Overview of the Differences Between Mainstream and Sidestream CO2 Monitoring
Overview of Differences Between Mainstream and Sidestream Capnography
Sidestream Capnography:
In sidestream capnography, the CO2 sensor is located inside the monitor (away from the airway), and a small pump draws a gas sample from the patient’s airway through a 6-foot capillary tube into the monitor. The sampling tube connects to a T-piece that is inserted at the connector of the endotracheal tube or anesthesia mask. The gas withdrawn from the patient often contains anesthetic gases, so the exhaust from the capnograph should be directed into a gas scavenging system or returned to the patient’s breathing circuit. Sampling flow rates can be high (>400 ml.min-1) or low (<400 ml.min-1). The optimal gas flow rate is considered to be 50-200 ml.min-1, which ensures reliable monitoring in both children and adults.1,2 Sidestream capnographs have a distinct advantage: they can monitor non-intubated patients, as exhaled gas sampling can be obtained from the nares via a nasal adapter.3-5 Additionally, with minor modifications, gas can also be sampled from the nares during oxygen administration via standard nasal cannulas.6,7 This feature enables monitoring of exhaled CO2 in patients receiving synchronized nasal intermittent positive pressure ventilation.
Mainstream Capnography.
In mainstream capnography, a sampling cell (airway adapter) is inserted directly into the airway between the breathing circuit and the endotracheal tube. A lightweight infrared sensor is then attached to the airway adapter. This sensor emits infrared light that passes through the adapter windows to a photodetector, typically located on the opposite side of the airway adapter. The intensity of light reaching the photodetector is used to measure ETCO2. Mainstream technology measures directly in the airway, thus eliminating the need for gas sampling and scavenging. This sampling technique produces cleaner waveforms that reflect real-time ETCO2 in the patient’s airway.
To prevent condensation of water vapor (which, if uncompensated, would cause falsely elevated CO2 readings), the mainstream sensor is heated to slightly above body temperature. This heating process helps keep the airway adapter windows clear and allows the sensor to tolerate high-humidity environments. Newer mainstream sensors incorporate current-limiting circuits that restrict output power, ensuring the sensor temperature never rises high enough to cause skin redness, thereby eliminating concerns about patient burns.
Over the years, mainstream technology has seen many advancements. Older generation mainstream analyzers were fragile and bulky, exerting traction on the ET tube and prone to damage. Newer generation mainstream sensors have addressed many of these issues. They are smaller, weighing less than 80 grams (2.8 ounces), and some feature a “solid-state” design with no moving parts, making them highly durable and resistant to damage. A variety of single-use airway adapters are now available, eliminating concerns about sterilization or cross-contamination. Additionally, low dead-space versions add less than 0.5cc of dead space, making this technology a viable option for ETCO2 monitoring in neonatal patients. In summary, recent technological advances have overcome some of the early drawbacks of mainstream sensors, making them comparable in weight and size to sidestream sensors.
References:
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Paloheimo M, Valli M, Ahjopalo H. A guide to CO2 monitoring. Finland: Datex Instrumentarium, 1988.
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.
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.
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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.
