Session 7 - Hypoxia And Respiratory Failure Flashcards Preview

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Flashcards in Session 7 - Hypoxia And Respiratory Failure Deck (32)
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What is the difference between hypoxaemia and hypoxia?

Hypoxaemia - low pO2 in the blood
Hypoxia - low O 2 at a tissue level


What is the normal range for oxygen saturation of the blood?



What is the normal range for the partial pressure of oxygen in the blood?

9.3-13.3 kPa


What is respiratory failure?

Impairment in gas exchange causing hypoxia with or without hypercapnia.


What is the difference between type 1 and type 2 respiratory failure?

Type 1 respiratory failure:
- low pO2
- normal pCO2 (or low)

Type 2 respiratory failure:
- low pO2
- high pCO2


What are the possible causes of hypoxaemia?

1. Low inspired pO2
2. Hypoventilation - respiratory pump failure (failure of muscles in chest wall)
3. Ventilation/Perfusion mismatch
4. Diffusion defect - problems of the alveolar-capillary membrane (e.g. pleural effusion)
5. Right to left shunt(e.g. cyanosis heart disease)


What is the optimal ventilation/perfusion ratio?



What is hypoventilation?

When the entire lung is poorly ventilated meaning alveolar ventilation (minute volume) is reduced.


What happens to pO2 and pCO2 of the blood in hypoventilation?

Alveolar pO2 falls —> arterial pO2 falls —> hypoxaemia
Alveolar pCO2 rises —> arterial pCO2 rises —> hypercapnia


Hypoventilation always causes hypercapnia. Therefore it always causes what type of respiratory failure?

Type 2 respiratory failure, as there will be both hypoxia and hypercapnia


Give some examples of causes of acute hypoventilation.

Opiate overdose
Head injury
Very severe asthma attack


Give an example of a cause of chronic hypoventilation.

Severe COPD
(Acute exacerbation of COPD may occur due to lower respiratory tract infection)


Why is chronic hypoventilation better tolerated than acute hypoventilation which requires immediate treatment?

Chronic hypoxia and chronic hypercapnia has a slow onset and progression. This allows time for compensation.


Give some causes of chronic type 2 respiratory failure.

Myopathy of muscles of respiration
Motor neurone disease
Severe obesity
Lung fibrosis
Late stages of COPD


What are the effects of hypoxaemia?

Impaired CNS function, confusion, irritability
Cardiac arrhythmias
Hypoxic vasoconstriction of pulmonary vessels


What is the difference between central and peripheral cyanosis?

Central cyanosis:
- seen in oral mucosa, tongue, lips
- indicates hypoxaemia

Peripheral cyanosis:
- seen in fingered, toes
- indicates poor local circulation


In chronic hypoxaemia, what compensatory mechanisms are used by the body to increase oxygen delivery?

Increased erythropoietin secretion by the kidney —> raises Hb (polycythemia)
Increased 2,3-BPG


What does chronic hypoxic vasoconstriction of pulmonary vessels result in?

Pulmonary hypertension
Right heart failure (Cor Pulmonale)


What are the effects of hypercapnia?

Respiratory acidosis
Impaired CNS function; drowsiness, confusion, coma, flapping tremors
Peripheral vasodilation - warm hands, bounding pulse
Cerebral vasodilation - headache


How is chronic respiratory acidosis compensated?

Retention of HCO3- by the kidneys


What is the effect of chronic hypercapnia on central chemoreceptors?

The central chemoreceptors ‘reset’ to the new higher CO2 level


How do the central chemoreceptors become ‘reset’ in chronic hypercapnia?

CO2 diffuses into the CSF —> CSF pH drops —> stimulates chemoreceptors.
Low CSF pH is corrected by choroid plexus cells which secrete [HCO3-] into the CSF
The CSF returns to normal meaning the central chemoreceptors are no longer stimulated
The pCO2 in the blood is still high but the central chemoreceptors and now unresponsive
Central chemoreceptors have ‘reset’ to a new higher CO2 level


Why might treatment for hypoxia worsen hypercapnia?

1. O2 therapy removes the stimulus for hypoxic respiratory drive. Alveolar ventilation drops —> causes worsening hypercapnia.
2. Correction of hypoxia removes pulmonary hypoxic vasoconstriction. This leads to increased perfusion of poorly ventilated alveoli, diverting blood away from better ventilated alveoli.


Describe how oxygen therapy should be given to patients suffering from type 2 respiratory failure.

Oxygen is life saving. It must be given, but pCO2 needs to be monitored. Controlled oxygen theraoy is given with a target saturation of 88-92%.
If oxygen therapy causes rise in pCO2 - need ventilatory support


What happens to the alveolar pO2 and pCO2 when the V/Q ration is <1?

Alveolar pO2 decreases and pCO2 increases


What happens to the alveolar pO2 and pCO2 when the V/Q ratio is >1?

Alveolar pO2 increases and pCO2 decreases


What happens in hypoxic vasoconstriction?

Blood is diverted from poorly ventilated areas to better ventilated areas.


Give examples of disorders that cause V/Q mismatch.

Occurs in disorders where some alveoli are being insufficiently ventilated. For example:
- asthma (variable airway narrowing)
- pneumonia (exudate in affected alveoli)
- RDS in newborn (some alveoli don’t expand)
- pulmonary oedema (fluid in alveoli)
- pulmonary embolism


Why does pulmonary embolism cause type 1 respiratory failure?

1. The embolus results in a redistribution of pulmonary blood flow
2. The blood is diverted to unaffected areas of the pulmonary circulation
3. Leads to V/Q ratio <1 if hyperventilation cannot match the increased perfusion
4. This causes hypoxaemia
5. The hyperventilation is sufficient to get rid of the excess CO2


Why do diffusion defects affect O2 more than CO2?

CO2 is more soluble than O2 so can diffuse more easily