Medical Formulas

Question 1

What is the Body Surface Area formula

Answer 1

Sqrt(Height (cm) x Weight (kg)/3600)

Question 2

What is Poiseuille’s Equation?

Answer 2

Question 3

How do you calculate Qp:Qs?

What about in single ventricle physiology?

Answer 3

(Sat aorta - Sat SVC) / (Sat PV - Sat PA)

(Sat aorta - Sat SVC) / (100 - Sat aorta)

Question 4

What is the Fick principle and formula? What about direct vs indirect Fick?

Answer 4

The Fick principle: We can measure blood flow if we know the amount of O2 taken up over time, the concentration of O2 in the arterial blood and the concentration of O2 in the venous blood.

Fick Equation CO=VO2/ (Ca-Cv)

Direct Fick is considered the gold standard, but rarely used.
Direct Fick is estimated to have 8% error compared to actual CO measures
Direct Fick requires direct measure of oxygen consumption (oxygen inhaled vs exhaled collected via Douglas bag), and sample blood from CvO2 (pulmonary artery), CaO2 (left atrium)

Indirect Fick uses substitutes which increase error:
Common substitutes:
VO2= 125mL O2/min x BSA (110mL O2/min if >70 yo)
Use peripheral ABG for SaO2 (little error compared to LA)
Use ScvO2 (central line) instead of SvO2 (little error compared to PA)

Indirect Fick Equation

CO = 125 x BSA/ [(SaO2-SvO2)xHb (g/dL) x1.34]/10
CO = 3 x wt (kg) / [(SaO2xHbx1.34) - (SvO2xHbx1.34)]x10

Question 5

What is the oxygen concentration equation? in arteries and veins?

Answer 5

Ca=SaO2 x Hb x 1.34
Cv=SvO2 x Hb x 1.34

Question 6

When is the Fick method inaccurate?

Answer 6

1. When systemic blood flow is not equal to pulmonary blood flow (e.g. heart failure or pulmonary hypertension)
2. States where estimated consumption is inaccurate
   a. Hyperdynamic consumption (e.g. sepsis)
   b. Hypodynamic consumption (e.g. hypothermia)
3. Sampling error (arterial or venous) (e.g. intracardiac shunt leading to mixing)
4. Narrow A-V gradient (e.g. high output states such as sepsis)

Question 7

What is the thermodilution method and what is the equation called?

Answer 7

Stewart-Hamilton equation

Question 8

Describe the Stewart-Hamilton equation

Answer 8

Question 9

When is thermodilution inaccurate?

Answer 9

1. Severe TR
2. Intracardiac shunt
3. Very low cardiac output
4. Pulmonary disease

Question 10

What are advantage of thermodilution over Fick?

Answer 10

1. Speed and convenience. Bedside application.
2. Continuous monitoring.
3. Less dependent on steady-state conditions.
4. Less reliance on oxygen consumption measurement
5. Comprehensive hemodynamic data

Question 11

How do you calculate systemic vascular resistance?
How do you calculate pulmonary vascular resistance?
What is the conversion from Woods units to dyne/CGS?

Answer 11

SVR (Woods units) = MAP - RAP / CO
PVR (Woods units) = MPAP - LAP / CO

Woods units x 80 = dyne/sec/cm^-5

Question 12

How do you calculate a transpulmonary gradient and what is a normal TPG?

Answer 12

TPG = PAP - PCWP
Normal TPG <15mmHg

Question 13

What are normal ranges for intracardiac pressures by chamber?

Answer 13

Question 14

What is the formula for relative risk?
What is the formula for relative risk reduction?
What is the formula for absolute risk reduction?
What is the formula for number needed to treat?

Answer 14

Question 15

What is Spinal cord perfusion pressure formula?

Answer 15

SCPP = MAP - CSFP
Normal is >80mmHg

Question 16

What is the Gorlin formula? What is it used to calculate?

Answer 16

Aortic valve area = CO (mL)/(SEP × HR × 44.3 × √mean gradient)
Mitral valve area = CO (mL)/(DFP × HR × 0.85 × 44.3 × √mean gradient)

Where SEP = systolic ejection period and DFP = diastolic filling period

Question 17

What is the Continuity equation?

Answer 17

A1V1=A2V2

Question 18

What is the Bernoulli equation?

Answer 18

Peak gradient=4V^2

Question 19

What are the common cardiac output and stroke volume equations?

Answer 19

CO=SV*HR
SV=LVEDV-LVESV

Question 20

What is the alveolar gas equation?

Answer 20

PAO2=FiO2*(Patm - Ph2o) - PaCO2/RQ RQ=0.8, Patm=760, Ph2o=47

Question 21

What is the LaPlace’s law?

Answer 21

Wall tension = Pressure x radius/ 2*wall thickness

Question 22

What is coronary perfusion pressure?

Answer 22

CPP = DBP - LVEDP

Question 23

How do you calculate Right Ventricular Stroke Work Index (RVSWI)?

Answer 23

RVSWI = SVI x (MPAP – RAP) x 0.0136

Question 24

How do you calculate mean arterial pressure?

Answer 24

MAP = 2/3DBP + 1/3SBP

Question 25

How do you calculate SVR?

Answer 25

SVR = (MAP-CVP)/CO * 80

units are in dynes/sec/cm-5

Question 26

How do you calculate PVR?

Answer 26

PVR = (MPAP-CVP)/CO * 80

units are in dynes/sec/cm-5

Question 27

What is the Hakki formula?

Answer 27

It is a simplified version of the Gorlin formula, used to calculate valve area

Valve area = CO/ sqrt(peak to peak gradient)

Question 28

How do you calculate the effective regurgitant orifice area (EROA)?

Answer 28

You need the PISA (proximal isosurface velocity area)

EROA = (2pir^2 * aliasing velocity) / maximal valve regurgitant velocity

Question 29

How do you calculate regurgitant volume?

Answer 29

Regurgitant volume can be calculated from the EROA and the VTI (velocity time integral). The VTI is calculated by the echo machine

Rvol = EROA * VTI

Question 30

What is an echo way to calculate the stroke volume?

Answer 30

SV=CSA * VTI

CSA is the cross-sectional area of the valve annulus

Question 31

How do you calculate the dimensionless index? What is the cutoff of the DI for severe AS?

Answer 31

Dimensionless index = Velocity in the LVOT / velocity in the AV

Dimensionless index <0.25 = severe AS

Question 32

What is a PAPI? (Pulmonary artery pulsatility index)

Answer 32

Systolic pulmonary artery pressure - diastolic pulmonary artery pressure / right atrial pressure. A PAPI under 1.85 is used to predict RV dysfunction post LVAD