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Bhavani Shankar Kodali MD

Effect of atmospheric pressure

Physics of capnography

Bhavani Shankar Kodali MD

Factors affecting IR Spectrography

Effect of Atmospheric Pressure
How does atmospheric pressure affect CO2?

Increases in atmospheric pressure result in an increase in the PETCO2 values by increasing number of IR absorbing molecules

and increasing intermolecular forces

The effect of Atmospheric pressure can be minimized by:-

Measuring CO2 as partial pressures

Calibrating with a known concentration of CO2 as partial pressure at the site of measurement

Keeping the sampling flow rate into the capnometer constant

Atmospheric pressure changes as a result of changes in the weather are of the order of 20 mm Hg which should result in a change of PCO2 of less than 0.5 to 0.8 mm Hg.

Changes in atmospheric pressure do not affect the C02 concentration (FC02) but rather what is interpreted by the detector as C02. A change in atmospheric pressure directly influences the reading of capnometers since C02 concentration is measured as partial pressure (direct effect). Further, an indirect effect is seen when a capnometer reports measurements in volume percent instead of partial pressure.1

Direct Effect:

This has two mechanisms.

(a) An increase in pressure proportionately increases the number of IR absorbing C02 molecules and thereby increases the C02 signal. A 1% increase in pressure causes a 1% relative increase in the C02 signal. This effect is eliminated by calibrating the capnometer with a known partial pressure of the C02 gas (mmHg = vol% * atmospheric pressure) using a commercially available calibrating gas. Given such a calibration, the capnometer will display CO2 concentrations within its measurement cuvette as partial pressure, regardless of changes in the atmospheric pressure.1 However, if the capnometer is calibrated in volume percent, then FC02 readings will need to be corrected for changes in atmospheric pressure (a 1% increase in pressure produces a 1% relative increase in FC02 value).2

(b) Changes in pressure also produce a second effect (alteration of intermolecular forces) exerted by C02 molecules, which alters the IR absorption. An increase in pressure by 1% results in a relative increase in the signal by 0.5 to 0.8% which can produce a small error.2,3

Maximal changes in the atmospheric pressure caused by changes in the weather are of the order of 20 mmHg. This would result in a change in PC02 of less than 0.5 to 0.8 mmHg (measurement range of PC02, 30-40 mmHg). Therefore, in routine clinical use, corrections for changes in atmospheric pressure are unnecessary.2 However, in studies in which precision is needed corrections for variations in barometric pressure are useful. Increases in the sampling flow rate of side-stream C02 sensors result in a reduction of pressure at the airway and lower apparent C02 measurements. However, if the unit is calibrated at the average prevailing atmospheric pressure and sampling flow does not change, the unit should be sufficiently accurate for clinical measurements. Further, application of PEEP (positive end expiratory pressure) increases the C02 reading. A PEEP of 20 cm H20 increases the C02 reading by 1.5 mmHg.1 Some units measure the pressure in their sensor and automatically adjust the C02 reading accordingly.1,3,4


The indirect effect of atmospheric pressure on the PCO2 values results when an analyzer reports measurements in volume percent. In conjunction with measurements of arterial blood gas tensions, it is preferable to record the readings as PCO2(mmHg) and not as volume percent. The atmospheric pressure at the time of measurement must be known to compute a PC02 value (mmHg = FC02 * atmospheric pressure).1 However, if the atmospheric pressure at the time of measurement differs from the atmospheric pressure at the time of calibration, then the observed FC02 readings should be corrected for the two components of "direct effect" of atmospheric pressure changes before computing the PC02 value.2,3

The effect of high altitude on the capnographs:

Pattinson, et al studied the the effect of high altitude on capnographs.5 They determined that pressure decreases at higher altitudes produced altered reading that may be unrelated to calibration of the instrument. The authors used Datex-S/5 portable critical care monitor, K4b2 (Cosmed), Datex Cardiocap, and Ohmeda 4700 in their study. Each instrument was assessed in the decompression chamber to simulated altitudes of 3600 m, 4800 m, and 5260 m. Despite calibration of these instruments according to the manufacturer's instructions, Datex-S/5 and Ohemda 4700 showed increased percentage CO2 readings as altitude increased. The Datex Cardiocap did not give any readings following decompression to 3600 m. The K4b2 readings increased slightly at higher altitudes but the results remained close to the 5% expected from the test gas. However, there was an error of nearly 7% in the CO2 reading at sea level.

The authors give following explanation for the observed altered readings:

1. Reduced gas flow rates through the sampling chamber due to increased difficulty encountered by the pump as a result of decreased density of the gases at lower pressures.

2. The second explanation for malfunction of the instrument is the effect of low barometric pressure on calibration. This influences the gas flow rate and the infra red spectroscopy measurements. These effects on the measurements can be corrected by re-calibration at the ambient pressures.

3. The third explanation is the effect of reduced barometric pressure on computer software within the instruments. Capnographs have an in-built barometer from which computer algorithms are used to compensate for changes in barometric pressures. According to the authors,5 Datex-Ohmeda have informed them that their machines have minimal barometric pressure limit, below which they become unreliable because the instruments assume the barometric pressure to be at that level and give incorrect values acccordingly.

Capnographs such as Datex-Ohemeda specifically excludes the use of their instruments at barometric pressures below 66.5 kPa.


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

2.Olsson SG, Fletcher R, Jonson B, Nordstrom L, Prakash O. Clinical studies of a gas exchange during ventilatory support - a method of using the Siemens-Elema CO2 analyzers. Br J Anaesth 1980;52:491-9.

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

4. Carbon dioxide Monitors. Health Devices 1986;15:255-85.

5. Pattinson K, Myers S, Gardiner-Thorpe C. Problems with capnography at high altitude. Anesthesia 2004;59:69-72.
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Chemical Method of Carbon dioxide Measurement

Physics of Capnography

Bhavani Shankar Kodali MD

Chemical method of CO2 measurement



A pH-sensitive chemical indicator changes color on exposure to CO2


FEF end-tidal detector:


A pH-sensitive chemical indicator is enclosed in a plastic housing and is connected to the gas stream between the endotracheal tube and the anesthesia circuit. The pH sensitive indicator changes color when exposed to C02.1 The color varies between expiration and inspiration, as C02 level increases or decreases. The color changes from purple (when exposed to room air or oxygen) to yellow (when exposed to 4% C02). The response time of the device is sufficiently fast to detect changes of C02 breath-by breath.1 However, this device is not very sensitive when CO2 output is low as is during CPR. Easy cap II is a an example of such pH sensitive indicator devices.

False negative results may occur even with correct endotracheal tube placement in patients in cardiac arrest, in whom sufficient CO2 may not be present in the lungs. It is also more relevant to point out the possibility of color change in the device due to agents other than exhaled carbon dioxide (false positive results). Gastric contents, mucus, and drugs such as epinephrine can cause false positive results. It is imperative that clinicians using these devices be aware of this limitation. One way to avoid this pitfall is to observe the change in color in the device with each breath. A false positive result causes a permanent color change in the device; hence, the color does not vary with ventilation.2,3

* Reference:

1.O'Flaherty D, Adams AP. The end-tidal carbon dioxide detector. Assessment of new method to distinguish oesophageal from tracheal intubation. Anaesthesia 1990;45:653-5.

2.Srinivasa V, Kodali BS. Video on orotrachal intubation. NEJM 2007;357:6:620

3.Srinivasa V, Kodali BS. Caution when using colorimetry to confirm endotracheal intubation. Anesth Analg 2007;104:738


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Types of Capnographs

Physics of Capnography

Bhavani Shankar Kodali MD

Types of Capnographs


Main-stream Capnographs Side-stream Capnographs

CO2 sensor located between endotracheal tube and breathing circuit Sensor is located in the main unit and CO2 is aspirated via a sampling tube connected to a
T-piece adapter located endotracheal tube and breathing circuit.


Side-stream Capnographs

Advantages Disadvantages

Easy to connect

No problems with sterilization

Can be used in awake patients

Easy to use when patient is in unusual positions such as in prone position

Can be used in collaboration with simultaneous oxygen administration via a nasal prong
Delay in recording due to movement of gases from the ET to the unit

Sampling tube obstruction

Water vapor pressure changes affect CO2 concentrations

Pressure drop along the sampling tube affects CO2 measurements

Deformity of capnograms in children due to dispersion of gases in sampling tubes

Main-stream Capnographs

Advantages Disadvantages
No sampling tube

No obstruction

No affect due to pressure drop

No affect due to changes in water vapor pressure

No pollution

No deformity of capnograms due to non dispersion of gases

No delay in recording

Suitable for neonates and children
Contrary to the earlier versions, the newer sensors are light weight minimizing traction on the endotracheal tube. (see below)

Long electrical cord, but it is lightweight.

Facial burns have been reported with earlier versions. This has been eliminated with newer sensors (see below)

Sensor windows may clog with secretions. However, they can be replaced easily as they are disposable.

Difficult to use in unusual patient positioning such as in prone positions.

The newer versions use disposable sensor windows thereby eliminating sterilization problem (see below)


Side-Steam Capnographs:

In side-stream capnography, the CO2 sensor is located in the main unit itself (away from the airway) and a tiny pump aspirates gas samples from the patient's airway through a 6 foot long capillary tube into the main unit. The sampling tube is connected to a T-piece inserted at the endotracheal tube or anesthesia mask connector. The gas that is withdrawn from the patients often contains anesthetic gases and so the exhausted gas from the capnograph should be routed to a gas scavenger or returned to the patient breathing system. The sampling flow rate may be high (>400 ml.min-1) or low (<400 ml.min-1). The optimal gas flow is considered to be 50-200 ml.min-1 which ensures that the capnographs are reliable in both children and adults.1,2 The side-stream capnographs have a unique advantage: they allows monitoring of non-intubated subjects, as sampling of the expiratory gases can be obtained from the nasal cavity using nasal adaptors.3-5 Further, gases can also be sampled from the nasal cavity during the administration of oxygen using a simple modification of the standard nasal cannulae.6,7 This feature enables monitoring of expired CO2 in subjects receiving simultaneous oxygen administration using nasal cannulae.

Main-stream capnographs:

In the mainstream capnograph, a sample cell or cuvette (airway adapter) is inserted directly in the airway between the breathing circuit and the endotracheal tube. A lightweight infrared sensor is then attached to the airway adapter. The sensor emits infrared light through the adapter windows to a photodetector typically located on the other side of the airway adapter. The light which reaches the photodetector is used to measure ETCO2. Mainstream technology eliminates the need for gas sampling and scavenging as the measurement is made directly in the airway. This sampling technique results in crisper waveforms which reflect real-time ETCO2 in the patient airway.

To prevent condensation of water vapor, which if not compensated for can cause falsely high CO2 readings, the mainstream sensor is heated to slightly above body temperature. This heating process helps keep the windows of the airway adapter clear so the sensor can tolerate high moisture environments. New mainstream sensors use circuitry, which limits the power delivered so the sensor never reaches a temperature high enough to cause even redness of the skin eliminating the concern of patient burns.

There have been many advances in mainstream technology over the years. Older generation mainstream analyzers have had the reputation of being fragile, bulky and heavy which put traction on the ET tube and made them prone to breakage. New generation mainstream sensor design addresses many of these issues. They are smaller and weigh less than 80 grams (2.8 ounces) and some utilize a “solid state” design, so there are no moving parts, which make them very durable and less prone to breakage. A variety of single patient use airway adapters are available eliminating the issue of sterilization or cross contamination. In addition, low deadspace versions which add less than 0.5cc of deadspace make the technology a viable ETCO2 monitor for the neonatal patient. In summary, recent technological advances have overcome some of earlier disadvantages of main stream sensors to match side stream sensors in terms of weight and size.


1. Kalenda Z. Mastering infrared Capnography. The Netherlands: Kerckebosch-Zeist 1989.
2. Carbon dioxide monitors. Health Devices 1986;15:255-85.
3. Paloheimo M, Valli M, Ahjopalo H. A guide to CO2 monitoring. Finland: Datex Instrumentarium, 1988.
4. Cambell FA, McLeod ME, Bissonette B, Swartz JS. End-tidal carbon dioxide measurements in infants and children during and after general anaesthesia. Canadian J Anaesth 1993;41;107-10
5. Iwasaki J, Vann WF Jr, Dilley DCH, Anderson JA. An investigation of capnography and pulse oximetry as monitors of pediatric patients sedated for dental treatment. Pediatric Dentistry 1989;11:111-7.
6. Roy J, McNulty SE,Torjman MC. An improved nasal prong apparatus for end-tidal carbon dioxide monitoring in awake, sedated patients. J Clin Monit 1991;7:249-52.
7. Tobias JD, Flanagan JF, Wheeler TJ, Garrett JS, Burney C. Noninvasive monitoring of end-tidal CO2 via nasal cannulas in spontaneously breathing children during the perioperative period. Crit Care Med 1994;22:11:1805-8.

For more information about Filterline Breathing Sample Capnograph products (side stream technology), click on the hyperlink-Close the new window to return back to this page

For more information about the current status of main stream technology, click on this hyperlink to open a new window. Close the new window to return back to this page

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Response Time of Analyzer

Physics of Capnography

Bhavani Shankar Kodali MD


Factors affecting IR Spectrography



Slow analyzers distort CO2 wave forms

slow response of capnographs can distort CO2 waveforms


2008-07-29Response time (Components) What is it? Affect on CO2
Transit time Time to move from point of sampling to point of measurement Underestimation due to dispersion of gases at longer transit times
Rise time (T70/T90) Time to change in response to a step changes in CO2. Abnormal wave forms, reduce slope of phase II at longer rise times


Response time can be reduced by

Use of more powerful amplifiers

Minimizing the volume of the sampling chamber and tubes

Use of relatively high sampling flow rates (150 ml-min-l)


For accurate measurements of CO2, a capnograph needs a rapid response time. There are two components of a response time. The transit time and the rise time.1 The transit time is the time required for the sample to move from the point of sampling to the detector cell. Prolonged transit time delays the appearance of the waveform at the detector, which causes a phase shift, but no distortion. However, a bolus of gas is subjected to dispersal caused by convection and diffusion during transit down the catheter. Such a dispersal converts a square wave front into a sigmoid shape, with loss of the highest and lowest gas concentration peaks. This results in an underestimation of PETCO2, particularly in children. The extent of the error increases with increased length and width of the sampling tube, reduced sample flow rate (50 ml.min-l) and higher breathing frequency of the patient (more than 31 breaths min-l ).2,3 The rise time (T90)is the time taken by the output from the capnometer to change from 10% of the final value to 90% of the final value in response to a step change in PC02. Alternatively, rise time may be specified as T70 which is the time taken to change from 10% to 70% of the final value.4 The rise time is dependent upon the size of the sample chamber and the gas flow.1 Slower flow rates increase the time required to flush the infra-red sample cell, which can increase the rise time. The rise times of capnographs for clinical use range from 50-600 ms. Carbon dioxide waveform is a function of rise time of the capnometer. Prolonged rise time can reduce the slope of phase II resulting in an underestimation of anatomical dead space.3,5 The rise time of commercially available CO2 analyzers is fast enough to measure PETCO2 in adults, with 5% accuracy (less than 30 breaths min-l). However, when the ventilatory rate is high, as in children, faster analyzers with rise time (T70) of 80 ms are necessary to measure PETCO2 values with 5% accuracy (at 100 breaths min-l and l:E ratios less than 2:1).4 The response time of the C02 monitors has been considerably reduced in newer units by (i) the use of more powerful amplifiers, (ii) minimising the volume of the sampling chamber and tubes and (iii) the use of relatively high sampling flow rates (150 ml-min-l). In order to achieve predictable PETCO2 values and C02 waveforms it is recommended that the response time of the analyzer be less than the respiratory cycle time of the patient.6

1. Parbrook GD, Davis PD, Parbrook EO. Gas chromatography and mass spectrometry. In: Basic Physics and Measurement in Anaesthesia. 3rd ed. London: Butterworth-Heinemann, 1990257-64.

2. From RP, Scamman FL. Ventilatory frequency influences accuracy of end-tidal CO2 measurements: analysis of seven capnometers. Anesth Analg 1988;67:884-6.

3. Pasucci RC, Schena JA, Thompson JE. Comparison of a sidestream and mainstream capnometer in infants. Crit Care Med 1989;17:560-2.

4. Brenner JX, Westenskow DR. How the rise time of carbon dioxide analysers influences in accuracy of carbon dioxide measurements. Br J Anaesth 1988;61:628-38.

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 CO2analyzer. Br J Anaesth 1983;55:177-85.

6. Schena J, Thompson, Crone RK. Mechanical influences on the capnogram. Crit Care Med 1984;12:672-4.

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