October 20, 2017

AVMA 2017: Anesthesia Monitoring With Capnography

Presenting at the 2017 AVMA Convention, Heidi Reuss-Lamky discussed the ways in which capnography provides important feedback about the severity of a patient’s condition and how patients respond to treatment.
By Nicola M. Parry, BVSc, MRCVS, MSc, DACVP, ELS
Capnography is an essential tool for monitoring anesthetized and critical care veterinary patients, according to Heidi Reuss-Lamky, LVT, VTS (Anesthesia & Analgesia), from Oakland Veterinary Referral Services in Bloomfield Hills, Michigan. Presenting at the 2017 American Veterinary Medical Association Convention in Indianapolis, Indiana, Reuss-Lamky discussed the ways in which capnography provides important feedback about the severity of a patient’s condition and how patients respond to treatment. 

Capnography refers to the measurement of CO2 in a patient’s exhaled breath, she said. The measurements are displayed as a capnogram, which is a graphic representation of exhaled CO2 as each breath is taken over time.

Reuss-Lamky highlighted the critical role of this tool in anesthetic monitoring by discussing findings from a key study from the human medical literature of more than 1000 anesthetic-related malpractice claims.1 Anesthesiologist reviewers analyzed the cases and concluded that “using capnography and pulse oximetry together could have prevented 93% of the anesthetic mishaps,” she said.

Key Aspects of Physiology
According to Reuss-Lamky,3 physiologic processes are essential for capnography use and interpretation: metabolism, circulation, and ventilation. Gas exchange occurs in alveolar capillary beds in the lung, she said. CO2 is produced in the body as a byproduct of tissue metabolism, transported to the lungs via perfusion, and then removed via alveolar ventilation. The exhaled CO2 is also known as end-tidal CO2 (ETCO2) and represents the concentration of CO2 in the lung alveoli that emptied last.

She explained that CO2 is transported in the body predominantly in 3 forms: 60% to 70% as bicarbonate ions, 20% to 30% bound to proteins, and 5% to 10% dissolved in arterial plasma. Bloodgas analysis measures the CO2 that is dissolved in plasma, said Reuss-Lamky, and this is known as the arterial partial pressure of CO2 (PaCO2).

She noted that, in a healthy patient, ETCO2 (normally 35-45 mm Hg) typically correlates with PaCO2 (Table) and is approximately 2 to 5 mm Hg lower. Because this gradient is considered clinically insignificant, she added that ETCO2, PaCO2, and alveolar CO2 (PACO2) levels are considered approximately equal.

Ideally, alveolar ventilation is matched to alveolar perfusion. Under normal conditions in the lung, the ratio of pulmonary ventilation to perfusion is close to 1. However, ventilation–perfusion mismatching may occur in some conditions, leading to abnormal ETCO2 levels (Table).

Why Use Capnography?
Capnography offers many benefits, said Reuss-Lamky. It allows noninvasive assessment of the patient’s systemic metabolism (acid-base status), pulmonary perfusion, and cardiac output.

Because ETCO2 , PaCO2 , and PACO2 levels are considered approximately equal, monitoring ETCO2 levels can help clinicians to detect trends or sudden changes in a patient’s PaCO2. Various clinical conditions, even anesthetic mishaps, may increase the gradient between ETCO2 and PaCO2, such that it becomes clinically significant, she said.

ETCO2 analysis can help evaluate a patient’s acid-base status, respiratory patterns, and adequacy of ventilation in a variety of clinical situations, said Reuss-Lamky. These include during anesthesia when anesthetic drugs and inhalants can reduce tidal volumes significantly, or during long-term ventilatory assistance, such as when a mechanical ventilator is used. Capnography can also help clinicians promptly identify airway mishaps, she added. One example comprises cases in which the endotracheal tube is placed in the esophagus instead of the trachea.

By monitoring ETCO2 using capnography, Reuss-Lamky explained that clinicians can identify and promptly respond to such conditions. For example, ETCO2 levels greater than 45 mm Hg in a patient indicate inadequate ventilation (Table), she said, requiring clinicians to provide ventilatory assistance via manual or mechanical means. Hypoventilation may occur in such condi- tions as pneumothorax or lung disease, she added.

Using capnography also helps clinicians maintain a stable plane of anesthesia. This can be especially useful in obese animals, during prolonged surgical procedures (>90 minutes), or when neuromuscular blocking agents are used. A sudden decrease in ETCO2 can also be an early and reliable indicator of impending cardiovascular collapse or cardiac arrest, she said.

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