Dihydropyridine calcium channel blockers (nicardipine, clevidipine) are arterial selective vasodilators routinely used for perioperative blood pressure control in patients undergoing cardiothoracic surgery. Clevidipine, prepared as a lipid emulsion, has a short half- life secondary to rapid metabolism by blood esterases, which facilitates its rapid titration. Clevidipine is infused initially at a rate of 1 to 2 mg/h, with the dose doubled until the desired effect is obtained, up to 16 mg/h. Because of its formulation as a lipid emulsion it is contraindicated in patients with soy or egg allergies and those with impaired lipid metabolism. Unlike verapamil and diltiazem, the dihydropyridine calcium channel blockers have minimal effects on cardiac conduction and ventricular contractility. These calcium channel blockers bind to L-type calcium channel and impair calcium entry into the vascular smooth muscle. L-type receptors are more prevalent on arterial than venous capacitance vessels. Consequently, cardiac filling and preload are less affected by these agents than by nitrates, which might dilate both arterial and venous systems. With preload maintained, cardiac output often increases when vascular tone is reduced by use of dihydropyridine calcium blockers. Nicardipine infusion is titrated to effect (5–15 mg/h).
Another intravenous agent that can produce hypotension perioperatively is the intravenous angiotensin-converting enzyme inhibitor enalaprilat (0.625–1.25 mg). The role of enalaprilat as a nondirect-acting agent in the acute treatment of a hypertensive crisis is limited.
CASE DISCUSSION Controlled Hypotension
A 59-year-old man is scheduled for total hip arthroplasty under general anesthesia. The surgeon requests a controlled hypotensive technique. What is controlled hypotension, and what are its advantages?
Controlled hypotension is the elective lowering of arterial blood pressure. The primary advantages of this technique are minimization of surgical blood loss and better surgical visualization. How is controlled hypotension achieved?
The primary methods of electively lowering blood pressure are use of hypotensive anesthetic techniques (eg, neuraxial anesthesia) and the administration of hypotensive drugs. Elevation of the surgical site can selectively reduce the blood pressure at the wound. During general anesthesia the increase in intrathoracic pressure that accompanies positive-pressure ventilation impedes venous return to the heart, lowering cardiac output and mean arterial pressure. Numerous pharmacological agents effectively lower blood pressure: volatile anesthetics, spinal and epidural anesthesia, sympathetic antagonists, calcium channel blockers, and the peripheral vasodilators discussed in this chapter. Which surgical procedures might benefit most from a controlled hypotensive technique?
Controlled hypotension has been successfully used during cerebral aneurysm repair, brain tumor resection, total hip arthroplasty, radical neck dissection, radical cystectomy, major spine surgery, and other operations associated with significant blood loss. Controlled hypotension may allow safer surgery of patients whose religious beliefs prohibit blood transfusions (eg, Jehovah’s Witnesses). What are some relative contraindications to controlled hypotension?
Some patients have predisposing illnesses that decrease the margin of safety for adequate organ perfusion: severe anemia, hypovolemia, atherosclerotic cardiovascular disease, renal or hepatic insufficiency, cerebrovascular disease, or uncontrolled glaucoma. What are the possible complications of controlled hypotension?
As the preceding list of contraindications suggests, the risks of low arterial blood pressure include cerebral thrombosis, hemiplegia (due to decreased spinal cord perfusion), acute tubular necrosis, massive hepatic necrosis, myocardial infarction, cardiac arrest, and blindness (from retinal artery thrombosis or ischemic optic neuropathy). These complications are more likely in patients with coexisting anemia. Consequently, the use of induced or controlled hypotension continues to decline. Patients requiring beach chair positioning for shoulder surgery or sitting position are at particular risk for cerebral hypoperfusion and perioperative cerebral infarction. What is a safe level of hypotension?
This depends on the patient. Healthy young individuals may tolerate mean arterial pressures as low as 50 to 60 mm Hg without complications. On the other hand, chronically hypertensive patients have altered autoregulation of cerebral blood flow and may tolerate a mean arterial pressure of no more than 20% to 30% lower than baseline. Patients with a history of transient ischemic attacks may not tolerate any decline in cerebral perfusion. Recent studies suggest that the lower limit of cerebral autoregulation may be at a much higher mean arterial pressure than long has been assumed. What special monitoring is indicated during controlled hypotension?
Intraarterial blood pressure monitoring correctly positioned to determine mean arterial pressure at the brain is suggested. Likewise, cerebral oximetry can be employed if hypotensive anesthesia techniques are to be considered.