The Effect of Thyroid Lobe Volume on the Common Carotid Artery Blood Flow in Thyroidectomy Position

Abstract

Background/Objectives: This study investigates the effect of thyroid lobe size on common carotid artery hemodynamics during thyroidectomy. While prior research has reported reduced carotid blood flow during the procedure, the impact of thyroid size remains unclear. We hypothesized that larger thyroid lobes may influence carotid flow dynamics via external compression. Methods: Adult patients undergoing elective thyroidectomy were prospectively included. Doppler ultrasonography measured carotid artery diameters and flow characteristics at three time points: before anesthesia induction, after induction, and after surgical positioning. Regional cerebral oximetry was recorded. Each carotid artery was analyzed separately. Results: Data from 202 carotid arteries (132 patients) were analyzed. Baseline carotid diameters and flow velocities were similar between patients with normal and large thyroid lobes. Anesthesia induction reduced flow velocities in all patients. After surgical positioning, patients with large thyroid lobes had significantly increased peak systolic velocity, leading to an overestimation of carotid blood flow, when using formula-based calculations. Manually traced Velocity Time Integral confirmed the increase in peak systolic velocity and a shortened systolic/diastolic ratio in these patients. Receiver operating characteristic analysis identified a thyroid lobe volume cutoff of 19.7 mL (AUC: 0.93, Sensitivity: 85%, Specificity: 98%). Regional cerebral oxygen saturation remained unchanged (p > 0.05). Conclusions: Larger thyroid lobes are associated with altered carotid flow dynamics during thyroidectomy, emphasizing diastolic flow. While these findings provide insight into thyroid-related hemodynamic changes, their applicability to patients with pre-existing carotid stenosis or peripheral artery disease remains uncertain, as our study population did not include such cases.

Keywords: Doppler ultrasonography; cerebral oxygenation; common carotid artery; common carotid artery blood flow; thyroid volume.

Figure 1

Timeline of data collection.

Figure 2

Consort diagram illustrating participant recruitment, exclusions, and study completion.

Figure 3

Example of velocity time integral (VTI) trace analysis with systolic and diastolic time annotations. The top panel illustrates a trace with a systolic/diastolic flow time ratio of 0.32 at a heart rate of 84 beats per minute, while the bottom panel shows a trace with a lower ratio of 0.15 at a heart rate of 61 beats per minute. Systolic (S) and diastolic (D) phases are labeled for clarity, and the corresponding time intervals are highlighted. These traces demonstrate how systolic and diastolic flow times are measured to calculate the systolic/diastolic flow time ratio used in the study.

Figure 4

Receiver operating characteristic (ROC) curve for determining the optimal thyroid lobe volume cutoff to identify a systolic/diastolic flow time ratio <0.3. The ROC analysis yielded an optimal thyroid lobe volume cutoff of 19.7 mL, with an area under the curve (AUC) of 0.93. At this threshold, the sensitivity was 81.82%, specificity was 96.88%, and overall accuracy was 92.14%. The curve demonstrates the trade-off between sensitivity and specificity, with the optimal point marked on the curve.

Figure 5

Boxplots show common carotid artery systolic diameter, diastolic diameter, peak systolic velocity (PSV), end-diastolic velocity (EDV), and mean velocity (MV) across different time points (T0, T1, T2) in patients with thyroid lobes ≤19.7 mL and >19.7 mL. Statistical comparisons within groups (p-values) are displayed, with Kruskal–Wallis tests used for overall group comparisons. Error bars indicate interquartile ranges. Large interquartile ranges in PSV, EDV, and MV at T2 primarily reflect inter-patient variability, with additional influence from a single extreme outlier per parameter.

Figure 6

Formula-derived and VTI-derived carotid blood flow at three time points during thyroidectomy (T0: baseline, T1: post-anesthesia induction, T2: post-surgical positioning), stratified by thyroid lobe volume (>19.7 mL vs. ≤19.7 mL). The left panel shows carotid blood flow (mL/min) calculated using PSV, EDV, and heart rate, which increased significantly after surgical positioning in patients with large thyroid lobes. The right panel illustrates carotid blood flow measured using VTI tracing. Formula-based calculations suggested an increase in blood flow due to elevated peak systolic velocity (PSV) in the large thyroid group, but VTI-derived measurements confirmed no significant difference in total blood flow, as systolic duration was significantly shorter in these patients. Statistical comparisons are indicated for each time point, with overall Kruskal–Wallis test results provided at the top. Error bars represent interquartile ranges, and group stratification illustrates differences based on thyroid lobe volume.