Thyroid function during coronary surgery with and without cardiopulmonary bypass

Cardiopulmonary bypass (CPB) is associated with thyroid hormone changes consistent with euthyroid sick syndrome. Similar changes have been observed after general surgical operations. Thyroid hormone changes and their association with global oxygen consumption were studied in low-risk patients undergoing coronary artery bypass grafting (CABG) with and without CPB.

Fifty-two patients undergoing primary CABG by the same surgeon were randomised into either on-pump (ONCAB, n=26) or off-pump (OPCAB, n=26) groups. Thyroid-stimulating hormone (TSH), free thyroxine (fT4) and free triiodothyronine (fT3) levels were measured at sequential time-points using chemiluminescence assays. Global oxygen consumption was measured at sequential time-points using a continuous cardiac output Swan-Ganz catheter.

In both groups TSH and fT4 remained within normal range throughout the study. There was a similar and progressive decline in fT3 levels with no significant difference between the groups over time (p=0.42). Mean fT3 levels at 24h were below the normal range and significantly lower than baseline values (ONCAB, 3.3+/-0.69 pmol/L vs 5.1+/-0.41 pmol/L, p<0.001; OPCAB, 3.3+/-0.51 pmol/L vs 5.0+/-0.46 pmol/L, p<0.001). There was a significant inverse relationship between fT3 levels and global oxygen consumption.

Off-pump surgery is associated with thyroid hormone changes similar to conventional surgical revascularisation. The data suggest that further studies into T3 administration during OPCAB may be warranted.