Volumetric Capnography: CO2 Against Volume, Not Time

Volumetric capnography is a more advanced form of carbon dioxide monitoring, and it answers questions an ordinary capnogram cannot. Instead of plotting CO2 against time, it plots CO2 against the volume of gas breathed out. That single change unlocks measurements of dead space and of the actual amount of CO2 the patient eliminates with each breath.

This page explains what volumetric capnography is, how it differs from the time capnography most clinicians know, what it measures, and where it is used. It is a reference, so keep it alongside the glossary.

Key takeaways

• Time capnography plots CO2 against time. Volumetric capnography plots it against exhaled volume.
• Volumetric capnography needs a flow sensor as well as a CO2 sensor.
• It measures dead space and the volume of CO2 eliminated per breath, which time capnography cannot.
• It is used mainly in intensive care and anaesthesia ventilation, on the ventilator.
• Most portable capnographs, including RespiCOz, perform time capnography.

Time capnography, the familiar kind

Almost every capnogram a clinician sees is a time capnogram.

It plots CO2 concentration on the vertical axis against time on the horizontal axis, producing the familiar near-rectangular waveform with its four phases. It tells you the end-tidal value, the respiratory rate and the shape of each breath, and it is the basis of everything in our guide to capnography waveforms and capnogram interpretation.

Time capnography is portable, simple and sufficient for the great majority of clinical needs: confirming a tube, monitoring ventilation, detecting apnoea and reading trends.

Volumetric capnography, the difference

Volumetric capnography changes the horizontal axis. Instead of time, it plots CO2 against the volume of gas exhaled during the breath.

To do this, the device needs two measurements at once: the CO2 concentration, from an infrared sensor, and the gas flow, from a flow sensor, so it can track how much volume has been breathed out at each moment. Multiplying and integrating the two gives a curve of CO2 against expired volume, sometimes called the single-breath CO2 curve.

That curve has three phases of its own: phase I is the CO2-free gas from the airways (anatomical dead space), phase II is the rising transition, and phase III is the alveolar plateau. Because the x-axis is now volume, the areas under and around the curve have real physical meaning that a time trace cannot give.

What volumetric capnography measures

This is why the extra complexity is worth it in the right setting.

Dead space. Because the curve is plotted against volume, it can separate and quantify the dead space of a breath, the airway (anatomical) portion and the alveolar portion, using established methods such as the Bohr and Enghoff approaches. Time capnography can only hint at dead space; volumetric capnography measures it. For why dead space matters, see EtCO2 vs PaCO2.

VCO2, the volume of CO2 eliminated. It measures the actual amount of carbon dioxide the patient breathes out per breath and per minute. This reflects metabolism and pulmonary blood flow, and it is used to track changes in a patient’s condition and in cardiac output.

Ventilation efficiency. It helps assess how effectively the lungs are clearing CO2, which is valuable when setting a ventilator in difficult cases.

Where it is used

Volumetric capnography is a specialist tool, not a bedside everywhere one.

It lives mainly in intensive care and anaesthesia, built into ventilators and anaesthesia workstations, where the flow sensor it needs is already part of the circuit. Clinicians use it to optimise lung-protective ventilation, to detect the effect of changes in PEEP, to monitor pulmonary embolism, and to follow dead space as a marker of a patient’s progress in conditions such as ARDS.

It is powerful, but it requires the flow-sensing hardware and the ventilator context. It is not something a small, portable, standalone device is designed to do.

Volumetric versus time capnography

Where RespiCOz fits

It is worth being clear and honest here, because the two kinds of capnography are easy to conflate.

RespiCOz performs time capnography, the kind used for the overwhelming majority of clinical needs: confirming an endotracheal tube, monitoring ventilation continuously, detecting apnoea, tracking EtCO2 and FiCO2, and reading the waveform. It is a portable mainstream capnograph, and it is designed for mobility and for the settings where a ventilator-based volumetric system is not present, theatre, recovery, the ward, emergency care and transport.

Volumetric capnography is a different tool for a different job, a ventilator-integrated measurement for specialist ICU use. A portable device like RespiCOz is not intended to replace it, and does not claim to. For what RespiCOz does, see the best handheld EtCO2 monitor guide.

Want to know more about portable capnography? Request a quote or a demonstration here.

Frequently asked questions

What is volumetric capnography? A form of capnography that plots CO2 against the volume of gas exhaled, rather than against time. It requires both a CO2 sensor and a flow sensor, and it can measure dead space and the volume of CO2 eliminated per breath.

How is volumetric capnography different from time capnography? Time capnography plots CO2 against time and gives the EtCO2, respiratory rate and waveform. Volumetric capnography plots CO2 against exhaled volume and additionally quantifies dead space and CO2 elimination, but needs a flow sensor and is usually tied to a ventilator.

What does volumetric capnography measure that time capnography cannot? Dead space, both anatomical and alveolar, and VCO2, the actual volume of carbon dioxide the patient eliminates per breath and per minute.

Where is volumetric capnography used? Mainly in intensive care and anaesthesia, built into ventilators and anaesthesia workstations, to optimise ventilation, follow dead space and monitor conditions such as ARDS.

Does RespiCOz do volumetric capnography? No. RespiCOz performs time capnography, which covers the great majority of clinical needs. Volumetric capnography is a ventilator-integrated specialist tool for the ICU.

Conclusion

Volumetric capnography is the advanced cousin of the everyday capnogram. By plotting CO2 against exhaled volume rather than time, and by adding a flow sensor, it measures dead space and CO2 elimination directly, which makes it valuable for fine-tuning ventilation in intensive care.

For almost everything else, confirming the airway, monitoring ventilation, detecting apnoea, reading trends, time capnography is what you need, and it is what a portable capnograph provides. Knowing the difference tells you which tool fits which job.

For the everyday waveform, see capnogram interpretation.