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Year : 2017  |  Volume : 10  |  Issue : 1  |  Page : 91-96

Controlled hypotension for functional endoscopic sinus surgery: a Comparative study between magnesium sulfate and nitroglycerin

1 Department of Anesthesia, Ain-Shams University, Cairo, Egypt
2 Department of Anesthesia and Intensive Care, Mansoura University, Mansoura, Egypt

Date of Web Publication3-Aug-2018

Correspondence Address:
Randa Ali Shoukry
Department of Anesthesia, Ain-Shams University, 8 Ismail Ghanim St., AlNozha, Cairo, 1184
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/asja.asja_46_16

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Background Intraoperative bleeding during functional endoscopic sinus surgery (FESS) leads to poor visibility of the surgical field, and is of major concern. Controlled hypotension, using a variety of pharmacological agents, during general anesthesia reduces blood loss and helps clear visibility of the surgical field during FESS. The aim of this study was to compare the surgical conditions for FESS during controlled hypotension provided by nitroglycerin (NTG) versus magnesium sulfate (MGS) under general anesthesia.
Patients and methods Fifty adult patients of both sexes requiring FESS under general anesthesia were randomly divided to receive either NTG infusion of 3–5 µg/kg/min (group NTG, n=25) or MGS (group MGS, n=25) 30 mg/kg, administered as a slow intravenous bolus and 10 mg/kg/h by continuous infusion during the operation, to provide controlled hypotension. In both the groups, the mean arterial blood pressure was reduced until the targeted mean arterial blood pressure (55–65 mmHg) was achieved.
Results Both drugs produced the desired hypotension, in the magnesium group there were better operative conditions, reduction in the duration of surgery (85.3±13.1 vs. 92.9±10.3 min) and reduced anesthetic requirements [average end-tidal sevoflurane concentration (vol %) and fentanyl consumption (μg); P<0.0001]. However, there was longer extubation time (10.0±2.9 vs. 5.5±2.3 min) and recovery time (16.7±4.4 vs. 9.8±2.3 min) in comparison with the NTG group. Heart rate values were significantly higher in the NTG group compared with the magnesium group (P<0.0004).
Conclusion Both NTG and MGS can be used safely to provide controlled hypotension during FESS. However, MGS was better as it provided optimum surgical condition and less tachycardia. In addition, it led to decreased anesthetic requirements.

Keywords: controlled hypotension, functional endoscopic sinus surgery, magnesium sulfate, nitroglycerin

How to cite this article:
Shoukry RA, Mahmoud AE. Controlled hypotension for functional endoscopic sinus surgery: a Comparative study between magnesium sulfate and nitroglycerin. Ain-Shams J Anaesthesiol 2017;10:91-6

How to cite this URL:
Shoukry RA, Mahmoud AE. Controlled hypotension for functional endoscopic sinus surgery: a Comparative study between magnesium sulfate and nitroglycerin. Ain-Shams J Anaesthesiol [serial online] 2017 [cited 2023 Dec 5];10:91-6. Available from:

  Introduction Top

Chronic sinusitis is a common disease affecting any age group and is defined as inflammation of the mucous membrane in the paranasal sinuses and fluid within the sinus cavity that lasts for more than 12 weeks. The main treatments for chronic sinusitis are antibiotics and topical nasal steroids and decongestants. If these methods are not successful, then sinus surgery is usually considered [1],[2].

Functional endoscopic sinus surgery (FESS) is the presently accepted surgical intervention for the treatment of refractory chronic sinusitis. Though relatively safe, there is possibility for both minor and major complications, including cerebrospinal fluid leak, orbital or intracranial injury, meningitis, synechiae formation, and bleeding. Minor complications have been noted to happen in less than 4%, but major complications occurred in ∼1% of cases [3],[4]. Intraoperative bleeding during FESS lessens visibility in the operative field which may lead to the occurrence of complications [5].

Controlled hypotension can reduce blood loss in many surgical procedures, including orthopedic surgery, maxillofacial surgery, and neurosurgery [6],[7]. Controlled hypotension in general anesthesia aims to lower the mean arterial blood pressure (MABP) to values between 50 and 65 mmHg in normotensive patients, with the goal of considerably reducing blood loss [8]. Controlled hypotension can decrease blood loss in FESS by 80–141 ml [8],[9]. However, deliberate hypotension has its potential complications, which include delayed recovery, cerebral thrombosis, brain ischemia, permanent cerebral damage, and death [10]. Many hypotensive medications have been used, such as β-blockers, magnesium sulfate (MGS), clonidine, and sodium nitroprusside [5],[11],[12] for attaining the desired hypotension.

MGS is a noncompetitive antagonist of N-methyl-d-aspartate receptors that has ananalgesic effect and is important for the release of acetylcholine from presynaptic terminals [13],[14] and as calcium channel blockers inhibit the entry of calcium into the cell. According to previous studies, magnesium can cause hypotension through a vasodilator effect. The vasodilator effect of this ion is caused by increased production of prostacyclin and inhibition of angiotensin-converting enzymes. So it seems that this product could be beneficial for lowering blood pressure during several surgical procedures [9],[15].

Nitroglycerin (NTG), a directly acting vasodilator, has been used to achieve induced hypotension as it has rapid onset, rapid offset, and titrability. However, it causes reflex tachycardia as well as venous congestion in the surgical site thus causing more blood loss [16].

To the best of our knowledge, there have been no studies comparing controlled hypotension by magnesium versus NTG for FESS. So, we decided to compare their efficacy in induced hypotension and likely hemodynamic effects in a prospective double-blind study in patients scheduled for FESS.

  Patients and methods Top

This work was performed in a randomized double-blind manner. The protocol was approved by the Institutional Medical Board, and the patients gave written informed consent. Fifty adult patients, American Society of Anaesthesiologists (ASA) physical status I and II, admitted for FESS under general anesthesia, were selected. The study was done at Ain-Shams University Hospitals during a 6-month period, from April to October 2015. Patients were allocated to one of the two parallel groups (NTG, n=25; MGS, n=25) intended for controlled hypotension. Patients with the following conditions were excluded from the study: hypermagnesemia, a known allergy to MGS or NTG, any degree of heart block, hypertension, cardiovascular or kidney disease, or endocrine or metabolic disease.

Upon arrival to the operating room, standard monitoring, including pulse oximetry, noninvasive blood pressure and five-lead ECG, was started. Establishing avenousline and administering 500 ml Ringer’s acetate solution were ensured before the induction of anesthesia. Patients were preoxygenated with 100% oxygen for 3 min. For induction of anesthesia, all patients in both groups received fentany l1.5 µg/kg and propofol 2.0 mg/kg, and cisatracurium 0.15 mg/kg was used for intubation and muscle relaxation. Anesthesia was maintained with sevoflurane.

After induction of anesthesia, a radial arterial line from the nondominant hand was inserted for continuous monitoring of MABP. All patients were mechanically ventilated, with a mixture of oxygen and air (the fraction of inspired oxygen was 0.5), at a tidal volume and respiratory rate adjusted to maintain an end-tidal concentration of carbon dioxide of between 30 and 40 mmHg.

Patients were randomized, by sealed envelopes technique, into two groups: the magnesium (MGS) group received MGS 30 mg/kg, in 100 ml normal saline, administered as a slow intravenous bolus over a 10 min period before the induction of anesthesia, and 10 mg/kg/h by continuous intravenous infusion during the operation. For the NTG group, normal saline 100 ml (placebo) was given as slow intravenous bolus before induction of anesthesia, NTG was infused at 3–5 µg/kg/min started after skin preparation. For all patients our target was to maintain a MABP of between 55 and 65 mmHg. If the MABP fell to below 50 mmHg, the infusion of either study drug should be stopped, a vasopressor should be given and the patient excluded from the study. If tachycardia [heart rate (HR)>100 bpm] occurred, sevoflurane concentration was increased by 0.5% and fentanyl 50 µg was given, fentanyl consumption was calculated as the fentanyl amount given after the induction dose.

MABP and HR were recorded at specific time intervals before induction of anesthesia (basal), after induction of anesthesia, and then 5, 15, 30, 45, and 60 min from the start of hypotensive infusion and 10 min later after discontinuing the hypotensive agent (end of surgery).

Both patient, surgeon, and the attending anesthetist who was assigned to record the patients’ parameters were blinded to the infused drugs. The randomization envelopes, the syringe pumps, and their code labels were prepared by an anesthetist independent in the study.

At the end of surgery, MgSO4 and NTG infusions were stopped, and muscle relaxant was reversed with neostigmine 0.04 mg/kg and atropine 0.02 mg/kg. The time between discontinuation of anesthesia and extubation (extubation time) was recorded in both groups. Patients were transferred to the postanesthesia care unit where modified Aldrete’s recovery score was obtained by an anesthetist blinded to the drug infused. Recovery time was defined as the period from extubation until patients reached a score of greater than or equal to 9, where the patients were ready to transfer to the ward.

In order to insure consistency, all patients were operated by the same surgeon who was asked about his assessment of the surgical field and recorded it using a six-point category scale (0–5: 0, no bleeding, almost bloodless field; 5, severe bleeding, constant suctioning is required) ([Table 1]).
Table 1 Category scale: assessment of intraoperative surgical field [5]

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Statistical methods

The required sample size has been calculated using the G*Power software version 3.1.9 (Universität Düsseldorf, Düsseldorf, Germany). The primary outcome measure is the proportion of patients with a surgical field assessment score of 2 or less. It has been estimated that a sample size of 25 patients in either group would have a power of 80% to detect a medium effect size (w) of 0.4 using a two-sided χ2-test and assuming a type I error of 0.05.

Data were analyzed using GraphPad Prism version 6 for Windows (GraphPad Software, La Jolla, California, USA). The D’Agostino–Pearson test was used to examine the normality of numerical data distribution. Normally distributed numerical variables were presented as mean±SD and intergroup differences were compared with the unpaired t-test. Categorical variables were presented as ratio or number (%) and between-group differences were compared using Fisher’s exact test. Ordinal data were compared using the χ2-test for trend. P value of less than 0.05 was considered statistically significant.

  Results Top

Fifty patients ASA physical status I and II (30 women) (20 men) undergoing FESS were included (NTG, n=25; MGS, n=25). All patients underwent the same type of surgery, performed by the same surgeon. No patients had to be excluded from the study as a result of severe hypotension that needs vasopressor or any other complications. Demographic variables (age, sex, ASA physical status, and BMI) were comparable in the MGS and NTG groups ([Table 2]).
Table 2 Characteristics of patients in the two study groups

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[Table 3] shows that the operative time was significantly lower in the MGS group compared with the NTG group; the P value is 0.027; the average end-tidal sevoflurane and fentanyl consumption were significantly lower in the MGS group compared with the NTG. However, extubation time and recovery time were significantly longer in the MGS group compared with the NTG group (P<0.0001).
Table 3 Operative time, anesthetic consumption, and extubation and recovery time in the two study groups

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Changes in MABP are presented in [Figure 1], MABP in both groups achieved the planned target values of the study (between 55 and 65 mmHg), values were comparable between the two groups at baseline, up to 30 min after starting hypotensive infusion, whereas the values were significantly higher in the NTG group compared with the MGS group at 30, 45, 60 min (P<0.0001, 0.004, 0.0005, respectively). At the end of surgery, specifically 10 min after discontinuing hypotensive infusion, MABP increased to 86.6±9.0 in the NTG group compared with 71.9±7.5 in the MGS group (P<0.0001).
Figure 1 Changes in mean arterial pressure in the two study groups. Markers represent the mean. Error bars represent the SD. MAP, mean arterial pressure; MgSO4, magnesium sulfate; NTG, nitroglycerin; pre-ind., preinduction; post-intub., postintubation. Data are represented as mean±SD.

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Statistically significant difference

HR variations are shown in [Figure 2], the baseline values showed no significant difference between the two groups, after anesthesia induction and intubation there was significant reduction in HR in the MGS group compared with the NTG group with a P value of 0.0004, subsequently HR values were significantly lower in MGS until the end of surgery.
Figure 2 Changes in heart rate in the two study groups. Markers represent the mean. Error bars represent the SD. MgSO4, magnesium sulfate; NTG, nitroglycerin; pre-ind., preinduction; post-intub., postintubation; bpm, beats per minute. Data are represented as mean±SD.

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[Table 4] represents the category scale for the assessment of intraoperative surgical field which was performed by the attending surgeon. There were more patients in the MGS group that had a surgical field assessment score of 1 and 2 compared with the NTG group, which means reduced operative field bleeding, so better operative conditions were achieved by MGS. The greatest surgeon satisfaction was achieved when he estimated a category scale of less than or equal to 2.
Table 4 Surgical field assessment score in the two study groups

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  Discussion Top

Hypotensive anesthesia is required by many surgeons during FESS. Our study evaluated NTG and MGS as a hypotensive anesthesia technique in endoscopic sinus surgery. In the magnesium group, an objectively better operative field, reduction in the duration of surgery, and reduced anesthetic requirements were noted. However, there was a longer extubation and emergence time in comparison with the NTG group. The possible mechanisms for decreasing the anesthetic requirements by MGS include antagonism of N-methyl-d-aspartate receptors in the CNS by magnesium [17] and reduction of catecholamine release as a result of sympathetic stimulation, thus reducing peripheral nociceptor sensitization and the stress response to surgery [17]. Additionally, the actions of magnesium at the neuromuscular junction include: reduction in acetylcholine release from motor nerve terminals, reduce the depolarizing action of acetylcholine at the end plate and diminution of muscle fiber membrane excitability [17],[18],[19] potentiating the effect of the muscle relaxant.

In the NTG group, reduction of MABP was accompanied by an increase in HR and rebound hypertension after stoppage of infusion. These results are in agreement with a study performed by Srivastava et al. [20], they compared NTG with esmolol, and they noted that when there is induced hypotension, it stimulates the release of endogenous catecholamines. NTG has direct action on vascular smooth muscles. This vasodilator effect produced more oozing at the surgical field. Reflex tachycardia may be a contributing factor to increased surgical bleeding by increasing the cardiac output [21].

Elsharnouby and Elsharnouby [9] used MgSO4 40 mg/kg intravenously over a period of 15 min before induction of anesthesia and 15 mg/kg/h by continuous infusion intraoperatively to induce hypotension. They noticed more episodes of severe hypotension using this dose of MgSO4. In our study, the magnesium group received MGS 30 mg/kg as a slow intravenous bolus in a 10 min period before the induction of anesthesia and 10 mg/kg/h by continuous intravenous infusion during the operation. This produced a steady and smooth reduction in MABP and reduced HR, with no episodes of severe hypotension that needs vasopressors, or significant bradycardia. Our finding was supported by studies conducted by many authors who used a similar dose of MgSO4. This dose led to significant reduction in anesthetic requirements [22],[23].

Invasive arterial pressure monitoring was used as a method of accurate and continuous monitoring of blood pressure. No patient had rebound hypertension when MGS infusion was stopped, in comparison to the NTG group in which an increase in blood pressure was noticed after discontinuing of infusion. This follows the use of arterial vasodilators as hypotensive anesthesia techniques. MGS was chosen to induce hypotension, as it is a hypotensive vasodilator with trivial myocardial depression [24],[25]. It leads to a dose-dependent depressant effect on cardiac contractility. This depressant effect on cardiac function is compensated by lowering the peripheral vascular resistance, thus maintaining cardiac pump function [26]. Magnesium did not have a deleterious effect on atrioventricular conduction time and surface ECG during 1 minimal alveolar concentration of sevoflurane [27]. Magnesium causes an increase in cerebral blood flow velocity [28], which would be useful in a hypotensive anesthesia technique. However, there are potential hazards of using magnesium, during anesthesia, as it has many sites of action. It potentiates the effects of opioids and neuromuscular blocking drugs, so can cause prolongation of emergence time [28].

Ryu et al. [29] compared magnesium with remifentanil for hypotensive anesthesia during middle ear surgery. They found that both drugs gave proper surgical conditions, and that patients in the magnesium group required lower concentration of sevoflurane and less postoperative rescue analgesics than in the remifentanil group. However, in contrary to this study, postanesthesia recovery duration was not different between the two groups, this may be explained by the usage of bispectral index to monitor anesthesia depth in Ryu study.

Many other studies used magnesium as a hypotensive agent. The role of magnesium, given before surgery, in controlling intraoperative hypertension has been investigated in hypertensive patients undergoing cataract surgery with local anesthesia [30], and was shown to reduce the intraoperative variability in arterial pressure. Magnesium also used to control BP during intubation in hypertensive patients in an optimal dose of 30 mg/kg [31]. In cardiac surgery, magnesium was as effective as nicardipine in controlling arterial pressure during cardiopulmonary bypass [32].

There is one limitation of the present study: magnesium is known to cause potentiation of the muscle relaxant and delayed recovery; peripheral nerve stimulation to monitor neuromuscular block or bispectral index to monitor depth of anesthesia were not used, instead we relied on measuring the extubation and recovery times, which was because our goal was to induce hypotension that will provide optimum surgical field conditions.

To conclude, controlled hypotension is vital during FESS for a clearer operative field. Both NTG and magnesium are effective and safe agents for this purpose. However, MGS was better as it provided optimum surgical field condition and less tachycardia. In addition, magnesium decreased anesthetic requirements; however, it led to longer emergence time. Further studies of this technique are warranted.

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Conflicts of interest

There are no conflicts of interest.

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  [Figure 1], [Figure 2]

  [Table 1], [Table 2], [Table 3], [Table 4]


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