The use of intravenous paracetamol (acetaminophen) infusion before the start of general anesthesia with spectral entropy monitoring decreases consumption of the inhalational anesthetic sevoflurane in patients undergoing thyroidectomy surgery.

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Proper monitoring of the depth of anesthesia is crucial for judicious titration of anesthetics to prevent awareness under general anesthesia as well as the side effects of anesthetic over-dose with the subsequent economic waste and environmental pollution. With awareness, the patient may exhibit symptoms ranging from mild anxiety to post traumatic stress disorder (sleep disturbances, nightmares and social difficulties). In the standard clinical practice, the depth of anesthesia is judged by the clinical experience of the anesthetist based on the patient’s vital signs and the hemodynamic responses. However the regular use of certain medications as beta-blockers and antihypertensive drugs render the hemodynamic signs unreliable for titration of anesthetics. At present; the electroencephalogram (EEG) based spectral entropy is increasingly being used for monitoring the depth of anesthesia and provides information regarding the cortical state of the patient and the level of hypnosis as well as an indirect measure of the adequacy of analgesia. The monitor uses different algorithms to calculate the level of consciousness index by processing the EEG signal measured over the forehead and drive the numeric index. The spectral entropy has 2 signals: State entropy (SE) which reflects the hypnotic level of the patient; computed from an EEG data from the previous 15 sec in the range of 0.8 to 32 Hz, and shows the value in the range of 0 – 91 and Response entropy (RE) that includes; in addition to the EEG, a forehead muscle electromyography component and reflects the patient arousal and response to painful stimuli. The latter is computed from an EEG data in the range of 0.8 to 47 Hz and shows the value in the range of 0 – 100. Entropy values between “40 – 60” are the recommended surgical level of anesthesia while “100” signifies awake state and “0” indicates suppression of the cortical neuronal activity. Intravenous (IV) paracetamol (acetaminophen in USA) is an effective analgesic and antipyretic agent acting at both the central and peripheral components of the pain pathway and devoid of the detrimental effects of opioids and non-steroidal anti-inflammatory drugs (NSAID) with a tolerability profile similar to placebo. The onset of paracetamol analgesia starts rapidly after 5 – 10 min of IV administration, with peak effect obtained within 1 hour and lasting 4 – 6 hours. Thus, IV paracetamol is a suitable medication for the treatment of postoperative pain when used either alone or as a part of a balanced analgesic regimen. In addition, several studies in the medical literature have demonstrated the opioid-sparing effect of IV paracetamol. In view of these reports, we hypothesized that preoperative infusion of IV paracetamol would decrease sevoflurane consumption during general anesthesia. To explore this; we utilized spectral entropy monitoring to evaluate the effect of the preoperative administration of IV paracetamol on sevoflurane consumption in patients undergoing thyroidectomy under general anesthesia.

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