Attaining Hypotension in Anesthesia

Attaining Hypotension

• Blood pressure can be lowered chemically by direct arterial or venous dilators or by ganglionic blocking drugs.
• Perfusion pressure drops in proportion to a decrease in vascular flow resistance, but adequate tissue blood flow exists.
• Fine adjustment of the desired level of hypotension can be achieved by mechanical maneuvers.

Techniques to Produce Hypotension

• Deep general anesthesia with halothane or isoflurane, followed by a vasodilator, produces the desired minute-to-minute effect.
• Sodium nitroprusside is a potent, fast-acting vasodilator that reduces virtually all resistance in vascular smooth muscle.
• Nitroglycerin directly dilates capacitance vessels, reduces preload, and improves myocardial perfusion during diastole.
• Trimethaphan (Arfonad) blocks sympathetic ganglia, resulting in relaxation of resistance and capacitance vessels and reducing arterial pressure.
• Fentanyl may be used as a basal anesthetic for hypotension, and the blood pressure can be maintained at the desired level by the addition of a small amount of a volatile agent.
• Other drugs such as verapamil, nifedipine, phentolamine, tetrodotoxin, or adenosine triphosphate may be used.

Precautions in the Use of Hypotension

• Safe lower limits of arterial pressure may vary, with a mean systolic value of 50 mm Hg and values ranging between 65 and 70 mm Hg.
• Potential complications of hypotensive anesthesia include cerebral or coronary ischemia or thrombosis, reactionary hemorrhage, anuria in acute renal failure, delayed awakening, and dermal ischemic lesions.
• Primary contraindications are vascular compromise to any vital organ system or the brain.
• Precautions include careful selection of the patient, preoperative cardiac, renal, and hepatic evaluation, selection of an appropriate level of blood pressure, administration and evaluation by expert anesthesia providers, and maintenance of blood volume at an optimal level.