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Physics of Capnography
Types of Capnographs
| Main-stream Capnographs | Side-stream Capnographs |
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| 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 The gas that is withdrawn from the patients often contains anesthetic gases and so the exhausted gas from the capnograph should be routed to a gas scavenger or returned to the patient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.
References:
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.