Is Waveform Capnography the Right Monitoring Tool?

There are several ways to measure end-tidal CO2 (the exhaled breath) with capnography monitoring.  Keep in mind that one option may be better than the other when it comes to your institution’s needs and goals.

In this article we’ll discuss the following differences:

  • Pulse oximetry and capnography
  • Mainstream versus sidestream capnography
  • Arterial blood gas (ABG) versus etCO2
  • Respiratory rate and waveform capnography
  • Colorimetric versus waveform capnography

What is the Difference Between Pulse Oximetry and Capnography?

To understand the difference between pulse oximetry (SpO2) and capnography (etCO2), it’s important to understand oxygenation and ventilation — two separate physiologic processes:

  1. Oxygenation is the process of getting O2 into the body
  2. Ventilation is the process of eliminating CO2 from the body

Pulse oximetry reflects oxygenation.  Capnography measures ventilation.  Combined, the two parameters offer you a comprehensive picture of respiratory status.1

The Joint Commission Sentinel Event Alert #49 recommends the following for use with patients on opioids:2

  • Do not rely on pulse oximetry alone (measures oxygenation)
  • Be aware that pulse oximetry may show a patient is sufficiently oxygenated during a respiratory compromise event, particularly when the patient has been prescribed oxygen

Conclusion: Capnography is valuable in measuring ventilation.

What is the Difference Between Mainstream and Sidestream Capnography? Does it Really Matter? 

A capnography monitor, by definition, is either diverting (sidestream) or non-diverting (mainstream). Sidestream capnography transports a portion of a patient’s expired gases from the sampling site, through a sampling tube, to the sensor. Mainstream capnography doesn’t transport the gas away from the sampling site. The difference can be seen as clinically measuring CO2 at the sample site versus measuring CO2 in the monitor distant from the sample site.

Mainstream technology may provide a shorter response time because the sensor is located right where the sample is collected.  Sidestream technology may be more comfortable for the patient because the sensor is away from the patient, therefore the patient only needs to wear a thin sampling tube.

Some disadvantages of mainstream capnography include the following:

  • Mainly developed for intubated patients
  • Often requires routine calibration
  • Requires sensor and cable at airway – weight and bulk may increase risk of accidental extubation
  • External cable/sensor prone to damage and very expensive to replace
  • Secretions can block sensor window
  • Difficult to use on non-intubated patients

Related: Learn more about capnography monitoring and Microstream™ technology  and how it is designed to help improve patient safety.

What is the Difference Between Arterial Blood Gas (ABG) and Capnography (etCO2)?

An arterial blood gas (ABG) test measures the amount of oxygen and carbon dioxide in the patient’s blood.  An ABG test requires that a small volume of blood be drawn from the radial artery with a syringe and a thin needle, but sometimes the femoral artery in the groin or another site is used.

Capnography, on the other hand, measures the exhaled breath (end-tidal carbon dioxide).  A sudden change in etCO2 can prompt the clinician to measure arterial carbon dioxide partial pressure (PaCO2) via an arterial blood gas sample.

Calculating the arterial gradient – the difference between PaCO2 and etCO2 values – requires obtaining a simultaneous arterial blood gas sample and an etCO2 measurement. Once the baseline carbon dioxide partial pressure gradient is determined, monitoring etCO2 could reduce the need for arterial blood gas sampling. This helps enables safe, comfortable, continuous monitoring with alarm limits that provide an early warning for intervention before the patient is compromised.

Related: Read an information sheet on the Clinical Overview of Arterial Gradient.

A study concluded that using the respiratory index, Integrated Pulmonary Index™ (IPI) algorithm, in Microstream™ technology was consistent with ABG values. Read the study.

IPI was developed to help you more easily monitor a patient’s complete respiratory status. It incorporates into a single number four real-time respiratory measurements:

  • End-tidal CO2 (etCO2)
  • Respiratory rate (RR)
  • Pulse oximetry (SpO2)
  • Pulse rate (PR)

Related: Download the Integrated Pulmonary Index™ Algorithm Guide.

What is the Difference Between Measuring Respiratory Rate and Waveform Capnography?

Clinical practice may inform that changes in respiratory rate is one of the first indicators of patient status change.  As you may know, respiratory rate is the number of breaths a person takes per minute.  The normal range is 12 to 20 breaths.  Rates under 12 and over 25 are considered abnormal. 

Accurately and objectively measuring respiratory rate can be challenging as it is normally done by manually counting the breaths a patient takes.  

Impedance monitoring is also challenging. Impedance monitors record chest wall or abdominal movement with ECG leads. It measures the changing impedance in a small electric current during chest wall expansion/contraction.  It requires the patient to be on an ECG monitor.  Accuracy is affected by motion artifact, ECG electrode placement, chest wall movement not related to RR (such as coughing, talking, eating, etc.).

Respiratory rate isn’t a measure of the adequacy of ventilation as it doesn’t measure CO2. Capnography measures exhaled breath — etCO2. In other words, capnography monitoring provides an assessment of  ventilation.  The normal etCO2 range is 35-45 mmHg.3  Capnography can also reveal the respiratory rate.  As the person’s respiratory rate and tidal volume changes, the waveform and etCO2 also changes. Exhaled CO2 waveforms can help provide an immediate indication of airway obstructions.  EtCO2 values help provide an objective piece of data to measure respiratory rate. Respiratory rate will increase as CO2 increases and vice versa.

Related: Visit the FREE Capnography Waveform Tutorial and Exercise to learn more about waveform capnography.

What is the Difference Between Colorimetric and Waveform Capnography?

Colorimetric etCO2 detectors provide verification of proper endotracheal tube placement throughout the resuscitation effort. They are designed to detect CO2 both at the start of intubation and during transport. They are simple and inexpensive compared to other devices. Colorimetric etCO2 detectors are single-use, disposable, and replaceable.  

It’s important to know that colorimetric capnography sensors only detect if CO2 is present; it doesn’t measure etCO2 values over a period of time or offer trend reports like continuous waveform capnography solutions.

Capnography is the noninvasive continuous measurement of CO2 concentration over time. It provides numerical etCO2data and graphic waveforms showing CO2 levels. Use this quick and free tool to gain an in-depth understanding of changes in the capnography waveform.

Related: We offer easy-to-use continuous monitoring solutions to help you keep your patients safe. Learn how you can gain more value from your medical devices using Vital Sync.

Learn more about capnography monitoring and Microstream™ technology.


1. Maddox RR, Williams CK, Oglesby H, Butler B, Colclasure B. Clinical experience with patient-controlled analgesia using continuous respiratory monitoring and a smart infusion system. Am J Health-Syst Pharm. 2006; 63(2):157–164.

2. The Joint Commission Sentinel Event Alert. Safe use of opioids in hospitals. Issue 49. 8 August 2012.

3.Understanding End Title CO2 Monitoring. American Nurse Today. November 2012 Vol. 7 No. 11.

About the Author

Michael Noble is a Senior Market Strategist for the Medical-Surgical and Procedural Sedation areas of care in the hospital at Medtronic. He develops strategies to help clinicians ensure patient safety with continuous monitoring solutions like Microstream™ capnography, Nellcor™ pulse oximetry, and Vital Sync™ clinical decision support.

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