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Bhavani Shankar Kodali MD

Effect of Inhalational Agents

Physics of capnography

Bhavani Shankar Kodali MD
Factors affecting IR Spectrography



Inhalational agents do not affect CO2 measurement

Inhalational agents absorb IR at lower frequency

The low concentrations of halogenated anaesthetic agents used during anaesthesia absorb IR energy at different wave lengths (around 3.3 milli microns) and their interference is not considered to be important.1

Carbon dioxide monitors. Health Devices;15:255-85.

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Microstream Technology Overview

Micro stream Technology Overview

    Oridion took a completely new approach to capnography in developing its Microstream® technology and FilterLine® components. In order to understand how these products conquer past problems with capnography, it helps to understand how conventional capnography works.

    Capnography is based on the principle that CO2 molecules absorb infrared radiation at specific wavelengths. The capnograph contains special photo detectors tuned to these wavelengths that enable the calculation of CO2 levels in the breath sample.

    Conventional capnographs typically use a heated element called a black body emitter for the infrared radiation source. Unfortunately, this type of emitter is both imprecise and inefficient because it produces a broad infrared spectrum. As a result, the capnograph requires a large sample cell and high flow rate, which causes occlusion and accuracy problems. Black body emitters also generate large amounts of heat, creating hardware challenges that restrict monitor portability and ruggedness.

Microstream ® - A Unique CO2 Emission Source

    Microstream ® employs a unique, laser-based technology called molecular correlation spectroscopy (MCS ™) as the infrared emission source. Operating at room temperature, the Microstream® emitter is electronically activated and self-modulating, which eliminates the need for moving parts.

    Unlike the broad infrared spectrum produced by a black body emitter, MCS ™ creates an infrared emission precisely matching the absorption spectrum of CO2. The Microstream® emitter radiates a focused beam of infrared energy characterized by the narrow region (0.15 µm wide) of the spectrum where CO2 molecules absorb infrared radiation. A black body emission is typically 135 times broader. Because MCS ™ is highly accurate with all gas samples, there is no need to create special algorithms within the monitor to correct for high concentrations of oxygen or anesthetic gases.

Small Sample Cell

    The highly efficient and CO2-specific emission source used in Microstream® technology results in an extremely short light path. This sets the stage for a number of technological advantages and clinical benefits. Because of the short light path, the breath sample cell can be greatly reduced in size (down to 15 µl) compared to sample cells used in conventional capnography.

Accuracy in Monitoring Neonates

    The advantage of a small sample cell is most apparent with neonatal patients who have high respiratory rates and small tidal volumes. A large sample cell can cause the inspired and expired breath to blend within the cell, resulting in slow response time, falsely low EtCO2 measurements and a distorted waveform shape. With Microstream®, a small sample cell designed for laminar flow, accurate monitoring can be attained with a much lower flow rate.

Minimal Flow Rate

    A low flow rate is important because it prevents moisture and humidity from entering the sample line and obstructing the pathway, a problem common in side stream technology. Microstream® operates at a flow rate of only 50 ml/min. Other capnography systems typically require flow rates two or three times as high. As with the small sample cell, the low flow rate ensures accurate and responsive monitoring for neonates and infants, despite their small tidal volumes.

For Further details, Go to Oridion website (opens a new window)

Effect of Oxygen

Physics of capnography

Bhavani Shankar Kodali MD


Factors affecting IR Spectrography



Oxygen does not absorb IR light

Oxygen does not absorb IR light
Oxygen does have affect via collision broadening effect
Oxygen has  collision broadening affect




How does oxygen affect CO2 measurements?

Oxygen does not absorb IR light
Oxygen produces collision broadening phenomena that can cause falsely low CO2 measurements.


Although oxygen does not absorb IR light it may indirectly affect the absorption by C02 by collision with the C02 molecules and broadening the absorption peak. While it does not cause as large an effect as N20, oxygen causes a falsely low C02 reading. Some units automatically correct or have a user-actuated electronic offset for the concentration oxygen encountered.1


1. Carbon dioxide monitors. Health Devices 1986;15:255-85.

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Effect of water vapor

Physics of Capnography

Bhavani Shankar Kodali MD


Factors affecting IR Spectrography



Water absorbs IR light but minimum at 4.3µ m

Water vapor iR light is minimal




How does water vapor affect CO2 readings?

Water can condense on IR cell and interfere with CO2 measurements

Decrease in partial pressure of water due to temperature differences between the patient's airway and the measuring unit can increase CO2 values.

Water can condense and clog the sampling line.

Changes in water vapor alter CO2 readings

Water vapor alters CO2 measurements




Water vapor can obstruct CO2 sampling tubes

Methods for decreasing contamination of sampling tubes by liquids or secretions

Position the sampling vertically upwards2 Use water filters at both ends of sampling tube

Oridion, Inc has incorporated a Filterline airway adaptor (Microstream airway adapter) coupled with low flow Microstream technology to minimize the water vapor problem.1 The airway adapter has three channels positioned in the center lumen of the adapter, facing different directions. This eliminates the problem associated with conventional sidestream airway adapters, whose upright orientation was mandatory for proper functioning, where any orientation other than upright may result in occlusions. Channel openings are made of small diameter hydrophobic material. These openings are specifically designed so that even when all but one channel is occluded, the system will continue to draw the sample breath, without secretion penetration into the sampling circuit. This allows for minimization of liquid and secretion blockage or penetration into the sampling circuit, thereby extending the life of the circuit. Most importantly, the multiple sample ports also permit the airway adapter to be inserted in any and all orientations.1


Water vapor can affect CO2 readings in two ways.

1. Effect of condensed water:

Water vapor may condense on the window of the sensor cell and absorb IR light, thereby produce falsely high C02 readings. This effect can be prevented by heating the sensor above body temperature (main-stream sensor units) or by removing the excess water vapor before it reaches the sensor (side-stream sensor units). Thus, some side stream-sensor units use a special sampling tube made of Nafion, a semipermeable polymer that selectively allows water vapor to pass from the interior of the tube to the exterior. Other side-stream units interpose liquid traps or moisture-absorbent filters between the sampling tube and the analyzer that help to remove excessive water and secretions, thus the optical system is protected.2-4

2. Effect of water vapor.

Main-stream IR analyzers measure the gas in the breathing circuit, which is generally saturated at body temperature. The exact water vapour pressure in the breathing circuit will depend on many factors including the use of heated humidification, fresh gas flow, length of time in use and temperature.3 In side-stream sensors the temperature of the sampling gases may decrease during the passage from the patient to the unit, resulting in a decrease in the partial pressure of water vapor. This can cause an apparent increase in C02 concentration of about 1.5-2%.4,5 Further, if Nafion tubing is used in the sample catheter, then it actually equilibrates the water vapor pressure inside the tubing to that of outside the tubing.3 Therefore, PETCO2 measurements should be corrected for the effects of water vapor, in accordance with the type of analyzer used, and the manufacturer's instructions.4,5


1. Coleman Y, Krauss B. Microstream capnography technology: A new approach to an old problem. J Clin Monit 1999;15:403-9.

2. Carbon dioxide monitors. Health Devices 1986;15:255-85.

3. Raemer DB, Calalang I. Accuracy of end-tidal carbon dioxide tension analyzers. J Clin Monit 1991;7:195-208.

4. Paloheimo M, Valli M, Ahjopolo H. A guide to CO2 monitoring. Finland: Datex Instrumentarium, 1988.

5. Fletcher R, Werner O, Nordstrom L, Jonson B. Sources of error and their correction in the measurement of carbon dioxide elimination using the Siemens-Elema CO2 analyzer. Br J Anaesth 1983;55:177-85.

6. Bhavani Shankar K. A method to prevent occlusion of CO2 sampling tubes. Can J Anaesth 1997:44.

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Effect of Nitrous Oxide

Physics of Capnography

Bhavani Shankar Kodali MD


Factors affecting IR Spectrography


Nitrous oxide absorbs IR light.

Nitrous oxide absorbs IR light

Absorption is minimal at IR 4.3 µ m


Nitrous oxide absorbs IR at 4.3 microns


Collision Broadening Phenomenon

Water vapor absorbs IR light



Effect of n2o on C02 measurements

Use of 4.3 µ m IR light does not affect C02 measurements.
Collision broadening phenomena does increase C02 values.


Correction factors for the presence of nitrous oxide.

Percent Nitrous oxide

Corrected reading =

70 %

Observed PC02 x 0.90


Observed PC02 x 0.94

Because nitrous oxide absorbs IR (IR absorption spectra of N20 = 4.5 µm whereas C02 = 4.3 µm), the presence of N20 therefore can give falsely high C02 readings. This problem can be eliminated by using a narrow band IR filter that only transmits the the wavelength most strongly absorbed by C02 (about 4.3 µm). Another problem relates to n2o concerns the interaction between N20 molecules and C02 molecules. This produces a "collision broadening effect" that affects the sensitivity of the IR analyzer and causes an apparent increase in C02 reading. "Collision broadening" is a phenomenon in which the spectral absorption peaks of a gas (C02) are broadened owing to the collision or proximity of molecules of another gas (N20).1 The correction factors for the presence of various concentrations of N20 have been studied and range from 0.90 at 70% n2o (corrected PC02 = observed PC02 x 0.90) to 0.94 at 50% n2o.2 Most monitors provide some system of electronic compensation to reduce this effect. Alternatively, the simplest method of eliminating this error is to calibrate the instrument with a gas mixture which contains the same background gas concentration as that to be analyzed.3,4


1. Raemer DB, Calalang I. Accuracy of end-tidal carbon dioxide tension analyzer. J Clin Monit 1991;7:19-208.

2. Kennell EM, Andrews RW, Wollman H. Correction factors for nitrous oxide in the infrared analysis of carbon dioxide. Anesthesiology 1973;39:441-3.

3. Carbon dioxide monitors. Health Devices 1986;15:255-85.

4. Paloheimo M, Valli M, Ahjopalo H. A guide to C02 monitoring. Finland:Datex Instrumentaitrium, 1988.

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