Clinical Gas Exchange

Question 1

What is respiratory failure?

Answer 1

failure to oxygenate properly leading to hypoxaemia

Question 2

What is ventilatory failure?

Answer 2

failure of the ventilatory pump mechanism leading to hypoxaemia

Question 3

Why do patients become hypoxic?

Answer 3

Hypoventilation
Ventilation perfusion (V/Q) mismatch (pathological vs. physiological)
Both of the above

Question 4

What is the relationship between CO2 and hypoventilation?

Answer 4

During normal ventilation CO2 diffuses out of blood into alveolus following a partial pressure gradient
CO2 is mostly dissolved in blood rather than bound to haemoglobin
If there is lower ventilation then CO2 accumulates in the alveolar space meaning less can be removed from the blood

Question 5

What are the arterial blood gases?

Answer 5

PaO2 lower than expected = respiratory failure
PaCO2
HCO3_
[H+]

Question 6

How do we diagnose the types of respiratory failure?

Answer 6

• PO2 is low= respiratory failure
• High PCO2= Type 2, ventilatory failure, may have V/Q mismatch
• Low PCO2= Type 1, respiratory failure, V/Q mismatch

Question 7

What causes the failure of the ventilatory pump?

Answer 7

Won’t breathe: control failure
-Brain failure to command e.g. drug overdose
-Sometimes in COPD
Can’t breathe: broken peripheral mechanism
-Nerves not working e.g. phrenic nerve cut
-Muscles not working e.g. muscular dystrophy
-Chest can’t move e.g. severe scoliosis
-Gas can’t get in and out e.g. asthma/COPD

Question 8

Describe Type 2 respiratory failure

Answer 8

Decrease in PO2
Increase in PCO2
Common causes in hospital:
-Severe COPD (can be acute or chronic)
-Acute Severe Asthma
-Pulmonary Oedema in acute Left Ventricular failure
Due to hypoventilation as main feature

Question 9

How do we treat type 2 respiratory failure?

Answer 9

Give oxygen
-Controlled in COPD patients with chronic respiratory failure
Treat the underlying cause to reverse hypoventilation e.g. bronchodilators for acute asthma or opiate antagonists for overdoses
Support ventilation
-Non-invasive ventilation
-Invasive ventilation

Question 10

Describe non-invasive ventilation for type 2 respiratory failure

Answer 10

Common treatment for COPD exacerbations with type 2 respiratory failure
Tight fitting mask, no need to sedate and intubate
Increases ventilation efficiency
Also useful in neuromuscular disease and thoracic wall disease

Question 11

What causes V/Q mismatch?

Answer 11

Most lung diseases effecting the airways and parenchyma
Lung infection such as pneumonia
Bronchial narrowing such as asthma and COPD (although they can also progress to type 2 resp failure)
Interstitial lung disease
Acute lung injury

Question 12

Why is there V/Q mismatch in pneumonia?

Answer 12

Creates a shunt leading to low PO2 because blood does not come into contact with adequate O2
Fluid in alveoli

Question 13

What happens to arterial CO2 in V/Q mismatch?

Answer 13

Blood leaving areas of low V/Q ratio has
-Low PaO2
-High PaCO2
High PaCO2 stimulates ventilation
‘Extra’ ventilation goes to areas of normal lung and areas with high V/Q ratio so get blood with low CO2
Blood from both areas mixes so overall CO2 is normal
But extra ventilation can’t push O2 content much higher than normal
Blood from both areas mixes but cannot overcome the low oxygen level

Question 14

How do we treat respiratory failure?

Answer 14

Give oxygen

Question 15

How can V/Q mismatch be caused by perfusion problems?

Answer 15

Pulmonary embolism
Can range form small PTE causing no problem with gas exchange ranging to massive PE with hypoxia
Emboli effectively cause areas of dead space where there is ventilation but no perfusion causing hypoxia
Massive emboli can cause circulatory failure and death

Question 16

How do we treat pulmonary emboli?

Answer 16

Oxygen in acute episode
Anticoagulation to stop further clot propagation
Thrombolysis in some cases where circulatory compromise

Question 17

How can asthma link to respiratory failure?

Answer 17

Hypoxaemia suggests significant asthma attack
Bronchospasm and mucous plugging causes ventilation defects and V/Q miss match
Type 2 resp failure develops when severe bronchospasm causes hypoventilation of alveoli or exhaustion
The patient needs oxygen to survive
Invasive ventilation may be required

Question 18

How can COPD be linked with respiratory failure?

Answer 18

COPD is a mixture of chronic airways inflammation and narrowing and emphysema
Problems with V/Q mismatch and hypoventilation
May present acutely with respiratory failure type 1 or type 2
May have chronic type 2 respiratory failure in advanced disease
Treat respiratory failure with oxygen but with caution in chronic type 2 respiratory failure

Question 19

Describe oxygen therapy through masks

Answer 19

Variable performance
-Cheap and cheerful
-Exact inspired O2 concentration not known
Fixed function
-Constant, known inspired concentration
Reservoir mask
-High inspired concentration of O2

Question 20

What are the types of Venturi masks?

Answer 20

• Pre-mixed gases= accelerate gas through nozzle, velocity increase: increased kinetic energy (negative pressure zone)
• Controlled oxygen therapy= aim to supply oxygen at flow rate faster than the patient can breathe, different oxygen percentages

Question 21

How do reservoir masks work?

Answer 21

Supplies maximum amount of oxygen
Store during expiration
Deliver during inspiration

Question 22

When might patients require ventilation?

Answer 22

Invasive Ventilation is required for severe respiratory failure not responding to oxygen therapy
Not a suitable treatment for all patients
Provided in intensive therapy units