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Hypotensive anesthesia

Hypotensive anesthesia, is a technique used intraoperatively that helps to minimize surgical blood loss, thereby decreasing the need for blood transfusion. Its careful application can be done safely in most patients, including children, and for a variety of surgical procedures. The technique entails the controlled lowering of blood pressure and is defined as a reduction of the systolic blood pressure to between 80-90 mmHg. An alternative definition is a decrease in mean arterial pressure (MAP) to 50-70 mmHg in a normotensive patient.

Inducing hypotension Deliberate hypotension is induced by a variety of pharmacological agents and nonpharmacological supplements. Since there is no single agent capable of safely and effectively lowering arterial pressure in all situations, the anesthetist may need to employ a variety of agents or techniques in order to achieve the target pressure.
Pharmacological agents can generally be divided into two categories: peripheral vasodilators and inhalation agents.
The three most commonly used vasodilators are: sodium nitroprusside (SNP), nitroglycerin (NTG), and trimethaphan.
SNP acts as a vascular smooth muscle relaxant and has a rapid onset but brief duration of action. Its primary influence is on arteriolar and venous vessels, but without significant myocardial effects.
NTG reduces blood pressure by relaxing venous smooth muscle and, like SNP, has rapid onset of action but short duration. NTG is less toxic than SNP; however, it is more difficult to fine-tune the degree of hypotension with NTG since it is less potent than SNP in its capacity to reduce blood pressure.
Trimethaphan produces hypotension through ganglionic blockade and direct vasodilator properties. It is also
short acting and provides tight control of blood pressure. Commonly used inhalation agents, or volatile anesthetic agents, include halothane, isoflurane and enflurane. The concentration of a volatile anesthetic agent produces a dose-dependent decrease in mean arterial pressure.
Spinal and epidural anesthesia can also be used to effect controlled hypotension. Unfortunately, these techniques require large infusions of fluids and the deliberate hypotension can be erratic and difficult to control.

Contraindications to Hypotensive Anesthesia

Congenital heart disease

Severe anemia

Coronary artery disease

Congestive heart failure

Poorly controlled hypertension

Increased intracranial pressure

Significant cerbro-vascular disease

Low flow states to the liver or kidney

LITERATURE

1. Sum DC, Chung PC, Chen WC: Deliberate hypotensive anesthesia with labetalol in reconstructive surgery for
scoliosis. Acta Anaesthesiol Sin 1996 Dec;34(4):203-207

Deliberate hypotensive anesthesia was found to significantly decrease the average blood loss

by nearly 55 per cent, reduce the need for transfusion by nearly 53 per cent, and shorten the average operating time by over one hour. Nocomplications attributable to the specific anesthetic technique occurred. The findings of this study suggest that deliberatehypotensive anesthesia with labetalol and isoflurane may be of benefit in scoliosis surgery by reducing blood loss, the need of bloodreplacement and operating time.

2. Precious DS, Splinter W, Bosco D: Induced hypotensive anesthesia for adolescent orthognathic surgery
patients. J Oral Maxillofac Surg 1996 Jun;54(6):680-683

Estimated blood loss was significantly less when induced hypotensive anesthesia was used. The surgical field was better, but there was no significant difference in duration of the procedure with induced hypotensive anesthesia. Induced hypotensive anesthesia results in both reduced blood loss and improvement in surgical field.

3.Nelson CL, Fontenot HJ: Ten strategies to reduce blood loss in orthopedic surgery Am J Surg 1995, N°6A (Suppl.), 170:64-68

In our studies using hypotensive anesthesia in 100 Jehovah's Witness patients who underwent primary total hipreplacement. the intraoperative blood loss was reduced by 40% compared with matched controls. In patients who had previous hip surgery before undergoing total hip replacement arthroplasty, hypotensive anesthesiaachieved a 30% reduction in intraoperative blood loss Of the 100 patients studied, none was given an allogeneic transtusion. No instances of myocardial infarction, cerebrovascular accident, or renal failure could be

attributed to hypotensive anesthesia. Also, there was no increased blood loss after surgery. In all cases, the blood

pressure was raised to prehypotensive levels before wound closure..