Respiratory failure Flashcards
Define respiratory failure?
- Impairment in gas exchange causing hypoxaemia with or without hypercapnia
- May be acute or acute on chronic condition
Outline type 1 respiratory failure
- Low PaO2 <8kPa
- Or O2 saturation <90%
- Breathing room air at sea level
- pCO2 normal or low
- Gas exchange is impaired at level of alveolar-capillary membrane
- Type 1 RF can progress to type 2
Outline type 2 respiratory failure
- Low PaO1 and high PaCO2 >6.5 kPa
- Breathing room air at sea level
- Reduced ventilatory effort (pump failure) or inability to overcome increased resistance to ventilate entore lung
Define hypoxaemia
- Low pO2 in arterial blood
Define hypoxia
- O2 deficiency at tissue level
- Tissues can be hypoxic without hypoxaemia
What are the normal ranges of O2 saturation and PaO2?
- O2 saturation should be 94-98%
- PaO2 should be 10.6-13.3 kPa
When is tissue damage most likely?
- O2 saturation <90%
- pO2 < 8kPa
Where is central cyanosis seen?
- Oral mucosa
- Tongue
- Lips
What does central cyanosis indicate?
- Hypoxaemia with oxygen saturation below 85%
Where is peripheral cyanosis seen?
- Fingers
- Toes
- If central cyanosis is present, peripheral cyanosis will also be present
What are the effects of hypoxaemia?
- Impaired CNS function
- Confusion
- Irritability
- Agitation
- Cardiac arrhythmias
- Cardiac ischaemia
- Hypoxic vasoconstriction of pulmonary vessels
What are the signs of hypoxaemia?
- Central cyanosis
- Tachypnoea/tachycardia
Why does central cyanosis occur?
- Bluish discolouration of skin and mucous membranes due to presence of 4-6 gm/dl of deoxyhaemoglobin (unsaturated Hb)
What happens in chronic hypoxaemia?
- Compensatory mechanisms increase oxygen delivery and therefore decrease hypoxia
- Chronic hypoxic vasoconstriction of pulmonary vessels
Which compensatory mechanisms increase oxygen delivery and decrease hypoxia during chronic hypoxaemia?
- Increased EPO secreted by kidney to raise Hb (polycythaemia)
- Increased 2,3 DPG to shift Hb/O2 saturation curve to right so oxygen is released more freely
- Increased capillary density
What does chronic hypoxic vasoconstriction of pulmonary vessels result in?
- Pulmonary hypertension
- Right heart failure
- Cor pulmonale
What are some causes of hypoxaemia?
- Low inspired pO2 - e.g. high altitude
- V:Q mismatch
- Diffusion defect - problems of alveolar capillary membrane
- Intra-lung shunt - ARDS
- Hypoventilation
How does low inspired pO2 result in hypoxaemia?
- Can occur due to high altitude
- pO2 = FiO2 x total atmospheric pressure
- Therefore partial pressure oxygen falls in alveoli at higher altitude
- Partial pressure oxygen in arterial blood is low - hypoxaemia
- Fully improves with O2
What are some causes of V:Q mismatches?
- Asthma (variable airway narrowing)
- COPD (variable airway narrowing/collapse, loss of some alveoli)
- Pneumonia (exudate in affected alveoli)
- RDS in newborn (some alveoli not expanded)
- Pulmonary oedema (fluid in alveoli)
How do we treat hypoxaemia caused by V:Q mismatch?
- Improves with O2 administration
- O2 will not completely correct hypoxaemia until underlying pathology is corrected
What happens if an alveolar unit is inadequately ventilated?
- Perfusion is still unchanged
- PaO2 falls
- PaCO2 rises
- Blood equilibrates to new alveolar paO2 and paCO2 i.e. plasma ends up with poor paO2 and high paCO2
What is the body’s response to hypoxaemia due to V:Q mismatch?
- Hyperventilation is induced by peripheral chemoreceptor firing
- Chemoreceptors in carotids detect that blood mixture is hypoxaemic and hypercapnic
- If lung disease is severe, hyperventilation may not be able to compensate for mismatch and CO2 remains elevated
- Pulmonary arteriole hypoxic vasoconstriction occurs to divert blood to better oxygenated areas of lung
Why does V:Q mismatch trigger hyperventilation?
- Output from poorly ventilated alveoli is mixed with output of healthy alveoli in blood
- Blood mixture is hypoxaemic and hypercapnic
How does pulmonary embolism lead to V:Q mismatch?
- Blood can’t get past blockage
- V/Q mismatch created - V>Q
- Backwards pressure is applied
- Blood redistributes to over-supply unaffected alveoli
- Hyperventilation occurs to try and increase V so that it matches Q
- Hyperventilation sufficient to get rid of CO2
What do diffusion defects lead to?
- Low pO2
- Normal or low pCO2
- Because CO2 is more soluble
- So CO2 diffusion is less affected in lung disease than diffusion of O2
- Initially leads to type 1 respiratory failure
- As disease progresses, leads to hypoventilation
- Results in hypercapnia
What problems with the alveolar capillary membrane can result in diffusion defects?
- Fibrotic lung disease: thickened alveolar membrane slows down gas exchange
- Pulmonary oedema: fluid in interstitial space increases diffusion distance
What are some causes of diffuse lung fibrosis?
- Idiopathic pulmonary fibrosis
- Asbestosis
- Extrinsic allergic alveolitis
- Pneumoconiosis
How do we improve diffuse lung fibrosis?
- Oxygen administration
What is an intra-pulmonary shunt?
- Shunting where some of blood flow through lungs is not properly oxygenated
What can cause intra-pulmonary shunting?
- ARDs
Outline ARDS
- ARDS can result in widespread acute alveolar injury
- Many types of injuries which lead to common pathway: damage to alveolar capillary unit
- Injury produces increased vascular permeability, oedema, fibrin-exudation
- Heavy, red lungs showing congestion and oedema - alveoli contain contain fluid and lined by hyaline membranes
How does ARDS affect the lungs?
- Diffuse loss of surfactant resulting in alveolar atelectasis
- Lung becomes stiff/less compliant
- Lung volumes decrease
- Loss of hypoxic pulmonary vasoconstriction mechanism
- Tremendous intrapulmonary shunt - no ventilation with respect to perfusion
How is ARDS managed?
- Hard to manage on a ventilator
- Even 100% O2 might not correct hypoxaemia
- Need to add positive pressure ventilation to open alveoli
Outline hypoventilation
- When entire lung is poorly ventilated due to inadequate respiratory rate or volume of alveolar ventilation
- Alveolar ventilation is reduced
- Alveolar pO2 falls so arterial pO2 falls, resulting in hypoxaemia
- Alveolar pCO2 rises so arterial pCO2 increases, resulting in hypercapnia
- Leads to type 2 respiratory respiratory failure
What does hypoventilation always lead to?
- Hypercapnia
- Therefore causes type 2 respiratory failure with both hypercapnia and hypoxaemia
- Hypoxaemia secondary to hypoventilation will correct with added oxygen
- Does not solve hypercapnia problem though as ventilation is impaired
Outline acute hypoventilation
- Need urgent treatment
- +/- artificial ventilation
- Commonly caused by opiate overdose, head injury, and very severe acute asthma
Outline chronic hypoventilation
- Chronic hypoxaemia and chronic hypercapnia
- Slow onset and progression
- Time for compensation
- Therefore is better tolerated than acute hypoventilation
- Common causes include severe COPD and LRT infection
What are some hypoventilation disorders?
- Central control due to opioid OD
- Motor neuron due to motor neuron disease
- Peripheral neuropathy due to Guillain Barre disease
- Muscle weakness due to Duchenne’s MD
- Chest wall disorders e.g. Kyphoscoliosis
- End stage COPD due to emphysema
- Severe asthma exacerbation
Which central disorders can cause hypoventilation?
- Central sleep apnoea
- Obesity hypoventilation syndrome
- Narcotic overdose
- Sedatives
- Hypothyroidism
Which motor disorders can cause hypoventilation/respiratory failure?
- Tetanus
- Amyotrophic lateral sclerosis
- Motor neurone disease
- Spinal cord injury at C3 level
Which disorders of the neuromuscular junction can result in hypoventilation/respiratory failure?
- Myasthenia gravis
- Organophosphate toxicity
- Botulism toxin
Which diseases/conditions can result in muscle fatigue or weakness and then hypoventilation?
- COPD
- Asthma
- Malnutrition
- Diaphragmic dysfunction
- Muscular dystrophy
What is scoliosis?
- Sideways curvature of the spine
What is kyphosis?
- Spinal disorder
- Excessive outward curve of spine results in abnormal rounding of upper back
What does kyphoscoliosis cause?
- Causes disordered movement of chest wall
- Respiratory system compliance reduced
- Due to reduction in chest wall compliance and a reduction in lung compliance due to microatelectasis
What are the effects of acute hypercapnia?
- Respiratory acidosis
- Impaired CNS function: drowsiness, confusion, coma etc.
- Peripheral vasodilation - warm hands, bounding pulse
- 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
Why might treatment of hypoxaemia worsen hypercapnia?
- Correction of hypoxia removes pulmonary arteriole hypoxic vasoconstriction
- Leads to increased perfusion of poorly ventilated alveoli
- Diverts blood away from better ventilated alveoli - worsens V/Q mismatch
- Haldane mechanism: oxygenated haemoglobin has low affinity for CO2 so CO2 dissociates from haemoglobin into blood
Should we give O2 to patients with respiratory failure?
- Giving O2 can resolve hypoxaemia but also worsen hypercapnia
- Must give O2 (it is life-saving) but need to monitor pCO2
- Give controlled O2 therapy with a target saturation of 88-92%
- If O2 therapy causes rise in pCO2, patient will need ventilatory support (assisted or mechanical ventilation)
