Carotid Artery Blood Flow Changes Associated with Head Positioning in Patients Undergoing Thyroidectomy.

Background and Aims: There are possibilities of insufficiency in blood flow through carotid arteries during head positioning in thyroid surgeries under general anesthesia which is usually compensated by collateral circulation in normal conditions. This compensation may be hampered in patients with congenital abnormalities or diseases such as atherosclerosis. We aimed to elucidate the changes in common carotid artery blood flow related to head positioning during thyroid surgery by Doppler examination.
Methods: In this observational prospective study, Doppler examination of both common carotid arteries including arterial diameter, peak systolic velocity, average velocity, and blood flow volume of forty patients who had undergone elective thyroidectomy under endotracheal anesthesia was done. Three sets of data (baseline, after induction, and after surgery) were collected and analyzed.
Results: There was a significant reduction in the diameter (P = 0.002) and the blood flow (P = 0.0001) in both carotid arteries and an increase in peak and mean velocity which was more pronounced immediately after head positioning and persisted till the end of the procedure. There was no correlation between the hemodynamic parameters with the carotid artery diameter, blood flow, and velocity. Conclusions: The head-and-neck positioning during thyroidectomy surgery reduces the blood flow through the carotid arteries which continued till the end of the procedure. Copyright:

INTRODUCTION

Thyroidectomy is one of the most common endocrine surgeries done worldwide. In thyroid surgeries, the positioning of the head is done for better surgical exposure and for the benefit of the procedure itself.[1] A shoulder roll is placed at the level of the acromion process of the scapula to help extend the neck. Due to the reduced neck muscle tone as a result of general anesthesia, the danger of hyperextension or overrotation of the neck is anticipated. The positioning of the head and neck during thyroid surgeries is performed with utmost care and caution in a step-by-step manner and in the presence of vigilant and skillful hands.

Carotid artery occlusions and dissections are reported when careful measures are not taken to avoid excessive head rotation or neck extension.[2] There are possibilities of insufficiency in blood flow through carotid arteries during head positioning which is usually compensated by collateral circulation in normal conditions. This compensation may be hampered in patients with congenital abnormalities or diseases such as atherosclerosis. Furthermore, sustained intraoperative hypotension may worsen the situation.

The contrasting outcomes shown in the studies which analyzed the association between the head positioning and the hemodynamics in the blood vessels of the head and neck[34] make this area more research worthy.[56] In the present study, we aimed to determine the changes in the common carotid artery blood flow on head positioning during thyroidectomy under general anesthesia by Doppler examination.

METHODS

The study was commenced after obtaining approval from the Institutional Ethics Committee (IEC/MES/54/2018 dated November 19, 2018). Written and informed consent was obtained from the patients who participated in the study. The study design was an observational prospective study.

A previous study by Saraçoğlu et al.[3] studied the effects of head rotation on carotid blood flow. Assuming 30% difference, at a two-sided Type 1 error of 0.05 and power of 90%, a sample size of 40 patients was arrived at. Forty adult patients of either gender between 18 and 50 years of age belonging to the American Society of Anesthesiologists (ASA) physical status I or II as outlined by the ASA, with normal levels of thyroid hormones coming for elective thyroidectomy requiring endotracheal anesthesia, were included in the study. Patients with anemia, uncontrolled hypertension, uncontrolled diabetes mellitus, metabolic disorders, vascular diseases such as atherosclerosis and stenosis, and intracranial pathologies were excluded from this study.

Preoperative assessment was performed by a senior resident in anesthesiology which included a detailed history, general physical examination, and systemic examination. Basic investigations were done which included complete blood count, blood grouping with Rh typing, random blood sugar, serum urea and creatinine, serum electrolytes, electrocardiography (ECG), chest X-ray, and an echocardiogram if indicated.

Patients were premedicated with oral diazepam (5 mg if they weighed <50 kg and 10 mg if they weighed more than 50 kg) on the night before and on the morning of surgery. The patients were kept nil per oral for both solids and liquids as per the ASA guidelines. After shifting the patients into the operating room, the following monitoring was established:

ECG

Noninvasive blood pressure

Pulse oximetry (SpO2).

Baseline values of heart rate, blood pressure, and SpO2 were recorded by the postgraduate performing the study (observer 1). An intravenous line was secured on the upper limb using an 18-gauge cannula. The carotid Doppler examination was done using a SonoSite M-Turbo ® ultrasound system with 13–6 MHz, HFL38 probe by the anesthesia consultant specialized in ultrasonography (observer 2). The patient was positioned supine with the head rotated 45° away from the side to be examined. The carotid measurements were carried out on the intraluminal vertical plane between the echogenic intimal layers of the common carotid artery 2 cm before its bifurcation on either side. The following Doppler data of the common carotid arteries were observed.

Arterial diameter (D)

Peak systolic velocity (PV)

Average velocity (TAMEAN, time-averaged mean velocity)

Blood flow volume (Vol).

After taking the above measurements, patients were premedicated with intravenous injections of midazolam (0.05 mg.kg−1) and glycopyrrolate (0.01 mg.kg−1). This was followed by preoxygenation with 100% oxygen for 3 min. Anesthesia was induced by the anesthesia consultant in charge of the case (observer 3) using fentanyl 2 μg.kg−1 (rounded off to the nearest multiple of 25 μg) and propofol 2.5 mg.kg−1 (rounded off to the nearest multiple of 10 mg). Patients were paralyzed with intravenous vecuronium bromide 0.08 mg/kg and tracheal intubation was performed 3 min later by observer 3. A shoulder roll was placed at the level of the acromion process of the scapula of the patient to help extend the neck. The head was adequately supported to avoid hyperextension of the neck.

The carotid Doppler examinations were repeated twice by observer 2.

After head positioning

At the end of the surgery before the thyroidectomy position was neutralized.

The Doppler data were recorded as mentioned earlier. Hemodynamic monitoring of the patient at regular intervals was done by observer 1.

The statistical analysis was performed using the SPSS 17.0 (Statistical Package for the Social Science Statistics for windows; Version 17.0, SPSS Inc., Chicago, Illinois, USA). The average, standard deviation, ratio, and frequency values were used for descriptive statistics; data distribution was examined using the Kolmogorov‒Smirnov test, and analysis of variance was used for the analysis of quantitative data. The correlation was analyzed with the Pearson correlation test. P < 0.05 will be considered statistically significant.

RESULTS

The data of 40 subjects were analyzed. Demographic data including the length of surgery and anesthesia are presented in Table 1. A decrease in mean arterial pressure and heart rate was observed compared to the baseline value which continued to the end of the operation [Table 2].

Table 1

Demographic data

Parameters	Minimum	Maximum	Mean (SD)
Age (years)	19	50	38.5 (8.178)
Weight (kg)	45	92	72 (10.8)
Height (m)	1.45	1.78	1.65 (0.08)
Body mass index (kg/m2)	20	34.21	26.37 (3.5)
Duration of anesthesia (min)	145	260	189.33 (21.95)
Duration of surgery (min)	100	215	137.25 (18.49)

SD=Standard deviation

Table 2

Hemodynamic data

Hemodynamics	Mean	SD	P (ANOVA)
MAP (mmHg)
Baseline	88.63	13.596	0.0001
Postinduction	70.43	8.212
End of surgery	80.48	13.097
HR (beat/min)
Baseline	89.95	11.858	0.0001
Postinduction	86.08	15.479
End of surgery	75.18	10.966

MAP=Mean arterial pressure, HR=Heart rate, SD=Standard deviation

The evaluation of the Doppler data revealed carotid diameter [Table 3] and volume of blood flow [Table 4] decreased significantly at the end of the operation compared to the baseline measurement. Whereas, both peak systolic [Figure 1] and average velocities [Figure 2] showed a slight, but significant raise (P < 0.05).

Table 3

Common carotid artery diameter in the preoperative period, postinduction, and postprocedure at the end of surgery

Diameter (cm)	Mean	SD	P (ANOVA)
Right
Baseline	0.712	0.058	0.002
Postinduction	0.665	0.064
End of surgery	0.694	0.055
Left
Baseline	0.703	0.057	0.002
Postinduction	0.654	0.07
End of surgery	0.686	0.059

SD=Standard deviation

Table 4

Volume of blood flow in the common carotid arteries in the preoperative period, postinduction, and postprocedure at the end of surgery

Volume of blood flow (mL/min)	Mean	SD	P (ANOVA)
Right
Baseline	646.293	108.261	0.0001
Postinduction	515.328	115.73
End of surgery	618.271	95.778
Left
Baseline	637.539	114.407	0.0001
Postinduction	495.289	95.716
End of surgery	596.576	107.164

SD=Standard deviation

Figure 1

Values of peak systolic velocity

Figure 2

Values of mean velocity

Mean arterial pressure and heart rate were not found to be correlated with the changes that occurred in the values of the diameter of the left and right common carotid arteries after induction and at the end of the surgery in comparison with the baseline values [Tables 5 and 6].

Table 5

The correlation between the changes in carotid artery diameters with blood pressure and heart rate at the preoperative period, postinduction, and at the end of surgery

Diameter (cm)	Correlation coefficient (r)	P
Baseline
MAP (mmHg)
Right	0.009	0.957
Left	0.073	0.653
HR (bpm)
Right	−0.083	0.612
Left	−0.063	0.701
Postinduction
MAP (mmHg)
Right	0.297	0.063
Left	−0.027	0.870
HR (bpm)
Right	0.170	0.217
Left	0.217	0.179
End of surgery
MAP (mmHg)
Right	−0.127	0.437
Left	−0.218	0.177
HR (bpm)
Right	−0.312	0.050
Left	−0.313	0.049

MAP=Mean arterial pressure, HR=Heart rate

Table 6

The correlation between the changes in blood flow volumes with blood pressure and heart rate at the preoperative period, postinduction, and at the end of surgery

Volume of blood flow (mL/min)	Correlation coefficient (r)	P
Baseline
MAP (mmHg)
Right	−0.048	0.770
Left	0.065	0.688
HR (bpm)
Right	−0.101	0.536
Left	−0.192	0.235
Postinduction
MAP (mmHg)
Right	0.321	0.043
Left	0.097	0.551
HR (bpm)
Right	−0.254	0.114
Left	0.041	0.802
End of surgery
MAP (mmHg)
Right	−0.169	0.298
Left	−0.106	0.515
HR (bpm)
Right	−0.311	0.050
Left	−0.252	0.117

MAP=Mean arterial pressure, HR=Heart rate

DISCUSSION

The present study was designed to examine the effects of head extension on the patients undergoing thyroidectomy operation. Here, we assessed the changes in the blood flow volume, carotid diameter, and mean velocities in the common carotid arteries on either side at different stages using Doppler ultrasound and found them to be adversely affected by the head positioning for thyroidectomy. After induction and positioning, a significant decrease was observed in these values compared to the baseline measurements. At the end of the surgery, the Doppler data of the carotid diameter and flow volume showed significantly decreased values. Whereas, both peak systolic and average velocities showed a slight, but significant raise. The results were similar to the study conducted in 28 patients at Istanbul School of Medicine by Ayten Saraçoğluetal who found out that, at the end of the operation, peak systolic velocity, average velocity, and blood flow volume of the common carotid artery decreased significantly compared to the baseline measurement (P < 0.001).[3]

No correlations were found between the changes in blood pressure, heart rate, and change in the carotid blood flow at the end of the surgery in comparison with the baseline values. This points toward the head positioning as the factor behind the decreased blood flow volumes rather than blood pressure changes.

The present study included forty adult patients of either gender between 18 and 50 years of age belonging to the ASA physical status I or II as outlined by the ASA, with normal levels of thyroid hormones coming for elective thyroidectomy requiring endotracheal anesthesia. They were free of systemic hypertension, hyperlipidemia, diabetes mellitus, cerebrovascular insufficiency, vertebrobasilar failure, metabolic diseases, or any intracranial pathology. The present study indicated that there was a 5% decrease (P < 0.05, which was significant enough) in carotid blood flow volume at the end of the surgery when compared to the baseline measurement. However, this magnitude of decrease in blood flow may lead to serious complications in elderly patients with hyperlipidemia, vertebrobasilar failure, or carotid plaques, so this should be taken into consideration.

Decreased carotid blood flow may lead to reduced cerebral blood flow and subsequently cerebral hypoxemia and ischemia.[4] This decrease has an exaggerated effect in cases of low cardiac output due to an unexpected intraoperative blood loss, or in an event of thromboembolism and in cases where both head extension and controlled hypotension are simultaneously applied again, if the patient has got low hemoglobin values, the already compromised oxygen-carrying capacity demanding higher arterial blood flow for adequate cerebral perfusion will further suffer.[37]

In elderly patients with atherosclerosis, carotid plaques, or stenosis, this fall in carotid blood flow following head positioning may further compound the deficiency of the cerebral perfusion.

Intravenous anesthetic agents such as thiopentone and propofol and volatile anesthetics have neuroprotective effects against the damage that may develop as a result of cerebral ischemia;[8] however, caution must be taken into consideration while giving anesthesia in high-risk surgical patients to protect the central nervous system,[9] especially when head positioning is done. Anesthesiologists should allow only a moderate neck extension in such high-risk patients. Further studies are needed on the choice of anesthesia or anesthetic agents in potentially high-risk cases where there is a requirement of intraoperative neck rotation or extension.

Limitations of the study include the restriction of the field of study to the flow changes within both common carotid arteries realizing the fact that the analysis is not complete with cerebral oxygenation. The usage of transcranial Doppler measurements[1011] of both anterior and posterior cerebral circulation gives a better assessment of cerebral blood flow. Furthermore, the postoperative cognitive functions of the patients were not assessed. Such studies could have given a better understanding of the potential clinical complications of cerebral blood flow deficiency.

CONCLUSIONS

The head-and-neck positioning during thyroidectomy surgery compromises the blood flow through the carotid arteries which continued till the end of the procedure irrespective of the hemodynamics.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.