Asian Cardiovasc Thorac Ann 2002;10:107-110
© 2002 Asia Publishing EXchange Pte Ltd
Coronary Bypass Surgery in Patients on Thyroxin Replacement Therapy
Aitizaz Uddin Syed, FRCS,
Ahmed F El Watidy, FRCS,
Akhlaque N Bhat, FRCS,
Ahmed Wahba, MBBS,
Reida M El Oakley, FRCS,
Imran Kiyani, MBBS,
Emad A Al Bukhari, FRCS,
Mohammed R Al Fagih, FRCS
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Prince Sultan Cardiac Center Armed Forces Hospital Riyadh, Saudi Arabia
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Aitizaz Uddin Syed, FRCS Tel: 1 513 636 4770 Fax: 1 513 636 3847 email: aitizaz{at}hotmail.com Department of Cardiothoracic Surgery, Children's Hospital Medical Center, 3333 Burnet Avenue, Cincinnati OH 45229-3039, USA.
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ABSTRACT
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The outcome of coronary bypass surgery was analyzed in 25 patients who were on thyroxin replacement therapy for chronic thyroid disorders at the time of operation. It was hypothesized that if such patients were given only their routine dose of thyroxin on the day of surgery, hemodynamic and cardiorespiratory recovery may be poor. All the patients on thyroxin replacement therapy were given their routine dose of thyroxin orally or via a nasogastric tube in the perioperative period. No supplemental dose was used. Based on preoperative levels of thyroid stimulating hormone, 68% of these patients were biochemically hypothyroid prior to surgery. Analysis of a large number of variables showed no difference in outcome against a control group who had no previous thyroid problems. We conclude that routine thyroxin administration is all that is required for a satisfactory outcome in patients undergoing coronary bypass surgery while on thyroxin replacement therapy.
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INTRODUCTION
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Patients with no previous thyroid problems are known to have depressed levels of total and free T3 (3,5,3'-triiodothyronine) with a concomitant rise in the level of reverse T3 (an inactive metabolite of thyroxin) in the first 24 hours after cardiopulmonary bypass.1 However, no studies have been reported dealing specifically with patients on long-term thyroxin replacement therapy (TRTx) who underwent coronary artery bypass grafting (CABG). Furthermore, the thyroid hormone status and regulation in these patients may not be at optimal levels at the time of surgery. Their hemodynamic and cardiorespiratory recovery after surgery was expected to be poor. In this study, we assessed the outcome of CABG in patients on TRTx.
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PATIENTS AND METHODS
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Between January 1985 and December 1998, a total of 3,047 patients underwent CABG at our center. Of these patients, 27 (0.9%) were found to be on long-term TRTx for chronic thyroid disorders of various causes at the time of surgery. Two of these patients were excluded from this study because of incomplete records. Based on their preoperative thyroid stimulating hormone (TSH) levels, these patients were divided into 2 groups: "euthyroid" group 1 (n = 8) and "hypothyroid" group 2 (n = 17). We compared the outcome of CABG in these 2 groups with group 3 (n = 20) consisting of randomly selected control patients with no previous thyroid problems.
Thyroid function tests were done preoperatively in all the patients on TRTx. All of them were under the regular supervision of the endocrinology department and had no clinical signs of hypothyroidism at the time of presentation for CABG. The results of these tests were not always available at the time of operation. No formal thyroid studies were carried out in the control group.
All the patients on TRTx were given their routine replacement dose (25 to 200 µg) orally on the morning of the operation. No additional intravenous or oral supple-ments of thyroxin were given. On the first postoperative day, the patients had their routine dose of thyroxin orally or via a nasogastric tube.
A Swan-Ganz catheter was inserted to monitor arterial and central venous pressures in all the patients. Cardiac output was measured in triplicates at 6-hourly intervals in the first 48 hours after the operation. Four consultant cardiac surgeons using different techniques of myocardial protection operated on these patients over the years. Distal anastomoses were performed on an arrested heart. The bypass technique (alpha-stat strategy), the use of inotropes, and postoperative management were fairly standardized over the years.
All data were statistically analyzed using SPSS release 6.1 for Windows. Data are expressed as mean ± standard deviation. Analysis of variance (ANOVA) F test was used to compare the 3 groups. A post-hoc multiple comparison method, modified Bonferroni test, was used to compare between individual groups if F < 0.05.
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RESULTS
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All preoperative patient characteristics were comparable in the study groups and the control group (Table 1
). All the patients were clinically euthyroid before the operation. TSH levels above 3.7 mIU•L-1 signify thyroid hormone deficiency or undertreatment. Eight patients on TRTx were identified as biochemically euthyroid based on preoperative serum TSH levels (3.04 ± 0.23 mIU•L-1) and were placed in group 1. The other 17 patients were biochemically hypothyroid with very high preoperative TSH levels (25.9 ± 15.3 mIU•L-1) and were assigned to group 2. The difference in preoperative TSH levels was statistically significant (p < 0.001). We did not identify any drug therapy in the patients that would interfere with the interpretation of TSH levels. All the patients had normal preoperative serum levels of urea, creatinine, electrolytes, and albumin.
The choice of temperature, type, and mode of delivery of cardioplegia varied. The surgeons also differed in their choice of topical and systemic hypothermia (Table 2). All 3 groups were comparable in terms of the number of grafts, crossclamp time, and bypass time. The amount of blood transfused, ventilation time (intermittent positive pressure ventilation), length of stay in the intensive care unit, and hospital stay of the 3 groups were not significantly different (Table 3). All the patients were weaned off mechanical ventilation as planned. No episode of signifi-cant hemodynamic instability was encountered. Following the procedures of our unit, all the patients were started on dopamine infusion at 10 µg•kg-1•min-1 in the operating room, which was tapered off within the first 12 hours. Cardiac output measurements in the intensive care unit showed no event of low cardiac output (i.e., cardiac index < 2.2 L•m-1•m-2). All patients had adequate urine output without infusions of diuretics or mechanical renal support. There was no significant difference between the 3 groups in the incidence of postoperative pulmonary complications, and there were no wound problems.
No mortality was reported. The mean follow-up of patients on TRTx was 23 months (range, 1 month to 6 years). Only 2 patients (1 each from groups 1 and 3) showed symptoms of heart failure late in the follow-up (after 5 years). The etiology of the heart failure was moderate mitral regurgitation of ischemic nature in the group 1 patient and myocardial ischemia following graft stenosis in the control patient. Both were successfully managed by medical therapy.
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DISCUSSION
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Cardiopulmonary bypass is known to induce a 75% and 50% decrease in total and free T3 levels, respectively. There is a concomitant 300% increase in reverse T3 levels, while TSH levels remained normal. This physiological state of low levels of active thyroid metabolites (total T3 and free T3) is known to persist for at least 24 hours in all patients following cardiopulmonary bypass.1 A similar situation is noted in patients with other chronic illnesses, renal failure, cirrhosis, malignancy, multiple organ failure, and sepsis. In all these conditions, the hypothalamic-thyroid axis is normal. This phenomenon has been termed "euthyroid sick syndrome" and is considered to be a protective homeostatic response.2
Triiodothyronine (T3) administration after normothermic ischemic arrest in rabbit hearts brought quicker recovery of left ventricular peak pressures.3 It has also been shown to produce a better response to ß adrenergic stimulation and increased velocity of contraction in porcine cardio-myocytes after hypothermic cardioplegic arrest for 2 hours.4 Triiodothyronine administration in dogs with pacing-induced chronic cardiomyopathy produced at least modest positive inotropic and lusitropic effects.5 However, numerous clinical trials have so far failed to produce conclusive evidence of beneficial effects of thyroxin supplementation in patients undergoing CABG.6 Triiodo-thyronine infusion to maintain normal T3 levels in patients undergoing CABG did not alter the incidence of post-operative complications, duration of stay in the intensive care unit and in the hospital, and mortality rate.7 In a large randomized controlled trial on patients with impaired myocardial function (preoperative ejection fraction below 40%), there was no significant difference in postoperative cardiac index or systemic vascular resistance between the groups on triiodothyronine infusion or dopamine infusion and a placebo control group.8
It is infrequent for patients undergoing CABG to be on TRTx. These patients do not undergo routine biochemical assay of thyroid functions before surgery. Our results demonstrate that supplemental thyroxin (i.e., in a dose in excess of routine replacement) is not mandatory even in patients who presumably have chronic thyroid hormone regulation disorder. A surprising finding was elevated preoperative TSH levels in 68% of our patients who were on oral thyroxin and had no clinical signs of hypo-thyroidism. Despite this, their postoperative outcome was similar to that of the euthyroid patients.
Two studies published in the 1980s concluded that patients with mild to moderate hypothyroidism tolerated surgery and anesthesia well.9,10 However, the number of patients in those studies was in single digit, and there were no data on preoperative cardiovascular status, operative variables, and postoperative cardiorespiratory function. Theoretically, increased sensitivity to respiratory depressant drugs, altered neuromuscular excitability, impaired ventilatory response to arterial hypoxemia, and multiple alterations in the adrenergic nervous system in hypothyroid patients can prolong their stay in the intensive care unit and make them a higher risk for anesthesia.11 However, we did not encounter such problems in our study population. Similarly, although a higher incidence of heart failure was reported in hypothyroid patients undergoing CABG,12 none of our patients had low cardiac output in the immediate postoperative period and the incidence of heart failure in the intermediate term did not differ between the study groups and the control group.
Estimating the level of tissue receptor-bound thyroxin may provide the answer to the relative ease with which patients tolerate CABG despite preoperative biochemical hormone deficiency as noted from plasma assays. Bound thyroxin has not been studied in patients undergoing CABG.
In conclusion, patients on chronic TRTx can be maintained on their routine thyroxin dose during CABG, without the need for supplemental doses. In the immediate and midterm period after CABG, hemodynamic and cardio-respiratory recovery in these patients is comparable to that in other bypass patients.
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ABSTRACT
INTRODUCTION
