Pathphysiology of Resp failure

Question 1

How can you measure O2 sats?

Answer 1

Pulse oximetry

Question 2

How is air in the trachea?

Answer 2

Gas is humidified in the trachea during inspiration

Question 3

What are the principles underlying the alveolar has equation?

Answer 3

PaO2 is a balance between 2 processes: the removal of O2 by the pulmonary capillaries and its continual supply by alveolar ventilation
Equation means that if PaCO2 goes up, then PaO2 comes down

Question 4

What are the partial pressures of the O2 in the alveoli compared to that of the blood?

Answer 4

O2 diffuses from the high pressure in the alveoli (15 kPa) to the area of lower pressure of the blood in pulmonary capillaries (5.3 kPa)
Normally O2 reaches diffusion equilibrium about 1/3 of the way along the capillary.

Question 5

What 3 factors affect diffusion according to Fick’s Law?

Answer 5

Concentration difference
Thickness of membrane
Surface area

Question 6

What does the V/Q ratio stand for? If the alveolar ventilation is 4L/min and cardiac output is 5L/min, what is the V/Q ratio?

Answer 6

Ventilation/Perfusion - ratio between the amount of air getting to the alveoli (alveolar ventialtion)and the amount of blood entering the lungs (cardiac output)
4/5 = 0.8

Question 7

Describe how alveolar ventilation varies between the apex and base of lungs.

Answer 7

Subject to effects of gravity
Alveoli at apex are exposed to greater distension - flatter part of compliance curve
Alveoli at base have greater change in volume and so are preferentially ventilated

Question 8

What part of the lungs are more perfused? (apex or base)

Answer 8

Base of lungs (same as ventilation), due to gravity

P arterial > P venous > P Alveoli

Question 9

How do Ventilation (V) and Perfusion (Q) change towards the base? and what which rib do they perfectly match (i.e 1)?

Answer 9

V and Q increase by different amounts towards the base. Match near to rib 3.

Question 10

What is the difference between shunt and dead space?

Answer 10

Shunt - highly perfused but not ventilated

Dead space - ventilated but not perfused

Question 11

What are examples of anatomical shunts?

Answer 11

Thesbian veins and bronchial arteries

Question 12

What can cause an increased Alveolar - arterial PO2 difference?

Answer 12

V/Q mismatch
Diffusion impairment
Anatomical shunt (pathalogical)

Question 13

What are the causes of hypoxaemia?

Answer 13

Low PiO2
Hypoventilation
V/Q mismatch
Diffusion abnormality
Low cardiac output
Low Hb

Question 14

What is the major difference between Type 1 and 2 respiratory failure?

Answer 14

Type 1 - one gas is wrong (Hypoxia)

Type 2 - two gases are wrong (Hypercapnia)

Question 15

What can cause Type 1 respiratory failure?

Answer 15

V/Q mismatch
Asthma
Pneumonia
PE
Pulmonary fibrosis
Pulmonary oedema
COPD
Pneumothorax
Atelectasis

Question 16

What can cause Type 2 respiratory failure?

Answer 16

Ventilatory failure - results in alveolar hypoventilation

• Dec resp drive
• Impaired lung movements
• Lung pathology (COPD/asthma)

Question 17

How can you distinguish between Type 1 and 2 respiratory failure?

Answer 17

Type 1 - low O2 with low/normal CO2

Type 2 - low O2 with high CO2

Question 18

If a patient presented with the below, what type of resp failure do they have?
pH: 7.12 (7.35-7.45)
PaO2: 6 (10–14)
PaCO2: 9.5 (4.5–6.5)

Answer 18

Type 2 respiratory failure (low O2 and high CO2)

Question 19

If a patient presented with the below, what type of resp failure do they have?
pH: 7.26 (7.35-7.45)
PaO2: 6.1 (10–14)
PaCO2: 3.5 (4.5–6.5)

Answer 19

Type 1 respiratory failure (low O2 and low CO2)

Question 20

What are the manifestations of respiratory failure?

Answer 20

Tissue hypoxia (red GCS, lactic acidosis, low BP and HR)
Haemoglobin (cyanosis)
Hypercarbia (resp acidosis, CO2 retention flap)

Question 21

What are the acute compensatory mechanisms of respiratory failure?

Answer 21

Use of accessory muscles
Tachypnoea
Sympathetic stimulation

Question 22

What are the chronic compensatory mechanisms of respiratory failure?

Answer 22

Hypoxia: Polycythaemia, Cardiovascular, pulmonary vasoconstriction, angiogenesis, cellular resp changes
Hypercarbia: renal compensation