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2 ANATOMY 2 lateral lobes connected by an isthmus, lie at the level C5-C7 Very vascular organ Surrounded by a sheath from pretracheal layer of deep fascia Closely attached to thyroid cartilage & to upper end of trachea – thus moves on swallowing Embryologically – originates from base of tongue & descends to middle of neck

3 Blood supply: I) superior thyroid artery ii) inferior thyroid artery iii) thyroidea ima

4 Normal function of thyroid gland – directed to secretion of T3 & T4 Insufficient hormone secretion – hypothyroidism /myxedema Excessive secretion – hyperthyroidism Hormone action:- - influence the growth & maturation of tissues - “ cell respiration & total energy expenditure - “ turnover of essentially all substrates, vitamins, & hormones

5 PHYSIOLOGY Recommended daily intake – 140ug The synthesis depends on: I) quantities of iodine ii) normal iodine metabolism in the gland iii) synthesis of thyroglobullin Dietary iodine – absorbed by GIT – converted to iodide ion – actively transported into thyroid gland Once inside – iodide is oxidized back to iodine, which is bound to tyrosine End results – triiodothyronine (T3) & thyroxine (T4)

6 T4 released more than T3, but T3 is more potent & < protein-bound Most T3 is formed peripherally from partial deiodination of T4 In plasma, >90% of T4 & T3 is bound to hormone-binding proteins Only free hormone available for tissue action

7 PHYSIOLOGY OF HYPOTHALAMIC- PITUITARY-THYROID AXIS 1- TRH released in hypothalamus – stimulates TSH release from pituitary 2- TSH stimulates TSH receptor in the thyroid, to  synthesis both T4, T3 & stored hormone  increased plasma levels of T4 & T3 3-  serum levels of T3 & T4 & conversion of T4 to T3 4- T3 & T4 will enter cells & bind to nuclear receptors & promote  metabolic & celular activity

8 Hypothalamus TRH Pituitary TSH Thyroid T4 T3 Thyroxine Tri-iodothyronine Peripheral Tissues Physiological Effects

9 Patients with thyroid disease can present for: i) surgery to the thyroid gland ii) Surgery to pituitary gland iii) Any incidental surgery

10 Problems in anaesthesia…. 1) Airway - tracheal compression/ deviation – difficult intubation - Infiltration by thyroid gland tumour - Tracheomalacia 2) Endocrine status - hyperthyroidism – thyroid crisis - Hypothyroidism -  sensitivity to anaesthetic agents with delayed recovery; poor tolerance to blood loss & other stresses

11 3) Surgery - head & neck surgery with  accessibility to airway - Injury to recurrent laryngeal nerve - Venous air embolism - Hypocalcaemia - Haematoma - hypothyroidism

12 HYPERTHYROIDISM Causes: - Grave’s disease, toxic multinodular goitre, thyroiditis, pituitary tumours, functioning thyroid adenomas, overdosage of thyroid replacement hormone Clinical manifestations: - weight loss, heat intolerance, muscle weakness, diarrhea, hyperactive reflexes, nervousness,fine tremor, exophthalmos, sinus tachycardia, atrial fibrillation, CCF Diagnosis:- abnormal TFT

13 1) Medical Tx - PTU, methimazole (inhibit hormone synthesis) - Potassium, sodium iodide (Prevent hormone release) - Propranolol (Mask signs of adrenergic overactivity) - Radioactive iodine 2) Surgery

14 Anaesthetic Considerations A) PREOPERATIVE - Postpone all elective cases till patient is rendered euthyroid with medical tx i) Airway - Determine ease of intubation - Compression Sx:- hoarseness of voice, stridor, dysphagia - Cervical x-ray – tracheal deviation / compression

15 ii) Cardiovascular system - heart rate & rhythm ( <85 bpm ), atrial fibrillation - Heart failure - Ischemic heart disease iii) Endocrine status - palpitations, tachycardia (awake & sleeping pulse) - Bruit over thyroid gland  Problems with CVS instability & thyroid storm  Latest thyroid function test

16 iv) Current treatment - continue medication & serve on morning of surgery v) Indirect laryngoscopy - ENT review on vocal cord function as a baseline finding Premedication - no premedication in pt with airway obstruction - Pt adequately sedated to prevent anxiety & apprehension ( BDZ / narcotic premedication ) - Emergency surgery – esmolol infusion - (50-150ug/kg/min)

17 B) INTRAOPERATIVE - Anaesthetic options: A- No difficulty anticipated: - usual iv induction & intubation (fentanyl, STP, non-depolarizing muscle relaxant B- possible difficulty in intubation: - iv induction, test ventilation when pt is unconscious, intubation +- suxamethonium C- definite intubation problem / evidence of airway obstruction - awake fibreoptic intubation - inhalational induction - choice of ETT- armoured ETT (< risk of kinking) important measures: - closely monitor pt’s CVS function & body temperature - eyes protection - to raise head of operating table 15-20 degrees to aid venous drainage (although  risk of venous air embolims)

18 - choice of anaesthetic agents: - induction agent – thiopentone - muscle relaxant – atracurium, vecuronium - volatile agent – isoflurane - narcotic analgesics – fentanyl, morphine - anaesthetic technique – balanced anaesthesia with N2O-O2- isoflurane-muscle relaxant-narcotic analgesics --- IPPV No controlled study has demonstrated clinical advantages of any anaesthetic drug over another – Miller * University of California (1968-1982)- all anaesthetic agents & techniques have been employed without adverse effects being even remotely attributable to agent / technique

19 Precautions: - avoid ketamine, pancuronium, indirect-acting adrenergic agonists & other drugs that stimulate the sympathetic nervous system - Prone to exaggerated hypotensive response on induction - Achieve adequate anaesthetic depth before laryngoscopy / any surgical stimulation - Administer neuromuscular blocking agent cautiously ( thyrotoxicosis a/w  incidence of MG & myopathies ) - Hyperthyroidism does not  anaesthetic requirements

20 Reversal: - uncomplicated cases: reverse & extubate as usual

21 C) POSTOPERATIVE Possible problems i) Thyroid crisis / storm - decompensated hyperthyroidism with excessive release of thyroid hormone - Onset – intraoperative / 6-24 hours after surgery - Sn & Sx:- hyperpyrexia, tachycardia or atrial fibrillation, hypotension, vomiting, dehydration, tachypnoea, acute abdominal pain simulating an acute abdomen, agitation, psychosis - May mimic malignant hyperthermia

22 Precipitants - infection, surgery, poorly prepared thyroid surgery, diabetic ketosis, radioiodine therapy in a poorly prepared pt, MI

23 - management: A) supportive B) medical Tx 1- investigate for precipitants – FBC, BUSE, blood glucose, FT4, FT3 2) hyperthyroidism: i- inhibition of thyroid hormone formation - PTU 900-1200mg/day orally / NG in 3-4 divided doses OR - carbimazole 60-120mg/day 3-4 divided doses orally / NG

24 ii) Inhibition of thyroid hormone release: - sodium iodide IV 1gm/24hr – slow infusion or - oral potassium iodide 100mg 6hrly - Given 1hr after 1 st dose PTU/carbimazole 3) Steroids - iv dexamethasone 2mg 6hrly - inhibits thyroid hormone release & peripheral conversion

25 4) Receptor blockade ( in the absence of HF) - Iv propranolol 1-2mg slowly 4-6hrly / oral propranolol 40- 80mg 6hrly 5) Cardiac failure - diuretics, digoxin, O2 +-propranolol if d/t uncontrolled AF with good LV function 5) Hyperpyrexia - fans, tepid sponge, PCM 6) Dehydration - IVD, CVP 7) Anticoagulation - heparin infusion in AF - Other pt – s/c heparin 5000U 2-3x dly 8) Severe agitation – chlorpromazine 150mg 8hrly PO / 25mg 8hrly IM 9) Exchange transfusion / PD/HD - If pt fails to improve within 24-48hrs

26 2) Airway obstruction Possible causes: - neck haematoma with tracheal compression - recurrent laryngeal nerve palsy - tracheomalacia - incomplete reversal - central depression

27 3) Tetany - clinical manifestations: circumoral tingling, paraesthesia, laryngeal spasm, (+)ve Chvostek & Trousseau signs - May result from respiratory alkalosis, d/t: - over-ventilation in immediate postoperative period - hypocalcemia from hypoparathyroidism Mx - calcium estimation - Slow injection of 10% calcium gluconate 10 mls IV

28 HYPOTHYROIDISM Causes - autoimmune disease, thyroidectomy, radioactive iodine, antithyroid medications, iodine deficiency, failure of hypothalamic-pituitary axis Clinical manifestations: - weight gain, cold intolerance, muscle fatigue, lethargy, constipation, hypoactive reflexes, depression, dull facial expression, -  HR, stroke volume, CO - Pleural, abdominal, pericardial effusion Dx: low free T4 level

29 Tx: - oral replacement therapy with a thyroid hormone preparation

30 Myxedema Coma - results from extreme hypothyroidism - Precipitated by – infection, surgery, trauma - C/f: - most pts are female, elderly - impaired mentation - hypoventilation - hypothermia - hypotension - bradycardia - comatose - hyporeflexia - hyponatremia

31 Management i) FT3, FT4, TSH, FBC, ii) Should start on clinical grounds iii) Thyroid hormone replacement - T4:- iv 200 mcg bolus, daily dose 100mcg till pt can take orally - T3:- iv/oral 10-20mcg bd till T4 can be given orally iv)steroids:- iv hydrocortisone 100mg stat, 50-100mg tds

32 v) ventilation: assisted ventilation if RF vi) hypothermia: - do not warm rapidly (>1C/hr)– CVS collapse - Blankets & close temperature monitoring vii) Hypotension viii) Hyponatremia - caused by dilution & redistribution - Fluid restriction ix) Tx of precipitating factors * Full recovery – replacement thyroxine dose titrated once / 2-3 weeks to maintain euthyroid state

33 A) PREOPERATIVE Severe hypothyroidism ( T4 <1mg/dL): Elective case – to correct first Emergency case – to treat with thyroid hormone prior to surgery Mild – moderate:- no absolute C/I

34 i- Airway ii- CVS Iii- endocrine status - coarse dry skin, slow mentation, cold intolerance, -  CO, hyporeflexia, hypoglycaemia  Increased sensitivity towards anaesthetic agents & central depressants  Hypotension & cardiac arrest following induction  Delayed recovery from GA

35 Premedication: Do not require much, prone to drug- induced respiratory depression Histamine H2 antagonists & metoclopramide – d/t slowed gastric emptying times

36 B- INTRAOPERATIVE - > susceptible to hypotensive effect of anaesthetic agents -  CO - blunted baroreceptor reflexes -  intravascular volume - induction agent of choice – ketamine - does not  MAC - Potential problems - hypoglycemia, anemia, hypoNa+ - difficult intubation d/t large tongue - hypothermia d/t low BMR

37 C) POSTOPERATIVE - delayed recovery – hypothermia, respiratory depression, slowed drug biotransformation - Should remain intubated till awake & close to normothermic - Postoperative pain relief – nonopiod (ketorolac)

38 References: i- Maged S. Mikhail: clinical anaesthesiology, Lange 2002 ii- Parveen Kumar: clinical medicine, W.B Saunders,1998 iii- Lee Choon Yee: manual of anaesthesia iv- Braunwald: Harrison’s principles of internal medicine, 1998 V- Soo Hua Huat: Handbook of medical emergencies

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