HYPERTENSION AND ANESTHESIA
Hypertension is an increasingly important medical and public health issue. The prevalence of hypertension increases with advancing age to the point where more than half of people 60–69 years of age and approximately three-fourths of those 70 years of age and older are affected. Arterial blood pressure remains the cornerstone of cardiovascular clinical evaluation and therapy. Hypertension represents a major risk factor for coronary artery disease, congestive heart failure, dementia, and renal and cerebrovascular disease, and is associated with dyslipidemia, diabetes, and obesity. The higher the arterial pressure, the higher the risk of myocardial infarction, heart failure, stroke, or kidney disease. Between the age of 40 and 70 yr, for each increment of 20 mm Hg in systolic or 10 mm Hg in diastolic arterial pressure, the chance of developing cardiovascular disease doubles across the arterial pressure range from 115/75 to 185/115 mm Hg. Therefore, the need for tight arterial pressure control and life-long treatment is undisputed.
The following are the three main reasons why anesthetists should remain wary of hypertension:
Hypertensive patients tend to be more hemodynamically unstable and prone to myocardial ischemia in the perioperative period. Several studies have demonstrated a significant association between perioperative myocardial ischemia and postoperative ischemic cardiac events, such as unstable angina, nonfatal myocardial infarction, and cardiac death.
Hypertension is a major risk factor for coronary artery disease, congestive heart failure, and renal and cerebrovascular disease. Any of these factors increase the likelihood of perioperative myocardial infarction or
Hypertension is associated with dyslipidemia, diabetes, and obesity, and the side-effects of drugs needed to treat these diseases.
The essential elements of perioperative management include:
Careful preoperative evaluation
Tight perioperative arterial pressure and heart rate control
Well trained, experienced and dedicated anesthetist.
Preoperative evaluation is a unique opportunity to identify patients with hypertension and evaluate them for appropriateness of therapy and the presence of end- organ damage. Given the current pressures to proceed expeditiously with scheduled surgery, it is important for the physicians evaluating such patients to understand the evidence regarding the value of delaying surgery and instituting additional pharmacologic therapies to reduce perioperative and long-term adverse outcomes.
The perioperative management of hypertensive patients is a complex issue that can be divided into a number of different questions.
a. Is having a diagnosis of hypertension itself associated
with increased perioperative risk, regardless of the arterial pressure at the time of admission to hospital for surgery?
b. Is elevated arterial pressure at the time of admission for surgery associated with increased perioperative cardiac risk?
c. What is the importance, if any, of poorly controlled hypertension in the perioperative setting? Is there any interaction between elevated admission arterial pressure and being diagnosed with hypertensive disease previously such that this increases perioperative risk?
d. Does the treatment of elevated admission arterial pressure before surgery reduce perioperative cardiac risk?
In the face of conflicting evidence, the seventh report of the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure (JNC- VII) recommends delay of surgery when the preoperative diastolic BP exceeds 110mmHg. In such patients, BP should be reduced slowly for fear of increasing morbidity and mortality secondary to a critical coronary or carotid arterial stenosis. The period of time required to adequately correct the hypertension is unknown, though amelioration of some of the myocardial and vascular changes related to severe hypertension may require a minimum of 6 to 8 weeks of therapy to reverse. The decision to delay surgery for long-term stabilization of BP, versus acutely correcting the hypertension, must take into account the potential benefits and the urgency of the surgical procedure.
Isolated systolic hypertension (systolic BP>160mmHg and diastolic BP<90 mm Hg) has been identified as a risk factor for cardiovascular complications in the general population and treatment reduces the future risk of stroke.1 Pulse pressure had been found to be strongly associated with cardiovascular risk. The benefits of treating isolated systolic hypertension are now clearly established.
End-organ Damage and Preoperative Outcome
Hypertensive comorbidities associated with adverse perioperative outcomes are given below:
Occult coronary artery disease (Q waves on the electrocardiogram)
Congestive heart failure
Left ventricular hypertrophy (voltage criteria)
Renal insufficiency (serum creatinine level >2.0 mg/dl)
Cerebrovascular disease (history of cerebrovascular accident or transient ischemic attack).
As a universally measured variable with a recognized association with coronary artery disease (CAD), hypertension serves as a useful marker for potential CAD. In addition, several investigators have demonstrated exaggerated intraoperative blood pressure fluctuation with associated ECG evidence of myocardial ischemia in patients with preoperative blood pressure elevation. This effect can be modified by treatment. Because intraoperative ischemia correlates with postoperative cardiac morbidity, it follows that control of blood pressure preoperatively may help reduce the tendency to perioperative ischemia. Patients with hypertension may have had a silent myocardial infarction (MI), evidenced by the presence of Q waves on the electrocardiogram. Congestive heart failure may occur secondary to hypertension-induced systolic and diastolic dysfunction. The presence of either of these risk factors places the patient at moderate- to high-clinical risk for perioperative MI or death; guidelines for diagnostic testing have recently been released by the American Heart Association and the American College of Cardiology.
In the absence of overt coronary artery disease, the presence of left ventricular hypertrophy defines the subset of ambulatory patients with an increased risk of cardiovascular complications. In a study of patients undergoing major vascular surgery, voltage criteria for left ventricular hypertrophy and ST segment depression of more than 0.5mm on preoperative electrocardiograms were both significantly associated with postoperative MI or cardiac death. Myocardial ischemia can result in patients with ventricular hypertrophy from supply and demand mismatches even in the absence of coronary artery stenoses; this supports better control of hypertension in these patients.
Chronic renal insufficiency is a common sequel of hypertension. Patients with hypertension should have a measurement of baseline serum creatinine. In the Revised Cardiac Risk Index, a preoperative serum creatinine level greater than 2.0 mg/dl (>176.8 μmol/l) is 1 of 6 independent factors that predicted increased cardiovascular risk. Hypertension is also a risk factor for cerebrovascular disease. Several studies have demonstrated the association between a preoperative history of a cerebrovascular accident and the occurrence of perioperative cardiac events.13 While acute hypertensive crises can lead to a stroke from an intracerebral bleed, abrupt decreases in BP can lead to injury from cerebral ischemia. It is therefore prudent to establish normal levels for maintenance of BP in a patient with hypertension and document that lowering BP in a patient with poorly controlled hypertension does not result in neurologic changes.
The physical examination and simple laboratory tests can rule out some of the rare but important causes of hypertension. Further evaluation to exclude secondary hypertension is rarely warranted before necessary surgery. If pheochromocytoma is a serious possibility, surgery should be delayed to permit its exclusion. A loud abdominal bruit may suggest renal artery stenosis. A radial to femoral artery pulse delay suggests coarctation of the aorta, whereas hypokalemia in the absence of diuretic therapy raises the possibility of hyperaldosteronism.
Perioperative Management of Patients with Hypertension or Elevated Arterial Pressure
High arterial pressures are associated with high levels of after load and cardiac work. This may predispose to myocardial ischemia and infarction, especially in the presence of coronary artery disease and left ventricular hypertrophy, and therefore simply ignoring markedly elevated arterial pressure may not be appropriate. Patients diagnosed as ‘hypertensive’ have a reputation for displaying increased cardiovascular lability during anesthesia. There is certainly a pathophysiological basis for such behaviour. Established hypertension is associated with an increased systemic vascular resistance. The systemic vasodilatation associated with anesthesia might well be expected to have profound effects on arterial pressure in such patients. For many anesthetists, however, cardiovascular lability implies something more than the decrease in arterial pressure seen at induction of anesthesia. It suggests swings in arterial pressure over a wide range of values. Interestingly, patients with preoperative hypertension appear more likely to develop intraoperative hypotension than non-hypertensive persons; this is particularly true for patients taking ACE inhibitors or angiotensin II receptor antagonists. In some patients, this may be related to a decrease in vascular volume.
With regard to the management of surgical patients with elevated admission arterial pressure, there are few substantive guidelines over which patients should be cancelled to allow treatment before surgery or the duration of such treatment before proceeding with surgery. Several investigators have demonstrated exaggerated intraoperative blood pressure fluctuation with associated ECG evidence of myocardial ischemia in patients with preoperative blood pressure elevation. This effect can be modified by treatment. Because intraoperative ischemia correlates with postoperative cardiac morbidity, it follows that control of blood pressure preoperatively may help reduce the tendency to perioperative ischemia. Although an elevated blood pressure on an initial recording in a patient with previously undiagnosed or untreated hypertension has been shown to correlate with blood pressure lability under anesthesia, the definition of the severity of hypertension rests with subsequent recordings in a non-stressful environment. In patients undergoing therapy for hypertension, a thorough review of current medications and dosages, along with awareness of known intolerance to previously prescribed drugs, is essential. The physical examination should include a search for target- organ damage and evidence of associated cardiovascular pathology. A fundoscopic examination may provide useful data regarding the severity and chronicity of hypertension.
If the initial evaluation establishes hypertension as mild or moderate, and there are no associated metabolic or cardiovascular abnormalities, there is no evidence that it is beneficial to delay surgery.18 Several investigators have established the value of effective preoperative blood pressure control among patients with established hypertension,8,10 and antihypertensive medications should be continued during the perioperative period. Particular care should be taken to avoid withdrawal of β-blockers and clonidine because of potential heart rate or blood pressure rebound. In patients unable to take oral medications, parenteral β-blockers and transdermal clonidine may be used. Medication selection and risks should be assessed on the basis of national guidelines.
For stage 3 hypertension, according to JNC VI classification, (systolic blood pressure greater than or equal to 180 mm Hg and diastolic blood pressure greater than or equal to 110 mm Hg), the potential benefits of delaying surgery to optimize the effects of antihypertensive medications should be weighed against the risk of delaying the surgical procedure. The evidence from medical studies suggests that patients with stage 3 hypertension are at significantly increased risk of target organ damage, whether or not this is clinically evident. There is certainly evidence to support a steadily increasing incidence of postoperative myocardial ischemia with increasing admission systolic arterial pressure.
However, there is evidence that very rapid control of arterial pressure with drugs such as sublingual nifedipine is associated with increased morbidity and mortality. Taken together, these concerns pose the dilemma that markedly raised arterial pressures and wide excursions of arterial pressure should be avoided in the perioperative period, but that dramatic acute reductions in arterial pressure may also be fraught with risk. Many patients with admission arterial pressures consistent with stage 3 hypertension will have isolated systolic hypertension. With rapidly acting intravenous agents, blood pressure can usually be controlled within a matter of several hours. If severe end-organ damage is present, the goal should be to normalize BP as much as possible before surgery.
Effective lowering of risk may require 6 to 8 weeks of therapy to allow regression of vascular and endothelial changes, but too rapid or extreme lowering of BP may increase cerebral and coronary ischemia; therefore, the benefits of delaying surgery for treatment must be weighed against the risks. Thus, if surgery cannot be postponed, the goal is to not decrease chronically increased BP too rapidly. Severely elevated BP should be lowered over a period of several weeks. Studies suggest that hypotension intraoperatively is far more dangerous than hypertension.
If the patient is considered fit for surgery in other respects, their operation should not be deferred simply on account of an elevated admission arterial pressure. If the arterial pressure is consistently elevated to levels of 180 mm Hg systolic or greater or 110 mm Hg diastolic or greater, surgery may proceed, but care should be taken to ensure perioperative cardiovascular stability. Invasive arterial pressure monitoring is indicated for major procedures, and the arterial pressure should be actively managed to prevent excursions of the mean arterial pressure of greater than 20% from baseline. Monitoring should continue into the postoperative period until it is clear that the patient is cardiovascularly stable. It may be appropriate to manage the patient in a high dependency area in the immediate postoperative period.
Several reports have shown that the introduction of preoperative β-adrenergic blockers leads to effective
modulation of severe blood pressure fluctuations and a reduction in the number and duration of perioperative
coronary ischemic episodes. The preoperative administration of b-adrenergic blocking drugs has been shown to decrease the incidence of postoperative atrial fibrillation, and in patients who have or are at risk for CAD who must undergo noncardiac surgery, treatment with beta blockers during hospitalization can reduce mortality and the incidence of cardiovascular complications. However, the Preoperative Ischemia Evaluation (POISE) trial has recently questioned the safety of starting these agents acutely. The optimal time to start b-blocker therapy to achieve its benefit while minimizing risk is unknown. Even though diuretics are first-line therapies in most circumstances, starting them in the preoperative period is not generally a good idea because of the alterations in potassium (both hypokalemia and hyperkalemia) that may occur and the need for close monitoring and replacement. Calcium channel blockers (e.g., amlodipine, 5 to 10 mg daily) can be very effective. Frequently, anxiety increases BP, and therefore antianxiolytics can be used as adjunctive therapy. BP should not be lowered too rapidly. Continuation of antihypertensive treatment preoperatively is critical.
Preoperative Administration of Antihypertensives
Most authors administer all antihypertensive drugs preoperatively, except ACE inhibitors or angiotensin II receptor blockers (ARBs), which needs to be tailored to the individual patient. It was demonstrated that more severe hypotensive episodes requiring vasoconstrictor treatment occur after induction of general anesthesia in patients chronically treated with an ARB and receiving the drug on the morning before surgery than in those in whom ARBs were discontinued on the day before surgery.25 If chronic ACE inhibitors/angiotensin receptor blockers are continued, vasopressin is the drug of choice for refractory hypotension. Most anesthesiologists even administer the patient’s chronic diuretics on the morning of surgery because the major effect of diuretics after 1 week of therapy is arteriolar vasodilation and assessment of urine output may be inaccurate if the diuretic is abruptly discontinued on the morning of surgery.
Intraoperative and postoperative treatment of hypertension can be achieved with multiple agents. Since several days may lapse before oral intake is resumed, parenteral treatment with diuretics, adrenergic inhibitors, vasodilators, angiotensin-converting enzyme inhibitors, or use of transdermal clonidine have all been proposed in the JNC-VII. It is critical to initiate some form of therapy to prevent rebound hypertension and tachycardia observed from withdrawal of these agents. Consideration should be given to restarting ACEI in the postoperative period only after the patient is euvolemic, to decrease the risk of perioperative renal dysfunction.
JNC 7 on Perioperative Hypertension
Uncontrolled hypertension is associated with wider fluctuations of BP during induction of anesthesia and intubation, and may increase the risk for perioperative ischemic events. BP levels of >180/110 mm Hg should be controlled prior to surgery. For elective surgery, effective BP control can be achieved over several days to weeks of outpatient treatment. In urgent situations, rapidly acting parenteral agents, such as sodium nitroprusside, nicardipine, and labetalol, can be utilized to attain effective control very rapidly.
Surgical candidates with controlled hypertension should maintain their medications until the time of surgery, and therapy should be reinstated as soon as possible postoperatively. Adequate potassium supplementation should be provided, if needed, to correct hypokalemia well in advance of surgery. Older patients may particularly benefit from treatment with β1 selective β-blockers before and during the perioperative period. Sudden intraoperative hypertension is managed by many of the same parenteral antihypertensive agents that are utilized in the management of hypertensive emergencies. Intravenous infusions of sodium nitroprusside, nicardipine, and labetalol can be effective. Nitroglycerin is often an agent of choice in patients with coronary ischemia, while the very short-acting b-blocker esmolol may be of benefit in managing intraoperative tachycardia.
Hypertension is very common in the early postoperative period and is related to increased sympathetic tone and vascular resistance. Contributing factors include pain and increased intravascular volume, which may require parenteral dosing with a loop diuretic such as furosemide. If resumption of oral treatment must be interrupted postoperatively, periodic dosing with intravenous enalaprilat or transdermal clonidine hydrochloride may be useful.
Key points in Anesthesia Management
Preoperatively all regular antihypertensive drugs to be continued (except ACEI, ARBs)
It is critical to initiate some form of therapy to prevent rebound hypertension and tachycardia observed from withdrawal of any hypertensive agents.
Sedative/anxiolytic premedication preferred.
No anesthetic technique is superior.
Good analgesia is important to minimize hypertensive swings.
Although effective, central neuraxial blocks can result in profound hypotension so frequent or continuousmeasurement of blood pressure is required.
If hypertension is severe or preoperative control is poor and major or emergency surgery is planned invasive monitoring should be instituted.
Induction and maintenance of anesthesia
Anticipate exaggerated blood pressure response to anesthetic drugs
Limit duration of direct laryngoscopy
Administer a balanced anesthetic to blunt hypertensive responses
Monitor for myocardial ischemia
Ensure adequate postoperative analgesia
Anticipate periods of systemic hypertension
Maintain monitoring of end-organ function.
General Anesthesia - Broad Outline
Induction of Anesthesia
Induction of anesthesia with rapidly acting intravenous drugs may produce an exaggerated decrease in blood pressure due to peripheral vasodilation in the presence of a decreased intravascular fluid volume, as is likely in the presence of diastolic hypertension. Hypotension during induction is more noticeable in patients continuing ACE inhibitor or ARBs therapy until the time of surgery.
Direct laryngoscopy and tracheal intubation can produce significant pressor response in patients with hypertension, even if these patients have been rendered normotensive preoperatively. Evidence of myocardial ischemia is likely to occur in association with the hypertension and tachycardia that accompany laryngoscopy and intubation. Intravenous induction drugs do not predictably suppress the circulatory responses evoked by tracheal intubation. Patients at high risk for developing myocardial ischemia may benefit from maneuvers that suppress tracheal reflexes and blunt the autonomic responses to tracheal manipulation such as deep inhalation anesthesia or injection of an opioid, lidocaine, b-blocker, or vasodilator. In addition, the duration of laryngoscopy is important in limiting the pressor response to this painful stimulus. Direct laryngoscopy that does not exceed 15 seconds in duration helps minimize blood pressure changes.
Maintenance of Anesthesia
The hemodynamic goal during maintenance of anesthesia is to minimize wide fluctuations in blood pressure. Management of intraoperative blood pressure lability is as important as preoperative control of hypertension in these patients.
The most likely intraoperative blood pressure change is hypertension produced by painful stimulation, i.e. light
anesthesia. Volatile anesthetics are useful in attenuating sympathetic nervous system activity responsible for pressor responses. Volatile anesthetics produce dose-dependent decreases in blood pressure, reflecting decreases in systemic vascular resistance and myocardial depression. A nitrous oxide—opioid technique can be used for maintenance of anesthesia, although it is likely that a volatile agent will be needed at times to control hypertension, for example, during periods of abrupt change in surgical stimulation. Antihypertensive medication by bolus or by continuous infusion is an alternative to the use of a volatile anesthetic for blood pressure control intraoperatively.
Hypotension during maintenance of anesthesia may be treated by decreasing the depth of anesthesia and/or by increasing fluid infusion rates. Sympathomimetic drugs such as ephedrine or phenylephrine may be necessary to restore vital organ perfusion pressures until the underlying cause of hypotension can be ascertained and corrected. Cardiac rhythm disturbances that result in loss of sequential atrioventricular contraction such as junctional rhythm and atrial fibrillation can also create hypotension and must be treated promptly.
Postoperative hypertension is common in patients with essential hypertension. This hypertension requires prompt assessment and treatment to decrease the risk of myocardial ischemia, cardiac dysrhythmias, congestive heart failure, stroke, and bleeding. Hypertension in postanesthesia care unit (PACU) is an independent predictor of morbidity and mortality. Hypertension that persists despite adequate treatment of postoperative pain may necessitate administration of an intravenous antihypertensive medication such as labetalol. Gradually, conversion can be made to the patient’s usual regimen of oral antihypertensive medication.
Regional anesthesia can certainly be used in hypertensive patients. However, a high sensory level of anesthesia with its associated sympathetic denervation can unmask unsuspected hypovolemia and may cause a precipitous fall in the blood pressure. Adequate preloading must be given before administering the neuraxial blockade. Vasopressor requirement intraoperatively may be high in hypertensive patients. It should be kept in mind that this subset of patients may show exaggerated response to the vasopressors and, hence, the vasopressors may be administered in small titrated doses.