What are the basics and recent developments in monitoring the depth of analgesia during surgery?

Monitoring of Analgesia Depth During Surgery: Basics and Recent Developments

Monitoring analgesia depth during surgery is essential for optimal patient outcomes, with processed electroencephalography (EEG) monitoring strongly recommended for patients over 60 years to reduce postoperative delirium and anesthesia-induced hypotension. 1

Basic Principles of Analgesia Monitoring

• Analgesia is one of the three major components of anesthesia, alongside amnesia and immobilization, and is continuously influenced by external stimuli and analgesic drug administration 2
• Traditional monitoring of analgesia relies on indirect autonomic responses such as tachycardia, hypertension, sweating, and lacrimation, which are non-specific but considered signs of inadequate analgesia 2
• Continual monitoring of ventilatory function through observation of qualitative clinical signs is recommended during procedural sedation and analgesia 3
• Regular assessment of a patient's response to verbal commands (e.g., at 5-minute intervals) is recommended during moderate sedation to gauge consciousness level 3

Advanced Monitoring Technologies

• Processed EEG monitoring, such as bispectral index (BIS), is strongly recommended for monitoring anesthesia depth, particularly in patients over 60 years of age 3, 1
• For elderly patients (>60 years), targeting a lighter level of anesthesia with a BIS of approximately 50 (versus deeper levels with BIS 35) can significantly reduce postoperative delirium 1, 4
• Avoiding burst suppression of the EEG is critical as it increases the risk of postoperative delirium in older patients 3, 1
• Autonomic monitoring techniques that help quantify reactions of the autonomic nervous system include:
  • Heart rate variability analysis
  • Laser Doppler flowmetry
  • Plethysmographically derived indices
  • Pupillary light reflex assessment 2

Surgical Stress Index (SSI)

• The Surgical Stress Index (SSI) was developed to correlate with total surgical stress and can be computed as a combination of normalized heart beat interval and plethysmographic pulse wave amplitude 5
• SSI increases at skin incision, stays higher during surgery than before surgery, and responds to changes in analgesic drug concentrations 5
• SSI formula: SSI = 100-(0.7×PPGA_norm+0.3×HBI_norm), where PPGA_norm is normalized plethysmographic pulse wave amplitude and HBI_norm is normalized heart beat interval 5

Clinical Recommendations for Specific Patient Populations

• For patients over 60 years at risk of postoperative delirium and anesthesia-induced hypotension, depth of anesthesia monitoring is strongly recommended 3
• Close monitoring of age-adjusted minimum alveolar concentration (MAC) is critical to avoid side effects such as hypotension in elderly patients 1
• For patients undergoing emergency laparotomy, there is a high incidence of frailty and old age which increases the risk of postoperative delirium, making depth of anesthesia monitoring particularly important 3

Recent Developments and Emerging Technologies

• Multimodal monitoring approaches that integrate information from the electroencephalogram, hemodynamic readings, and pulse plethysmography are being developed to better monitor nociception during anesthesia 2, 6
• Different physiological bases for nociception monitoring have led to multiple valid approaches for estimating a patient's nociception under general anesthesia 6
• Processed EEG-guided anesthesia care may facilitate rapid emergence and recovery, according to recent reviews 3

Implementation Considerations and Limitations

• Currently available monitoring algorithms do not account for all anesthetic drugs, including ketamine, nitrous oxide, and halothane 7
• EMG and other high-frequency electrical artifacts are common and can interfere with EEG interpretation 7
• Data processing time produces a lag in the computation of depth-of-anesthesia monitoring indices 7
• EEG effects of anesthetic drugs may not always predict movement in response to surgical stimulus, as the main site of action for preventing movement is the spinal cord 7

Documentation Requirements

• Patient's level of consciousness, ventilatory and oxygenation status, and hemodynamic variables should be recorded at a frequency dependent on medication administered, procedure length, and patient condition 3
• At minimum, documentation should occur: before administration of sedative/analgesic agents, after administration, at regular intervals during the procedure, during initial recovery, and just before discharge 3
• Device alarms should be set to alert the care team to critical changes in patient status 3

By implementing these monitoring strategies and considering the recent developments in analgesia depth assessment, anesthesia providers can optimize patient outcomes while minimizing complications related to inadequate or excessive anesthesia depth.