S7) Respiratory Failure Flashcards
What is respiratory failure?
Respiratory failure is a condition which results from inadequate gas exchange by the respiratory system so that arterial oxygen, carbon dioxide or both cannot be kept at normal levels
What is Type 1 Respiratory Failure?
- Type 1 Respiratory Failure is defined as a low pO2 (< 8 kPa) or O2 saturation (<90%) in breathing room air at sea level
- pCO2 might be normal or low
What is Type 2 Respiratory Failure?
Type 2 Respiratory Failure is defined as a low pO2 (< 8 kPa) and high pCO2
Identify 5 conditions which might cause hypoxia
- Low inspired pO2
- Hypoventilation
- Ventilation/perfusion mismatch
- Diffusion defect in the alveolar capillary membrane
- Right to left shunt eg. cyanotic heart disease
What is hypoventilation?
Hypoventilation is when the entire lung is poorly ventilated and alveolar ventilation (minute volume) is reduced
State 2 effects of hypoventilation
- Hypoxaemia as alveolar pO2 falls → arterial pO2 falls
- Hypercapnia as alveolar pCO2 rises → arterial pCO2 increases
Acute hypoventilation requires urgent treatment and possible artificial ventilation.
Identify 3 conditions where this can occur
- Opiate overdose
- Head injury
- Very severe acute asthma
Describe 3 features of chronic hypoventilation
- Chronic hypoxia and chronic hypercapnia
- Slow onset and progression
- Time for compensation so better tolerated
Identify a condition where chronic hypoventilation is observed
Severe COPD – most common cause of chronic type 2 respiratory failure
Identify a condition involving the respiratory centre in the brainstem that results in hypoventilation
Head injury
Identify a condition involving the spinal cord that results in hypoventilation
Trauma to the cervical spinal cord (phrenic and intercostal nerves)
Identify a condition involving the phrenic and intercostal nerves that results in hypoventilation
Guillain-Barré syndrome
What is Guillain-Barré syndrome?
Guillain–Barré syndrome is a rare and serious condition causing rapid-onset muscle weakness due to the immune system damaging the peripheral nervous system
Identify a condition involving the neuromuscular junction that results in hypoventilation
Myasthenia gravis
Identify 2 conditions involving the muscles of respiration that result in hypoventilation
- Myopathy
- Motor neurone disease
Identify 2 conditions involving the chest wall that result in hypoventilation
- Severe obesity
- Kyphoscoliosis
What is kyphoscoliosis?
Kyphoscoliosis is a deformity of the spine characterised by abnormal curvature of the vertebral column in the lateral and anterior planes
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Identify 2 conditions involving the pleural cavity that result in hypoventilation
- Pneumothorax
- Pleural effusion (large)
Identify 2 conditions involving poor lung compliance that result in hypoventilation
- Respiratory distress of the newborn
- Lung fibrosis
Identify 2 conditions involving high airway resistance that result in hypoventilation
- Very severe acute asthma
- COPD (late stages)
Identify a condition involving upper respiratory failure that results in hypoventilation
Laryngeal oedema
Identify 5 conditions where chronic Type II Respiratory Failure is observed
- Myopathy
- Severe obesity
- Lung fibrosis
- Kyphoscoliosis
- COPD
State 4 effects of hypoxaemia
- Impaired CNS function (confusion & irritability)
- Cyanosis
- Cardiac arrhythmias
- Hypoxic vasoconstriction of pulmonary vessels
What is cyanosis?
Cyanosis is the bluish discolouration of the skin and mucous membranes due to presence of >50gm/litre of unsaturated Hb
Where is central cyanosis seen and what does it indicate?
- Seen in oral mucosa, tongue and lips
- Indicates hypoxaemia
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Where is peripheral cyanosis seen and what does it indicate?
- Seen in fingers, toes
- Indicates poor local circulation
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State the relationship between central and peripheral cyanosis
If central cyanosis is present, peripheral cyanosis will also be present
State the compensatory mechanisms seen in chronic hypoxaemia to increase oxygen delivery
- Increased EPO secreted by kidney → raised Hb (polycythaemia)
- Increased 2,3, BPG
State 3 results of the chronic hypoxic vasoconstriction of pulmonary vessels seen in chronic hypoxaemia
- Pulmonary hypertension
- Cor pulmonale (right heart failure)
State 4 effects of hypercapnia
- Respiratory acidosis
- Impaired CNS function (drowsiness, confusion, coma, tremors)
- Peripheral vasodilatation (warm hands, bounding pulse)
- Cerebral vasodilation (headache)
What is the compensatory mechanism seen in chronic hypercapnia?
Respiratory acidosis compensated by retention of HCO3- by kidney
In 5 steps, outline the impact of chronic CO2 retention on the central chemoreceptors
⇒ CO2 diffuses into CSF and pH drops
⇒ Central chemoreceptors are stimulated
⇒ Choroid plexus cells secrete HCO3- into CSF to correct low pH
⇒ CSF pH normalises despite high blood pCO2
⇒ Central chemoreceptors now unresponsive to this pCO2
Explain why the treatment of hypoxia may worsen hypercapnia
- O2 removes stimulus for the hypoxic respiratory drive and alveolar ventilation falls, worsening hypercapnia
- Removes pulmonary hypoxic vasoconstriction, increasing perfusion of poorly ventilated alveoli & diverting blood away from better ventilated alveoli
How can one treat hypercapnia?
- Controlled oxygen therapy with a target saturation of 88-92%
- Ventilatory support if oxygen therapy causes rise in pCO2
V/Q matching happens at alveolar level.
When does optimal gas exchange occur?
V/Q ratio = 1
What happens when V/Q ratio < 1? (hypoventilation)
- Alveolar pO2 falls
- Alveolar pCO2 rises
What happens when V/Q ratio > 1? (hyperventilation)
- Alveolar pO2 rises
- Alveolar pCO2 falls
V/Q mismatches occur in disorders where some alveoli are being poorly ventilated.
Identify and describe 5 conditions
- Asthma (variable airway narrowing)
- Pneumonia (exudate in affected alveoli)
- RDS in newborn (some alveoli not expanded)
- Pulmonary oedema (fluid in alveoli)
- Pulmonary embolism
In 4 steps, explain how the body responds to a V/Q ratio < 1
⇒ Hypoxic vasoconstriction occurs to diverts some blood to better ventilated areas
⇒ Mixed blood in left atrium (low arterial pO2 and high arterial pCO2)
⇒ Central and peripheral chemoreceptors are stimulated
⇒ Hyperventilation occurs
In 4 steps, explain how a pulmonary embolism can lead to hypoxaemia
⇒ Embolus redistributes of pulmonary blood flow
⇒ Blood is diverted to unaffected areas of pulmonary circulation
⇒ V/Q ratio < 1 if hyperventilation cannot match the increased perfusion
⇒ Hypoxaemia
Fibrotic lung disease is when the thickened alveolar membrane slows gas exchange.
Identify 4 conditions causing lung fibrosis
- Idiopathic Fibrosing alveolitis
- Asbestosis
- Extrinsic allergic alveolitis
- Pneumoconiosis
How do diffusion defects cause Type I respiratory failure?
- Poor diffusion across alveolar capillary membrane
- Leads to low O2 and normal/low pCO2
Why is a low pO2 observed in diffusion defects but pCO2 is normal(/low)?
CO2 is more soluble so its diffusion less affected than O2 diffusion