Formulas

Question 1

Fractional excretion of sodium (FENa)

Answer 1

[(Serum Cr × Urine Na) / (Serum Na × Urine Cr)] × 100%

Urinary flow rate X Urinary Na) / (GFR X Serum Na

Question 2

Volume of Distribution (Vd)

Answer 2

amount of drug in the body / plasma drug concentration

Question 3

Bioavailability

Answer 3

(AUC oral / AUC iv) X 100

Question 4

Clearance (CL)

Answer 4

rate of elimination of drug / plasma drug concentration

Vd × Ke

(0.7 X Vd) / half-life

Ke = elimination constant
Can be impaired in patients with cardiac, hepatic, or renal dysfunction.

Question 5

Half-life (t 1/2)

Answer 5

0.7 × Vd / CL

In first-order elimination

Question 6

Loading Dose

Answer 6

(Cp X Vd) / F

Cp = target plasma concentration at steady state

Question 7

Maintenance Dose

Answer 7

(Cp X CL X τ) / F

Cp = target plasma concentration at steady state
τ = dosage interval (time between doses), if not
administered continuously

Question 8

Therapeutic Index

Answer 8

Median toxic dose / Median effective dose

Question 9

Cardiac Output (CO)

Answer 9

Rate of O2 consumption / (arterial O2 content − venous O2 content)

SV X HR

MAP / TPR

The oxygen consumption rate can either be measured using a spirometer or by using an assumed value (usually 250 mL O2/min (or 3.5–4.0 mL/kg/min).

Question 10

Mean arterial pressure (MAP)

Answer 10

CO × TPR

2 ⁄3 DBP + 1 ⁄3 SBP = (SBP + 2 x DBP) / 3

DBP + 1/3 PP

A MAP of > 60 mmHg is necessary to ensure the perfusion of vital organs.

Question 11

Stroke volume (SV)

Answer 11

EDV – ESV

CO / HR

Question 12

Ejection fraction (EF)

Answer 12

SV / EDV

(ESV - EDV) / EDV

Question 13

Resistance

Answer 13

driving pressure (ΔP) / flow (Q)

[8η (viscosity) × length] / πr4

Q ∝ r4
R ∝ 1/r4

Question 14

Capillary Fluid Exchange

Answer 14

Kf [ (Pc − Pi) − σ (πc − πi) ]

Question 15

Reticulocyte Production Index

Answer 15

Reticulocyte % × (Actual Hct / Normal Hct)

Normal Hct ≈ 45%

Question 16

Renal Clearance (Cx)

Answer 16

(Ux V) / Px

Question 17

Glomerular Filtration Rate (GFR)

Answer 17

(U inulin × V) / P inulin

Kf [ (P GC – P BS) – (π GC – π BS) ]

Question 18

Effective Renal Plasma Flow (eRPF)

Answer 18

(U PAH × V) / P PAH

Question 19

Renal blood flow (RBF)

Answer 19

RPF / 1 − Hct

Question 20

Filtration fraction (FF)

Answer 20

GFR / RPF

Question 21

Henderson-Hasselbalch Equation (for extracellular pH)

Answer 21

6.1 + log [HCO3−] / (0.03 Pco2)

Question 22

Winters Formula

Answer 22

1.5 [HCO3–] + 8 ± 2

Question 23

Anion Gap

Answer 23

Na+ − (Cl– + HCO3–)

Question 24

Physiologic Dead Space (VD)

Answer 24

VT × [ (PaCO2 − PECO2) / PaCO2 ]

Anatomic dead space (conducting airways) + Alveolar dead space

Question 25

Pulmonary Vascular Resistance (PVR)

Answer 25

(P pulm artery – P L atrium) / CO

Question 26

Alveolar gas equation (PAO2)

Answer 26

PIo2 – (Paco2 / R)

Question 27

Wall Tension

Answer 27

Pressure X Radius

Question 28

Wall Stress

Answer 28

(Pressure X Radius) / (2 X Wall Thickness)

Question 29

Pulse Pressure

Answer 29

SBP - DBP

Question 30

Total Peripheral Resistance (TPR)

Answer 30

(MAP - CVP) / CO

In practice CVP is 0

Question 31

Volumetric Flow Rate (Q)

Answer 31

Flow velocity (v) X Cross-sectional area (A)

Question 32

Free Water Clearance (C H2O)

Answer 32

V ( [ 1 - U osm ] / P osm)

Positive values mean water excretion, while negative values mean net water retention.

Question 33

Vascular Elastance

Answer 33

ΔP / ΔV

• ΔP = change in pressure (mm Hg)
• ΔV = change in volume (mL)

Greater elastance → greater change in blood pressure during blood volume change
Less elastance → less change in blood pressure during blood volume change

Question 34

Collapsing pressure (P)

Answer 34

(2 X surface radius) / radius