Equations To Know

Question 1

Obligatory Urine Volume

Answer 1

Excretion amount (mOsm/day)/ maximal urine concentrating ability (mOsm/L)

If less than calculated amount then the pt has oliguria

Question 2

Free water clearance

Answer 2

C(H20)= V-C(osm) = (Uosm x V)/ P(osm)

Uosm = urine osmolarity
V = urine flow rate
P = plasma osmolarity 

CH20 needs to be in minutes

Question 3

What is the henderson-hasselbach equation in relation to carbonic acid/bicarbonate?

Answer 3

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

Question 4

If you are given pH and PCO2, how do you find HCO3?

Answer 4

HCO3- = 10 ^ (pH-6.1) * 0.03 * PCO2

Question 5

If you are given pH and HCO3-, how do you find PCO2?

Answer 5

PCO2 = HCO3/ [0.03 *(10 ^ (pH-6.1))]

Question 6

How to calculate metabolic acidosis?

Answer 6

PaCO2 = 40- (1.2 x (24-HCO3))

Question 7

How to calculate metabolic alkalosis?

Answer 7

PaCO2 = 40 + (0.7 x ((HCO3 - 24))

If chloride-responsive, urine Cl- will be <20 mEq/L

If chloride-resistant, hypokalemia and urine Cl- is >20 mEq/L

Question 8

How to calculate respiratory acidosis?

Answer 8

Acute: HCO3 = 24 + (0.1 x (PaCO2-40))

Chronic: HCO3 = 24 + (0.4 x (PaCO2 - 40))

Question 9

How to calculate respiratory alkalosis?

Answer 9

Acute: HCO3 = 24 - (0.2 x (40 - PaCO2))

Chronic: HCO3 = 24 - (0.5 x (40-PaCO2))

Question 10

How do you calculate a serum anion gap?

Answer 10

Na - (Cl + HCO3) = X

Range is 8-16

Question 11

How do you calculate a urine anion gap?

Answer 11

Na + K - Cl = 8 (normal value)

• Usually becomes negative (e.g. -20) in metabolic acidosis

Question 12

How to calculate osmolal gap?

Answer 12

Serum Osm (measured) - [(2*Na) + (glucose/18) + (BUN/2.8)] = 0 (normal value)

Question 13

How to determine high versus normal anion gap?

Answer 13

Delta Gap/ Delta [HCO3]

Delta gap = anion gap - 12
Delta [HCO3] = 24- [HCO3]

If < 0.4 = normal anion gap
0.4-0.8 = mixed
0.8- 2.0 = high anion gap
>2.0 = mix of high anion gap and metabolic alkalosis

Question 14

Filtered load equation

Answer 14

Filtered load = GFR * plasma concentration * % filterability

Question 15

How do you calculate net acid excretion?

Answer 15

NAE = (urinary NH4+ * urine flow rate) + (urinary titratable acids * urine flow rate) - (urinary bicarbonate * urine flow rate)