Ventilation-perfusion relationships

Question 1

What should ventilation/perfusion be in a healthy person?

Answer 1

Approximately 1 would be ideal

In reality, it is around 0.8

Question 2

Where must ventilation/perfusion be matched?

Answer 2

At the ALVEOLAR level
- that is where gas exchange occurs
Overall VA/Q may be misleading

Question 3

What is the V/Q ratio of a patient with a shunt?

Answer 3

Tends towards 0

Question 4

What is the V/Q ratio of a patient with lots of dead space? (e.g. emphysema)

Answer 4

Tends towards affinity

Question 5

What are normal right to left shunts?

Answer 5

Venous blood from bronchial veins and a few small veins draining the wall of the LV (Thebesian veins)
- added to left-sided blood without going via lungs

Question 6

When might you have an abnormal right to left shunt?

Answer 6

Lobar pneumonia
Collapsed lung
Some congenital heart diseases

Question 7

What are the two types of heart defect that initially cause L to R shunts?

Answer 7

Atrial septal defect

Ventricular septal defect

Question 8

What happens after time with a ventricular septal defect?

Answer 8

The pressure in the RV increases causing hypertrophy of the RV muscle - becomes a right to left shunt
e.g. Fallot’s tetralogy

Question 9

What does a moderate fall in O2 content do to the PO2?

Answer 9

Large decrease

Question 10

What does a moderate rise in CO2 content do to the PO2?

Answer 10

V small increase

Question 11

What is the effect of increased ventilation in a patient with a right to left shunt?

Answer 11

PO2 will remain low

PCO2 will decrease or even become low due to increased ventilation

Question 12

Is breathing 100% oxygen helpful for a right to left shunt patient?

Answer 12

Only has a mild effect

This is because it does not reach the shunted blood and the ventilated blood is already near saturation

Question 13

High Co2 in an asthma patient indicates what?

Answer 13

Ominous finding

Acute respiratory acidosis

Question 14

What does the kidney do in chronic hypoventilation?

Answer 14

Renal compensation normalises pH (increase HCO3-)

Question 15

Can high V/Q areas balance out low V/Q areas?

Answer 15

NO

• more blood comes from the low V/Q areas
• high V/Q areas do not have high oxygen content

Question 16

Where is ventilation greatest in the lung?

Answer 16

At the bottom

- these alveoli are smaller (because of gravity) and have a bigger capacity to expand

Question 17

Where is V/Q highest in the lung?

Answer 17

Top

Effect on perfusion is larger than ventilation at the bottom of the lung

Question 18

What accounts for the PO2 A-a difference? (PO2 slightly lower in arterial than alveolar)

Answer 18

1. The fact that most of the lung has a low V/Q (effects of gravity on VA/Q
2. Physiological shunts

Question 19

Where in the lung is TB seen?

Answer 19

Top (apices) - highest V/Q ratios, most O2

Obligate aerobes

Question 20

What is hypoxic pulmonary vasoconstriction?

Answer 20

BVs constrict regionally when exposed to hypoxic conditions
Moves blood to better ventilated parts of the lung
Therefore improves V/Q matching

Question 21

When is Hypoxic pulmonary vasoconstriction not helpful?

Answer 21

Total hypoxia (hypobaric hypoxia) e.g. at altitude
Or respiratory failure
Can lead to right heart failure

Question 22

How can you treat hypoxic pulmonary vasoconstriction?

Answer 22

Use a vasodilator (NO) - inhaled (gets to lungs specifically)

Question 23

How do we assess ventilation-perfusion mismatching?

Answer 23

1. Isotope ventilation
2. Ventilation/ Perfusion scans
3. Measure alveolar dead space/shunt effect
4. Measure A-a PO2 gradient

Question 24

What can pneumonia cause?

Answer 24

Right to left shunt

Question 25

What are the 5 potential causes of arterial hypoxia? (Low PaO2)

Answer 25

1. Low inspired oxygen (hypobaric hypoxia)
2. hypoventilation
3. Diffusion impairment (fibrosis)
4. Right to left shunt
5. Ventilation-perfusion mismatch (MOST COMMON)

Question 26

Why does PO2 decrease when PCO2 increases?

Answer 26

Alveolar gas equation

Question 27

Which is the only mechanism causing hypoxia that leads to an increased pCO2?

Answer 27

Hypoventilation

Question 28

Which factors causing hypoxia increase the A-a PO2 gradient?

Answer 28

Diffusion impairment
Right to left shunt
Ventilation-perfusion mismatch

Question 29

What is the normal A-a difference?

Answer 29

10-15 mmHg