Capnography

Capnography

• Capnography is a non-invasive method of monitoring a graphical display of CO2 throughout the respiratory cycle.
• Capnography directly reflects the elimination of CO2 by the lungs (ventilation). 
• Capnography has been the standard of care for monitoring patient safety during anesthesia because it provides a quick and reliable method for detecting life-threatening conditions like malposition of tracheal tubes, ventilatory failure, circulatory failure, and defective breathing circuits.

Why use Capnography?

• Pulse oximetry monitoring is not enough to ensure patient safety.
• The patients CO2 will rise well before a fall in SpO2, especially when supplemental oxygen is administered. 
• In addition, capnography can provide information about pulmonary perfusion, alveolar ventilation, and respiratory patterns. 

Types of capnograph

Side-stream capnographs

• Sensor is located in the main unit and CO2 is aspirated via a sampling tube connected to a T-piece adapter located between endotracheal tube and breathing circuit.
• Tiny pump aspirates gas samples from the patients airway through a capillary tube into the main unit. 

Main-stream capnographs

• CO2 sensor located between endotracheal tube and breathing circuit.
• 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. 

Side-stream Capnographs

Advantages

• 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
• Monitoring of non-intubated subjects, as sampling of the expiratory gases can be obtained from the nasal cavity using nasal adaptors.

Disadvantages

• 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

• 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

Disadvantages

• Contrary to the earlier versions, the newer sensors are light weight minimizing traction on the endotracheal tube
• Long electrical cord, but it is lightweight
• Facial burns have been reported with earlier versions. This has been eliminated with newer sensors 
• 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