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Why monitor End-tidal Carbon Dioxide?

   |  July 17, 2020

1. What is End-Tidal Carbon Dioxide?

End-Tidal Carbon Dioxide refers to the partial pressure or concentration of carbon dioxide (CO2) at the end of exhalation, which is normally 35-45 mm Hg.


Phase I: Happens suddenly with an inspiration.

Phase II: Beginning of expiration, represents anatomical dead space of the respiratory tract and is not discernible from the inspiratory phase before it (PECO2 = 0 mmHg),

Phase III: A very rapid increase in ETCO2, which represents exhalation of mixed air.

Phase IV: Reflects the alveolar expiratory flow (a small increase in ETCO2), which happens the peak at the end of tidal expiration (ETCO2). In this phase PECO2 is close to alveolar carbon dioxide tension (PACO2).

CO2 produced by tissue and cell metabolism is transported to the lungs through capillaries and veins, and finally expelled through exhalation. The body’s carbon dioxide output (VCO2) and alveolar ventilation value (VA) determine the end-tidal carbon dioxide (ETCO2). ETCO2 = VCO2 × 0.863 / VA (0.863 is a constant that converts gas volume to pressure). The ETCO2 of normal animals ≈PA CO2 (partial pressure of carbon dioxide in alveolus) ≈Pa CO2 (partial pressure of carbon dioxide in arteries), which is slightly lower than PaCO2.

2. Why monitor End-tidal Carbon Dioxide?

End-tidal carbon dioxide (ETCO2)-the sixth vital sign

Besides body temperature, respiration, pulse, blood pressure and SpO2, ETCO2 is the sixth vital sign. It has special clinical significance for judging body metabolism, pulmonary ventilation, and pulmonary blood flow changes. It is widely applied in clinical anesthesia, cardiopulmonary cerebral resuscitation, pre-hospital first aid, intensive care post-anesthesia.

3. What can ETCO2 monitoring provide for the veterinarians?

a. Confirming the correct location for tracheal intubation. 

ETCO2 is a recognized gold standard for confirming the correct location of the endotracheal tube. As long as the continuous ETCO2 waveform shows, it’s confirmed that the tracheal tube is in the trachea. The ETCO2 waveform can help vets make the final judgment when intubating cats, rabbits, and rodents that are difficult to intubate.

b. Evaluating animal ventilation function

For animals without obvious cardiopulmonary disease, ETCO2 is equal to Pa CO2 in a certain extent. When the ventilator is used during general anesthesia or with respiratory insufficiency, continuous ETCO2 monitoring helps vets to find out whether the animal is insufficiently or excessively ventilated and adjust the relevant parameters to maintain normal ventilation.

c. Diagnosing and handling the mechanical failure of anesthesia machine and ventilator in time.

For animals undergoing mechanical ventilation, through observing the change of ETCO2 waveform, vets can duly diagnose and handle the mechanical failure such as joint fall-off, circuit leakage, twisted catheter, tracheal obstruction.

d. Guiding the safe removal of the ventilator

ETCO2 monitoring is continuous and non-invasive, which can be used to guide the temporary suspension of the ventilator. When SpO2 and ETCO2 remain normal during spontaneous breathing, the ventilator can be removed. Attention should be paid to the presence of abnormal ETCO2, and blood gas examination should be applied if necessary.

e. Detecting cardiac arrest in time and guiding CPR

During anesthesia, cardiac arrest is what vets mainly concern about. During the surgical operation, the use of electrocautery always strongly interferes with the ECG monitoring, which leads to false alarms. When ETCO2 waveform disappears, the connection is correct and the tube is not prolapsed, it can be diagnosed as cardiac arrest, for which duly cardiopulmonary resuscitation is needed. If ETCO2 goes higher than 10mmHg, the resuscitation success rate is higher. ETCO2 is an important non-invasive monitoring indicator to judge whether the precordial compression is effective during resuscitation.

f. Metabolic monitoring to assist in the early diagnosis of malignant hyperthermia

Monitoring the emission of CO2 can assess the metabolic rate of the body and assist in diagnosing malignant hyperthermia, which is characterized by a large increase in CO2 occurs earlier than the increase of body temperature.

4. How to choose a capnography monitor?

There are currently two types: 1. multi-parameter monitors that includes capnography 2. Stand-alone capnography monitors

For newly opened pet hospitals, it is recommended to directly choose the first type; for old hospitals, if already have multi-parameter monitors without capnography, the second type is recommended or expand the original monitors function if possible.

The advantages of stand-alone ETCO2 monitors: 1.The greatest advantage is good compatibility, which means a stand-alone capnography monitor can be matched with other monitors. 2. In addition, they are compact, lightweight, space-saving, easy to move, and applicable for more diverse scenarios.

RWD RM-C1 Veterinary Capnograph belongs to the second type.



• Mainstream CO2 monitoring, real-time measurement of end-tidal carbon dioxide in animals.

• Fast response, end-tidal carbon dioxide (ETCO2) and real-time ETCO2 waveform displayed in 5 seconds.

• Audible and visual alarm system for No Adapter, Clogged Adapter, No Breath (Apnea), Low Battery and adjustable High and Low ETCO2 alarm.

• Clear display of waveform and value to provide doctors with comprehensive interpretation.

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