effect of chronic hypercapnia on the central chemoreceptors
- CO2 diffuses into CSF -> CSF pH drops -> stimulates central chemoreceptors
- Persistent CSF acidity harmful to neurons
- Low CSF pH corrected by choroid plexus cells which secrete HCO3 into CSF
- CSF pH returns to normal, central chemoreceptors no longer stimulated
- pCO2 in the blood is still high but central chemoreceptors now unresponsive to this pCO2
- Central chemoreceptors have reset to a new higher CO2 level
- Persistent hypoxia stimulates peripheral chemoreceptors
- Respiratory drive now driven by low oxygen
How can treatment of chronic type 2 respiratory failure with oxygen be a problem
- Treatment of hypoxia may worsen hypercapnia
- Increasing oxygen removes the stimulus for hypoxic respiratory drive
- Alveolar ventilation drops as person breathes less, leading to worsening hypercapnia
- Correction of hypoxia removes pulmonary hypoxic vasoconstriction
- Leads to increased perfusion of poorly ventilated alveoli, diverting blood away from better ventilated alveoli
- When giving oxygen, pCO2 levels need to be monitored
- Controlled oxygen therapy with a target saturation of 88-92%
- If oxygen therapy causes rise in pCO2, need ventilation support
