Heart Rate Variability to Quantify General Anesthesia Depth

Anesthesia is a state of reversible unconsciousness induced by one or more pharmacological agents. The three main anesthesia components are analgesia (pain relief), amnesia (memory loss), and immobilization, including autonomic reflexes' inhibition for harmful stimuli.1 Drugs used to induce anesthesia, in general, have different effects on each of these three components mentioned above.

Anesthesia depth assessment ( DoA: from English, Depth of Anesthesia ) is essential for anesthetic practice. The DoA depends on two opposing factors: the amount of anesthetic agent administered to the patient and the stimuli produced during surgery, increasing the level of awareness, and the patient's nociception. The ideal level d the DoA involves the guaranteed unconsciousness and anti-nociception adequate during surgery without compromise the functions of vital organs. In other words, one of the main challenges for anesthetists is to identify the appropriate amount of anesthetic to be administered to avoid the unwanted effects of anesthetic shortages (leading, for example, to intraoperative awareness) or anesthetic overdose, which can cause severe problems during intra and postoperative periods. 2 Before using muscle relaxants, the appropriate level of DoA could be monitored by the absence of movements to painful stimuli; once, the surgical incision's lack of movements was a sure sign that the patient was not superficially anesthetized. However, with the muscle relaxants' adoption in the anesthetics protocols, other measures to ensure that anesthetic agents' concentrations were administered properly were necessary.

Becoming conscious during surgery is a major concern for patients and anesthesiologists. It is estimated that the incidence of consciousness during general anesthesia is low, around 0.04% to 0.3%. However, considering the high number of surgical interventions, accidental awareness during surgery represents thousands of cases worldwide. Therefore, despite being an old problem, awareness during anesthesia is still a very relevant issue in public health.

DoA inefficient may also lead to an imbalance nociception x anti-nociception intraoperatively, even without patient conscious pain. Nociceptive stimuli can substantially affect the physiological state, inducing, for example, tachycardia, hypertension, nausea, fainting, and, therefore, negatively influencing patients' postoperative period 1 Thus, careful and efficient monitoring of DoA is the key to minimizing both the possibility of accidental awareness in surgeries and the overdose of anesthetic agents. 3 However, despite the importance of monitoring d the DoA, it is not easy to quantify this parameter during general anesthesia, and several approaches have been proposed to accomplish this task.

Quantification of DoA:

The anesthetic depth could be identified by monitoring the sympathetic activity of the patient. However, many anesthetics substantially alter autonomic activity, both sympathetic and parasympathetic. The observation of autonomic responses such as sweating, tear formation, hypertension, tachycardia, and pupil dilation are also important indicators of adequate unconsciousness levels. However, these signals cannot guide the anesthetist, considering that the patient cannot react due to neuromuscular blockade. 3 On the other hand, some authors suggest monitoring the concentration of inspired and expired anesthetic gases as markers of DoA.4 However, to date, there is no standardized and universally accepted method for quantifying DoA, and the anesthesiologist's experience is still the most important and effective factor in determining DoA.

DoA's monitoring techniques have advanced considerably in recent years. The nature of changes in electroencephalographic patterns during the stages of anesthesia was studied in detail, opening up possibilities for monitoring DoA by identifying electroencephalogram (EEG) patterns with crucial clinical significance for anesthesia.5 The use of EEG devices made monitoring of DoA potentially easier and was recommended by NIH - Care Excellence.

Although the progress brought by EEG-based monitors to anesthetic practice is indisputable, they still have significant limitations: 1) the algorithms created to derive a single DoA index from the EEG are exclusive (owned by their creators) for each device; 2) The EEG obtained in these devices is limited, exclusively, to the activity of the frontal lobe, lacking the representation of deeper subcortical structures ; 3) EEG is recognized for being highly sensitive to a variety of sources of interference during monitoring; 4) it is high cost restricts the availability of such equipment. Several monitors have prices ranging from £ 4,687 to £ 10,285 for the initial purchase, followed by the acquisition of single-use sensors ranging from £ 0.56 to £ 14.08. (3) Also, recent studies have pointed to substantial divergences between the most common commercial displays at DoA, making the application of these electronic devices still controversial and not universally recommended. 4 Specific limitations to the use of BIS, such as the inconsistency of this index in some situations, have also been reported. 6 BIS's particular challenge involves the administration of ketamine, nitric oxide, and xenon, which do not produce the typical EEG patterns observed during general anesthesia with other anesthetic agents. Besides, as the BIS was created based on a database of volunteers under specific conditions, it must be revalidated whenever used with a new drug or even a unique patient population, whose characteristics differ from the original population database data.7