Light Absorbance Monitors (Capnometer, Anesthetic Analyzer)

Capnographs and anesthetic analyzers both use the Beer law by analyzing constituents of the respiratory gas stream. The physical design of most capnometers is divided into two categories: mainstream and sidestream. In mainstream capnometers, the light absorption chamber is placed directly in the airway, and the light source shines throughout the chamber, with CO₂ being measured during inspiration and expiration directly. Advantages of this technique are a very fast response time and no problems with clogging of the sampling tubes, which is a disadvantage of sidestream capnometers. Disadvantages include the necessity of having a potentially heavy, expensive infrared measurement device placed directly in the endotracheal tube and the unavailability of devices that can measure both anesthetic agents and CO₂ . The most commonly used devices in the operating room are sidestream capnometers. A small capillary sampling tube is attached to the airway, and samples are aspirated at 200 to 300 mL/min into the measurement chamber, which is within the monitor. The advantages of this method are just the opposite of the disadvantages of the mainstream capnometers, that is, the sampling tube is lightweight, and the device can be used to measure carbon dioxide and anesthetic agents. Disadvantages relate mostly to the delayed response time and potential clogging of the aspiration tubing.^[18]

Agent analyzers and capnography function by the same physical principles, but with different wavelengths of light being used ( Fig. 30-33 ). The mixture of gases can interfere with each other, so compensations are built into current devices.