QUARTERLY BEAT / DECEMBER 2025
REBREATHING CARBON DIOXIDE (CO 2 )
IDENTIFYING CAUSES
IN VETERINARY MEDICINE:
Ever glance at your monitor and wonder why your capnograph is tracing high, and what to do about it? You’re not alone! In this VETgirl article, Amanda Shelby, RVT, VTS (Anesthesia & Analgesia), breaks down the basics of carbon dioxide monitoring and reviews the key causes of rebreathing. Read on to learn how to recognize the patterns, troubleshoot equipment, and keep your patients safer under anesthesia.
Amanda M. Shelby, RVT, VTS (Anesthesia & Analgesia) Senior CE Specialist, VETgirl
Hopefully in the video below, you recognize quickly that this patient has significant rebreathing of CO 2 occurring! But don’t worry, after reading this blog, you’ll be able to recognize rebreathing of CO 2 and understand the common causes related to patient anesthesia breathing systems.
Figure 2. A capnograph; inspiration is represented by the yellow line and the expiratory phase is represented by the red line. Image courtesy of Amanda M. Shelby CONSEQUENCES OF HYPERCAPNIA (E.G., ELEVATED ETCO 2 ) But first, before we identify the causes for increased FiCO 2 during anesthesia, we should review the consequence of hypercapnia (e.g., elevated EtCO 2 ). CO 2 is a weak acid; increases in the partial pressure of CO 2 in the blood (PaCO 2 ) causes a decrease in pH which could result in a respiratory acidosis. Elevations in CO 2 also can do the following: • Influence vascular tone (resulting in vasodilation)
A REVIEW OF THE FUNDAMENTALS Capnography measures CO 2 , the by-product of cellular metabolism, as it is removed from the body via the respiratory system during exhalation. The measurement of expired CO 2 with capnography allows for a non-invasive, real-time assessment of ventilation and circulation. Use of capnography in intubated, anesthetized, patients have demonstrated decreased morbidity and mortality in human and veterinary species. 2-5 For these reasons, capnography is a monitoring favorite by anesthetists and others. In fact, it’s my go-to (and favorite) monitoring device as a veterinary anesthetist! In most mammalian species, normal end-tidal CO 2 (EtCO 2 ) is 35-45 mmHg. Normal fraction of inspired CO 2 (FiCO 2 ) is 0 mmHg. Looking at Figure 2, expiration is from A to C, represented by the red line and inspiration is represented as the yellow line. The presence of CO 2 during the inspiratory phase in intubated patients indicates inappropriate ventilation and/or a mechanical failure of anesthesia equipment. By understanding causes of rebreathing CO 2 as they relate to the breathing circuit in use allows the anesthetist to quickly resolve the issue. Figure 1. Video: Rebreathing of CO 2 from a low FGF rate on a non- rebreathing circuit
• Potentially increase intercranial pressure (ICP) through an increase in cerebral perfusion pressure (CPP)
• Decrease myocardial contractility, further perpetuating hypotension,
• When extremely excessive can result in death.
However, monitoring ventilation with capnography increases awareness of EtCO 2 and FiCO 2 values allowing the anesthetist to assist or control ventilation where necessary. In the absence of ventilation to perfusion (V/Q) mismatching, EtCO 2 should reflect PaCO 2 , albeit at a slightly lower value.
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