Key cardiology equations and laws

Question 1

Fick principle and equation

Answer 1

Question 2

Describe the Fick principle in words where oxygen is the substrate

Answer 2

• Total uptake of oxygen by the body is equal to the product of the cardiac output and the arterial-venous oygen content difference

Question 3

FICK EQUATION

Answer 3

• CO = VO2/ Ca - Cv
  ◦ Blood flow to an organ = rate of uptake or excretion of a substance / arterio-venous concentration difference

Question 4

How is VO2 measured in the direct Fick method

Answer 4

• VO2 measurement
  ◦ patients breaths through a spirometer containing a known volume of 100% oxygen and a CO2 absorbed, after a minute the volume of O2 remaining in the spirometer allows the calculation of O2 uptake

Question 5

What is a typical VO2 normal value

Answer 5

250ml/min

Question 6

What is a typical CaO2 value in the direct Fick equation

Answer 6

0.2mL O2 per mL of blood

Question 7

What is a typical CvO2 in the Fick equation

Answer 7

0.15mL of O2 per mL of venous blood

Question 8

What is the Stewart Hamilton equation

Answer 8

Question 9

How is the Fick principle used indirectly?

Answer 9

• Measured of cardiac outptu using the Fick equation but substituting estimated values for the some of the measured variables
• Estimations
  ◦ Uses age/weight and sex based nomogram to estimate VO2 - especially inaccurate if morbidly obese, paralysed, thyrotoxicosis, burns, sepsis, hypothermia where metabolically not normal patients. Additionally pulmonary O2 consumption can be dramatically increased in pnumonia overestimating cardiac output
  ◦ Mixed venous blood assumed on the basis of normal vlues or estimated from CVC samplws; or from end tidal
  ◦ Arterial oxygen content can be estimated from pulse oximetry

Question 10

What different methods of indicator dilution are there? What underlying prinicple is there?

Answer 10

Stewart Hamilton equation underlies

Methods
* Thermodilution by PAC or PICCO
* Lithium dilution
* Conductivity dilution using saline
* Indicator dye dilution

Question 11

What is the equation for cardiac output when an indicator dye is used?

Answer 11

Question 12

What is the equationf or cardiac output when temperature change is used?

Answer 12

Question 13

How is stroke volume derived from pulse contour analysis? How is it calibrated? What is the calibration factor?

Answer 13

Question 14

What is the doppler equation for measuring velocity? How does velocity relate to flow?

Answer 14

◦ V = F (d)c / 2 F(O) cos (theta)
‣ V = velocity of blood in descending aorta
‣ F(d)c = change in frequency of the reflected ultrasound x speed of ultrasound in tissue
‣ F(O) = transmitted ultrasound freqeuncy
◦ Blood flow is then determined by velocity x cross sectional area of the descending aorta (thoracic) estimated from patients height and weight

Question 15

What is the equation for SVR

Answer 15

Question 16

What is a normal BSA for males and females

Answer 16

These are indexed to BSA- The normal average adult BSA is 1.9 m2 for males and 1.6 m2 for females. Thus, a normal adult male with a cardiac output of 5.0 L/min would score a cardiac index (CI) of 2.6 L/min/m2.

Question 17

BSA calculation

Answer 17

Question 18

When does BSA measurement break down

Answer 18

• Morbidly obese - BSA higher than actual required output, so required index may be below expected
• Children - BSA and metabolic rate do not correlate as well, so even with normal index can be insufficent

Question 19

What is cardiac index? How is it calculated? What are its normal values?

Answer 19

Cardiac output / BSA

COmparison between cardiac output of patients of a different size

Normal 2.5 - 4 L/min/metre squared

Question 20

Define stroke volume

Answer 20

the volume of blood pumped out fo the L of the heart during each systolic contraction

Question 21

How is stroke volume determined using cardiac measurement devices

Answer 21

cardiac output / HR –> i.e. average SV over 1 minute

Question 22

Normal stroke volume values

Answer 22

60 -100mL/beat

Question 23

SVI define? Normal values

Answer 23

CI / HR x 1000

Indexed for body size

33 - 47 mL/metre squared / beat

Question 24

SVR calculation and normal values

Answer 24

80 x (MAP - CVP) / cardiac output

Normal 800 - 1200 dynes-sec/cm ^ -5

Question 25

SVRI - Define?

Answer 25

SVR indxed to body size

80 x (MAP - CVP) / CI

Normal vlue 1970 - 2390 dynes-sec/cm^-5 x metres squared

Question 26

Pulmonary vascular resistance calculation

Answer 26

80 x (MPAP - PAWP) / cardiac output

Normal value <250 dynes - sec / cm^ -5