Hypoxia and Respiratory Failure

Question 1

Define hypoxaemia

Answer 1

Low pO2 in blood

Question 2

Define hypoxia

Answer 2

O2 deficiency at tissue level

Question 3

What are the normal levels of O2 saturation?

Answer 3

94-98%

Question 4

Tissue damage occurs at what oxygen levels?

Answer 4

O2 saturation <90%

PO2 <8 kPa

Question 5

Outline the characteristics of T1 respiratory failure

Answer 5

Low pO2, <8 kPa, or O2 sat <90% breathing room air at sea level

PCO2 normal or low

Question 6

Outline the characteristics of T2 respiratory failure

Answer 6

Low pO2 + high pCO2

Question 7

What are the causes of hypoxia?

Answer 7

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

Question 8

What is hypoventilation and its result?

Answer 8

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

Question 9

What are some causes of acute hypoventilation?

Answer 9

Opiate overdose

Head injury

Very severe acute asthma

Question 10

What are some causes of chronic hypoventilation?

Answer 10

COPD

Lung fibrosis

Question 11

What are the causes of hypoventilation?

Answer 11

Narcotics

Stroke

Trauma to C spine

Guillain barre syndrome

Myasthenia gravis

Myopathiy

Kyphoscoliosis

Question 12

Outline the effects of hypoxaemia

Answer 12

Impaired CNS function, confusion, irritability

Cyanosis (>50gm/litre of unsaturated Hb)

Cardiac arrhythmias

Hypoxic vasoconstriction of pulmonary vessels

Question 13

How can central and peripheral cyanosis be distinguished?

Answer 13

Central = oral mucosa, tongue, lips = hypoxaemia

Peripheral = fingers, toes = poor local circulation

Question 14

What compensatory mechanism occurs to due chronic hypoxaemia?

Answer 14

Increased EPO = raised Hb

Increased 2,3, DPG

Question 15

What can result from chronic hypoxaemia?

Answer 15

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

Question 16

What are the effects of hypercapnia?

Answer 16

Respiratory acidosis

Impaired CNS function: drowsiness, confusion, coma, flapping, tremors

Peripheral vasodilatation –warm hands, bounding pulse

Cerebral vasodilation – headache

Question 17

Outline the effect chronic CO2 retention has on the central chemoreceptors

Answer 17

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

Question 18

Why might treatment of hypoxia worsen hypercapnia?

Answer 18

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

Question 19

Outline ventilation-perfusion mismatch (V/Q)

Answer 19

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

Question 20

Name some causes of ventilation-perfusion mismatch

Answer 20

Asthma (variable airway narrowing )

Pneumonia (exudate in affected alveoli)

RDS in newborn (some alveoli not expanded)

Pulmonary oedema ( fluid in alveoli)

Pulmonary embolism

Question 21

Name some causes of lung fibrosis

Answer 21

Idiopathic Fibrosing alveolitis

Asbestosis

Extrinsic allergic alveolitis

Pneumoconiosis