**Volumetric Capnography...
Highlights
- Volumetric capnography: data points & equipment
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There are three major reasons for limited utilization of volumetric capnography (VCap):
(1) Lack of education/training
(2) Lack of understanding of the data
(3) (Perceived) lack of equipment
Lets try to address 2 & 3 (View Tweet)
- Let's start with a TL;DR of what VCap is:
Conventional capnography is 'time-based': Time on the X-axis and pCO2 on the Y-axis.
VCap has volume on the Y-axis.
VCap requires simultanoues measurement of exhaled pCO2 and expiratory flow.
(View Tweet)
- VCap data points
VCap provides several data points: some more useful than others.
(i) PECO2: Mixed expired CO2 pressure (mmHg):
This is simply the “volume-averaged” CO2 of the pCO2-volume curve: the mean pCO2 of expired gas.
(ii) VTCO2: Volume of CO2 expired in a breath (cc):
(View Tweet)
- Note that (i) and (ii) are exactly the same data point but with different units.
Each can be converted to the other by application of the gas laws.
E.g. If the monitor only provides VTCO2 (cc), it can be converted to PECO2 by doing the analysis detailed in the attached images.
(View Tweet)
- (iii) V̇CO2: Volume of CO2 expired per minute (cc/min):
This is simply VTCO2 x RR.
However, a special feature of V̇CO2 is that at steady state (stable respiratory pattern), V̇CO2 represents CO2 production per minute.
Hence, V̇CO2 reflects BMR and V̇O2 (see image).
(View Tweet)
- Clinically, (i), (ii), & (iii) are the most important data points in VCap as they allows real-time detection of a sudden change in dead space.
Whenever an intervention causes increase in dead space, PECO2/VTCO2/V̇CO2 will instantly⬇️ (dead space causes reduced CO2 elimination). (View Tweet)
- Since CO2 production remains the same, this eventually results in increased PaCO2 and PvCO2 --> ⬆️gradient between capillary & alveolar CO2 --> PECO2 eventually returns to baseline over the next few minutes.
This way, the impact on dead space with the following can be measured:-
(View Tweet)
- a) PEEP titration: ⬆️dead space with overinflation
b) Recruitment maneuvers: ⬇️dead space with recruitment
c) Application of post-inspiratory pause: may ⬇️dead space. See:
https://t.co/LVVrGDOPdK
With any change, a steady-state will be achieved again in a few minutes (image)
(View Tweet)
- Some of the other data points:
(iv) VD/VT: Physiological dead space fraction:
‘Physiological dead space’ is a “gobal” metric of dead space and includes factors causing actual dead space, as well as those causing a dead space effect.
For more details:
https://t.co/KyOU57c8wf (View Tweet)
- Clinically, the most popular way to calculate this is the “Enghoff's method” (PMID: 25395032):
VT/VD = (PaCO2/PECO2) ÷ PaCO2
Typically, PaCO2 (from ABG) is fed into the monitor and it calculates VT/VD based on the PECO2 at the time of collection.
(Alternatively, see image 👇)
(View Tweet)
- VD/VT cannot be obtained in real-time as ABG data (PaCO2) is needed. Hence, it can't be used for the aforementioned applications.
The only additional clinical importance of knowing the precise VD/VT is that it provides prognostic information in ARDS patients (image).
(View Tweet)
- (v) VDaw: Airway dead space
Airway dead space = volume of conducting airways + apparatus dead space.
This can be estimated in real-time by doing a graphical analysis on the VCap curve using Fowler’s method (PMID: 25395032):
Clinical utility:
Higher PEEP can increase VDaw... (View Tweet)
- ...by (slightly) expanding the conducting airways. This effect can be detected in real-time during PEEP titration.
However, this effect is only really relevant when PEEP is set higher than end-expiratory pleural pressure (image)
VDaw can also be used to fractionate VD (image)
(View Tweet)
- (v) VCap Graph & SIII:
Apart from the pCO2-volume graph, some devices objectify the slope of phase-III (SIII).
VCap graphical analysis provides the same information as conventional cap.
However, ⬇️ in SIII has been shown to correlate with optimal PEEP (PMID: 21342153, 19535705)
(View Tweet)
- Whew. With that out of the way. Let’s talk about equipment.
NICO (now NM3) monitor is a dedicated VCap device. It measures flow and pressure using a sensor attached at the wye. Capnographic data is obtained using a maintream sensor: Capnostat 5.
It displays all data except SIII
(View Tweet)
- However, the NM3 is hard to find and may actually have been discontinued.
The good news is, our ventilators can (often) be used instead. The major exception is Puritan Bennett. If you have these, you'd need the NM3.
Remember, for VCap, we need two pieces of hardware: (View Tweet)
- (a) Flow sensor: Vents already have that
(b) Mainstream CO2 sensor: E.g. the Capnostat-5
The Capnostat-5 is commonly available as it is part of the Zoll monitor system on our crash carts. Also, most vents can receive data from it* (exception: Drager has its proprietary sensor).
(View Tweet)
- Of note, the vent may require an additional module &/or a software update - which may already be available without us realizing its Vcap potential
E.g our Servo-Us have the CO2 module that is primarily used for conventional capnography. I recently discovered the hidden Vcap data
(View Tweet)
- Although VCap data points on some vents is more comprehensive than others, they ALL provide the most important ones: V̇CO2, PECO2 &/or VTCO2.
The table attached shows a comparison of data available.
(View Tweet)
- The ability to use the vent for VCap makes it more accessible than relying on dedicated monitors
If you use VCap, do share your setup!
cc
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