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
Types of Capnography
Two types of capnographs are available, said Reuss-Lamky: mainstream and sidestream devices.

Mainstream Devices
Mainstream, or nondiverting, devices analyze the respiratory gases locally (at the interface between the endotracheal tube and breathing circuit) and provide instant (<100 ms) results. Samples of the patient’s exhaled gases are obtained from the anesthetic circuit using an adapter that attaches to the end of the endotracheal tube. This adapter should be attached at the point of the patient’s nose to avoid excessive dead space and prevent rebreathing of CO2, said Reuss-Lamky.

Mainstream technology is subject to fewer problems secondary to secretions or moisture compared with sidestream devices, involves fewer disposable supplies, and does not require scavenging of sampled waste anesthetic gases, she said. However, mainstream monitors have several disadvantages. Because of their weight and location, they are prone to accidental disconnection, leaks, and damage, and they may cause kinking of the endotracheal tube.

Sidestream Devices
Sidestream, or diverting, devices extract a sample of respiratory gases via tubing into a measurement chamber for analysis and can also be used in patients that are not intubated. These monitors use small, lightweight sensing tees attached at the interface between the endotracheal tube and breathing system. They are also useful for remote monitoring (such as during magnetic resonance imaging).

Sidestream devices, however, have some disadvantages. In small patients, high fresh gas flow rates may produce falsely low ETCO2 readings and waveform changes as a result of sample dilution. Sidestream monitors also have a 2- to 3-second delay in response time and require periodic calibration and replacement of disposable supplies (such as sensing tees). Sampling tubes may become occluded due to buildup of respiratory moisture. This technology also requires scavenging of sampled waste anesthetic gases.



The Capnogram
The waveform of the capnogram represents the changes in a patient’s ETCO2 levels over time, said Reuss-Lamky. The upward slope and the top of the waveform represent the exhalation phase, and the downward slope and the bottom of the waveform represent the inhalation phase.

In a normal capnogram (Figure 1) the baseline of the waveform should reach a zero level because the patient should not be inhaling CO2 , the expiratory upstroke of the wave should be steep, the expiratory plateau should be relatively flat, and the inspiratory downstroke should be an almost vertical drop to baseline. The highest point of the plateau represents the actual ETCO2 value. The shape of the capnogram waveform can help clinicians identify problems with the patient’s ventilation, perfusion, or metabolism.

Some Common Abnormal Capnograms
ETCO2 Level Abnormalities
Increased ETCO2 levels (>45 mm Hg) may occur as a result of hypoventilation (Figure 2). This may be due to problems such as airway obstruction, pneumothorax, lung disease, or acutely increased metabolism (eg, in malignant hyperthermia). Decreased ETCO2 levels (<35 mm Hg) may occur as a result of hyperventilation (Figure 3). This may be due to problems such as airway occlusion, endotracheal tube dislodgement, or cardiac arrest.

Other Abnormalities
If the baseline does not return to zero during inspiration, this indicates that the patient is rebreathing exhaled CO2 (Figure 4). Possible causes include an exhausted soda lime absorber or faulty valves in the breathing circuit.

A ripple effect at the end of the expiratory phase represents cardiogenic oscillations as the heart contracts and relaxes against the diaphragm when the lungs are nearly deflated (Figure 5). Cardiogenic oscillations may be seen at low respiratory rates and are considered physiologically insignificant.

A notch (curare cleft) in the top of the waveform in the expiratory plateau may be seen in cases when a patient is receiving a neuromuscular blocking agent and is being ventilated mechanically. These notches indicate that the effect of the neuromuscular blocking agent is wearing off.

A waveform that shows an elevation, or peak, at the end of exhalation just before inhalation—often described as a “shark fin” because of its distinct shape—indicates an airway obstruction (Figure 6). This may be due to such problems as kinks or mucus plugs in the endotracheal tube.

A weak inhalation phase showing decreasing ETCO2 with loss of the expiratory plateau indicates an endotracheal tube cuff leak or deflated cuff.

Conclusion
Capnography represents a valuable tool to help clinicians evaluate and monitor anesthetized veterinary patients. By familiarizing themselves with the waveform of the normal capnogram, clinicians can learn to recognize abnormal waveforms and thus identify underlying clinical situations or technical problems that may affect a patient during anesthesia.

 
Dr. Parry, a board-certified veterinary pathologist, graduated from the University of Liverpool in 1997. After 13 years in academia, she founded Midwest Veterinary Pathology, LLC, where she now works as a private consultant. Dr. Parry writes regularly for veterinary organizations and publications.
Reference:
  1. Tinker JH, Dull DL, Caplan RA, Ward RJ, Cheney FW. Role of monitoring devices in prevention of anesthetic mishaps: a closed claims analysis. Anesthesiology. 1989;71(4):541-546.


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