The advent of the exhaled breath carbon dioxide monitoring curve is another major improvement in the use of non-invasive technology to monitor lung function, especially lung ventilation function, making it possible to monitor patients continuously and quantitatively at the bedside, especially for anesthesia patients, ICUs, and respiratory departments. Respiratory support and respiratory management provide clear indicators.


During the breathing process, the measured carbon dioxide concentration and the corresponding time are traced one-to-one to obtain the so-called carbon dioxide curve. The standard curve is divided into four parts, namely the ascending branch, alveolar plateau, descending branch, and baseline. The exhalation starts from the ascending branch point P and passes through Q to point R. The QR represents the alveolar plateau (also called peak phase), and the R point is the peak value of the alveolar plateau. This point represents the end-tidal (also called end-tidal) carbon dioxide concentration. The beginning of the descending branch means the start of inhalation. With the inhalation of fresh gas, the carbon dioxide concentration gradually returns to the baseline. Therefore, P.Q.R is the expiratory phase and R.S.P is the inspiration phase. The area between the curve and the baseline can be compared to carbon dioxide emissions.


The most commonly used method is infrared absorption spectroscopy, which is based on the principle that when infrared light passes through a gas sample, its absorption rate is related to the concentration of carbon dioxide (CO2 mainly absorbs infrared light with a wavelength of 4260nm). The reaction is rapid and the measurement is convenient. At the same time, there are other methods such as mass spectrometry, Roman spectroscopy, photoacoustic spectroscopy, carbon dioxide chemical electrode method and so on.


Depending on the position of the sensor in the airflow, there are two commonly used sampling methods: mainstream and side hole sampling. Mainstream sampling is to connect the sensor in the patient's airway. The advantage is that it is directly in contact with the airflow, and the recognition response is fast; airway secretions or water vapor have little effect on the monitoring effect; no gas is lost. The disadvantage is that the weight of the sensor is relatively large; an additional dead space (about 20ml) is added; it is not suitable for patients without tracheal catheters. Side-hole sampling is to continuously suck part of the gas from the airway through the sampling tube for measurement. The sensor is not directly connected to the ventilation circuit, and does not increase the dead space of the circuit; does not increase the weight of the components; for patients without tracheal catheters, The modified sampling tube can still make accurate measurements through the nasal cavity. The disadvantage is that the recognition response is slightly slow; sampling is affected by water vapor or airway secretions; care should be taken to supplement the amount of gas lost due to sampling during low-flow anesthesia or pediatric anesthesia. At present, most monitors adopt the side hole sampling method.