Critical Care

Question 1

How does an acid differ from a base?

Answer 1

Acids donate protons

Question 2

The equilibrium between what two compounds is most important to the human body’s ability to buffer?

Answer 2

Carbon Dioxide and Water

H2O + CO2 ↔H2CO3 ↔ H+ + HCO3-

Question 3

In the human body, what ratio determines the extracellular pH?

Answer 3

ratio of the bicarbonate concentration to the PaCO2

Question 4

Metabolic acids are largely produced through what kind of metabolism?

Answer 4

anaerobic metabolism

Question 5

What comprises the most abundant and hence most important buffering substrate in the human body.

Answer 5

Proteins, in particular - Hemoglobin

Question 6

How does the respiratory system eliminate acid?

Answer 6

medullary chemoreceptors –> affects the pH of the ECF through regulation of the PaCO2

Question 7

How does the renal system eliminate acid?

Answer 7

actively excrete H+ into the urine and reclaim Na+ from the filtrate via the Na+/H+ transport pump.

Cytoplasmic carbonic anhydrase converts the CO2 and H2O –> H2CO3.
H2CO3 –> H+ and HCO3.
HCO3- –> out of the cell’s basolateral membrane and conserved.

Question 8

What five (5) values are present on an arterial blood gas report?

Answer 8

◾pH (Normal range: 7.35 - 7.45)
◾PaO2 - partial pressure of arterial oxygen. (Normal 75 - 100 mmHg)
◾PaCO2 - partial pressure of arterial CO2. (Normal 35 - 45 mmHg)
◾HCO3- - concentration of bicarbonate ion. (Normal range: 22 - 26 mEq/L)*
◾Base Excess - calculated measure of metabolic component (Normal range: -2 - +2)

Question 9

What is a normal PaO2 at sea level?

Answer 9

~100mmHg

Question 10

What is the alveolar gas equation?

Answer 10

PAO2 = FIO2(PB – PH2O) – (1/RQ)PaCO2

Question 11

What is a normal arterial to alveolar oxygen tension: PaO2/ PAO2?

Answer 11

~0.85

Question 12

Acutely, what is the expected rise in HCO3- for a patient with a pCO2 of 70 mmHg?

Answer 12

3mEq/L

Respiratory acidosis: For every 10mmHg rise in PaCO2 above 40:

• Acute: HCO3- increases by 1mEq/L.
• Chronic (compensated): HCO3- increases by 4mEq/L.

Respiratory alkalosis: For every 10 mmHg fall in PaCO2 below 40:

• Acute: HCO3- decreases by 2 mEq/L.
• Chronic (compensated): HCO3- decreases by 3 mEq/L.

Question 13

How do you calculate base excess?

Answer 13

pH = 7.1, PaCO2 = 35, HCO3- = 10

pH is 0.3 units from normal (from 7.1 to 7.4)

• bicarbonate will decrease by 3 mEq/L (0.3 times 10) –> new value of 7 (10 minus 3)
• The BE = 7 – 24, or -14.

pH = 7.0, HCO3 = 8
pH 7.0 or 0.4 units from 7.4
- bicarbonate will decrease by 4 (10 x 0.4).
- HCO3 of 8 - 4 = 4.
- The BE is thus 4 – 24 = -20

Question 14

Calculate the amount of NaHCO3- required to correct the metabolic acidosis of an adult 100kg man with a base deficit of 10.

Answer 14

First calculate the ECF fraction which

• 0.5 (premature baby)
• 0.4 (term baby)
• 0.3 (2 year old)
• 0.25 (older child)
• 0.2 (adult).

BE x wt (kg) x ECF fraction = bicarbonate deficit

• Bicarb merely converts metabolic acid to respiratory acid that must be removed, as CO2, by increased minute ventilation.
• Hyperosmolar and can cause cerebral hemorrhage, particularly in children.
• correcting the extracellular acidosis with exogenous bicarbonate may worsen intracellular acidosis.