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Why Does Accurate etCO2 Measurement Begin with Advanced Sampling?

Obtaining an accurate end-tidal carbon dioxide (etCO2) measurement and quality waveform with any capnograph begins with the sampling line. And, if the sampling line isn’t providing a representative CO2 sample, it may impact the accuracy of the measurement. 

Considerate sampling line design considers how patients breathe — through the nose, through the mouth or both. In this blog post we discuss the impact of accurate sampling on patient safety by assessing:

  • How CO2 sampling works
  • The role of design in sampling accuracy
  • Uni-junction™ technology

Related: Get to know more about key features of capnography sampling lines currently on the market. Check out our blog for an insightful overview. 

How CO2 sampling works 

A capnogram is a graphical representation of the concentration of exhaled CO2. The graph is considered a waveform. A capnogram is obtained by a capnograph. And, both the numerical measurement and capnogram are necessary to develop a complete picture of the patient’s ventilation for clinical evaluation.

Capnography refers to a device that provides both the numeric end tidal CO2 value, a respiration rate, and a capnogram or waveform. It also offers waveform tracings, so a patient’s respiratory status can be visualized and analyzed over time.

Sampling line technology that offers accurate waveforms by providing a breath-by-breath representation of the patient’s airway status:

  • Normal breath
  • No breath
  • Hypoventilation
  • Hyperventilation and partial airway obstruction

The role sampling line design plays in breath sample accuracy

Sampling line designs vary, but one of the more common designs splits sampling and oxygen delivery between two nares. This design delivers oxygen to one nostril while sampling CO2 is taken from the other. There are limitations with this design for oxygen delivery specifically — if one or both nares are blocked. For example, waveforms and etCO2 measurements may be impacted if a patient has a deviated septum or sinus congestion.  

Some sampling line designs may impact quality breath samples from mouth breathers. Mouth breathing may happen if a patient experiences respiratory distress or while under sedation. Both are common scenarios for monitoring etCO2. Sampling lines designed with consideration for mouth breathers, or for patients who switch between nasal and mouth breathing, may help capture more accurate breath samples.2

Another consideration of sampling line design involves oxygen delivery. It’s important to ensure the sampling line provides both a quality sample of exhaled gas and consistent levels of prescribed oxygen. CO2 samples may be diluted by oxygen or other supplemental airflow.3  

With Microstream™ sampling lines, oxygen is delivered in a cloud from pinholes around the mouth and nose — ensuring delivery without diluting readings or waveforms.2,3 

Related: Are all capnography sampling lines the same? Read this white paper about how accurate capnography is highly dependent on quality sampling lines.

Intended benefits of Microstream™ monitoring solutions 

The Microstream™ capnography sampling line portfolio offers nonintubated, oral/nasal sampling, engineered to minimize dilution of CO2 samples when oxygen is prescribed. Here’s how:

Uni-junction™ technology enables etCO2 sampling from the oral or nasal source with the greatest pressure — without capturing ambient air. That means it permits sampling when a patient alternates between mouth and nose breathing. This helps ensure CO2 sampling accuracy. The technology has:

  • An oral scoop which offers a broad surface area to capture CO2 samples, helping to enhance accuracy, even in the presence of low tidal volume. 
  • Dual-nare sampling for sampling from either nare, important when patients experience alternating congestion. With dual nare sampling, the nasal airflow shifts back and forth intermittently between nares during alternating congestion.
  • Integrated oxygen delivery capability without diluting the CO2 sample, so if oxygen is prescribed, it’s then delivered through holes in front of the nasal prongs.  Microstream™ capnography sampling lines are designed to deliver up to 5 L/m of O2 without diluting the CO2 sample. CO2 sampling may be obtained via nonintubated sampling line under a mask if the patient needs CPAP intervention.

Related:  What do etCO2 sampling lines cost and are they worth the price? Get the information you need in our blog post.

1. Duckworth RL. How to read and interpret end-tidal capnography waveforms. J Emerg Med Serv. 2017; 42(8).
2. Colman Y, David U. Comparison of capnography filter lines for nose and mouth breathing of end tidal carbon dioxide sampling with and without supplemental oxygen. Milwaukee, WI: Society for Technology in Anesthesia. Abstract 43; Jan. 13–14, 2009.
3. Dungan G, Colman J, Lain D. Evaluation of oxygen delivery via a novel smart CapnoLine delivery system during simultaneous oxygen therapy and carbon dioxide monitoring: Milwaukee, WI: Society for Technology in Anesthesia. Abstract 20; Jan. 18–21, 2012. 

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TOPIC: Microstream™ Capnography Monitoring

About the Author

Gina Farquharson is a senior technical consultant for global marketing/market development at Medtronic specializing in capnography. Originally from Oridion Medical, she began her career at Medtronic as a ventilator account manager.

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