eLFH - Fick principle and Input-Output Principle

Question 1

Fick principle

Answer 1

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

Question 2

Use of the Fick principle

Answer 2

Measure cardiac output or specific organ blood flows

Question 3

Difference between Fick principle and the input-output principle (IOP)

Answer 3

Fick principle is a more specific case of the IOP and relates specifically to blood flows

Question 4

Methods for measuring cardiac output using Fick principle

Answer 4

Fluxoids and Oxygen uptake

Dye dilution

Thermodilution (most commonly used now)

Question 5

Fluxoid definition

Answer 5

Any 3 dimensional structure in a concentration-flow-time axis system

Question 6

What does volume of a Fluxoid represent

Answer 6

Volume represents the mass of the fluxoid substance being plotted - eg. Oxygen

Question 7

How to calculate Oxygen uptake using fluxoids

Answer 7

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

Question 8

How to calculate cardiac output from oxygen uptake

Answer 8

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

Question 9

Flow equation from fluxoid with time of 1 minute

(also represents an expression of the Fick principle)

Answer 9

Question 10

Assumptions made when determining cardiac output through fluxoids

Answer 10

Constant flow
and
Constant mixed venous and arterial oxygen contents

Over time period used

Question 11

Situations where assumptions made for CO measurement through fluxoids may not be the case

Answer 11

Sick patients with unstable CO or developing hypoxia

Over short time periods in healthy patients then can assume that assumptions are valid

Question 12

How to calculate cardiac output from CO2 fluxoids

Answer 12

Similar to calculating from from Oxygen

Question 13

Why is CO2 less reliable to calculate cardiac output than oxygen

Answer 13

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

Question 14

Three alveolar compartments of pulmonary pathophysiology modelling

Answer 14

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)

Question 15

Derivation of the fluxoids required for the shunt equation using two compartment model (Normal alveoli and shunt alveoli)

Answer 15

QN = flow through normal alveoli
QS = flow through shunt alveoli
QT = total flow = cardiac output

Question 16

Which 5 fluxoids are used to represent the shunt equation

Answer 16

Pulmonary artery - total afferent cardiac output

Afferent to normal alveoli

Afferent and efferent to shunt alveoli

Efferent from normal alveoli

Total efferent cardiac output

Question 17

Pulmonary artery - total afferent cardiac output fluxoid

Answer 17

Question 18

Afferent to normal alveoli fluxoid

Answer 18

Question 19

Afferent and efferent to shunt alveoli fluxoid

Answer 19

Question 20

Efferent from normal alveoli fluxoid

Answer 20

Question 21

Total efferent cardiac output fluxoid

Answer 21

Question 22

5 fluxoids of shunt equation represented together in flow diagram

Answer 22

Question 23

How to derive shunt equation from fluxoids

Answer 23

Input-Output for the left side of the heart

Question 24

How is Cc’O2 estimated

Answer 24

From PAO2 (alveolar gas equation) and Hb-O2 dissociation curve

Question 25

Shunt equation

Answer 25

Qs / QT = (Cc’O2 - CaO2) / (Cc’)2 - CvO2)

Question 26

Dye which was typically used for the dye dilution measurement of cardiac output

Answer 26

Indocyanine green

Question 27

Method of cardiac output measurement using dye dilution

Answer 27

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

Question 28

How to calculate cardiac output from dye dilution

Answer 28

Imagine curve is made up of multiple thin fluxoids representing mass of dye
Sum of fluxoids (minus the recirculating peak) = mass of injected dye

Question 29

Calculation for cardiac output (Q) from dye dilution

Answer 29

Question 30

How thermodilution calculates cardiac output

Answer 30

Same process as dye dilution but substitutes dye concentration with temperature (as a measure of energy content)

Question 31

Why is thermodilution preferred to dye dilution

Answer 31

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

Question 32

Method of CO measurement via thermodilution - formation of fluxoid

Answer 32

Volume of cold saline injected
Warmed to body temperature by energy flow from blood
Total energy required to do this calculated

Question 33

Calculation for total energy required to warm saline to body temperature

Answer 33

Total energy content = Volume x Specific heat x Change in temperature

Question 34

Calculation for CO from thermodilution

Answer 34

Total energy content represents the volume of the fluxoids
S = specific heat (of saline or blood)

Question 35

Creatinine clearance for renal blood flow fluxoid

Answer 35

Question 36

Measurement of renal blood flow in fluxoid form

Answer 36

Question 37

Calculation for renal blood flow

Answer 37

Renal blood flow = Minute excretion of creatinine / (Creat clearance of renal artery - Creat clearance of renal vein)

Question 38

Creatinine clearance definition

Answer 38

Volume of blood cleared of creatinine in one minute

Question 39

Minute excretion of creatinine (M) equation

Answer 39

M = (Urinary concentration of creat x Volume) / Time

Question 40

Creatinine clearance equation

Answer 40

Using fluxoid:

Creat clearance = M / Plasma creatinine

or

Creat clearance = (Urinary concentration creat x Volume) / Plasma creat concentration