S3C20 - Blood gases

Question 1

How much does 1L of O2 by nasal canula increase the FI02 by?

Answer 1

4%

-normal room air FI02 is 21%

Question 2

How much FI02 can be delivered through a simple mask?

Answer 2

35-60^ at 10-15L/min flow

Question 3

How much FIO2 can a non-rebreather supply?

Answer 3

95% at 10-12L/min

Question 4

Estimation of PaO2

Answer 4

• can be estimated by multiplying the actual delivered percentage of O2 by 6
  eg. a pt receiving 60% O2 would have an approx PaO2 of 60x6 = 360mmHg

Question 5

Gas diffusion in the lungs - CO2 vs O2

Answer 5

CO2 is 20 times more soluble than O2 therefore it diffuses much more easily and isn’t as affected by membrane thickness
-therefore conditions that increase the diffusion distance such as pulmonary edema affect O2 much more than CO2

Question 6

Things that affect the surface area of the lungs:

Answer 6

• PNA
• atelectasis
• resection
• emphysema

Question 7

A-a gradient = alveolar-arterial oxygen gradient

Answer 7

A-a is a pressure difference b/w the partial pressures of gas in the alveoli (PAO2) and arterial blood (PaO2)
PA02 - PaO2 = {[PB-47)xFIO2]-(PaCO2/0.8)} - PaO2

PB= barometric pressure = 760 at sea level
47 = partial pressure of water
FIO2= 21%  (0.21)  
(PB-47)xFIO2 = 147

=147 - (PaCO2x1.25) - PaO2
Normal A-a gradient is

Question 8

What is a P/F ratio?

Answer 8

PaO2/FIO2

-P/F ratio

Question 9

Oxyhemoglobin dissociation curve

Answer 9

Factors that shift it right and thus decrease Hb affinity for O2 = increase in acidity, PCO2, temperature, 2,3-DPG

-shifting the curve left increases Hb affinity

Question 10

Lactate levels

Answer 10

Lactate=lactic acid

lactate >4 mmo/dL is associated with a 28% mortality rate in pts with signs/symptoms of infection

Question 11

Mixed venous blood

Answer 11

Can only be sampled with a swan-ganz catheter

Question 12

End-tidal CO2

Answer 12

Capnography shows a wave of CO2 tension curve as it is exhaled from the lungs

• at the end of the curve just before inspiration occurs is where the end-tidal CO2 concentration is taken (PETCO2)
• conditions that increase V/Q mismatch decrease PETCO2
• PETCO2 is usually about 3mmHg less than PaCO2