ABG

Question 1

Factors that increase O2 off-loading

Answer 1

Rightward shift:

Decreased pH < 7.4 (Bohr Effect)
Increased PCO2 > 40 Torr
Increased temperature
Increased 2,3-DPG

Question 2

Factors that decrease O2 off-loading

Answer 2

Increased pH
Decreased PCO2
Decreased Temperature
Decreased 2,3-DPG

Question 3

Calculate O2 delivery to tissues as a function of CO and arterial O2 content

Answer 3

DO2 = Q x (SaO2 x 1.39[Hb]) + (0.003 x PaO2)

Question 4

Calculate O2 consumption from CO and the difference in O2 saturation between arterial and venous blood

Answer 4

VO2 = Q x (SaO2 - SVO2) x 1.39[Hb]

Question 5

Hypoxemia - Cut off values

Answer 5

PaO2 < 80 Torr at sea level

PaO2 < 65 torr in Denver

Question 6

5 causes of hypoxemia

Answer 6

Low ambient PO2 in inspired air (altitude)
Reduced ventilation 
Diffusion limitations
V/Q mismatch
Shunt

Question 7

3 ways CO2 is carried in the blood

Answer 7

Freely dissolved CO2 (~1.2mM at typical PCO2 of 40Torr)

HCO3- (~24mM)

Carbamino compounds - CO2 bound to Hb (1.2mM)

Question 8

Haldane effect

Answer 8

Deoxygenated Hb binds CO2 better than oxygenated Hb; therefore, there is enhanced carriage of CO2 by Hb in venous blood

Question 9

Henderseon-Hasselbach Equation for Bicarbonate

Answer 9

pH = 6.1 + log ([HCO3-] / 0.03 x PCO2)

Question 10

Normal arterial blood gas values for pH, PaCO2, and [HCO3-]

Answer 10

pH = 7.4 (7.38 - 7.43)

PaCO2 = 40 Torr

[HCO3-] = 24mM

Question 11

Respiratory acidosis

Answer 11

Increased PaCO2 leading to decreased pH

Caused by hypoventilation

Compensation via conservation of bicarbonate in the kidney; bicarbonate buffers by “absorbing” extra protons

Question 12

Respiratory alkalosis

Answer 12

Decreased PaCO2 leading to increased pH

Caused by excessive ventilation in relation to CO2 production

Compensation via increased excretion of bicarbonate by the kidney

Question 13

Anion gap metabolic acidosis

Answer 13

Low pH due to the presence of additional, unmeasured acids in the blood (AG > 14)

Caused by: MUDPILES

Methanol
Uremia
Diabetic Ketoacidosis (and other causes of ketoacidosis: starvation, alcoholism)
Propylene glycol 
Isoniazid
Lactate
Ethylene Glycol
Salicylates

Question 14

Non-gap metabolic acidosis

Answer 14

Low pH without the presence of additional, unmeasured acids in the blood; AG within normal range of 12 +/- 2

Caused by loss of HCO3- due to GI loss or renal loss, or by giving large quantities of saline

Question 15

Compensation of metabolic acidosis

Answer 15

Increased ventilation to “blow off” CO2

Expected pCO2 = 1.5[HCO3-] + 8 +/- 2 (Winter’s Formula)

if the pCO2 measured on ABG is within this range, then the compensation is considered complete; if pCO2 is higher than expected then compensation is incomplete

Question 16

Metabolic alkalosis

Answer 16

Increased pH caused by an increase in base such as HCO3- or a decrease in acid other than CO2

Acid loss during vomiting
Ingestion of antacids or baking soda
Hypovolemia causing re-absorption of bicarbonate in the kidney (contraction alkalosis)

Compensation involves decreased ventilation to increase PaCO2; however, the brain will not allow hypoventilation to the point of hypoxemia and so compensation is usually incomplete

Question 17

Compensation rule - acute respiratory disturbances

Answer 17

An acute change in PaCO2 of 10 Torr yields a pH change of 0.08 units

This is the uncompensated situation

Question 18

Compensation rule - chronic respiratory disturbances

Answer 18

A change in PaCO2 of 1 Torr yields a compensatory change in [HCO3-] of 0.4 in the same direction

This is the compensated situation

Question 19

Metabolic disturbances - compensation rule

Answer 19

A decrease in [HCO3-] of 1mEq results in a decrease in PaCO2 of 1.3 Torr

An increase in [HCO3-] of 1mEq results in an increase in PaCO2 of 0.7 Torr

Question 20

Calculate the anion gap

Answer 20

AG = [Na+] - ([Cl-] + [HCO3-]) = 12 +/- 2