Hypotensive anesthesia in maxillofacial surgeries: Current concepts

Abstract

Hypotensive anesthesia is a commonly used technique that aims to reduce intraoperative blood loss, consequently improving surgical field visibility and the need for blood transfusions post-operatively. It is widely used in major maxillofacial surgeries with a high risk of intraoperative bleeding. The aim is to reduce the patient’s systolic blood pressure to 80 to 90 mm Hg and mean arterial pressure (MAP) by at least 30% of preoperative blood pressure or keeping a minimum MAP of 50–65 mmHg. Hypotensive anesthesia not only provides a bloodless surgical field but also helps to identify various anatomical structures. There are various pharmacological and non-pharmacological methods to induce hypotensive anesthesia. The decision to induce hypotensive anesthesia should be based according to the general condition of the patient, the extent of the surgery, and in coordination with the operating surgeon. The target blood pressure should be adjusted according to the patient’s preoperative status and coexisting illness. The risk of organ hypoperfusion should be kept in mind. Close intraoperative monitoring with optimal patient selection is important for good patient outcomes.

INTRODUCTION

One of the key components of balanced anesthesia is maintaining the perfusion of vital organs during surgery. Blood pressure is an important vital sign that determines organ perfusion and cardiac output. Hence, normotensive anesthesia is the routine standard practice. However, in some scenarios, hypotension is induced deliberately. This technique is known as hypotensive or controlled anesthesia.[1]

Initially proposed by Gardner in the 20^th century, the concept of hypotensive anesthesia has revolutionized anesthesia practice.[2] Hypotensive anesthesia is a commonly used technique that aims to reduce intraoperative blood loss, consequently improving surgical field visibility and the need for blood transfusions post-operatively.

This technique is used in many fields, like middle ear surgeries, oro-maxillary surgeries, spinal surgeries, orthopedic surgery, neuro surgeries, cardiovascular, and liver transplant surgery. It not only provides a bloodless surgical field but also helps to identify various anatomical structures.[3] The aim is to reduce the patient’s systolic blood pressure to 80 to 90 mm Hg and mean arterial pressure (MAP) by at least 30% of preoperative blood pressure or keeping a minimum MAP of 50–65 mmHg.[4] This article mainly focuses on hypotensive anesthesia in maxillofacial surgeries with major intraoperative bleeding.

Effect of hypotension on various organs

1. Lungs

Pulmonary Alveolar macrophages – These are phagocytes, which are located in the alveolar compartment of the lungs, protecting against inhaled pathogens and debris. They modulate inflammation by producing anti-inflammatory cytokines like interleukin-10 and promote blood flow in the lungs. They also release vasoactive mediators, nitric oxide, and prostaglandins which help to regulate blood flow and vascular tone in the lungs.

Pulmonary alveolar macrophage (PAM) reduces with the anti-Trendelenburg position or sitting position and increases dead alveolar space with the risk of hypoxia and hypercapnia.[5]

Pulmonary blood flow gravitates to the dependent areas, particularly in the head up position. Thus, non-dependent lungs are ventilated but not perfused adequately, hence increasing dead space. Further use of vasodilators to induce hypotension abolishes the hypoxic pulmonary vasoconstriction response, thus increasing intra-pulmonary shunt. All these factors combined, result in hypercarbia, an increase in arterial-end tidal carbon dioxide (etCO2) gradient and hypoxaemia. Therefore, it is important to have regular measurements of partial pressure of carbon dioxide (PaCO2) during controlled hypotension.[6]

For the same reason, the arterial oxygen tension (PaO2) may also be reduced during hypotension. Thus, a higher fraction of inspired oxygen (FiO2) may be necessary to maintain the oxygen saturation within normal limits, during hypotensive anesthesia.

CNS

Cerebral blood flow normally remains constant over a MAP range of 60 to 150 mm Hg. In patients with chronic hypertension, the autoregulation curve is shift to the right. While in patients with controlled blood pressure on antihypertensive treatment, the curve tends to shift back toward normal. It is recommended to keep the patient in normocapnia status [CO₂ partial pressure (PaCO₂) between 35–40 mmHg][7]

Renal

As per the recent studies, there is a strong relation between intraoperative hypotension and postoperative acute kidney injury (AKI). Moreover, autoregulation of renal blood flow is impaired during general anesthesia, and renal blood flow is compromised with even moderate decreases in arterial blood pressure. Hence, it is recommended to maintain MAP >65 mmhg.[8]

CVS

Increase myocardial oxygen demand due to any reason requires simultaneous increase in the coronary artery blood flow. This may be hampered by hypotensive anesthesia which may reduce coronary blood flow due to the decrease in afterload or/and preload.

Hence, hypotensive anesthesia is associated with significant intraoperative risk of myocardial infarction.[6]

Pros of hypotensive anesthesia

The main goal of hypotensive anesthesia is to decrease the MAP either by reducing the cardiac output or by reducing the systemic vascular resistance or both. However, inducing hypotension purely by decreasing cardiac output is not an ideal method, because the maintenance of adequate blood flow to organ is essential. Systemic vascular resistance can be decreased by peripheral vasodilatation of the resistance blood vessels.[9]

The maxillofacial region is highly delicate and vascular, which requires quite precise and accurate surgery. There is a risk of a generous amount of blood loss, which demands the instant availability of various blood products.[6] This is not only a concern in terms of availability issues but also the risk of transmission of infectious diseases and other complications associated with blood transfusion.[10] Besides this, uncontrolled bleeding in head and neck surgeries can lead to hematoma formation which can further compromise the airway.

Therefore, induced hypotension becomes important in such a scenario. It decreases the overall duration of surgery thus leading to improved quality of the surgical field[11] while avoiding major complications. The integrity of vital structures is maintained, and tissue regeneration and healing are also quick.

Cons of hypotensive anesthesia

Not every patient planned for surgery can tolerate hypotensive anesthesia; therefore, careful patient selection is a must. The risk of hypotension-associated hypoperfusion of vital organs like the brain, heart, and kidney is well known. Patients with ischemic heart disease, fixed cardiac output state, and vascular disease are more susceptible to this.[12] Moreover, patients with known hypertension have less tolerance for low blood pressure due to their higher set autoregulation point. In addition, they have contracted blood volume which can further exaggerate hypotension. Hence, modification of the hypotensive technique by keeping a higher safety margin along with adequate volume replacement should be performed in these patients.[1]

If the decrease in cerebral blood flow is more than the decrease in the cerebral metabolic requirement for oxygen, cerebral ischemia may develop. In normotensive adult patients, cerebral ischemia can arise at a MAP threshold of 40–50 mmHg in the orthostatic position and 45–55 mmHg in the supine position.[13]

Cerebral ischemia may be detected by the clinical availability of several non-invasive near-infrared spectroscopy (NIRS)-based cerebral oximetry devices that can give early warning of reduced oxygen delivery. Blasi et al.[14] conducted a study and found that remifentanil-based general anesthesia with propofol or sevoflurane changed the muscle microcirculation in different ways. These changes were measured by non-invasive quantitative NIRS effectively, which is a technique that considers the optical tissue properties of an individual.

Zhang conducted a randomized control trial in FESS patients and found that a 30% decrease from the baseline MAP leads to a decrease in the intraoperative blood loss, improved quality of the surgical field, and the shortening of the duration of surgery in the hypotensive group. However, the fall in regional cerebral oxygen saturation (monitored by NIRS) from the baseline was only ~ 5%, which was within safe levels, and no cerebral desaturation events were noted in the intervention group.[15]

Tang et al.[16] conducted a study in patients (<60 years age) undergoing noncardiac surgery to note the association of intraoperative hypotension with postoperative AKI and observed that there was a significant risk of postoperative AKI when intraoperative MAP was less than 55 mm Hg for more than 20 min.

Similarly, Walsh et al.[17] found a graded relationship between more than 5 min spent with a MAP of less than 55 mm Hg or a MAP of 55 mm Hg to 59 mm Hg and stage I AKI and myocardial injury.

In an observational study of 5,127 patients, Sun et al.[18] found that postoperative AKIN stage I AKI was associated with an intraoperative MAP of less than 55 mm Hg for more than 10 min and a MAP of less than 60 mm Hg for 11 min to 20 min.

A retrospective cohort analysis was performed by Salmasi et al.[19] to assess relationship between various absolute and relative characterizations of intraoperative hypotension and myocardial injury after noncardiac surgery (MINS) as well as AKI. They found that even a MAP of 65 mm Hg which is a clinically acceptable pressures correlated with both MINS and AKI. At lower pressures, the association was stronger, and even a low pressure for a short time was associated with poor outcomes.

The goal is to keep the target blood pressure moderately low to allow a reduction in bleeding without disturbing the microcirculatory autoregulation of the vital organs, i.e. heart, brain, or kidney.[20]

Contraindications of hypotensive anesthesia

Controlled hypotension is not safe to practice in certain conditions. Those can either be related to patient, drug, or anesthesia limitations.

Absolute contraindications

• include cerebrovascular disease with severe carotid stenosis,[21]

• symptomatic or severe aortic or mitral stenosis

• stage IV chronic kidney disease

• known drug allergy

• lack of experience or knowledge in the subject

Relative

• Cerebrovascular disease

• Uncontrolled Diabetes mellitus

• Severe hypertension

• Coronary artery disease

• Anemia, hemoglobinopathies, polycythaemia

• Hepatic disease

• Renal disease

• Severe lung disease

• Pregnancy

• Inadequate monitoring

• Hypovolemia

Monitoring and perioperative preparation

Hypotensive anesthesia demands meticulous monitoring of the patient. This includes arterial oxygen saturation, electrocardiography, end tidal carbon dioxide monitoring, temperature, urine output, and non-invasive or invasive measurement of blood pressure (depending on the procedure and patient’s comorbidities).

Before the induction of hypotension, a period of cardiovascular stability must be ensured by giving titrated dose of anesthetic drugs for induction and maintenance. Since non-invasive blood pressure is intermittent, episodes of hypotension may be missed. Hence, invasive blood pressure monitoring is necessary especially if a rapid-onset hypotensive agent is used. Radial artery pressure is usually preferred since it accurately measures central arterial pressure and is easy to cannulate.[22]

To ensure adequate perfusion of vital organs during hypotensive anesthesia, cardiac output (CO) monitoring can also be done if available. Few methods for CO monitoring are described below:

• Lithium dilution cardiac output (LiDCO): Where lithium chloride is injected and CO is calculated by measuring the concentration change over time.

• Flowtrac – It is a minimally invasive method to measure CO. It utilizes a proprietary algorithm to analyze the pulse pressure waveform obtained from an arterial catheter and provide real-time CO monitoring.[23]

The patient should be positioned carefully. The nasotracheal tube should be fixed gently with minimum pressure on the nares to avoid skin necrosis. Similarly, air cushion should be placed in pressure areas like sacrum to prevent pressure sores.

Pharmacological methods

There are various strategies for achieving hypotensive anesthesia. Newer techniques rely on the natural hypotensive effect of the anesthetic drugs. The ideal drug should be short-acting, dose-dependent, with minimal effect on vital organs, and quick excretion without any residual toxic metabolites.[24] This comprises mainly drugs which are either used alone or as an adjunct to limit dose requirement and side effects of other drugs.[25]

Inhalational anesthetic agents include isoflurane, sevoflurane, and desflurane which are used commonly. They are primarily vasodilators, thus decreasing systemic vascular resistance. However, volatile anesthetics when used alone in high doses for controlled hypotension can cause hepatic or renal injury and delayed recovery from anesthesia.[26]

Recent studies have combined inhalation agents with drugs like nitro-glycerine, adenosine, morphine, labetalol, and esmolol to reduce the concentration and side effects of each drug. A study conducted by Rossi et al.[27] shows that desflurane reduces blood loss in comparison with sevoflurane and contributes to a better surgical field. Overall, the use of inhalation anesthetic agents solely as hypotensive agent is controversial, and an adjuvant is required to induce truly controlled hypotension.

Propofol

It is one of the most commonly used intravenous anesthetic agent which acts by enhancing the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) through GABA type A (GABAA) receptors. Hypotension is a well-known side effect associated with the use of propofol which is often desirable in surgeries requiring deliberate hypotension. It has rapid onset and recovery, hence, mostly used as a part of total intravenous anesthesia (TIVA). Complete and smooth recovery without any residual effects makes it a good choice for day-care surgery.[28,29]

Opioids – Popularly used in anesthesia for their analgesic effects, few opioids can induce hypotension which may not be desirable routinely. However, they have been used successfully as adjuvant to other hypotensive agents where controlled hypotension is indicated. Out of the commonly available opioids like morphine, fentanyl, alfentanil, sufentanil, and remifentanil, the latter has gained popularity due to its rapid onset and long elimination half-life.[30]

Remifentanil is an ultra-short-acting mu receptor agonist. It has context sensitive half-life of about 5 min. It is used mostly as an adjuvant with propofol in TIVA. Degoute et al.[24] conducted a study using remifentanil and propofol in TIVA and found that they provided good surgical conditions and desired hypotension for tympanoplasty.

Vasodilators – like nitro-glycerine (NTG) and sodium nitroprusside rapidly control BP because of their short onset and duration of action. They produce nitric oxide which causes vascular smooth muscle relaxation. Among these, sodium nitroprusside for a long time has been a drug of choice for hypotensive anesthesia. However, side effects like tachyphylaxis and cyanide toxicity impose challenges in the administration of these drugs. Nitro-glycerine in comparison with sodium nitroprusside has a slightly slower action, but it does not cause myocardial ischemia, rebound hypertension, or any toxic metabolites.[31]

Betablocker – They have been used efficiently for reducing blood pressure during surgery either alone or in combination with other agents. Based on their duration of action and selectivity, many β-blockers are in clinical use.

Labetalol blocks both α and β receptors. α-receptors blockage leads to vasodilatation, of blood vessels, thus causing hypotension, while β-receptor blockage prevents reflex increase in heart rate. Studies have demonstrated better hemodynamic, decrease bleeding, and improved surgical visibility with the use of labetalol as compared to other agents. It is generally given in a dose of 0.25 mg/kg followed by continuous infusion of 0.5–2 mg/min which can be titrated up to 10 mg/min. It has a long elimination half-life of 5.5 hours which may cause delayed postoperative hypotension that maybe a cause of concern.[32]

Esmolol is a short-acting cardio-selective drug with antagonist action on the β-1 receptor. It has negative chronotropic and inotropic action, thus reducing CO, and heart rate, eventually decreasing blood pressure. It can be given intravenously as a bolus or infusion and the blood pressure returns to normal once the infusion is stopped. Dose is 1000 mcg/kg bolus over 30 seconds, followed by 150 mcg/kg per min infusion, with a maximum dose of 300 mcg/kg per min. Its ability to prevent reflex tachycardia and rebound hypertension makes it a good choice among β-blockers for hypotensive anesthesia. However, β-blockers need to be avoided in patients with a history of asthma due to the risk of bronchospasm associated with them.[33]

Dexmedetomidine, an adrenergic receptor (α-2-AR) agonist, offers a promising role in hypotensive anesthesia. This is due to its wide spectrum of properties which includes sedation, analgesia, anxiolysis, and sympatholytic action at both central and peripheral levels. It preserves cardiovascular and respiratory functions and lacks any major side effects. Besides this, it is easy to administer and quite predictable with other anesthetic agents.[34] Various studies have proven that dexmedetomidine decreases surgical bleeding while maintaining hemodynamic stability, thus providing desirable surgical field.[35,36]

This makes it a near-ideal agent for hypotensive anesthesia. However, it may cause bradycardia which may limit its use in certain circumstances. Postoperative sedation and dry mouth are a few side effects associated with the use of dexmedetomidine. It can be given in a loading dose 1 mcg/kg/I.V infusion over 10 min followed by a maintenance dose of 0–0.2 0.7 mcg/kg/hour.[37] Sajan et al.[38] compared dexmedetomidine and labetalol for induced hypotension in ear nose and throat surgeries and found better hemodynamic with dexmedetomidine and faster recovery with labetalol.

Clonidine is another α-2 adrenoceptor agonist which is commonly used in clinical practice to treat hypertension, migraine and menopausal flushing. It is less selective to α-2 receptor as compared to dexmedetomidine (220:1 vs. 1620:1), hence a less potent hypotensive agent.

Goswami et al.[39] compared the efficacy of dexmedetomidine and clonidine infusion to produce hypotensive anesthesia in patients undergoing orthognathic surgery and surprisingly found no significant difference between the two groups. However, dexmedetomidine provided better surgical field and surgeon satisfaction and reduced the need of rescue analgesia which was an additional advantage.

Magnesium sulfate

It has been well-recognized as a versatile drug for a long time. Its mechanism of action includes antagonism of the non-competitive N-methyl-D-aspartate receptors, decrease catecholamine release, and anti-nociceptor action, thus decreasing response to noxious stimuli. It is a vasodilator and decreases systemic vascular resistance, thus producing hypotensive effects. Besides this, it has also anti-arrhythmic action. Hence, it can be used as an adjuvant for controlled hypotension. Elsharnouby et al.[40] conducted a study to study the hypotensive effect of magnesium sulfate in FESS patients where it was given in a dose 40 mg kg⁻¹ i.v. as a bolus preinduction and 15 mg kg⁻¹ h⁻¹ by continuous i.v. infusion intraoperatively.

Lidocaine is a commonly used amide local anesthetic agent. It is also used to treat ventricular arrythmia and blunt the hypertensive response of laryngoscopy. Its negative inotropic action on the heart along with the sympatholytic effect may contribute to hypotension.

Karami et al.[41] recently conducted a study to compare labetalol and lidocaine for the induction of controlled hypotension in tympanoplasty. They noted that both the drugs effectively achieved controlled hypotension during surgery with improved surgical conditions.

Similarly, Hamed M et al. compared magnesium sulfate and lidocaine for controlled hypotension in patients undergoing FEES and found that lidocaine provided better surgical field clarity and shorter extubation time.[42]

Few other studies in past have also reported successful use of lidocaine to achieve controlled hypotension.[43] However, more studies with large sample size are needed to establish its role in hypotensive anesthesia.

Newer drugs, like fenoldopam, adenosine and alprostadil, are presently being evaluated for controlled hypotension. However, a higher treatment cost and associated side effects limit their development in this field.

Table 1 Summarizes the commonly used drugs for hypotensive anesthesia.

Table 1.

Commonly used drugs for hypotensive anesthesia

Drug	Onset time	Duration of action	Side effects	Patient contraindications	Adjustment for special population
Propofol	30 sec-1 min	5–10 min	Propofol infusion syndrome after prolong infusion, bradycardia	Severe renal or hepatic disease	Dose reduction in elderly patients, liver disease, heart failure
Remifentanil	1–3 min	5–10 min	Respiratory depression (dose-dependent)	Obstructive sleep apnea	Dose reduction in elderly patients, and respiratory disease
NTG	1–2 min	10–30 min	reflex tachycardia, venous congestion methemoglobinemia and cyanide toxicity (rare)	Patients with increase intracranial tension, Severe anemia	Decrease dose in patients with heart failure, chronic obstructive airway disease (COPD)
Labetalol	2-5 min	4-8 hours	Bradycardia, heart block	Severe bradycardia, Bronchial asthma, heart block	May have long duration of action in patients with liver and renal dysfunction due to slow metabolism Caution in asthma
Esmolol	1–2 min	10–20 min	Bradycardia, heart block	Severe asthmatics	Caution in asthma
Dexmedetomidine	5–10 min	15–30 min	Hypotension, bradycardia, dry mouth, and nausea decrease lacrimation	Current high cost relative to generic medications	In patients with heart block or ventricular dysfunction (ejection fraction <30) – continuous monitoring of ECG, BP, SPO2 is essential
Magnesium sulfate	1–2 min Peak effect within 5-10 min	30 min-1 h after a single iv dose	Bradycardia, Respiratory depression Muscle weakness	Severe renal impairment Myasthenia gravis Heart block	Reduced dose in renal disease, elderly patient
Lidocaine	3–5 min	1–2 hours	C.N.S and cardiovascular toxicity	Severe heart block and liver disease	Reduced dose in liver disease, elderly patient

Non-pharmacological methods

There are several maneuvers to control bleeding. Commonly used is reverse Trendelenburg or head up position where head is reclined 15 degrees from horizontal to allow venous drainage from the surgical field. The blood pressure falls by 2 mm hg for every inch of head raise.[20] However, this technique may be associated with a risk of air embolism. Valsalva maneuver is another method that can help to identify bleeding points, aids in postoperative drainage, and also maintains hemostasis. Pre-donation of autologous blood may decrease the requirement for homologous blood, but this requires extensive preparation and does not reduce the infection risk.[11]

Checklist summarizing the key steps[44]

Preoperative preparation

• Thorough knowledge and familiarity of anesthesia team planning the case under hypotensive anesthesia

• Check essential investigations, including hemoglobin (Hb), urea, and electrolytes. Ensure minimum Hb of 10g/dL

• Adequate premedication can be used to assist induction

• ABG sampling in major surgery (both preop and postop)

• Ensure patient is normovolumic

• Rule out drug allergy

Intraoperative preparation

• Ensure smooth induction and intubation

• Adequate iv access (especially if plan to start hypotensive agent through infusion)

• Maintain adequate depth of anesthesia

• Proper padding of pressure areas to prevent skin necrosis

• Patient should be positioned with the head elevated about 20º above the heart

• Ideally intraarterial blood pressure monitoring should be done

• Non-invasive monitoring is sufficient in short and minor surgery

• Careful estimation of the blood loss

• Discontinue hypotensive anesthesia before wound closure

CONCLUSION

Since each individual is unique, being familiar with the specificity of a patient and modifying anesthesia practice according to his requirements is a component of good anesthesia practice. The hypotensive anesthesia technique is widely used in major maxillofacial surgeries with a high risk of intraoperative bleeding. The decision to induce hypotensive anesthesia should be based according to the general condition of the patient, the extent of the surgery, and in coordination with the operating surgeon. Mere deepening of the anesthesia plane by increasing the concentration of inhalation agents or intravenous agents is not a reasonable practice as it could delay a patient’s arousal from anesthesia and have adverse cardiovascular effects. The target blood pressure should be adjusted according to the patient’s preoperative status and coexisting illness. The risk of organ hypoperfusion should be kept in mind. Close intraoperative monitoring with optimal patient selection is important for good patient outcomes. Other strategies to minimize blood loss can be considered in patients not suitable for hypotensive anesthesia. A risk-benefit analysis should be performed. Once the desired bloodless field is achieved, further drop in blood pressure should be avoided.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.