Ventilation-perfusion relationship

Question 1

Why could overall VA/Q be misleading?

Answer 1

Because one side of the heart may have ventilation and no perfusion vice versa but overall looks alright. Should be looked at by alveolar capillary level not overall

Question 2

VA/Q ratio for shunt and dead space and its defintions

Answer 2

Shunt=perfusion fine but no ventilation aka no diffusion gradient (0). PCO2 increases PO2 decreases. But increase in ventilation normalise or increases PCO2
Dead space=no perfusion but ventilation fine (infinity) PCO2 decreases PO2 increases but not O2 content because O2 saturated

Question 3

Normal right to left shunt exceptions

Answer 3

venous blood from bronchial veins and veins draining wall of left ventricle added straight to left side of heart

Question 4

Abnormal right to left shunt examples

Answer 4

Collapsed (atelectasis), consolidated (pneumonia), congienital heart disease (fallot’s tetralogy)

Question 5

Atrial and ventricular septal defects

Answer 5

Initially cause L toR shunts-this doesn’t cause low arterial oxygen content and PO2. For VSD high pressure in pulmonary circulation causes remodelling and a R to L shunt. Pressure in RV>LV deoxygenated blood goes directly to LHS

Question 6

How to calculate effect of 20% shunts on arterial O2 and CO2

Answer 6

O2 content=80/100 x arterial content+20/100 xvenous content

same for CO2

Question 7

How do O2 and CO2 content differ? Draw it

Answer 7

O2 is sigmoidal, moderate fall in O2 content causes large fall PO2. CO2 is linear. Moderate rise in CO2 causes very small rise in PCO2

Question 8

Effect of increased ventilation in right to left shunt

Answer 8

Low PO2 and high PCO2 stimulates chemoreceptors->increases ventilation->ventilated areas lose more CO2 but gain little extra O2. Final blood gases=low PaCO2 (remains hypoxaemic), normal (blow off little CO2 to normalise) or lowe PaCO2 (if hypoxic and blow off too much CO2, hypercapnic and alkalotic). Breathing 100% O2 limited effect because doesn’t reach shunted blood

Question 9

Can’t effect of high VA/Q areas balance effect of low VA/Q areas

Answer 9

No, larger flow dominates. More blood tends to come from low VA/Q areas as high VA/Q areas caused by poor perfusion. High VA/Q areas do not have high O2 content

Question 10

Response to O2 enriched air in R to L shunts and ventilation-perfusion mismatching

Answer 10

R to L shunt-very small improvement in PO2

VA/Q mismatching-improvement marked because PO2 in underventilated lung areas improved

Question 11

Why mismatching in normal upright lung. Draw graph and explain

Answer 11

Before inspiration, top alveoli>bottom alveoli because bottom squashed by gravity. After inhalation, proportion of expansion bigger in bottom than top. Gravity improves perfusion and ventilation towards bottom of lung so better matched. greater effect on perfusion at bottom

Question 12

Hypoxic pulmonary vasocontriction. What to do and not do

Answer 12

To do: inhalation vasodilator

Not to do: injection vasodilator because overall vasodilator allocation undo lung work and make VA/Q worse

Question 13

Mechanisms leading to arterial hypoxia

Answer 13

low inspired PO2, hypoventilation, diffusion impairment, R to L shunt, VA/Q mismatch

Question 14

What increases PaCO2

Answer 14

hypoventilation

Question 15

Effect of hypoxic pulmonary vasoconstriction

Answer 15

diverts blood from poorly ventilated areas to well ventilated areas improving VA/Q ratio. Increases resistance in pulmonary artery which increases pressure so load on right heart increases. NB not useful for global hypoxia

Question 16

Global hypoxia what

Answer 16

Hypoxic lung disease, respiratory failure, altitude

Question 17

Assessment methods VA/Q mismatching

Answer 17

Isotope ventilation-perfusion scans, Measure dead space and shunt effect using inert gases. Use Bohr and shunt equations. A-a gradient