Volumetric Capnography Flashcards
Capnometry
The measurement of CO2 in respiratory gases, will just show you a number
Measured through Infrared Spectroscopy
PET Vs. PaCO2
PetCO2 usually 3 to 5 mmHg less than PaCO2 (healthy)
This is because the concentration in alveoli and in the blood should be the same in a healthy individual, however we are measuring arterial blood in the radial artery and by this time there has been mixed venous blood and the venous blood from the heart (which will not go through the lungs) will join into the arterial circulation, as well blood from the bronchocapillary tree will also be mixed into this blood and this venous admixture will decrease the PaCO2
FeCO2
Fraction of expired oxygen
Approximately 0.05 – 0.06 (5 – 6%)
There is not a lot of CO2 in the gas that we breath out
Capnography is Influenced By
Ventilatory Variables (Vt, RR, PEEP, I:E, PIP)-The way air moves in and out of the lungs
Hemodynamic Variables (SBP, PBP, CO, Shunt, V/Q mismatch)-The way the blood is moving the air
Capnograph
Will give you a measure of the highest concentration of CO2 rather than the total amount of CO2 curve
From the diagram below is a normal single breath capnogram
Initially, the expired PCO2 is 0 mm Hg, indicating exhalation of pure dead space gas (A, phase I).
Soon after, alveolar gas begins mixing with dead space gas, causing a rapid increase in expired PCO2 (A to B, phase II).
Later in expiration, the CO2 concentration begins leveling off.
This plateau indicates exhalation of gas coming mainly from ventilated alveoli (B to C, phase III).
Gas sampled at the end of exhalation is called end-tidal gas, with its partial pressure of CO2 abbreviated as PetCO2.
In healthy individuals, PetCO2 averages 3 to 5 mm Hg less than PaCO2, or 35 to 43 mm Hg (approximately 5% to 6% CO2).
The sharp downstroke and return to baseline that normally occurs after the end-tidal point indicates inhalation of fresh gas with zero CO2.
Vd/Vt
Enghoff Approach
Vd/Vt= (PaCO2-PeCO2)/PaCO2
Enghoff Approach (Adaptation of Bohr Approach)
This is a common equation but at best it is an approximation
Non specific for deadspace alone
Afffectedby other causes of V/Q mismatch, including shunting
Overestimates alveolar deadspace
Bohr Equation
Vd/Vt = (PACO2-PeCO2)/PACO2
Considered to be “true deadspace”
The biggest difference is PAO2 use instead of PaO2 so we art looking at the alveoli instead of arterial
Not widely used because PACO2 has been difficult to measure
…Until now!
Volumetric Capnography Graph
Graph of exhaled volume (x axis) versus % CO2(y axis).
A horizontal line drawn at the top of the curve represents the % CO2in arterial blood.
Area X represents actual CO2 exhaled in one breath
Area Y is the amount of CO2not eliminated because of alveolar dead space
Area Z is the amount of CO2 not eliminated because of anatomic dead space.
You will never get 100% CO2 number because of venous admixture
You can see in the graph that it takes a while for CO2 to go up because it is not the first gas that is exhaled and rather the air in the conducting airway will come out before the air in the alveoli
The midpoint of the ling dividing y and x will give us are PACO2
Volumetric Capnography
Combines Capnometry with pneumotachometry (flow sensors) we will know the size of the breath and the amount of CO2 that came out
Volumetric capnography – CO2 measurement combined with volume of each breath.
◦Now …instead of PetCO2, VtCO2 can be obtained
VtCO2 (mL/Breath) X RR (breaths /minute) = VCO2 (mL/min)
PETCO2
CO2 measurement without quantifying the volume of the exhaled breath.
PECO2 Calculation
PECO2 = VtCO2(Pbaro-47) or PECO2 = (VCO2 ÷MVe) (Pbaro-47)
VCO2 is your CO2 production per minute
VtCO2 is your fractional CO@ (percent of CO2 exhaled) and will often be expressed as a percent
Volumetric Capnography and V/Q Mismatchig
Can help in order to try and determine optimal PEEP in order to recruit collapsed lung units
When these collapse lung units participate in gas exchange there will be an increase in VtCO2 and a decrease in PaCO2
Deadspace Ratio and Volumetric Capnography
When you decrease Vt you want to know your deadspace as you will be changing the deadspace ratio
The deadspace ratio (Vd/Vt) will increase when you decrease Vt as you are loosing more of your tidal volume to deadspace
When you decrease your Vt at the same time you increase your RR MV may not stay the same as Vt tends to have a larger effect on MV than RR due to deadspace
So if we have 500 ml set on the vent and the alveolar Vt is 250
With lung volume recruitment there is an increase in VtCO2 and a decrease in PaCO2 because now we are better at ventilation
Capnography and Weaning from the Mechanical Ventilator
During weaning, you may see a patient increase their RR during the SBT and the common interpretation is that the patient is failing their SBT and you should discontinue the trial
However if you see that VCO2 is also increasing then the increase in RR is appropraite and you should continue the SBT
If VCO2 does not increase then the patient has failed the SBT and you should discontinue the trial
