Chapter 19 & 20 - Acid Base, ABG

Question 1

Describe renal bicarbonate handling.

Answer 1

Bicarb is filtered by glomerulus

80% is resorbed in proximal tubule

• Hydrogen ions combine with HCO3 in the lumen to form H2CO3
• Carbonic anhydrase dissociates H2CO3 into H2O and CO2
• CO2 diffuses accross membrane into cell
• CA reforms H2CO3, which dissociates into HCO3 and H+
• H+ is secreted as in first step, while HCO3 is transported into the peritubular capillary via Na cotransporter

Remainder is collected in distal tubule

• CA forms H+ and HCO3
• H+ is secreted by ATPase and sequestered by NH3 and PO4 in the lumen
• Glutamine dissociates into NH4 and HCO3
• NH4 secreted by Na exchanger
• HCO3 from above mechanisms diffuses via Cl exchanger.
• Generation of HCO3 takes 3-5 days

Question 2

What is the A-a gradient equation?

Answer 2

PAO2 - PaO2

[(PB - 47)x FiO2 - paCO2/0.8] - PaO2

In Calgary PB = 667mmHg

(620xFiO2 - PaCO2x1.25) - PaO2

Normal A-a gradient is <15, increases with age.

Question 3

What is the formula for arterial oxygen content?

Answer 3

(Hb x 1.34 x SaO2) + (PaO2 x 0.003)

Question 4

What causes the oxygen dissociation curve to shift left? Right?

Answer 4

Left (increasing affinity, decreasing delivery to tissues)

Alkalosis, low CO2, hypothermia, CO, MetHb

Right (decrease affinity, increase delivery)

Acidosis

High CO2

Hyperthermia

High 2,3 DPG

Question 5

What is DO2

Answer 5

Delivered oxygen

= CaO2 x CO

Question 6

What accounts for the differences in central venous vs mixed venous oxygenation?

Answer 6

Central venous does not include contribution from coronary sinus (Sa 50%). Therefore it has a ~5% higher Sa than mixed venous.

Question 7

What is the O2 extraction ratio, and what formula estimates it?

Answer 7

O2ER = VO2/DO2

= SaO2 - (SmvO2/SaO2)

Normally 25% of DO2 is used. >35% indicates tissue hypoxia likely.

3mL O2/kg/min consumption

VO2 = (CaO2 - CmvO2) x CO

Ca = arterial oxygen content

Cmv = mixed venous oxygen content

(For CVO2 >30% ER/ 70% SCVO2)

Question 8

What are the normal values of these parameters for a healthy adult?

Tidal Volume

RR

Minute ventilation

Dead space (%)

Answer 8

TV = 7cc/kg

RR = 12

MV = 6L/min

DS = 30% TV

Question 9

What increases EtCO2?

What decreases EtCO2?

Answer 9

Increase -

Increased CO, decreased RR, hyperthermia, HCO3 administration, insufflation for surgery

Decrease -

Reduced CO, increase RR, hypothermia, arrest, PE, AFE, equipment failure/obstruction

Question 10

What are the classes of lactic acidosis and underlying causes?

Answer 10

Type A - reduced DO2

Type B

1. underlying disease (liver, DKA, leukemia, lymphoma)
2. toxin (metformin, CN, nitroprusside, EtOH, metHb, EtOH)
3. IEM - pyruvate dehyrdrogenase deficiency