Hypoxia and Respiratory Failure Flashcards
Define hypoxaemia
Low pO2 in blood
Define hypoxia
O2 deficiency at tissue level
What are the normal levels of O2 saturation?
94-98%
Tissue damage occurs at what oxygen levels?
O2 saturation <90%
PO2 <8 kPa
Outline the characteristics of T1 respiratory failure
Low pO2, <8 kPa, or O2 sat <90% breathing room air at sea level
PCO2 normal or low
Outline the characteristics of T2 respiratory failure
Low pO2 + high pCO2
What are the causes of hypoxia?
Low inspired pO2
Hypoventilation = respiratory pump failure
Ventilation/perfusion mismatch = ratio should be 1
Diffusion defect = problems of the alveolar capillary mem
R to L shunt = cyanotic heart disease
What is hypoventilation and its result?
Low breathing rate
Entire lung poor ventilated
Amount of O2 entering blood and CO2 entering alveolus per min remains unchanged (met rate is the same)
Alveolar pO2 falls = arterial pO2 falls = hypoxaemia
Alveolar pCo2 rises = arterial pCO2 increases = hypercapnia
What are some causes of acute hypoventilation?
Opiate overdose
Head injury
Very severe acute asthma
What are some causes of chronic hypoventilation?
COPD
Lung fibrosis
What are the causes of hypoventilation?
Narcotics
Stroke
Trauma to C spine
Guillain barre syndrome
Myasthenia gravis
Myopathiy
Kyphoscoliosis
Outline the effects of hypoxaemia
Impaired CNS function, confusion, irritability
Cyanosis (>50gm/litre of unsaturated Hb)
Cardiac arrhythmias
Hypoxic vasoconstriction of pulmonary vessels
How can central and peripheral cyanosis be distinguished?
Central = oral mucosa, tongue, lips = hypoxaemia
Peripheral = fingers, toes = poor local circulation
What compensatory mechanism occurs to due chronic hypoxaemia?
Increased EPO = raised Hb
Increased 2,3, DPG
What can result from chronic hypoxaemia?
Vasoconstriction of pulmonary vessels =
Pulmonary hypertension
R heart failure
Cor pulmonale = R side of the heart enlargement due to disease of the lungs/ pulmonary blood vessel
What are the effects of hypercapnia?
Respiratory acidosis
Impaired CNS function: drowsiness, confusion, coma, flapping, tremors
Peripheral vasodilatation –warm hands, bounding pulse
Cerebral vasodilation – headache
Outline the effect chronic CO2 retention has on the central chemoreceptors
CO2 diffuses to CSF = CSF pH drops = stim central chemoreceptors
Persistently 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 stim
PCO2 in blood still high but chemoreceptors now unresponsive to this pCO2 as they have reset to new higher CO2 level
Respiratory drive = now driven by hypoxia via peripheral chemoreceptors
Why might treatment of hypoxia worsen hypercapnia?
1) O2 removes stim for hypoxic resp drive = alveolar ventilation drops = worsening hypercapnia
2) correction of hypoxia removes pulmonary hypoxic vasoconstriction = increased perfusion of poorly ventilated alveoli = diverting blood away from better ventilated alveoli
Outline ventilation-perfusion mismatch (V/Q)
Some alveoli are poorly perfused
V/Q ratio = <1
Alveolar pO2 falls and pCO2 rises = vasoconstriction = diverts blood to better ventilated alveoli
Central/peripheral chemoreceptors stim = hyperventilation
Ventilation still doesn’t meet demand of the blood supply = mismatch
Name some causes of ventilation-perfusion mismatch
Asthma (variable airway narrowing )
Pneumonia (exudate in affected alveoli)
RDS in newborn (some alveoli not expanded)
Pulmonary oedema ( fluid in alveoli)
Pulmonary embolism
Name some causes of lung fibrosis
Idiopathic Fibrosing alveolitis
Asbestosis
Extrinsic allergic alveolitis
Pneumoconiosis