Oxygenation

Question 1

What is the VCO₂/VO₂ ratio for the following:

carbohydrates

proteins

fats

Answer 1

carbohydrates = 1.0

proteins = 0.8

fats = 0.7

Question 2

What is the normal PvO₂ and saturation?

Answer 2

PVO₂ is around 40-45 mmHg

saturation is around 100 mmHg

Question 3

What is a normal A-a difference?

Answer 3

A-a difference = 2.5 + 0.25(age)

Question 4

PF ratio

Answer 4

PaO₂/FiO₂

typically used to measure the severity of hypoxemia

regardless of FiO2, a P/F ratio of greater than 450 is generally viewed as normal

Question 5

What does a swidened A-a difference suggest?

Answer 5

hypoxemia

Question 6

What are the three main pathophysiologic states that cause hypoxemia?

Answer 6

shunt

low V/Q

diffusion limitation

Question 7

shunts

Answer 7

intrapulmonary shunt (pus, water, blood, atelectasis)

right to left intracardiac shunt

ratio of ventilation to perfusion is zero in lung units that are not ventilated

Question 8

Low V/Q

Answer 8

ventilation is decrease but not absent to perfused lung units (asthma attack or COPD)

Q is increased for a given V (as in pulmonary embolism)

increasing the fraction of inspired oxygen readily increases the PaO₂

Question 9

diffusion limitation

Answer 9

increased interstitial thickening such as in pulmonary fibrosis

decreased area available for diffusion as in emphysema

Question 10

causes for lowered M_VO₂

Answer 10

low cardiac output

high oxygen consumption

anemia

low SaO₂

Question 11

oxygen delivery (DO₂) equation

Answer 11

DO₂ = CO x arterial oxygen content

DO₂ = CO x [(1.34 x Hg x SaO₂) + 0.003 x PaO₂]

Question 12

ScvO₂

Answer 12

central venous oxygen saturation

measurement done through a central venous catheter via the internal jugular or subclavian vein

Question 13

causes of hypoxemia that do no involve an elevated A-a difference

Answer 13

hypoventilation

low barometric pressure

low FiO₂

low RQ ratio

Question 14

hypoventilation

Answer 14

overdose of heroin suppresses respiration

decreases minute ventilation and acutely elevates PaCO₂ to 60 mmHg

PAO₂ drops to 75 mmHg

PaO₂ in response drops to 68 mmHg

A-a difference is preserved, but hypoxemia results

Question 15

treatment of hypoventilation from heroin overdose

Answer 15

increase minute ventilation with the drug naloxone

stop heroin use

Question 16

low barometric pressure

Answer 16

decreased P_B will explain lowered PAO₂ and PaO₂ values

Question 17

low FiO₂ (below 0.21)

Answer 17

rare but can occur in inhalation injuries/fires where other gases “dilute” the fraction of inspired oxygen