respiratory faliure

Question 1

what is hypoxia?

Answer 1

-reduced level of tissue oxygenation

Question 2

what is hypoxaemia?

Answer 2

decrease in partial pressure of oxygen in the blood (PaO2)

Question 3

what happens in cyanide poisoning?

Answer 3

• cells are unable to utilise O2 despite having normal blood and tissue oxygen levels

Question 4

name 2 ways in which we can measure hypoxaemia?

Answer 4

• arterial oxygen tension- partial pressure of oxygen that indicates the dissolved oxygen in the plasma (measured by arterial blood gas analyser)
• arterial oxygen saturation- percentage of Hb saturated with oxygen (measured with a pulse oximeter and arterial blood gas analyser)

Question 5

what are the two types of respiratory failure?

Answer 5

• type 1-reduction in PaO2 but not change in PaCO2- V/Q mismatch
• type 2 increase in PaCO2 and a reduction in the PAO2- underventilation

Question 6

what are the mechanisms of hypoxaemia? 5

Answer 6

• V/Q mismatch
• right-left shunt
• diffusion impairment
• hypoventilation
• low inspired pO2

Question 7

describe the A-a gradient? 4

Answer 7

• difference between alveolar O2 level (PAO2) and the arterial oxygen level (PaO2)
• A-a gradient indicates the integrity of the alveolocapillary membrane and the effectiveness of gas exchange-pathology of alveolocapillary unit widens the gradient
• hypoxaemia caused by a V/Q mismatch, diffusion limitation and shunt widen the A-a gradient
• hypoxaemia caused by hypoventilation have a normal gradient

Question 8

explain a V/Q mismatch? 6

Answer 8

• most common cause of hypoxaemia
• regional heterogeneity of V/Q- sub atmospheric intrapleural pressure and gravity
• ventilation and perfusion are higher at the bases and lower at the apex
• V/Q ratio is higher at the apex and lower at the base
• A low V/Q ratio produced hypoxaemia by decreasing the PAO2 and subsequent PaO2
• Compensatory mechanisms there is a restriction in perfusion in areas of the lung with reduced ventilation (hypoxic pulmonary vasoconstriction)

Question 9

explain a high V/Q ratio? 3

Answer 9

• ventilation is in excess of perfusion-pulmonary embolism
• in PE, there is less perfusion- high V/Q ratio
• hypoxaemia is caused if the compensatory rise in the total ventilation is absent

Question 10

what are the characteristics of a V/Q mismatch? 3

Answer 10

• hypoxaemia due to a V/Q mismatch can be easily corrected by supplemental oxygen
• widened A-a gradient
• common causes: asthma, COPD, cystic fibrosis

Question 11

what is a shunt?

Answer 11

• blood from the right side of the heart enters the left side without taking part in gas exchange
• extreme degree of V/Q mismatch where there is no ventilation
• poor response to oxygen therapy is a feature that distinguishes shunt from other mechanisms
• hypoxaemia is uncommon in shunt until the shunt fraction reaches 50%
• lack of hypercapnia is due to stimulation of the respiratory centre by chemoreceptor
• A-a gradient is elevated
• pCO2 is normal
• common causes- pneumonia, pulmonary oedema

Question 12

describe diffusion limitation? 7

Answer 12

• transport across the alveolocapillary membrane is impaired
• decrease in lung surface area for diffusion, inflammation and fibrosis, low alveolar oxygen and reduced capillary transmit time
• since O2 and CO2 occur across the alveolocapillary membrane, theoretically it should cause hypoxaemia and hypercapnia
• hypercapnia is uncommon- CO2 is 20x more soluble than O2 and is less likely to be affected by diffusion limitation
• hypoxia shows a good response to oxygen therapy
• A-a gradient is elevated
• PaCO2 is normal

Question 13

what is hypoventilation? 5

Answer 13

• high PaCO2
• leads to low PAO2 and PaO2
• normal A-a gradient
• doesn’t cause significant hypoxaemia in healthy lungs but does in the presence of lung disease
• oxygen therapy corrects hypoxaemia

Question 14

what are the causes of hypoventilation? 4

Answer 14

• impaired central drive
• damage to spinal cord
• damage to neuromuscular junction
• damage to respiratory muscles