eLFH - Fick principle and Input-Output Principle Flashcards
Fick principle
During any time interval, the quantity of a substance entering a compartment in the inflowing blood must equal the sum of the accumulation in the compartment and the quantity leaving in the efferent blood
Use of the Fick principle
Measure cardiac output or specific organ blood flows
Difference between Fick principle and the input-output principle (IOP)
Fick principle is a more specific case of the IOP and relates specifically to blood flows
Methods for measuring cardiac output using Fick principle
Fluxoids and Oxygen uptake
Dye dilution
Thermodilution (most commonly used now)
Fluxoid definition
Any 3 dimensional structure in a concentration-flow-time axis system
What does volume of a Fluxoid represent
Volume represents the mass of the fluxoid substance being plotted - eg. Oxygen
How to calculate Oxygen uptake using fluxoids
Draw fluxoid representations of Oxygen content of mexed venous blood (pulmonary artery) and Oxygen content of arterial blood
(represents afferent and efferent content)
Difference between two fluxoids represents Oxygen uptake / consumption
How to calculate cardiac output from oxygen uptake
We have measured oxygen uptake from blood samples so the area of the top block is known
Only unknown is the flow axis - which is cardiac output
Flow equation from fluxoid with time of 1 minute
(also represents an expression of the Fick principle)
Assumptions made when determining cardiac output through fluxoids
Constant flow
and
Constant mixed venous and arterial oxygen contents
Over time period used
Situations where assumptions made for CO measurement through fluxoids may not be the case
Sick patients with unstable CO or developing hypoxia
Over short time periods in healthy patients then can assume that assumptions are valid
How to calculate cardiac output from CO2 fluxoids
Similar to calculating from from Oxygen
Why is CO2 less reliable to calculate cardiac output than oxygen
CO2 can be easily altered with minute ventilation therefore less likely to remain constant over short period of time and fluxoids less likely to be rectangular in shape
Three alveolar compartments of pulmonary pathophysiology modelling
Alveoli that are both ventilated and perfused optimally
Alveoli that are unventilated but still perfused (shunt)
Alveoli that are ventilated but not perfused (dead space)
Derivation of the fluxoids required for the shunt equation using two compartment model (Normal alveoli and shunt alveoli)
QN = flow through normal alveoli
QS = flow through shunt alveoli
QT = total flow = cardiac output
Which 5 fluxoids are used to represent the shunt equation
Pulmonary artery - total afferent cardiac output
Afferent to normal alveoli
Afferent and efferent to shunt alveoli
Efferent from normal alveoli
Total efferent cardiac output
Pulmonary artery - total afferent cardiac output fluxoid
Afferent to normal alveoli fluxoid
Afferent and efferent to shunt alveoli fluxoid
Efferent from normal alveoli fluxoid
Total efferent cardiac output fluxoid
5 fluxoids of shunt equation represented together in flow diagram
How to derive shunt equation from fluxoids
Input-Output for the left side of the heart
How is Cc’O2 estimated
From PAO2 (alveolar gas equation) and Hb-O2 dissociation curve
Shunt equation
Qs / QT = (Cc’O2 - CaO2) / (Cc’)2 - CvO2)
Dye which was typically used for the dye dilution measurement of cardiac output
Indocyanine green
Method of cardiac output measurement using dye dilution
Dye injected IV
Serial measurements of arterial dye concentration
Concentration gradually tailed off as per graph
Second peak due to dye recirculation
Relies on constant cardiac output during measurement interval
How to calculate cardiac output from dye dilution
Imagine curve is made up of multiple thin fluxoids representing mass of dye
Sum of fluxoids (minus the recirculating peak) = mass of injected dye
Calculation for cardiac output (Q) from dye dilution
How thermodilution calculates cardiac output
Same process as dye dilution but substitutes dye concentration with temperature (as a measure of energy content)
Why is thermodilution preferred to dye dilution
Temperature measurement is quicker and can therefore be measured more frequently
Therefore gives smoother graph curve
Eliminates issue of recirculation of dye
Can be frequently repeated as clinical situation changes
Method of CO measurement via thermodilution - formation of fluxoid
Volume of cold saline injected
Warmed to body temperature by energy flow from blood
Total energy required to do this calculated
Calculation for total energy required to warm saline to body temperature
Total energy content = Volume x Specific heat x Change in temperature
Calculation for CO from thermodilution
Total energy content represents the volume of the fluxoids
S = specific heat (of saline or blood)
Creatinine clearance for renal blood flow fluxoid
Measurement of renal blood flow in fluxoid form
Calculation for renal blood flow
Renal blood flow = Minute excretion of creatinine / (Creat clearance of renal artery - Creat clearance of renal vein)
Creatinine clearance definition
Volume of blood cleared of creatinine in one minute
Minute excretion of creatinine (M) equation
M = (Urinary concentration of creat x Volume) / Time
Creatinine clearance equation
Using fluxoid:
Creat clearance = M / Plasma creatinine
or
Creat clearance = (Urinary concentration creat x Volume) / Plasma creat concentration
