7. Respiratory failure

Question 1

what is the difference between hypoxaemia and hypoxia

Answer 1

• hypoxaemia = decreased plasma [O2]
• hypoxia = decreased [O2] at tissue level (can be hypoxic without hypoxaemia, e.g. severe anaemia or poor tissue perfusion)

Question 2

define respiratory failure (inc. O2 sat and pO2) - what are the 2 types

Answer 2

RF = impairment in gas exchange causing hypoxia, with or without hypercapnia - O2 sat <90% and pO2 <8kPa

• T1 = low pO2 with normal or low pCO2
• T2 = low pO2 with high pCO2 (>6.7kPa)

Question 3

what are the normal ranges for O2 saturation and pO2

Answer 3

• O2 sats: 94-98%

- pO2: 9.3-13.3 kPa

Question 4

what type of RF is caused by hypoventilation - explain why

Answer 4

T2RF - entire lung is poorly ventilated due to insufficient mov. of air in/out of lungs

• decreased alveolar pO2… decreased arterial pO2: hypoxaemia
• increased alveolar pCO2… increased arterial pCO2: hypercapnia

Question 5

what is the difference between acute and chronic hypoventilation

Answer 5

Acute (eg opiate overdose, very severe asthma, upper airway obstruction): requires urgent treatment +/- artificial ventilation.

Chronic (eg COPD, lung fibrosis): slow onset and progression so time for compensation of hypoxia and hypercapnia (better tolerated).

Question 6

explain the difference between the 2 types of V/Q mismatch and give examples of causes

Answer 6

1. reduced ventilation but normal perfusion: V/Q <1. Eg.
   - early stage asthma (variable airway narrowing)
   - early stage COPD
   - pneumonia (exudate in affected alveoli)
   - neonatal resp. distress syndrome (some alveoli not expanded due to lack of surfactant)
2. Normal ventilation but reduced perfusion: V/Q>1. Eg. PE

Question 7

why does V/Q mismatch result in type 1 RF (ie low pO2 and normal/low pCO2)

Answer 7

1. V/Q mismatch causes pO2 decrease and initial pCO2 increase… hypoxic vasoconstriction that diverts some blood to better ventilated areas… but mixed blood from affected and unaffected alveoli still has low pO2 and high pCO2… stimulates central and peripheral chemoRs…
2. ChemoRs stimulate hyperventilation… increase CO2 removal from unaffected alveoli (where V/Q now >1)… normal or low pCO2.

However, hyperventilation of unaffected areas cannot compensate for hypoxaemia as Hb already 100% saturated at normal resp. rates, so hyperventilation can only increase amount of dissolved O2 in blood (v. small).
Results in low pO2 and normal or low pCO2.

Question 8

why is PE a T1RF

Answer 8

i) embolus causes blood diversion to unaffected areas of pulmonary circulation…
ii) V/Q >1 in affected alveoli and V/Q <1 in unaffected alveoli if hyperventilation cannot match increased perfusion…
iii) low pO2 but normal pCO2 (due to hyperventilation)

Question 9

why is hypoxia due to diffusion impairment a T1RF

Answer 9

O2 diffuses much less readily than CO2 so is always more affected by any change to the diffusion barrier.

So diffusion impairment causes low pO2 with normal or low pCO2

Question 10

describe 2 compensatory mechanisms to increase O2 delivery in chronic hypoxaemia

Answer 10

1. increased EPO secretion by kidney… increased Hb

2. increased 2,3-BPG

Question 11

what is the effect of chronic hypoxic vasoconstriction in hypoxaemia on the heart

Answer 11

cor pulmonale: chronic hypoxic vasoconstriction… pulmonary hypertension… RH failure

Question 12

describe the effect of chronic hypercapnia on central chemoRs

Answer 12

1. CO2 diffuses into CSF, dropping CSF pH… stimulates central chemoRs…
2. persistent CSF acidity harmful to neurones… choroid plexus cells secrete HCO3- into CSF…
3. CSF pH returns to normal… central chemoRs no longer stimulates - pCO2 in blood still high but central chemoRs no unresponsive to this pCO2
4. associated persistent hypoxia stimulates peripheral chemoRs… resp. drive now driven by hypoxia

Question 13

why does treatment of hypoxia in T2RF worsen

Answer 13

1. O2 treatment removes stimulus for hypoxic resp. drive… decreased alveolar ventilation rate… worsened hypercapnia
2. correction of hypoxia removes pulmonary hypoxic vasoconstriction… increased perfusion of poorly ventilated alveoli… diverts blood away from better ventilated alveoli

Question 14

how should hypoxia be treated in chronic T2RF

Answer 14

Controlled O2 (24-28%) with target saturation of 88-92%.

pCO2 needs to be monitored. If O2 therapy causes pCO2 increases - need ventilatory support.