{"id":34092,"date":"2024-06-14T17:07:44","date_gmt":"2024-06-14T17:07:44","guid":{"rendered":"https:\/\/todaysveterinarypractice.com\/?p=34092"},"modified":"2026-02-12T14:23:21","modified_gmt":"2026-02-12T14:23:21","slug":"capnography-assessing-ventilation-during-anesthesia","status":"publish","type":"post","link":"https:\/\/navc.sitepreview.app\/todaysveterinarypractice.com\/anesthesiology\/capnography-assessing-ventilation-during-anesthesia\/","title":{"rendered":"Capnography: Assessing Ventilation During Anesthesia"},"content":{"rendered":"

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Abstract<\/strong><\/p>\n

Capnography measures the concentration or partial pressure of carbon dioxide in expired gases, which provides valuable information about a patient\u2019s ventilation and cardiovascular status. Anesthetic agents used for sedation and induction of general anesthesia often cause central nervous system depression, which can lead to hypoventilation and hypoxemia. Monitoring an anesthetized patient with capnography helps minimize the risk for a general anesthesia adverse event. This article discusses types of capnography systems, common capnograph waveforms, and ways to troubleshoot anesthesia complications associated with expired carbon dioxide.<\/p>\n

Take-Home Points<\/strong><\/p>\n

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  • Many anesthetic agents cause respiratory depression, which commonly leads to hypoventilation, hypercapnia, and potentially hypoxemia.<\/li>\n
  • Hypoventilation, defined as elevated expired carbon dioxide, may be compounded by anesthesia drugs, obesity, and endocrinopathies.<\/li>\n
  • Adding capnography to a general anesthesia monitoring protocol can aid in early identification of complications, such as hypoventilation, and guide necessary interventions.<\/li>\n
  • Capnography waveforms can indicate if a patient is rebreathing carbon dioxide, apneic, or in cardiopulmonary arrest. Capnography waveforms can also indicate an airway obstruction and\/or a leak in an endotracheal tube.<\/li>\n
  • The necessary adjustment for a patient that is rebreathing carbon dioxide depends on the type of patient circuit (rebreathing or nonrebreathing circuit).<\/li>\n<\/ul>\n

    <\/div><\/div><\/p>\n

    Capnography is the measurement and graphical recording of carbon dioxide (CO2<\/sub>) concentration in exhaled gases. CO2<\/sub> is the major waste product of aerobic metabolism; it is circulated to the lungs for extraction and exhalation. Because CO2 <\/sub>is highly soluble, it diffuses readily from the blood into alveoli, making arterial (Paco2<\/sub>) and alveolar (PAco2<\/sub>) concentrations almost equivalent in healthy patients. Those concentrations provide information about a patient\u2019s metabolism, ventilation, and perfusion.1<\/sup> Ventilation is coordinated by the central respiratory center, central and peripheral chemoreceptors, and stretch receptors in the lungs. As Paco2<\/sub> increases, neural outputs from the respiratory center increase alveolar ventilation to compensate, encouraging the exhalation of excess CO2.<\/sub> Alveolar ventilation is inversely proportional to Paco2 <\/sub>concentrations. As alveolar ventilation doubles, more CO2 <\/sub>is expired because of increased gas exchange, thus Paco2<\/sub> is halved.<\/span><\/p>\n

    When a patient is anesthetized with inhalant anesthesia, normal feedback mechanisms are often impaired, which affects ventilation. Many anesthetic agents cause central nervous system depression and muscle relaxation, ultimately decreasing respiration. A patient\u2019s ventilation can be assessed by monitoring end-tidal CO2<\/sub> (ETco2<\/sub>) concentrations.<\/span><\/p>\n

    Other respiration and oxygenation monitoring modalities, such as visual observation and pulse oximetry, are inadequate for assessing ventilation and oxygenation status. Visual monitoring of chest excursions is extremely subjective and indicates only the presence of spontaneous breathing, not the quality of respiration. Pulse oximetry measures hemoglobin saturation but not ventilation. For example, in small dogs and cats that are breathing shallowly, pulse oximetry measurement may be within normal limits but the ETco2<\/sub> concentration may be high due to hypoventilation.<\/span><\/p>\n

    Numerous complications can arise from hypoventilation and inadequate gas exchange. CO2<\/sub> acts as a sympathomimetic and can increase heart rate and blood pressure. At high levels (><\/span>\u2009<\/span>70 mm Hg), CO2<\/sub> acts as a depressant and can decrease the amount of inhalant anesthetic needed. If a patient is poorly ventilated and supplemental oxygen is not provided, it will often become hypoxemic from increased PAco2<\/sub>. In addition, hypoventilation leads to respiratory acidosis, which causes cerebral vasodilation and increases the likelihood of cardiac arrhythmias, central nervous system depression, and organ dysfunction.<\/span><\/p>\n