Sidestream capnography is a non-invasive medical monitoring technique that measures carbon dioxide (CO2) levels in exhaled breath. It helps healthcare professionals assess a patient’s breathing and ventilation status.
Understanding Sidestream Capnography
This technique operates on the principle that carbon dioxide molecules absorb infrared radiation at specific wavelengths. A sidestream capnograph works by aspirating a small, continuous sample of exhaled air from the patient’s airway through a thin sampling tube. This tube carries the gas sample away from the patient to a measurement chamber located within the main monitoring unit.
Inside the measurement chamber, an infrared light source emits radiation through the sampled gas. Detectors on the opposite side measure the amount of infrared light that passes through. The more CO2 present in the sample, the more infrared light is absorbed, resulting in less light reaching the detector. This inverse relationship allows the device to precisely calculate the CO2 concentration, which is then displayed as a numerical value and a waveform on the monitor.
Advantages and Clinical Applications
Sidestream capnography offers numerous benefits due to its adaptability. Its portability allows for use across diverse clinical environments, including emergency departments and various ambulatory care settings. This versatility means it can be readily moved between patient rooms or during transport.
The method is suitable for monitoring both intubated and non-intubated patients, broadening its applicability in different medical scenarios. For children, this technique is beneficial because it does not involve placing a heavy device directly at the airway, which could cause discomfort or dislodgement. Furthermore, sidestream capnography can be used simultaneously with oxygen administration through nasal tubes or masks.
Potential Drawbacks and Considerations
Despite its benefits, sidestream capnography has certain limitations inherent in its diverting design. A time delay occurs as the gas sample travels from the patient’s airway through the sampling tube to the external measurement unit. This delay means the displayed CO2 readings are not truly instantaneous.
Measurements can also be influenced by physical factors within the sampling system. Pressure drops along the length of the sampling tube can affect the gas flow and concentration readings. Additionally, changes in water vapor pressure within the exhaled breath sample may alter the accuracy of CO2 measurements. The sampling tube itself is susceptible to obstruction by respiratory secretions or condensation from moisture, which can impede gas flow and lead to inaccurate or absent readings. In patients with very shallow breathing or rapid respiratory rates, the small sample volume aspirated by the device may not capture a representative breath, potentially leading to less accurate CO2 measurements.
Sidestream Capnography Compared to Other Methods
Mainstream capnography presents a contrast by measuring carbon dioxide directly at the patient’s airway, through a sensor placed between the endotracheal tube and the breathing circuit. This direct placement provides immediate, real-time feedback on CO2 levels with high accuracy, beneficial for intubated patients requiring constant respiratory monitoring. However, the mainstream sensor can be cumbersome, potentially causing discomfort or dislodgement, and its weight may pose a challenge, especially for pediatric patients.
A more refined version of sidestream technology is Microstream™ capnography, which employs laser-based molecular correlation spectroscopy. This advanced technique requires a small breath sample to accurately measure CO2. The minimal sample volume and high precision make Microstream™ suitable for neonates and other younger patients who have very small tidal volumes. This innovation improves the reliability of CO2 monitoring in delicate patient populations.