The advent of the capnography for monitoring end-tidal carbon dioxide (ETCO2) is a major advancement in the use of non-invasive technology to monitor PF (pulmonary function), especially pulmonary ventilation function. It makes it possible to monitor patients continuously and quantitatively at the bedside to provide clear indicators for respiratory support management, especially for anesthesia patients, ICUs, and respiratory departments.
The capnograph represents continuous monitoring of the partial pressure of CO2 in a circuit and has four main phases:
- Phase Ⅰ：The gas comes from the dead space and doesn’t contain CO2.
- Phase Ⅱ: The mixed gas from the dead space and alveoli starts being exhaled.
- Phase Ⅲ: This phase is known as the “alveolar plateau” where rich gas all comes from the alveoli.
- Phase 0: Inhalation starts, ETCO2 extremely drops to baseline.
Measurement of ETCO2
There are three methods for measuring the ETCO2: infrared spectrography, mass spectrography, and colorimetric capnometry. The infrared spectrography commonly used in clinical practice is divided into two types: main-stream capnograph and side-stream capnograph.
Application of ETCO2
(1) Monitoring ventilation function
In anesthesia, the clinical manifestations of hypoventilation are not sensitive and specific due to improper VT setting and air leakage in the circuit most of the time, so the ETCO2 waveform is of higher diagnostic value.
(2) Maintain normal ventilation
During general anesthesia or when using a ventilator with respiratory insufficiency, the ventilation volume can be adjusted according to ETCO2 to avoid hypoventilation and hyperventilation that may cause hypercapnia or hypocapnia.
(3) Determine the position of the trachea
There are currently three recognized methods to prove that the tracheal tube is in the trachea:
- The duct must be seen in the glottis.
- The clinical use of fiberoptic bronchoscopy technology is the “gold standard” for judging the catheter position, but it is inconvenient to use. ETCO2 is quick and intuitive to judge the catheter position.
- Normal end-tidal waveform and PV ring to avoid the misjudgment of the tracheal tube entering the esophagus by mistake.
(4) Timely discover the mechanical failure of the ventilator
If the joint falls off, the circuit leaks, the catheter is twisted, the trachea is blocked, the valve fails, and other mechanical failures occur during the operation, the capnograph is able to change clinically. The operation of the head and face surgery is easy to cause the joints to fall off, which is often difficult for the observer to find due to obstruction. ETCO2 monitoring helps find that the carbon dioxide waveform disappears in time, accompanied by a sudden drop in tracheal pressure. In general anesthesia for tracheal intubation, continuous ETCO2 monitoring is better than other monitoring methods such as SpO2, expiratory tidal volume, etc. especially if there is no expiratory tidal volume monitoring during the operation and the anesthetizer is far away from the patient’s head. ETCO2 monitoring can promptly find and deal with poor airways.
(5) Adjust the parameters of the ventilator and guide the removal of the ventilator
- Adjust ventilation.
- Choose the best PEEP value. Generally speaking, the PEEP with the minimum ETCO2 value is the best PEEP value.
- ETCO2 is continuous non-invasive monitoring that used to guide the temporary deactivation of the ventilator. When SpO2 and ETCO2 remain normal while breathing spontaneously, the ventilator can be removed.
(6) Monitoring changes in CO2 production in the body
Intravenous injection of a large amount of NaHCO3 will significantly increase ETCO2; repeated inhalation, increase in body temperature, sudden release of the tourniquet, and malignant hyperthermia(MH) all increase CO2 production; and rapid increase of ETCO2 is an early indicator of malignant hyperthermia susceptibility.
(7) Monitoring cycle function
Cardiac arrest is manifested in shock, pulmonary embolism, reduction or cessation of pulmonary blood flow, CO2 concentration rapidly becomes 0, CO2 waveform disappears, ETCO2 disappears and ETCO2 rapidly drops for more than 30 seconds. ETCO2 as an important non-invasive monitoring index for whether the precordial area compression is effective during resuscitation and first aid, its prognostic value is greater. At this time, ETCO2 levels and cardiac output are associated with changes.
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In short, ETCO2 monitoring is a very valuable alarm system in clinical anesthesia which involves a wide range of areas, complex conditions, and many complications. With ETCO2, changes in the condition can be detected timely and accurately, so as to avoid severe hypoxic damage and greatly improve the safety of surgical anesthesia.