Gianluca Bini, DVM MRCVS DACVAA

Monitoring anesthetized patients is critical for ensuring safety and optimizing outcomes. This lecture explores the reliability of various monitoring equipment, potential pitfalls, and practical considerations for veterinary anesthetic monitoring.

1. Importance of Anesthetic Monitoring

Minimum technical standards for monitoring anesthetized patients were established by the ASA in 1985 and are periodically updated.
Anesthesia mortality rates in human improved from the 1 in 3000 deaths in 1985 to 1 in 400000 in 2021
Anesthesia mortality rates are 1 in 600 dogs, 1 in 400 cats and 1 in 100 horses, those rates in veterinary medicine have remained stagnant over the years highlighting the need for more vigilant monitoring.

2. Electrocardiography (ECG)

Function: Detects electrical activity of the heart.
Common Issues:

Double/triple counting of HR due to tall P or T waves.
Artifacts that may mask arrhythmias.
Low frequency interference, caused by:

Patient respiration
Patient movement

High frequency interference, caused by:

muscle fasciculations
Shivering
Electrical equipment in the area:
Electrosurgery units
Cellphones
MRIs
Flexible bronchoscopes or endoscopes
Nerve stimulators

Risk of burns from improper grounding when using electrosurgical units, when using esophageal ECG probes

Best Practices:
Use pulse oximeter or arterial line to confirm pulse rate.
Ensure electrodes are correctly placed and hydrated.
Be cautious when interpreting results from multi-parametric monitors, they are not made to accurately diagnose abnormal electrical activity.
Studies show that arrhythmias can be common in anesthetized animals, even in healthy individuals.

3. Blood Pressure Monitoring

Methods:

Invasive blood pressure: Gold standard, invasive because need an arterial catheter

Overdamping

Caused by: Air bubbles, clots or kinks in the tubing
Results in: Underestimation of systolic arterial pressure (SAP), overestimation of diastolic arterila pressure (DIA), narrowed pulse pressure but mean arterial pressure (MAP) is not impacted

Underdamping

Caused by: An arterial catheter that is too rigid or tachydysrhythmias
Results in: Overestimation of systolic arterial pressure (SAP), underestimation of diastolic arterila pressure (DIA), wider pulse pressure but mean arterial pressure (MAP) is not impacted

Oscillometric Devices: Provide estimates of MAP with variable accuracy depending on the device.
Doppler Ultrasonography: Generally measures systolic pressure but may approximate MAP in cats. Doppler provided values that are not accurate in dogs weighing < 5 kg, while in cats doppler was found to be a poor predictor of both direct systolic and mean arterial BP.

Best Practices:

Ensure cuff placement is appropriate: medial front limb, lateral above the hock, or ventral tail.
Cuff width should be ~40% of limb circumference.
Verify readings with direct arterial measurement when possible.

4. Pulse Oximetry

Mechanism: Measures arterial oxygen saturation (SpO2) using light absorption differences between oxyhemoglobin and deoxyhemoglobin.
Most common probes: transmission (light crosses the tissue and is detected on the other side) and reflectance (light is reflected and then detected)
Common sources of errors:

Poor peripheral perfusion due to hypotension or hypothermia.
Arrhythmias or venous pulsations leading to false readings.
Movement artifacts and improper probe positioning.

Arterial compression caused by the sphygmomanometer cuff.
Best Practices:

Ensure proper probe placement with good tissue contact.
Recognize that SpO2 readings lag behind actual blood oxygen changes due to the oxyhemoglobin dissociation curve.

5. Capnography

Function: Measures end-tidal CO2 (EtCO2) to assess ventilation and metabolic status.
Clinical Importance:

Detects hypercapnia/hypocapnia and esophageal intubation.
Allows to detect if the ET-Tube is too deep
In humans esophageal intubation occurs up to 68% of the times, leading to cardiovascular instability, hypoxemia and death
In humans, professional liability insurance closed-claims analysis found that capnometry plus pulse-ox could potentially prevent 93% of avoidable anesthetic mishaps

Common Issues

Can identify increasing mechanical dead space, exhausted CO2 absorbent, or faulty unidirectional valves.
Too much moisture in the line can lead to wrong readings
Needs to be calibrated every year with your vaporizer

Best Practices:

Calibrate equipment regularly.
Empty water traps daily and replace moisture tubing as needed.

6. Practical Considerations for Reliable Monitoring

Always correlate machine readings with clinical assessment.
Regularly maintain and calibrate equipment to ensure accuracy.
Train staff to recognize false readings and troubleshoot common issues.
Have backup monitoring methods available in case of equipment failure.

Conclusion

While monitoring equipment is invaluable in anesthesia, it is not infallible. Understanding the limitations of each device and integrating clinical assessment with technology ensures the safest anesthetic management for veterinary patients.

References

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