7.1 Hypoxaemia And Respiratory Failure

Question 1

Define hypoxaemia

Answer 1

Low pO2 in the blood

Question 2

Define hypoxia

Answer 2

O2 deficiency at the tissue level

Question 3

Can tissues be hypoxic without hypoxaemia? Why?

Answer 3

Yes

Anaemia, poor circulation

Question 4

What is the normal range for O2 saturation?

Answer 4

94-98%

Question 5

What is the normal range for pO2?

Answer 5

9.3 - 13.3kPa

Question 6

In terms of oxygen, when is tissue damage most likely and what is this used to diagnose?

Answer 6

Respiratory failure
O2 sats <90%
pO2 <8kPa

Question 7

Define respiratory failure

Answer 7

Impairment in gas exchange causing hypoxia with or without hypercapnia

Question 8

Describe blood gases in type 1 respiratory failure

Answer 8

Low pO2 (<8kPa or oxygen sats <90%)
pCO2 normal or low

Question 9

Describe blood gases in type 2 respiratory failure

Answer 9

Low pO2

High pCO2

Question 10

Name 5 causes of hypoxia

Answer 10

Low inspired oxygen
Hypoventilation (due to respiratory pump failure)
Ventilation/Perfusion mismatch
Disruption defect (problem with alveolar capillary membrane)
Right to left shunt (cyanotic heart disease)

Question 11

What should alveolar ventilation be?

Answer 11

350ml x 15 (RR) = 5250ml/min

Question 12

What should pulmonary perfusion be (pulmonary blood flow)?

Answer 12

SV X HR

70 X 70 = 4900 ml/min

Question 13

What is reduced in hypoventilation?

Answer 13

Alveolar ventilation

Question 14

Why does the amount of oxygen entering the blood and carbon dioxide entering the alveolus per min remain unchanged in hypoventilation?

Answer 14

Metabolic rate remains unchanged

Question 15

What happens to blood gases in hypoventilation?

Answer 15

Hypoxaemia

Hypercapnia

Question 16

Hypoventilation always causes hypercapnia, true or false?

Answer 16

True

Question 17

What type of respiratory failure does hypoventilation fall into?

Answer 17

Type 2 respiratory failure

Question 18

What can cause acute hypoventilation?

Answer 18

Opiate overdose
Head injury
Very severe acute asthma

Question 19

How should acute hypoventilation be treated?

Answer 19

Urgent treatment

Ventilation

Question 20

Why is chronic hypoventilation better tolerated?

Answer 20

Slow onset and progression allows time for compensation

Question 21

What disease can cause chronic hypoventilation?

Answer 21

Severe COPD

Question 22

What respiratory centre causes can lead to hypoventilation?

Answer 22

Opiates

Head injury

Question 23

Damage to phrenic and intercostal nerves can cause hypoventilation in which pathology?

Answer 23

Guillain-Barre Syndrome

Question 24

What NMJ problem causes hypoventilation?

Answer 24

Myasthenia gravis

Question 25

What are some chronic causes of hypoventilation?

Answer 25

Muscles of respiration - myopathy, MND Chest wall - severe obesity, kyphoscoliosis Lung fibrosis Late stage COPD

Question 26

What can effect the pleural cavity to cause hypoventilation?

Answer 26

Pneumothorax | Large pleural effusion

Question 27

What are the effects of hypoxaemia?

Answer 27

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

Question 28

Clinical definition for central cyanosis

Answer 28

>50gm/L of desaturated Hb in blood

Question 29

What does central cyanosis indicate?

Answer 29

Hypoxaemia | Seen in oral mucosa, tongue, lips

Question 30

What does peripheral cyanosis indicate?

Answer 30

Poor local circulation

Question 31

What compensatory mechanisms are there in chronic hypoxaemia?

Answer 31

Increased EPR secreted by kidney -> Polycythemia | Increased 2,3-DPG -> Shift curve to right

Question 32

What can result from chronic hypoxic vasoconstriction?

Answer 32

Pulmonary hypertension Right HF Cor pulmonale

Question 33

What is cor pulmonale?

Answer 33

HF secondary to pulmonary hypertension

Question 34

What are the effects of hypercapnia?

Answer 34

Respiratory acidosis Impaired CNS function: drowsy, confused, coma, flapping tremors Peripheral dilation: warm hands, bounding pulse Cerebral vasodilation: headache

Question 35

How can chronic hypercapnia be compensated for?

Answer 35

Respiratory acidosis compensated by retention of HCO3 by kidney

Question 36

Describe chronic CO2 retention in brain

Answer 36

CO2 diffuses into CSF CSF pH drops Stimulates central chemoreceptors Choroid plexus cells secrete HCO3 into CSF Central chemoreceptors reset to a new higher level of CO2

Question 37

Why may treatment of hypoxia worsen hypercapnia in chronic type 2 respiratory failure?

Answer 37

O2 removes stimulus for hypoxic respiratory drive | Removes pulmonary hypoxic vasoconstriction

Question 38

Describe blood gases when V/Q <1

Answer 38

Hypoventilation pO2 falls pCO2 rises

Question 39

Describe blood gases when V/Q >1

Answer 39

Hyperventilation pO2 rises pCO2 falls

Question 40

How are central and peripheral chemoreceptors stimulated in hyperventilation?

Answer 40

V/Q ration < 1 in affected alveoli ALVEOLAR oxygen falls and cardboard dioxide rises Causes hypoxic vasoconstriction, diverting blood to better ventilated areas If V/Q still <1 then that alveolar blood will mix with blood from functioning alveoli - ARTERIAL low O2, high pCO2 Stimulates receptors

Question 41

How can V/Q mismatch lead to type 1 respiratory failure?

Answer 41

Hyperventilation occurs Unaffected segments have increased ventilation V/Q>1 - O2 content not significantly increased because the rise in pO2 only causes a small increase in dissolved oxygen and Hb is fully saturated above 10kPa (insufficient to compensate) Drop in pCO2 and reducing in total CO2 in blood enough to compensate for any CO2 retention

Question 42

Why do V/Q mismatches arise?

Answer 42

Occurs in disorders where SOME alveoli are being poorly ventilated

Question 43

What are some disorders that may cause a V/Q mismatch?

Answer 43

``` Asthma Pneumonia RDS in newborn Pulmonary oedema Pulmonary embolism (Early stage COPD) ```

Question 44

Why does asthma cause V/Q mismatch?

Answer 44

Variable airway narrowing

Question 45

Why does pneumonia cause V/Q mismatch?

Answer 45

Exudate in affected alveoli

Question 46

Why does RDS in newborn cause V/Q mismatch?

Answer 46

Some alveoli not expanded

Question 47

Why does pulmonary oedema cause V/Q mismatch?

Answer 47

Fluid in alveoli

Question 48

Why does pulmonary embolism cause V/Q mismatch?

Answer 48

Embolus results in redistribution of blood flow Need to hyperventilate to meet needs If hyperventilation cant match increased perfusion then V/Q >1 Causes hypoxaemia (Ie lung cant ventilate the rest of the lung enough to oxygenate the extra blood flow)

Question 49

Why do you get type 1 respiratory failure if there is poor diffusion across the alveolar membrane?

Answer 49

CO2 is more soluble so its diffusion is affected less than oxygen

Question 50

Name some causes of diffuse lung fibrosis?

Answer 50

Idiopathic fibrosis alveolitis Asbestosis Extrinsic allergic alveolitis Pneumoconiosis