Ventilation-Perfusion Matching

Question 1

Normal V/Q

Answer 1

V/Q = (4 L/min)/(5 L/min) = 0.8

Question 2

Pulmonary embolism/pulmonary dead space

Answer 2

Lung area is ventilated but not perfused –> V/Q = infinity

Question 3

Shunt

Answer 3

Lung area is perfused but not ventilated (bronchial obstruction) –> V/Q = 0

Ex: pneumonia

Question 4

Equation to calculate vascular resistance

Answer 4

R = P/Q (from V = IR)

SVR = (P_aorta - P_ra)/QdotC

PVR = (P_pa - P_la)/QdotC

Remember QdotC is CO

Question 5

Fick Principle equation

Answer 5

VdotO2 = QdotC x (CaO2 - CvbarO2)

Main idea: difference between what is flowing in and what is flowing out is what is taken up by the lung

Question 6

Mixed venous and arterial O2 content

Answer 6

CvbarO2 = 150 ml/L

CaO2 = 200 ml/L

Question 7

Factors determining fluid movement across pulmonary capillaries

Answer 7

Hydrostatic pressure

Oncotic pressure

Alveolar pressure

Question 8

How do we get pulmonary edema?

Answer 8

1) Increased mean pulmonary capillary pressure (left heart failure)
2) Reduced plasma oncotic pressure (hypoproteinemia)
3) Leaky alveolar-capillary membranes which allow plasma proteins out of capillaries

Question 9

What are some things that can prevent edema?

Answer 9

Lymphatic drainage keeps lung dry

Positive end-expiratory pressure (PEEP) which generates positive alveolar pressure that turns into positive interstitial space pressure

Question 10

Ventilation and perfusion in 3 zones of the lung

Answer 10

Zone 1: PA > Pa > PV; least overall ventilation and blood flow; highest Vdot/Qdot; capillaries will collapse

Zone 2: Pa > PA > PV; blood flow driven by difference between Pa and PA, not arterial and venous pressure like usual; some capillaries may collapse

Zone 3: Pa > PV > PA; most overall ventilation and blood flow; lowest Vdot/Qdot; blood flow driven by difference between arterial and venous pressure

Note: blood flow is more affected by gravity than ventilation

Question 11

What happens to PO2 and CaO2 at low V/Q regions and high V/Q regions?

Answer 11

This is all because of sigmoid shape of Hb-O2 dissociation curve

Low V/Q regions: low PO2 and low O2 content

High V/Q regions: high PO2 but only slightly increased O2 content because dissociation curve is FLAT in this range

Question 12

What happens to PCO2 in high and low V/Q regions?

Answer 12

CO2 dissociation curve is linear in the physiological range

Hyperventilatory effects of high V/Q regions counterbalance hypoventilatory effects of low V/Q regions, so PaCO2 and PACO2 are not affected (?)

Question 13

What happens when you have low V/Q?

Answer 13

Hypoxic pulmonary vasoconstriction

Low PAO2 triggers reflex vasoconstriction to close blood vessels there

This is good because you’re not getting ventilation so want to shunt blood into regions that ARE getting ventilation

This increases pulmonary arterial pressures

(Note: NO is involved in this vasoconstriction)

Question 14

What happens when you have high V/Q?

Answer 14

Reflex bronchoconstriction

Low PACO2 causes airways to restrict

This is good because want to get ventilation to areas that are more perfused

Question 15

Shunt fraction

Answer 15

Physiological shunt calculation: some mixed venous blood goes through areas of lung that are perfused but not ventilated

QdotShunt/QdotTotal = (Cc’O2 - CaO2)/(Cc’O2 - CvbarO2)

Can calculate Cc’O2 from PAO2 and assuming a lung exchanges gases ideally

Cc’O2 is blood that goes through part of lung with full ventilation and CvbarO2 is before and CaO2 is after

Question 16

How can you distinguish V/Q mismatch from shunt?

Answer 16

Give person 100% O2:

V/Q mismatch person has SOME ventilation, so their PO2 would increase

Shunt person would have NO ventilation, so PO2 would not increase

Question 17

What is an extra-pulmonary shunt? (right to left shunt)

Answer 17

1) Ventricular atrial septal defect–goes right heart to left heart (skips lungs!), introducing deoxygenated blood into systemic circulation
2) Thebesian veins drain myocardium by draining venous blood into left heart
3) Pulmonary artery to vein fistula

Question 18

Why is it easy to compensate for increased CO2 but hard to compensate for decreased O2 that results from physiological shunt?

Answer 18

With CO2, places in the lung with high V/Q (lower CO2 content) can make up for areas of low V/Q (higher CO2 content)

With O2, nonlinear curve. Areas of low V/Q (low O2 content) can’t be compensated by overventilating high V/Q areas because hemoglobin is already fully saturated, and blood only carries a TINY proportion of dissolved O2 (low solubility)

Question 19

4 causes of hypoxemia (low PaO2)

Answer 19

Alveolar hypoventilation (normal PA-aO2 because low PA also!)

Diffusion impairment

Maldistribution of V/Q

Right to left shunt