eLFH - Fick principle and Input-Output Principle Flashcards

1
Q

Fick principle

A

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

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2
Q

Use of the Fick principle

A

Measure cardiac output or specific organ blood flows

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3
Q

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

A

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

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4
Q

Methods for measuring cardiac output using Fick principle

A

Fluxoids and Oxygen uptake

Dye dilution

Thermodilution (most commonly used now)

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5
Q

Fluxoid definition

A

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

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6
Q

What does volume of a Fluxoid represent

A

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

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7
Q

How to calculate Oxygen uptake using fluxoids

A

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

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8
Q

How to calculate cardiac output from oxygen uptake

A

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

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9
Q

Flow equation from fluxoid with time of 1 minute

(also represents an expression of the Fick principle)

A
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10
Q

Assumptions made when determining cardiac output through fluxoids

A

Constant flow
and
Constant mixed venous and arterial oxygen contents

Over time period used

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11
Q

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

A

Sick patients with unstable CO or developing hypoxia

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

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12
Q

How to calculate cardiac output from CO2 fluxoids

A

Similar to calculating from from Oxygen

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13
Q

Why is CO2 less reliable to calculate cardiac output than oxygen

A

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

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14
Q

Three alveolar compartments of pulmonary pathophysiology modelling

A

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)

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15
Q

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

A

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

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16
Q

Which 5 fluxoids are used to represent the shunt equation

A

Pulmonary artery - total afferent cardiac output

Afferent to normal alveoli

Afferent and efferent to shunt alveoli

Efferent from normal alveoli

Total efferent cardiac output

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17
Q

Pulmonary artery - total afferent cardiac output fluxoid

18
Q

Afferent to normal alveoli fluxoid

19
Q

Afferent and efferent to shunt alveoli fluxoid

20
Q

Efferent from normal alveoli fluxoid

21
Q

Total efferent cardiac output fluxoid

22
Q

5 fluxoids of shunt equation represented together in flow diagram

23
Q

How to derive shunt equation from fluxoids

A

Input-Output for the left side of the heart

24
Q

How is Cc’O2 estimated

A

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

25
Shunt equation
Qs / QT = (Cc'O2 - CaO2) / (Cc')2 - CvO2)
26
Dye which was typically used for the dye dilution measurement of cardiac output
Indocyanine green
27
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
28
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
29
Calculation for cardiac output (Q) from dye dilution
30
How thermodilution calculates cardiac output
Same process as dye dilution but substitutes dye concentration with temperature (as a measure of energy content)
31
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
32
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
33
Calculation for total energy required to warm saline to body temperature
Total energy content = Volume x Specific heat x Change in temperature
34
Calculation for CO from thermodilution
Total energy content represents the volume of the fluxoids S = specific heat (of saline or blood)
35
Creatinine clearance for renal blood flow fluxoid
36
Measurement of renal blood flow in fluxoid form
37
Calculation for renal blood flow
Renal blood flow = Minute excretion of creatinine / (Creat clearance of renal artery - Creat clearance of renal vein)
38
Creatinine clearance definition
Volume of blood cleared of creatinine in one minute
39
Minute excretion of creatinine (M) equation
M = (Urinary concentration of creat x Volume) / Time
40
Creatinine clearance equation
Using fluxoid: Creat clearance = M / Plasma creatinine or Creat clearance = (Urinary concentration creat x Volume) / Plasma creat concentration