Physiology of Respiratory Failure

Question 1

What is type 1 respiratory failure?

Answer 1

Hypoxic respiratory failure

Question 2

What is type 2 respiratory failure?

Answer 2

Hypercapnic respiratory failure

Question 3

What does lung failure lead to?

Answer 3

Type 1 respiratory failure

Question 4

What does pump (heart?) failure lead to?

Answer 4

Type 2 respiratory failure

Question 5

What is the problem in type 1 respiratory failure?

Answer 5

Gas exchange

Question 6

What is the problem in type 2 respiratory failure?

Answer 6

Ventilation

Question 7

What is the definition of hypoxaemia?

Answer 7

PaO2 < 8kPa

Question 8

What are the 5 causes of hypoxaemia?

Answer 8

1) VQ mismatch (main reasons)
2) Alveolar hypoventilation e.g. GBS (neuromuscular diseases)
3) Impaired diffusion e.g. pulmonary fibrosis (can’t get gas across BM)
4) Low partial pressure of inspired oxygen (low PiO2) e.g. flying
5) Anatomical R-L shunt e.g. PAVM lobar pneumonia

Question 9

What is the most common cause of arterial hypoxaemia?

Answer 9

VQ mismatch

Question 10

Describe V/Q matching

Answer 10

• Ventilation and perfusion must be exactly matched
• Va = Q
• Va/Q is the critical factor governing gas exchange
• Regions of high ventilation should have high blood flows i.e. base of lung
• Regions of low ventilation should have low blood flows i.e. apex of lung

Question 11

What does inadequate gas exchange occur?

Answer 11

When regional Va &laquo_space;1 or Va/Q&raquo_space; 1

Question 12

What happens in shunt?

Answer 12

• Va/Q = 0
• Q&raquo_space; Va
• There is wasted perfusion so the blood is perfusing an underventilated lung
• So low PAO2 (40) and high PACO2 (46)

Question 13

What happens in dead space?

Answer 13

• Va/Q = infinity
• Q &laquo_space;Va
• There is wasted ventilation as we are ventilating and underperfused lung
• So PAO2 = 150 (v high) and PACO2 = 0

Question 14

What happens in an ideal VA/Q match?

Answer 14

• Va/Q = 1
• Q = Va
• PAO2 = 100 and PACO2 = 40

Question 15

What happens to blood flow in a healthy lung?

Answer 15

There are important reflex mechanisms that increase blood flow to well ventilated alveoli and reduce blood flow to poorly ventilated alveoli

Question 16

What happens in lung disease?

Answer 16

There is an imbalance between Va and Q resulting in hypoxaemia and hypercapnia

Question 17

What is the definition of hypercapnia?

Answer 17

PaCO2 > 6 kPa

Question 18

What is the equation for overall minute ventilation (VE)?

Answer 18

Tidal volume (VT) x RR

Question 19

What is PaCO2 inversely proportional to?

Answer 19

Alveolar ventilation

Question 20

What is essential to maintain CO2 homeostasis?

Answer 20

Adequate alveolar ventilation

Question 21

What determines alveolar ventilation?

Answer 21

RR and VT

Question 22

What are the two possible responses to oxygen therapy?

Answer 22

1) Patient’s PaCO2 and clinical state may improve or not change
2) Patient’s PaCO2 rises and they become drowsy or unconscious

Question 23

What 3 characters need to be balanced to avoid (?) hypercapnia?

Answer 23

1) Drive
2) Load
3) Capacity (ability to transmit information through nervous system)

Question 24

What causes hypercapnia?

Answer 24

Reduced drive, causing low capacity 
High load (resistive)

Question 25

What two areas are involved in drive to breathe?

Answer 25

Cortex and brain stem

Question 26

What are causes of drive failure?

Answer 26

1) Cortical and brainstem lesions → trauma, encephalitis, ischaemia, haemorrhage, Cheyne-Stokes Respiration (linked to CHF)
2) Drugs → sedatives, opioids
3) Metabolic alkalosis → loop diuretics, blunts respiratory drive (hypercapnia?)

Question 27

What are causes of low capacity?

Answer 27

Problems with spinal cord, peripheral nerves, NMJ and muscles

Question 28

What can lead to problems with nerves and the NMJ (transmission failure from nerves) leading to problems with capacity?

Answer 28

1) Spinal cord lesion
2) Polio
3) MND
4) Phrenic nerve injury
5) GBS
6) CINMA
7) NMB agents
8) Aminoglycosides esp. gentamicin
9) Myasthenia gravis

Question 29

What can lead to action failure (problems with muscles and the diaphragm) leading to problems with capacity?

Answer 29

1) Muscular dystrophies (Duchenne = most common)
2) Inflammatory myopathies
3) Malnutrition myopathy
4) Acid maltase deficiency
5) Thyroid myopathy
6) Biochemical anomalies → hypokalaemia, hypophosphataemia

Question 30

What are causes of high load (impedance)

Answer 30

1) Elastic load → pulmonary infarction, alveolar oedema, atelectasis, pleural effusion, obesity, abdominal distension
2) Resistive load → bronchospasm, UAO, OSA, secretion retention in ET tube
3) Threshold → intrinsic PEEP, dynamic hyperinflation

Question 31

What does load outweigh?

Answer 31

What the muscles can do

Question 32

What do a ⅓ of patients with chronic respiratory disease have it secondary to?

Answer 32

Obesity

Question 33

What is hypercapnic respiratory failure?

Answer 33

• Acute (on chronic) imbalance in load-capacity-drive relationship
• Caused by a defect in each area or combination

Question 34

In summary, what are causes of hypoxic respiratory failure?

Answer 34

• V/Q mismatch
• R-L shunt
• Impaired diffusion
• Hypoventilation
• Low FiO2 (low fraction of oxygen in the air, e.g. high altitudes)

Question 35

What is the cause of hypercapnic respiratory failure?

Answer 35

Imbalance in load, capacity or drive