Capnography is the measurement of expired carbon dioxide at the end of exhalation, where the concentration is highest. Knowing the amount of carbon dioxide in the lungs and blood stream is critical when determining whether or not a patient is breathing adequately. CO2 is a byproduct of cellular metabolism; it’s transported by the circulatory system to the lungs where it is excreted through the process of exhalation. In addition to monitoring respiratory status, capnography can be used to evaluate a patient’s metabolic and circulatory status as well.
In the ICU capnography (abbreviated ETCO2) is used to evaluate the severity of a pulmonary disease and the effectiveness of the plan of therapy. The normal range of CO2 in the arterial blood is between 35 to 45 mm Hg; the capnography range is expected to be 3-8 mm Hg lower. The capnography monitor can provide a continuous sample range and prevent the patient from having excessive arterial blood gas (ABG) sample sticks.
Indications of capnography
Procedures that require moderate sedation can depress the rate and depth of breathing and lead to carbon dioxide retention. A high CO2 level represents inadequate or cessation of breathing and therefore poor air exchange in the lungs. An abnormal ETCO2 can be noticed before the patient is in acute distress, and therefore prompt treatment can be started.
ETCO2 aids in the determination of effectiveness of mechanical ventilation. ETCO2 provides a continuous analysis of exhaled and inhaled CO2 while being ventilated. These numbers will determine any changes that are needed to maintain therapeutic levels of carbon dioxide and assess metabolic rate or alveolar ventilation. Primarily, adjustments in the set respiratory rate, tidal volume, or pressure will adjust the level of exhaled CO2.
During intubation, ETCO2 monitoring is used to determine the correct placement of the endotracheal tube. The monitor is immediately attached to the end of the endotracheal tube while manually resuscitating. A number above 5 mm HG along with bilateral breath sounds will confirm the tube placement in the lungs and not in the esophagus. This monitoring is particularly helpful during emergent intubations requiring CPR and when other forms of monitoring are not readily available.
In the intensive care unit (ICU) continuous bedside monitoring of ETCO2 is displayed on the cardiac monitor along with other vital measurements such as oxygen saturation, blood pressure and heart rate.
Sensors that are used on mechanically ventilated intubated patients are called “mainstream” CO2 sampling devices. These are small, clear plastic devices that fit into the ventilator circuit in an area where exhaled gas can be detected. These sensors must be placed where there is very little condensation or humidification since they unable to function when wet. The sensor is usually connected to a wire that leads to either the ventilator monitor or the cardiac monitor. The senor must be calibrated before use and as needed for accuracy.
The “sidestream” method of sampling allows the user to place the sensor on a non-intubated patient through a nasal device that is attached to an oxygen nasal cannula. The sensor is placed just outside of the nostrils r mouth and analyses the ETCO2 and the respiratory rate when exhalation takes place. This device works well for babies since they primarily breathe through their nose. It’s also used for short-term procedures that require minimal sedation.
Capnography procedures in the ICU may vary depending upon the type or brand of equipment used but the application of set up is generally simple to do. The monitoring devices include step-by-step procedures for calibration and patient use. The sensors made today are disposable and intended for single patient use only. The manufacturer will recommend the frequency of calibrations needed while in use and the disinfectant procedures for non-disposable units. Some common trouble shooting problems include: an obstruction or moisture in the probe or disconnection of any part of the system. The patient’s respiratory or cardiac status should always be assessed when abnormalities are seen on the capnography monitor.