Published on 07/02/2015 by admin
Filed under Anesthesiology
Last modified 22/04/2025
This article have been viewed 1102 times
Prith Peiris, MD
Although thyroid operations are often viewed as routine procedures, they can present a unique combination of problems for the anesthesia provider. For example, difficulties securing the airway in the presence of a large goiter and surgical trauma to the recurrent laryngeal nerves (RLN) may cause dysphonia and stridor after extubation. The presence of coexisting thyroid hyperfunction or hypofunction, particularly when poorly controlled, can impact morbidity and mortality. Anesthesia can precipitate thyroid storm in patients with hyperfunction of the thyroid; hypofunction, especially of unknown severity, can present as multisystem clinical challenges during both the intraoperative and postoperative periods. Close cooperation between the anesthesia provider and the surgeon is imperative in achieving optimal outcomes in patients undergoing thyroid operations.
Thyroid operations have been successfully completed under local, regional, and intravenous anesthesia; however, most thyroid operations require general inhalation anesthesia. For these patients, a reinforced wire spiral tracheal tube may be necessary to maintain airway patency. (Nerve integrity monitor [NIM] tracheal tubes, which will be discussed in the section Preserving and Assessing the Function of the RLN, are reinforced.)
Intubation issues and potential difficulty in securing the airway in patients with goiters should be anticipated. Positional dyspnea and hypotension may suggest that the patient’s airway or great vein is compressed by a goiter. If the results of the preoperative examination and a review of the patient’s medical records, including the surgeon’s outpatient examination records, suggest that intubation may be difficult in any patient undergoing a thyroid operation, the results of ultrasonography or computerized tomography may assist the anesthesia provider in deciding whether a surgical airway or an awake intubation would be the safer option. In patients with goiters, a radiograph of the chest may reveal any tracheal deviation and airway collapse, but a computerized tomographic scan is better for assessing retrosternal extension, tracheal ring compression, and airway tortuosity.
The overall incidence of RLN damage during thyroid operations is 2% to 5%. Surgical identification and preservation of the RLNs remains the gold standard for protecting the RLN, but real-time intraoperative monitoring of RLN function may also help to reduce the incidence of damage.
To intraoperatively monitor RLN function, the anesthesia provider uses a tracheal tube with an embedded NIM to intubate the trachea under direct vision. When this tube is properly aligned, the electrodes in the NIM come into contact with both vocal cords. The anesthesia provider verifies the integrity of contact using a small stimulating current. The device provides both an audible alarm and a visual display of the action potential on a monitor whenever the RLN is stimulated. Studies have shown that such monitors detect RLN stimulation more than 70% of the time, making this technique useful in those cases in which difficulty is anticipated, such as in patients who are undergoing repeat operations or operations to excise cancer. During minimally invasive operations, monitoring of the external branch of the superior laryngeal nerve may serve as a surrogate to monitoring the RLN.
At the end of the operation, direct laryngoscopy not only will define vocal cord movement, but also may help identify factors that may cause stridor, such as glottic edema. Unilateral RLN paralysis, which occurs more often than bilateral paralysis, results in a midline ipsilateral cord on inspiration. Bilateral RLN paralysis, in which both cords are midline and is associated aphonia and stridor, requires reintubation.
Postoperative hypocalcemia secondary to parathyroid gland removal or devascularization occurs less than 2% of the time but may occur with a technically difficult operation. It may present as laryngospasm but, unlike RLN paralysis, usually occurs 24 hours or more after the operation. Excessive bleeding is rare but should be recognized early because hemorrhage into a confined space could lead to early airway compromise.
Up to 20% of patients with hyperthyroidism may present with atrial fibrillation as the first sign. Patients with postoperative agitation, restlessness, and tachycardia may not have anxiety or pain but, instead, impending thyroid crisis. Awareness of the possibility of thyroid dysfunction is the key to diagnosis and management.
In patients with any thyroid dysfunction, the goal prior to elective operations is attaining a euthyroid state. In the past, surgery was usually postponed in patients with all but mild cases of hypothyroidism, but it now appears that there may not be a difference in outcome postoperatively even in patients with moderate disease. However, the anesthesia provider should always remain vigilant because the clinical expression of hypothyroidism shows much individual variation. Potential problem areas include increased aspiration risk, increased sensitivity to opioids and anesthetic agents, hypoglycemia, hypothermia, and intraoperative hypotension. Thyroxine (T4) has a half-life of approximately 7 days, and triiodothyronine (T3), of 1.5 days; therefore, even if both are administered preoperatively for the treatment of severe myxedema, the clinical benefit may be delayed. The cardiac effect of administered T3/T4 may have an onset faster than that of its other actions. Whether this is a benefit or increases the risk of the patient developing arrhythmias and acute coronary syndrome is uncertain. The risk of preoperative treatment in severe cases needs to be weighed on an individual basis and in coordination with advice from an endocrinologist and intensivist. These patients are at risk for also developing adrenal insufficiency and, therefore, should be given prophylactic steroids intraoperatively—100 mg of hydrocortisone or its equivalent every 8 h.
In patients with hyperthyroidism, the thyroid gland produces excess hormone, the effects of which may be subclinical or overt. In thyrotoxicosis, as the name suggests, there is excess circulating hormone, resulting in major end-organ effects. For example, patients with normal coronary vasculature may have angina pectoris. β-Blockade will relieve the tremor, palpitations, and anxiety associated with hyperthyroidism but none of the metabolic manifestations. In the presence of moderate to severe hyperthyroidism, surgery should be postponed until the patient achieves a euthyroid state, usually a period between 3 and 6 weeks. The thioamide drugs, methimazole and propylthiouracil, are the primary drugs used to treat thyrotoxicosis, but their action stops when the drug is not taken, so patient compliance should be reviewed before surgery. Pharmacologic treatment involves the use of thioamides that block the synthesis but not the release of stored hormone, stores of which may be significant in goitrous disease. In patients with Graves disease, the presence of exophthalmos requires careful eye care with lubricant and taping to avoid corneal-conjunctival desiccation and damage.
Thyroid storm is a clinical diagnosis in which manifestations of preexisting hyperthyroidism are exacerbated into a life-threatening condition by the sudden release of T3 and T4. It can be induced by pregnancy, surgery, trauma, or severe illness and can take up to 24 h to develop. A chemical euthyroid state lessens the chance of a storm occurring but does not prevent it. Thyroid storm that initially manifests in the operating room may be confused with malignant hyperthermia, neuroleptic malignant syndrome, or the signs manifested by a pheochromocytoma. The signs and symptoms of a thyroid storm include confusion, high fever, tachycardia, metabolic acidosis, and congestive cardiac failure.
Once the diagnosis is made, the goal is to treat the underlying cause and to provide simultaneous supportive therapy. The patient’s body should be cooled with ice packs and intravenously infused cold fluids. Propranolol has been the β-adrenergic receptor blocking agent of choice for heart rate control and inhibition of peripheral T4 to T3 conversion; however, intravenously administered esmolol titrated up to 300 μg·kg−1·min−1 has been shown to be effective. Vasopressors for hypotension and inotropes for heart failure may be indicated, and magnesium has been shown to be helpful in reducing catecholamine-induced arrhythmias provoked by thyroxine. Adrenal hypofunction may coexist, as in hypothyroidism; therefore, it is prudent to intravenously administer hydrocortisone. The main role of agents that inhibit hormone secretion and block their peripheral action is in their use as a part of a preoperative plan. Carbimazole, methimazole, and propylthiouracil have been used successfully in this capacity. In the operating room, their use is better postponed until after discussion with an endocrinologist. Patients with thyroid storm, which carries a significant mortality risk, should be monitored in the intensive care unit postoperatively until their condition stabilizes.
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