Hypertension

HYPERTENSION

Patients with hypertension frequently present for elective surgical procedures. Some will have been effectively managed, but unfortunately, many others will not have been. Hypertension is a leading cause of death and disability in most Western societies and the most prevalent preoperative medical abnormal-ity in surgical patients, with an overall prevalence of 20% to 25%. Long-standing uncontrolled hyper-tension accelerates atherosclerosis and hypertensive organ damage. Hypertension is a major risk factor for cardiac, cerebral, renal, and vascular disease.

Complications of hypertension include MI, con-gestive heart failure, stroke, renal failure, periph-eral occlusive disease, and aortic dissection. The presence of left ventricular hypertrophy (LVH) in hypertensive patients may be an important predic-tor of cardiac mortality. However, systolic blood pressures below 180 mm Hg, and diastolic pressures below 110 mm Hg, have not been associated with increased perioperative risks. When patients present with systolic blood pressures greater than 180 mm Hg and diastolic pressures greater than 110 mm Hg, anesthesiologists face the dilemma of delaying sur-gery to allow optimization of oral antihypertensive therapy, but adding the risk of a surgical delay versus proceeding with surgery and achieving blood pres-sure control with rapidly acting intravenous agents. Intravenous β-blockers can be useful to treat preop-erative hypertension. Of note, patients with preop-erative hypertension are more likely than others to develop intraoperative hypotension. This is particu-larly frequent in patients treated with angiotensin receptor blockers and/or angiotensin-converting enzyme (ACE) inhibitors.

Blood pressure measurements are affected by many variables, including posture, time of day or night, emotional state, recent activity, and drug intake, as well as the equipment and technique used. A diagnosis of hypertension cannot be made by one preoperative reading, but requires confirmation by a history of consistently elevated measurements. Although preoperative anxiety or pain may produce some degree of hypertension in normal patients, patients with a history of hypertension generally exhibit greater preoperative elevations in blood pressure.

Epidemiological studies demonstrate a direct and continuous correlation between both diastolic and systolic blood pressures and mortality rates. The definition of systemic hypertension is arbitrary: a consistently elevated diastolic blood pressure greater than 90 mm Hg or a systolic pressure greater than 140 mm Hg. A common classification scheme is listed in Table 21–4. Borderline hypertension is said to exist when the diastolic pressure is 85–89 mm Hg or the systolic pressure is 130–139 mm Hg. Whether patients with borderline hypertension are at some increased risk for cardiovascular complications remains unclear. Accelerated, or severe hypertension (stage 3), is defined as a recent, sustained, and pro-gressive increase in blood pressure, usually with dia-stolic blood pressures in excess of 110–119 mm Hg. Renal dysfunction is often present in such patients. Malignant hypertension is a true medical emer-gency characterized by severe hypertension (>210/ 120 mm Hg) often associated with papilledema and encephalopathy.

Pathophysiology

Hypertension can be either idiopathic (essential), or, less commonly, secondary to other medical con-ditions such as renal disease, renal artery stenosis, primary hyperaldosteronism, Cushing’s disease, acromegaly, pheochromocytoma, pregnancy, or estrogen therapy. Essential hypertension accountsfor 80% to 95% of cases and may be associated with an abnormal baseline elevation of cardiac output, systemic vascular resistance (SVR), or both. An evolving pattern is commonly seen over the course of the disease, where cardiac output returns to (or remains) normal, but SVR becomes abnormally high. The chronic increase in cardiac afterload results in concentric LVH and altered diastolic function. Hypertension also alters cerebral autoregulation, such that normal cerebral blood flow is maintained in the face of high blood pressures; autoregulation limits may be in the range of mean blood pressures of 110–180 mm Hg.

The mechanisms responsible for the changes observed in hypertensive patients seem to involve vascular hypertrophy, hyperinsulinemia, abnormal increases in intracellular calcium, and increased intracellular sodium concentrations in vascu-lar smooth muscle and renal tubular cells. The increased intracellular calcium presumably results in increased arteriolar tone, whereas the increased sodium concentration impairs renal excretion of sodium. Sympathetic nervous system overactivity and enhanced responses to sympathetic agonists are present in some patients. Hypertensive patients sometimes display an exaggerated response to vasopressors and vasodilators. Overactivity of the renin–angiotensin–aldosterone system seems to play an important role in patients with accelerated hypertension.

Long-Term Treatment

Effective drug therapy reduces the progression of hypertension and the incidence of stroke, conges-tive heart failure, CAD, and renal damage. Effective treatment can also delay and sometimes reverse con-comitant pathophysiological changes, such as LVH and altered cerebral autoregulation.

Some patients with mild hypertension require only single-drug therapy, which may consist of a thi-azide diuretic, ACE inhibitor, angiotensin-receptor blocker (ARB), β-adrenergic blocker, or calcium channel blocker, although guidelines and outcome studies favor the first three options. Concomitant illnesses should guide drug selection. All patients with a prior MI should receive a β-adrenergic blocker and an ACE inhibitor (or ARB) to improve outcomes, irrespective of the presence of hyperten-sion. In many patients, the “guideline specified” agents will also be more than sufficient to control hypertension.

Patients with moderate to severe hyperten-sion often require two or three drugs for control. The combination of a diuretic with a β-adrenergic blocker and an ACE inhibitor is often effective when single-drug therapy is not. As previously noted, ACE inhibitors (or ARBs) prolong survival in patients with congestive heart failure, left ventricular dys-function, or a prior MI. Familiarity with the names, mechanisms of action, and side effects of commonly used antihypertensive agents is important for anes-thesiologists (Table 21–5).