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Perioperative Hypotension and Depth of Anesthesia: Finding the Sweet Spot that Saves Lives

Because of their potent vasodilating function, hypnotic overdoses can result in perioperative hypotension. That’s why physicians are increasingly turning to entropy in the O.R. to improve patient care and reduce risks.

Anesthetic agents, such as hypnotics, are potent vasodilators. In fact, studies have found that anesthetics can threaten cardiovascular stability through alterations in cardiac function. Specifically, use of these agents can lead to issues including myocardial depression, peripheral vasodilation, and attenuated sympathetic nervous system activity.[1]

This is why the overall impact of anesthetics is often systemic hypotension and why unnecessarily deep anesthesia only increases this risk[2].
This makes monitoring the depth of anesthesia vital to reduce the risk of intra-operative hypotension and improve overall patient outcomes.

Entropy is a widely studied measurement for assessing depth of anesthesia. It is also recommended and accepted by several industry organizations since it has been found to reduce the use of certain hypnotic drugs, helping to achieve that “sweet spot” where a patient is given enough anesthesia to prevent awareness, while lowering the risk of hypotension.

This article explores not only the risk of hypotension that comes with hypnotics overdoses, but also the value of entropy in patient care and risk reduction.

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The Consequences of Prolonged Hypotension

Since anesthetics act at multiple levels on the cardiovascular system, with action occurring directly on vascular smooth muscle, as well as on the endothelial cells of numerous vascular beds, they demonstrate a profound influence on total peripheral and regional vascular resistance[1].

This means that the agents’ actions affect organ blood flow.

It also explains why prolonged intraoperative hypotension has been associated with increased risk of myocardial infarction and acute kidney injury in the post-operative period. It has been demonstrated that the more cumulative time the patient spends in hypotensive state, the higher the risk of these consequences.

One study of over 9,000 patients found that clinically important hypotension occurred in 42% of the patients. This hypotension was significantly associated with 30-day myocardial infarction and mortality in each of three periods monitored: (1) intraoperative, (2) remaining day of surgery and (3) during the initial four postoperative days.[3].

Another multicenter review of more than 300,000 patients discovered that prolonged exposure (defined as 10 minutes) to hypotension with a mean arterial pressure (MAP) of less than 80 mm Hg, as well as shorter durations at less than 70 mm Hg, were associated with mildly elevated risks of any end-organ injury[4].

This review also found that increased durations for MAP <65-60 mm Hg or for any exposure <55-50 mm Hg were associated with moderately or highly elevated risks of any end-organ injury.

Therefore, we must look at options to avoid hypotension due to anesthetic exposure, especially prolonged durations of low mean arterial pressure, in order to prevent the deleterious sequela that can result.

Reducing the Risk of Hypotension by Avoiding Unnecessarily Deep Anesthesia

Professor Katie Leslie recently delivered a TopMed Talk on her 10 year study delving into depth of anesthesia and long-term outcomes. Dr. Leslie is an Honorary Professor at Monash University, Melbourne, VIC, Australia, Specialist Anaesthetist, The Royal Melbourne Hospital & North Western Mental Health and Non-Executive Director for the Australian Medical Council.

This Balanced Anesthesia Study was able to define a broad range over which anesthesia may be safely delivered in older, comorbid patients. The study findings support the safety of anesthetic titration using processed EEG monitoring, such as entropy, in order to avoid risks such as hypotension[5].

In fact, the study showed that with volatile anesthetics there was a wide range of MAC values with adequate (EEG -based) depth of anesthesia, and that even in the case of “light” anesthesia, no patient awareness or additional adverse events resulted.

The results of this decade long research back up previous study findings including:

  • The results of a 2008 study which found that “entropy monitoring provides more reliable hemodynamic control, and fewer occurrences of hemodynamic fluctuations”.[6]
  • A 2013 trial which discovered that “episode of hypotension occurred in three patients in the control group, whereas no episodes of hypotension occurred in the study (entropy-guided anesthesia) group”.[7]
  • A study by Gruenewald et al. that found that propofol-remifentanil Entropy-guided anesthesia may lead to a lower frequency of hemodynamic unwanted events such as hypertension/hypotension, tachycardia and bradycardia.[8]

Individualized Anesthesia Care to Improve Patient Outcomes

All of the above studies demonstrate the hypotensive dangers of overdosing intraoperative hypnotics, as well as the benefits that can be achieved through monitoring the depth of anesthesia via entropy. Since different patients react differently to similar doses of anesthetic agents, the direct view into the state of the brain provided by EEG monitoring is a valuable approach for optimizing patient wellbeing and safety. While research into depth of anesthesia is ongoing, it is clear that individualized anesthesia is necessary to reduce drug usage, avoid unwanted events, and improve patient outcomes.


[1] Takashi Akata, David C. Warltier; General Anesthetics and Vascular Smooth Muscle: Direct Actions of General Anesthetics on Cellular Mechanisms Regulating Vascular Tone. Anesthesiology 2007; 106:365–391 doi:

[2] Musialowiczet al., 2014, Current Status of EEG-Based Depth-of-Consciousness Monitoring During General Anesthesia

[3] Sessler et al., 2018, Period-dependent Associations between Hypotension during and for Four Days after Noncardiac Surgery and a Composite of Myocardial Infarction and Death: A Substudy of the POISE-2 Trial

[4] Wesselink et al., 2018, Intraoperative hypotension and the risk of postoperative adverse outcomes: a systematic review

[5] Short TG, Leslie K, Chan MT, Campbell D, Frampton C, Myles P. Rationale and Design of the Balanced Anesthesia Study: A Prospective Randomized Clinical Trial of Two Levels of Anesthetic Depth on Patient Outcome After Major Surgery. Anesth Analg. 2015 Aug;121(2):357-65. doi: 10.1213/ANE.0000000000000797. PMID: 25993386.

[6] Wuet al, 2008, Use of Spectral Entropy Monitoring in Reducing the Quantity of Sevoflurane as Sole Inhalational Anesthetic and in Decreasing the Need for Antihypertensive Drugs in Total Knee Replacement Surgery

[7] El Hor et al., 2013, Impact of entropy monitoring on volatile anesthetic uptake

[8] 1. M-Entropy guidance vs Standard Practice during propofol-remifentanil anaesthesia: a randomised controlled trial. Gruenewald et al. Anaesthesia 2007 Dec; 62(12): 1224-9

  • Neurology
  • Entropy