[Pet Diagnosis and Treatment] Why can't we ignore the monitoring of ETCO2 during anesthesia?
author: Karry Sun
2023-10-25
What is ETCO2?
End expiratory carbon dioxide (ETCO2) : refers to the partial pressure or concentration of carbon dioxide contained in the mixed alveolar air exhaled at the end of the expiratory period, and the normal value is 35--45mmHg. End-expiratory carbon dioxide and its graph have special clinical significance for determining the changes of body metabolism, pulmonary ventilation and pulmonary blood flow. It is the sixth basic vital sign besides body temperature, respiration, pulse, blood pressure and oxygen saturation, and has important application value in clinical anesthesia, cardiopulmonary resuscitation, pre-hospital emergency care, and anesthesia (to understand the related advantages of inhalation anesthesia).
The monitoring principle of end-expiratory carbon dioxide (ETCO2)
CO2 produced by the metabolism of tissue cells is transported to the lungs through capillaries and veins and expelled during exhalation. The internal carbon dioxide production (VCO2) and pulmonary ventilation volume (VA) determine the alveolar partial pressure of carbon dioxide (ETCO2), that is, ETCO2=VCO2×0.863/VA, and 0.863 is the constant at which the gas capacity is converted into pressure.
CO2 dispersion ability is very strong, easily from the pulmonary capillaries into the alveoli. The alveolar and arterial CO2 are completely balanced, and the last exhaled gas should be alveolar gas, with ETCO2≈PACO2 (partial pressure of carbon dioxide in alveoli) ≈PaCO2 (partial pressure of arterial carbon dioxide) under normal circumstances. The determination of end-expiratory carbon dioxide has three kinds of infrared method, mass spectrometer method and colorimetric method, and the infrared method commonly used in clinical practice is divided into two categories according to the way of gas sampling: side-flow type and mainstream type.
End expiratory carbon dioxide (ETCO2) waveform and significance
The normal CO2 waveform can generally be divided into four phases and four segments:
Phase Ⅰ : A→B, the inspiratory baseline, should be in the zero position, is the beginning of the expiratory part of the respiratory tract for the dead chamber gas, basically no carbon dioxide.
Phase II: B→C, ascending branch of expiratory, steeper, mixed air of alveoli and ineffective chamber.
Phase III: C→D, the carbon dioxide curve is horizontal or slightly upward sloping, called the expiratory platform, is the mixed alveolar gas, the end of the platform is the end-expiratory air flow, is the ETCO2 value.
Phase Ⅵ : D→E, inspiratory descent branch, carbon dioxide rapidly and sharply drops to the baseline fresh gas into the airway.
ETCO2 curves of common anomalies
1. Reduced ETCO2 (normal expiratory platform)
Causes: a, hyperventilation; b, dead cavity gas increase.
2. Elevated ETCO2 (normal expiratory platform)
3. There is spontaneous breathing in mechanical ventilation (small respiratory waves appear in the inhaling and exhaling phase)
Causes: a, poor ventilator regulation; b, the muscle is not satisfied; c, severe hypoxia; d, animals wake up; e. The chest is pressed.
4. Expiratory ascending branch is prolonged
Causes: a trachea catheter partial obstruction; b Animal airway obstruction.
5, ETCO2 decreased (suddenly dropped to zero)
(What does the graph look like? Imagine it!)
Causes: a, tracheal catheter protrusion; b, ventilation system disconnected; C. Complete tracheal catheter obstruction; d, gas sampling tube plug; e. The CO2 measuring instrument is faulty.
How to use an oximeter correctly
Practical application of oximeter and PI value
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