Arterial Blood Gases - O2 Carriage/CO2 Carriage

Question 1

What are the different modes of CO2 carriage and what are the approximate values?

Answer 1

1. Freely dissolved: 1.2 mM
2. HCO3-: 24 mM
3. Carbamino (mainly Hb): 1.2 mM

Question 2

What enzyme catalyzes bicarb formation from CO2 and H2O?

Answer 2

Carbonic anhydrase

Question 3

Explain the Bohr effect? What about the Haldane effect?

Answer 3

Bohr: CO2 binding to Hb reduces O2 affinity

Haldane: O2 binding reduces CO2 affinity (not as important)

• Two CO2 binding sites on one hemoglobin

Question 4

So muscles can only take up freely dissolved O2. So what factors increase O2 offloading?

Answer 4

Reduction of Hemoglobin binding infinity:

• High temperatures
• High CO2
• Low PH
• 2,3-diphosphoglycerate (from hypoxia)

Question 5

What is the O2 carrying capacity? How do you solve for it?

Answer 5

The maximum amount of O2 that can be carried by a particular amount of Hb

• = [Hb] x 1.39
• multiply this by SaO2 (saturation) to get arterial oxygen content
• multiply arterial oxygen content by Q (flow) to get total volume of O2 delivered to tissues/ minute

Question 6

T/F: your tissues usually consume most of the arterial O2 delivered?

Answer 6

FALSE

• Your body usually delivers like 1000 ml O2 at rest
• You consume ~240 ml O2 at rest

Question 7

Try to reason through how you could calculate O2 consumption going through this capillary

Answer 7

VO2 = Q x (SaO2 - SvO2) x Hb x 1.39

1. Find the difference between O2 sat in the arteries and veins
2. multiply by the O2 carrying capacity
3. multiply that by the flow

Question 8

What is D02?

Answer 8

DO2 = amount of blood delivered to oxygen in one minute

• Calculated by flow (Q) x arterial oxygen content (Ca02)

Question 9

How is hypoxia defined? What is the difference with hypoxemia?

Answer 9

Hypoxia: Low O2 in tissues ( P02 <1-2 torr in mitochondria)

• More general term

Hypoxemia: Low Sa02 (saturation) with low Pa02

• ​​More specific

Question 10

What are some causes of hypoxemia?

Answer 10

1. Low PiO2
2. Low PaO2
3. Diffusion problems
4. Shunt
5. V/Q mismatch

• (<80 Torr at sea level)
• (<65 Torr in Denver)

Question 11

What are the causes of hypoxia?

Answer 11

Question 12

What are three problems caused by CO binding to Hb?

Answer 12

1. Competition with O2 reduces SaO2
2. CO binds extremely tightly (makes that site unavailable for months)
3. Increases O2 affinity (reducing off-loading)

Question 13

For the following questions, describe what you would expect to see in your blood gases (PaO2, SaO2, PaCO2, A-a gradient) in each situation:

Low PiO2 (high altitude)

Answer 13

PaO2: decreased

SaO2: decreased

PaCO2: decreased (hyperventilating)

A-a gradient: normal (no diffusion problem)

Test: Measure PaCO2

Question 14

For the following questions, describe what you would expect to see in your blood gases (PaO2, SaO2, PaCO2, A-a gradient) in each situation:

Low PaO2 (severe COPD)

Answer 14

PaO2: decreased

SaO2: decreased

PaCO2: increased (hypoventilating)

A-a gradient: normal (no diffusion problem)

Test: measure PaCO2

Question 15

For the following questions, describe what you would expect to see in your blood gases (PaO2, SaO2, PaCO2, A-a gradient) in each situation:

Diffusion (interstitial lung disease)

Answer 15

PaO2: decreased

SaO2: decreased

PaCO2: normal (CO2 is too efficient to be affected)

A-a gradient: increased (diffusion problem)

**CO2 single breath test

Question 16

For the following questions, describe what you would expect to see in your blood gases (PaO2, SaO2, PaCO2, A-a gradient) in each situation:

V/Q mismatch (moderate COPD)

Answer 16

PaO2: decreased

SaO2: decreased

PaCO2: normal

A-a gradient: increased (alveolar doesn’t change but arterial decreases)

Question 17

For the following questions, describe what you would expect to see in your blood gases (PaO2, SaO2, PaCO2, A-a gradient) in each situation:

Shunt (pneumonia)

Answer 17

PaO2: decreased

SaO2: decreased

PaCO2: normal

A-a gradient: increased

**Same as V/Q mismatch- distinguish with 100% O2

Question 18

For the following questions, describe what you would expect to see in your blood gases (PaO2, SaO2, PaCO2, A-a gradient) in each situation:

Low Hb

Answer 18

PaO2: normal

SaO2: normal

PaCO2: normal

A-a gradient: normal

*there will be the same O2 just no enhanced effect from Hb

**Test: measure hemoglobin

Question 19

For the following questions, describe what you would expect to see in your blood gases (PaO2, SaO2, PaCO2, A-a gradient) in each situation:

CO poisoning

Answer 19

PaO2: normal

SaO2: decreased

PaCO2: normal

A-a gradient: normal

Test: measure CO bound to Hb