Chapter 13

• Clinical trials have shown that inhaled nitric oxide is a selective pulmonary vasodilator that may be beneficial in the treatment of reversible pulmonary hypertension. By improving perfusion only in ventilated areas of the lung, inhaled nitric oxide may improve oxygenation in patients with acute respiratory distress syndrome or during one-lung ventilation.
• Acute cyanide toxicity is characterized by metabolic acidosis, cardiac arrhythmias, and increased venous oxygen content (as a result of the inability to utilize oxygen). Another early sign of cyanide toxicity is the acute resistance to the hypotensive effects of increasing doses of sodium nitroprusside (tachyphylaxis).
• By dilating pulmonary vessels, sodium nitroprusside may prevent the normal vasoconstrictive response of the pulmonary vasculature to hypoxia (hypoxic pulmonary vasoconstriction).
• Preload reduction makes nitroglycerin an excellent drug for the relief of cardiogenic pulmonary edema.
• Hydralazine relaxes arteriolar smooth muscle, causing dilatation of precapillary resistance vessels.
• The body reacts to a hydralazine-induced fall in blood pressure by increasing heart rate, myocardial contractility, and cardiac output. These compensatory responses can be detrimental to patients with coronary artery disease and are minimized by the concurrent administration of a β-adrenergic antagonist.
• Adenosine slows atrioventricular (AV) conduction (increases the P–R interval) and can interrupt reentrant dysrhythmias that involve the AV node.
• Fenoldopam mesylate (infusion rates studied in clinical trials range from 0.01–1.6 μg/kg/min) reduces systolic and diastolic blood pressure in patients with malignant hypertension to an extent comparable to nitroprusside.

A multitude of drugs are capable of lowering blood pressure, including volatile anesthetics (see Chapter 7), sympathetic antagonists and agonists (see Chapter 12), and calcium channel blockers and angiotensin-converting enzyme inhibitors (see Chapter 20). This chapter examines additional agents that may be useful to the anesthesiologist for intraoperative control of arterial blood pressure: nitrates, adenosine, and fenoldopam (Figure 13–1 and Table 13–1). Although all these drugs lower blood pressure by dilating peripheral vessels, they are not identical in their mechanisms of action, clinical uses, routes of metabolism, effects on organ systems, or drug interactions. Trimethaphan, the only ganglionic blocker, is no longer being manufactured and will not be discussed.

Table 13–1. Comparative Pharmacology of Hypotensive Agents.1

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