Principles of Anesthesia

Sestamibi accumulates in the thyroid and parathyroid tissues within minutes after IV administration, but it has a different washout rate from these two tissues. It is released faster from the thyroid than from the parathyroid. The presence of large numbers of mitochondria-rich cells in parathyroid adenomas is thought to be responsible for their slower release of ^99m Tc-sestamibi from hy- perfunctioning parathyroid tissue versus the adjacent thyroid tissue. Thus, on 2- to 3-hour washout images, after thyroid uptake has dissipated, the presence of a retained area of activity allows one to identify and localize a parathyroid adenoma. Overall, ^99m Tc-sestamibi parathyroid scintigraphy has good sensitivity for the detection and localization of a single adenoma in patients with primary hyperparathyroidism. Correlation with ultrasound findings can also be helpful.

Single Proton Emission Computed Tomography

SPECT scanning for parathyroid disease allows increased accuracy of routine sestamibi scanning by ⬃2 to 3%. SPECT scanning can be performed within the first several hours after a patient is injected with the sestamibi. During the scan, multiple images are taken of the patient’s head and neck. These images are assimilated to provide a three-dimensional picture. SPECT scanning is typically used when ordinary planar sestamibi scans are inconclusive.

Factors of Anesthesia

An ideal anesthetic strikes a balance between the following four essential factors, which in turn are influenced by independent patient risk factors, unique surgical requirements, and circumstances under which recovery is to occur.

● Amnesia/anxiolysis : management of preoperative anxiety and control of intraoperative awareness is a cornerstone of anesthetic care. Various oral, IV, and/or inhalational agents may be used, depending on the desired effect.

● Analgesia : multimodal therapy is the preferred method of achieving intra- and postoperative pain control with a combination of narcotics, nonsteroidal antiinflammatory drugs (NSAIDs), local anesthetics, and/or adjutants [e.g., gabapentin, Ketamine (Pfizer Pharmaceuticals, New York, NY), clonidine].

● Muscle relaxation : depolarizing and/or nondepolarizing muscle relax- ants facilitate optimal airway conditions for laryngoscopy and surgical manipulation.

● Antiemetics : postoperative nausea and vomiting (PONV) is a strongly undesirable complication in patients following head and neck surgery.

High-risk procedures include otology surgery, adenotonsillectomy, thyroidectomy, as well as head and neck cancer surgery. PONV influ- ences patient satisfaction, length of stay, recovery, and surgical outcome.

Multimodal and prophylactic therapy improves outcomes. Emesis may predispose development of wound hematoma, which can become an airway emergency in the presence of a neck wound.

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Phases of Anesthesia

● Preinduction : begins in the preoperative area and continues to the point where a patient is positioned on the operating room table with standard monitors (electrocardiogram [ECG], noninvasive blood pressure, pulse oximetry, capnography) and has been adequately preoxygenated.

● Induction : general anesthesia in patients with established IV access is initiated with rapid-acting parenteral drugs to facilitate airway control.

Otherwise, inhalational anesthetics are administered via a face-mask;

once an adequate depth of anesthesia is achieved, IV access is established and is followed by the placement of an endotracheal tube or supraglottic device (unless a very brief procedure is planned). For sedation cases, IV agents are titrated to effect while maintaining spontaneous ventilation.

For difficult airway situations, awake fiberoptic nasotracheal intubation or awake tracheotomy may be indicated to secure the airway, prior to induction of general anesthesia.

● Maintenance : global amnesia is maintained during surgery with inhalational agents and/or parenteral drug infusions. Surgical analgesia is maintained either with a well-established regional block, generous local anesthetic infiltration, intermittent opioid boluses, and/or IV narcotic infu- sion. Duration of required muscle paralysis depends on several factors, and may be just long enough to facilitate endotracheal tube placement. In some instances (e.g., cases involving extensive neuromonitoring), anesthesia may be maintained through the predominant or exclusive use of parenteral infusions (total intravenous anesthesia [TIVA]). TIVA is sometimes useful to reduce vasodilation and bleeding, i.e., in endoscopic sinus surgery.

● Emergence : extubation upon meeting criteria

Table 1.10 provides an overview of the key aspects of each phase of anesthesia.

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Stages of Anesthesia

● Stage 1 : the period between initial delivery of induction agents and loss of consciousness. Patients experience analgesia without amnesia and can often carry on a conversation during this time.

● Stage 2 : more commonly known as the “excitement stage,” follows loss of consciousness and is characterized by delirious, uninhibited, and often spastic activity. Cardiovascular and respiratory patterns are erratic.

There is also an increased risk for aspiration. The goal of any anesthetic induction is thus to minimize time spent in this stage.

Table 1.10 Key Areas in Each Phase of Anesthesia

Preinduction Induction Maintenance Emergence Airway assessment

Intravenous access Anxiolysis Aspiration precautions Positioning for safety and patient comfort

Baseline vital signs Preoxygenation

Adequate depth of anesthesia (to prevent awareness or laryngospasm) Aspiration precaution (cricoid pressure, rapid sequence induction) Positioning for optimal view of the airway upon direct laryngoscopy Securing the airway (confirmed by auscultation, inspection, and capnography)

Maintenance of end organ perfusion Adequate fluid administration Normothermia Appropriate anesthetic depth Procedural analgesia Facilitation of optimal surgical field (e.g., controlled hypotension)

Extubation upon meeting criteria (spontaneous ventilation, reversal of neuromuscular blockade, suctioning of airway)

● Stage 3 : also known as “surgical plane,” where procedural stimulation causes minimal, if any, cardiovascular and/or respiratory changes.

● Stage 4 : massive anesthetic overdose that causes severe depression of brainstem activity leading to respiratory and/or cardiovascular collapse.

This stage should never be reached as it may be lethal even with appropriate cardiovascular and/or respiratory support.

Further Reading

1.4 Anesthesia

1.4.1 Principles of Anesthesia

Further Reading

Factors of Anesthesia

Phases of Anesthesia

Stages of Anesthesia

Further Reading