Fick Principle and Alveolar gas equation Flashcards
What is Fick’s Principle?
Amount in = Amount taken up by organ + amount out
Utilising Fick’s Principle and a bell-spirometer, and invasive lines how can you calculate cardiac output?
What assumptions are needed?
If you give 100% O2 via a bell spirometer over a time period we can measure oxygen uptake over time. If we assume that this uptake is constant we can derive a value for VO2 per minute.
If you assume that all the O2 taken into the lungs is passed onto to the arterial blood.
You can measure un-oxygenated venous blood utilising a pulmonary artery catheter and arterial blood using an arterial line.
VO2 = (CaO2-CvO2) x Q
(Q=perfusion in this case Cardiac output)
You can therefore rearrange this formula to read:
Q= VO2/(CaO2-CvO2)
This method presumes the following:
+ CaO2 and CVO2 is constant (This is false in developing hypoxia)
+ Q is constant (This is false in unstable cardiac output)
Why is CO2 production and removal a much worse method of calculating CO than utilising O2 uptake?
CO2 removal is much less reliable than utilising O2, as CO2 removal will vary greatly with alveolar ventilation.
Whereas O2 uptake is dependent on metabolism and will not vary as greatly with an increase in FiO2 or change in alveolar ventilation.
Thinking about mismatched ventilation and perfusion draw a diagram to represent shunt?
Draw 2 alveoli, one should be ventilated and perfused, the other should be perfused but not ventilated.
The total efferent flow is Qt and oxygen content is CvO2.
The total afferent flow is Qt and oxygen content is CaO2.
The flow through the normal alveoli is QN and oxygen content will be CcO2.
The flow through the shunt alveoli will be Qs and the oxygen content will remain CvO2 (as it is not ventilated.)
Qn= Qt -Qs
Using input/output principle you can state:
Qn.CcO2 + Qs.CvO2 = Qt.CaO2
Derive the shunt equation?
Start by drawing a diagram of a partially ventilated lung in terms of flow and 2 content.
Qn.CcO2 + QsCvO2 = Qt.CaO2
Qn = Qt-Qs
Qt.CcO2 -Qs.CcO2 + QsCvO2 = Qt.CaO2
Qt.CcO2 - Qt.CaO2= Qs.CcO2 - Qs.CvO2
Qt (CcO2 -CaO2) = Qs (CcO2 - CvO2)
(CcO2 -CaO2) / (CcO2 -CvO2) = Qs/Qt
CCO2 must be estimated from the alveolar gas equation and haemoglobin-O2 dissociation curve.
Describe how cardiac output can be calculated using dye dilution?
A known mass of dye is injected intravenously.
Serial measurements of arterial dye concentration Are made in the ensuing minutes. This results in a rise before the attainment of a peak concentration after which concentration tailed off progressively.
A second peak then occurs as the dye recirculates.
If you plotted this all on a graph, and then made it 3D and considered the fluxoid model.
With dye concentration on the y axis, time on the x axis and flow (Q) on the z axis.
If we ignore the second peak which is secondary to the recirculation, we can state that Q x Area under the curve (as this accounts for both the x and y axis) = mass of dye.
We can therefore state Q= mass/area under the curve
Where Q = flow over the set time
What is a fluxoid model?
A fluxoid is any 3-dimensional structure in a concentration-flow-time axis system. The volume of a fluxoid represents a mass.
Describe how thermodilution can be used to calculate cardiac output?
Cold saline is injected via a pulmonary artery catheter.
A thermometer is attached to the end of the pulmonary artery catheter. The greater the flow the less the temperature changes.
You can express this in a formula:
volume of injectate (m) x specific heat capacity (of injectate) x Temp change = AUC x specific heat capacity of blood x Q
If we assume the specific heat capacity of blood is the same as saline then we get,
m x ΔT = AUC x Q
Q= M.ΔT/AUC
How is creatinine clearance caluclated?
Renal blood flow x (Creatinine content renal aa - Creatinine content renal vv = Creatinine clearance
Aka: Qr.(CrRa-CrRv) = CrCl
However this is highly invasive and therefore an impractical method of measuring CrCl.
Mass of urinary excretion of creatinine over a time period =M
M=(Urine volume x urine creatinine concentration)/Time
If we assume all of that creatinine is totally cleared from renal blood without being reabsorbed then we can state that a change in plasma creatinine concentration will be reflected in urine creatinine concentration.
Creatinine clearance = (Urinary concentration x volume)/ plasma creatinine concentration
State the alveolar gas equation?
PAO2 = PiO2 - PaCo2/R
What are the potential variables in the alveolar gas equation?
PiO2
PaCo2
Alveolar ventilation
Describe the shape of the graph of the alveolar gas equation if PiO2 is the variable?
y axis PAO2
x axis PiO2
The graph will follow the shape of y= mx - c
m = 1
c= PaCo2/R
Therefore it will be a linear curve with a gradient of 1 and will dissect the y axis at the value of - c
Describe the shape of the graph of the alveolar gas equation if PaCO2 is the variable?
y axis PAO2
x axis PaCo2
The graph will follow the shape of y= mx + c
m = -1/R
c= PiO2
Therefore it will be a linear curve with a gradient of -1/R and will dissect the y axis at the value of PiO2
Demonstrates that hypoventilation will result in rise in PaCO2 and a decrease in PAO2
Describe the shape of the graph of the alveolar gas equation if Alveolar ventilation is the variable?
y axis PAO2
x axis VA
PaCO2 = (VCO2 x Pi)/VA
If VA is the x variable
The shape of the graph will therefore follow the shape of:
y= c- k/x
k= VCo2 x Pi
c= PiO2
Therefore it will be a reciprocal curve. As alveolar ventilation drops there will be a steep drop in PAO2. But a small increase in Alveolar ventilation will not greatly increase your PAO2.
Demonstrates that hypoventilation is an important cause of hypoxia and it is why post op patients receive high FiO2 to offset there hypoventilation.
