Respiratory Failure

Question 1

What is hypoxaemia?

Answer 1

Low pO2 in arterial blood
<10.6kPa

Question 2

What is hypoxia?

Answer 2

O2 deficiency at tissue level

Question 3

What is normal O2 saturation?

Answer 3

94-98%

Question 4

What is the normal range for PaO2?

Answer 4

10.6-13.3kPa

Question 5

At what O2 saturation is tissue damage most likely?

Answer 5

<90%

Question 6

At what pO2 is tissue damage most likely?

Answer 6

<8kPa

Question 7

What is respiratory failure?

Answer 7

Impairment in gas exchange causing hypoxaemia +/- hypercapnia
Type 1 without
Type 2 with

Question 8

What is type 1 respiratory failure?

Answer 8

• impairment of gas exchange causing hypoxaemia WITHOUT hypercapnia
• low PaO2 <8kPa or O2 sats <90%
• pCO2 normal or low

Question 9

What is type 2 respiratory failure?

Answer 9

• impairment of gas exchange causing hypoxaemia WITH hypercapnia
• low PaO2 <8kPa AND high PaCO2 >6.5kPa
• respiratory pump failure

Question 10

Signs of hypoxaemia

Answer 10

• impaired CNS function > confusion + irritability
• central cyanosis
• Tachypnoea
• tachycardia
• cardiac arrhythmias + cardiac ischaemia
• hypoxic vasoconstriction of pulmonary vessels (can cause pulmonary hypertension)

Question 11

What are the oxygen stats of a patient with central cyanosis?

Answer 11

<85%

Question 12

What are compensatory mechanisms to increase oxygen delivery (+ decrease hypoxia)?

Answer 12

• increased EPO secreted by kidney > raised Hb
• increased 2,3 DPG > shifts Hb/O2 dissociation curve to right > O2 released more freely
• increased capillary density

Question 13

What can chronic hypoxic vasoconstriction of pulmonary vessels result in?

Answer 13

• Pulmonary hypertension
• Right heart failure
• Cor pulmonale: hypertrophy of right side of heart

Question 14

Causes of hypoxaemia

Answer 14

• low inspired FiO2 e.g. high altitude
• V/Q mismatch
• diffusion defect (problems of alveolar capillary membrane)
• intra-lung shunt - acute respiratory distress syndrome
• hypoventilation

Question 15

What is the most common cause of hypoxaemia?

Answer 15

V/Q mismatch

Question 16

List some causes of V/Q mismatch

Answer 16

• asthma
• COPD
• pnemonia
• respiratory distress syndrome in newborn
• pulmonary oedema
• pulmonary embolism

Question 17

Treatment of V/Q mismatch

Answer 17

• improves with O2 administration
• underlying pathology must be corrected for O2 to completely correct hypoxaemia

Question 18

What happens to PaO2 and PaCO2 in V/Q mismatch due to inadequate ventilation?

Answer 18

• PaO2 falls
• PaCO2 rises

Blood equilibrates to new alveolar PaO2 + PaCO2

Question 19

What is the V/Q ratio in a pulmonary embolism?

Answer 19

> 1
Due to poor perfusion

Question 20

What does poor diffusion across the alveolar membrane cause in terms of respiratory failure?

Answer 20

• initally type 1 respiratory failure pCO2 normal or low + pO2 low
• as disease progresses, restrictive lung disease leads to hypoventilation > hypercapnia: pCO2 high (type 2 respiratory failure)

Question 21

Causes of diffuse lung fibrosis

Answer 21

• idiopathic
• asbestosis: exposure to asbestos
• extrinsic allergic alveolitis
• pneumoconiosis: inhalation of dust particles

Question 22

Reasons for intrapulmonary shunts

Answer 22

• collapsed alveolus
• fluid filled alveolus

no gas exchange in that area

Question 23

Example of intrapulmonary shunt

Answer 23

Acute respiratory distress syndrome

Question 24

What occurs in acute respiratory distress syndrome?

Answer 24

• loss of surfactant > causes alveolar atelectasis
• exudate in lungs
• lung becomes stiff (less compliant) > lung volume decreases
• loss of hypoxic pulmonary vasoconstriction mechanism
• intrapulmonary shunt due to no ventilation with respect to perfusion

Question 25

Treatment to acute respiratory distress syndrome

Answer 25

- hard to manage on ventilator - even 100% O2 may not correct hypoxaemia - need to add positive pressure ventilation to open the alveoli

Question 26

What is hypoventilation?

Answer 26

The entire lung is poorly ventilated due to inadequate respiratory rate or volume of alveolar ventilation

Question 27

What does hypoventilation cause in terms of respiratory failure?

Answer 27

- ventilation reduced - alveolar pO2 falls > arterial pO2 falls > **hypoxaemia** - alveolar pCO2 rises > artieral pCO2 rises > **hypercapnia** - **Type 2 respiratory failure**

Question 28

Causes of acute hypoventilation

Answer 28

- opiate overdose - head injury - very severe acute asthma

Question 29

Causes of chronic hypoventilation

Answer 29

Severe COPD

Question 30

Is acute or chronic hypoventilation tolerated better? Why?

Answer 30

**Chronic** Slow onset + progression so there is time for compensation

Question 31

Effects of acute hypercapnia

Answer 31

- **respiratory acidosis** - **impaired CN function** > drowsy, confusion, flapping tremor, coma - **peripheral vasodilation** >warm hands + bounding pulse - **cerebral vasodilation** > headache

Question 32

Why can treatment of hypoaxemia worsen hypercapnia?

Answer 32

1. **Correction of hypoxia removes pulmonary arteriole hypoxic vasoconstriction** - this leads to increased perfusion of poorly ventilated alveoli - blood diverted away from better ventilated alveoli - V/Q mismatch worsens 2. **Haldane mechanism** - oxygenated haemoglobin has low affiliated for CO2 > CO2 dissociates from Hb into blood

Question 33

How does pneumonia cause V/Q mismatch?

Answer 33

Exudate in affected alveoli Reduced ventilation

Question 34

What is atelectasis?

Answer 34

Partial collapse or incomplete inflation of lung

Question 35

Effects of chronic hypercapnia

Answer 35

- respiratory acidosis compensated by retention of HCO3- by kidney - vasodilation mild but still present

Question 36

What is scoliosis?

Answer 36

Sideways curvature of the spine

Question 37

What is kyphosis?

Answer 37

Excessive outward curve of spine Abnormal rounding of upper back

Question 38

What is kyphoscolosis?

Answer 38

Both scoliosis + kyphosis Sideways curvature + excessive outward curve of spine

Question 39

What conditions can cause hypoventilation?

Answer 39

- gullien barre syndrome - kypohoscoliosis - myasthenia gravis - COPD + asthma due to muscle weakness/fatigue - motor neurone disease

Question 40

What is the mechanism explains why a patient with kyphoscoliosis can develop respiratory failure?

Answer 40

- Hypoventilation - Due to inability to adequately expand the thoracic cavity during inspiration - Cannot adequately decrease intrapulmonary pressure relative to atm - inspiration is compromised