Quantitative study of Lowe's square-root-of-time method of closed-system anaesthesia

Area of Science:

• Anesthesiology
• Pharmacology
• Critical Care Medicine

Background:

• Intermittent liquid anesthetic injection is valuable in resource-limited settings.
• Lowe's method for calculating anesthetic doses requires rigorous assessment.

Purpose of the Study:

• To rigorously assess Lowe's method for calculating intermittent anesthetic doses.
• To compare halothane, enflurane, and isoflurane using this technique.

Main Methods:

• Double-blind, randomized comparison of three anesthetics (halothane, enflurane, isoflurane) in oxygen-air.
• Doses adjusted to maintain systolic arterial pressure within 20% of baseline.
• Partial pressures monitored but concealed from the anesthetist.

Main Results:

• Mean doses were within 33% of calculated 1.3 MAC, but end-tidal pressures stabilized lower (0.97, 0.42, 0.77 MAC).
• Recovery from enflurane was significantly faster than halothane or isoflurane.
• No patients reported dreams, suggesting adequate anesthesia depth despite lower MAC.

Conclusions:

• Anesthetic uptake declines slower than predicted by standard models.
• Enflurane's hypotensive effect necessitates smaller doses, resulting in less deep anesthesia and quicker recovery.
• The assessed method provides a viable, albeit imprecise, approach to anesthetic management in certain contexts.