Key cardiology equations and laws Flashcards
Fick principle and equation
Describe the Fick principle in words where oxygen is the substrate
- Total uptake of oxygen by the body is equal to the product of the cardiac output and the arterial-venous oygen content difference
FICK EQUATION
- CO = VO2/ Ca - Cv
◦ Blood flow to an organ = rate of uptake or excretion of a substance / arterio-venous concentration difference
How is VO2 measured in the direct Fick method
- 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
What is a typical VO2 normal value
250ml/min
What is a typical CaO2 value in the direct Fick equation
0.2mL O2 per mL of blood
What is a typical CvO2 in the Fick equation
0.15mL of O2 per mL of venous blood
What is the Stewart Hamilton equation
How is the Fick principle used indirectly?
- 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
What different methods of indicator dilution are there? What underlying prinicple is there?
Stewart Hamilton equation underlies
Methods
* Thermodilution by PAC or PICCO
* Lithium dilution
* Conductivity dilution using saline
* Indicator dye dilution
What is the equation for cardiac output when an indicator dye is used?
What is the equationf or cardiac output when temperature change is used?
How is stroke volume derived from pulse contour analysis? How is it calibrated? What is the calibration factor?
What is the doppler equation for measuring velocity? How does velocity relate to flow?
◦ 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
What is the equation for SVR
What is a normal BSA for males and females
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.
BSA calculation
When does BSA measurement break down
- 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
What is cardiac index? How is it calculated? What are its normal values?
Cardiac output / BSA
COmparison between cardiac output of patients of a different size
Normal 2.5 - 4 L/min/metre squared
Define stroke volume
the volume of blood pumped out fo the L of the heart during each systolic contraction
How is stroke volume determined using cardiac measurement devices
cardiac output / HR –> i.e. average SV over 1 minute
Normal stroke volume values
60 -100mL/beat
SVI define? Normal values
CI / HR x 1000
Indexed for body size
33 - 47 mL/metre squared / beat
SVR calculation and normal values
80 x (MAP - CVP) / cardiac output
Normal 800 - 1200 dynes-sec/cm ^ -5
SVRI - Define?
SVR indxed to body size
80 x (MAP - CVP) / CI
Normal vlue 1970 - 2390 dynes-sec/cm^-5 x metres squared
Pulmonary vascular resistance calculation
80 x (MPAP - PAWP) / cardiac output
Normal value <250 dynes - sec / cm^ -5
