Assessment of V/Q

Question 1

What is the ideal V/Q ratio?

Answer 1

1

Question 2

True or False: With a V/Q of 1, blood gases equilibrate with alveolar gases

Answer 2

True

Question 3

When is V/Q equal to infinity and what does this mean for gas exchange?

Answer 3

V/Q is equal to infinity when ventilation is normal but perfusion is occluded (denominator approaches 0). In this situation, the alveolar gas equilibrates with inspired, humidified air and there is no oxygenation of blood.

Question 4

When is V/Q equal to zero and what does this mean for gas exchange?

Answer 4

V/Q is equal to zero when the ventilation is zero but perfusion is normal. In this situation, alveolar gas equilibrates with blood gases but there is no oxygenation of blood.

Question 5

Generally, what is a normal V/Q level? What is V/Q in the upper lobes? Lower lobes?

Answer 5

Normal V/Q ratio = 0.8 Upper lobes V/Q ratio = 2.5 (well ventilated but relatively under perfused) Lower lobes V/Q ratio = 0.6 (perfused but relatively under ventilated)

Question 6

What happens with local regulation of V/Q when the V/Q ratio is too high?

Answer 6

In situations with high V/Q ratio, the ventilation of CO₂ is happening faster than the blood (perfusion) can bring the CO₂. This results in a drop in alveolar P_CO2. The drop in alveolar P_CO2 causes bronchoconstriction to increase local airway resistance. This decreases ventilation and lowers the V/Q ratio.

Question 7

What happens with local regulation of V/Q when the V/Q ratio is too low?

Answer 7

In situations where the V/Q ratio is too low, alveolar P_O2 drops because the blood is carrying oxygen away faster than the ventilation can bring oxygen into the alveoli. The drop in P_O2 in the alveoli causes hypoxic vasoconstriction which decreases perfusion and increases V/Q.

Question 8

What is normal P_aCO2 at sea-level vs. denver?

Answer 8

Sea-level = 40 mmHg

Denver = 35 mmHg

Question 9

What is normal O₂ saturation at sea-level vs. Denver?

Answer 9

Sea-level = 97.5%

Denver = 95%

Question 10

What is normal C_aO2 at sea-level vs. denver?

Answer 10

Sea-level = 20.7 ml O₂/100 ml blood

Denver = 19 ml O₂/100 ml blood

Question 11

What is normal C_aCO2 at sea-level vs. Denver?

Answer 11

Sea-level = 44 ml CO₂/100 ml blood

Denver = 42 ml CO₂/100 ml blood

Question 12

What is anatomic dead space?

Answer 12

This is the part of the respiratory system that doesn’t participate in gas exchange (the first 16 branch points of the airway tree/trachea, bronchi, bronchioles). This makes up about a third of your whole airway.

Question 13

What is alveolar dead space?

Answer 13

These are alveoli that aren’t being perfused.

Question 14

How do you calculate physiological dead space?

Answer 14

Physiological dead space is the total dead space. To calculate this, you just add together anatomical dead space and alveolar dead space

Question 15

Why is dead space bad?

Answer 15

Dead space is bad because it is ventilation that isn’t being used. Your body is working to cause ventilation but without any benefit.

Question 16

True or False: Dead space is a common contributor to hypoxemia

Answer 16

False. Dead space doesn’t generally cause hypoxemia unless very severe.

Question 17

Does dead space increase or decrease with exercise?

Answer 17

Dead space decreases with exercise because exercise increases perfusion (Q).

Question 18

P_aCO2 is regulated by what 3 things?

Answer 18

Question 19

What 5 things can increase dead space?

Answer 19

1. rapid shallow breathing
2. pulmonary embolus
3. decreased cardiac output
4. mechanical ventilation (e.g. snorkeling)
5. emphysema

Question 20

How can you differentiate between low V/Q and shunt in patients?

Answer 20

Low V/Q responds to increased F_iO2 but shunts do not.

Question 21

What are some common things that cause shunts? (name 3)

Answer 21

1. Transudate from heart failure
2. Exudate from pneumonia
3. Exudate from ARDS

Question 22

Name 3 anatomic issues that can cause V/Q mismatch

Answer 22

1. congenital heart disease
2. pulmonary fistula
3. vascular lung tumor

Question 23

What is the most commonly used calculation to estimate V/Q mismatch?

Answer 23

A-a gradient

Question 24

Blood gas is typically presented in this format. _ /_ /_ /_ /_ on _.

What does each space stand for?

Answer 24

pH/P_aCO2/P_aO2/S_aO2/HCO₃^- on how much O₂

Question 25

In a normal patient, A-a gradient is typically due to ____ and ____

Answer 25

V/Q mismatch, shunt

Question 26

How does pulse oximetry (S_pO2) work?

Answer 26

Pulse oximeters measure the ratio of deoxy-Hb and oxy-Hb to give an estimate of % oxy-Hb as S_pO2.

It is able to do this because deoxy-Hb absorbs maximally in visible red band and oxy-Hb absorbs maximally in infrared band.

Question 27

What is the main issue with pulse oximetry? (1 main issue and 4 minor)

Answer 27

Pulse oximeters only measure Hb saturation but not what they are bound to. So, if the Hb is bound to CO or methylene blue, the pulse oximeter would still give a normal value. Thus, it’s important to understand that S_pO2 is different from P_aO2 or S_aO2. Clinical examples would be someone who was in a house fire may have CO poisoning and low O₂ saturation but their pulse ox reading would falsely show that they are normal. Or, patients could be on topical anesthetics which would cause methylene blue to bind to Hg but the pulse oximeter would falsely show a normal reading.

Also, pulse oximeters may have falsely high or low measurements due to lighting, nail polish, movement, or temperature.

Question 28

What is met-hemoglobin and how is it caused?

Answer 28

Met-hemoglobin is hemoglobin that is bound to methylene blue. Some anesthetics can cause this so hemoglobin is bound to methylene blue which causes a decrease in S_aO2 but the S_pO2 (from pulse ox) still reads normal. In clinic, you might increase F_iO2 when the patient has low S_pO2 but with Met-hemoglobin, the F_iO2 increase won’t help