Providing a safe anesthetic in the hypertensive patient can be challenging. The goal is to maintain blood pressure within 20% of baseline and and minimize transient hypotension and hypertension that frequently occur during induction and emergence of anesthesia. Heart rate control is important to decrease myocardial oxygen demand, maintain adequate ejection fraction and prevent ischemia. Hypertensive patients are at risk for myocardial ischemia even in the absence of coronary artery disease.
As discussed in the previous chapter, optimization of blood pressure preoperatively decreases the risk of large changes in blood pressure intraoperatively. This decreases incidence of subsequent cardiac and cerebral ischemia. Difficulty often arises in elderly patients with isolated systolic hypertension and normal to low diastolic blood pressure. Diastolic blood pressure is responsible for coronary perfusion. Optimization of systolic blood pressure with antihypertensive therapy in these patients can reduce diastolic blood pressure to a point that results in myocardial ischemia. Systolic hypertension can be tolerated in these patients. Asymptomatic patients with mild-moderate hypertension and absence of end organ involvement can undergo elective surgery without increased cardiovascular risk.
Standard ASA monitors are applied prior to induction in all patients. This allows for a baseline evaluation of the patient’s vital signs. A five lead EKG allows the anesthesiologist to evaluate lead II for arrhythmias or changes in rhythm and V5 for ischemia with ST analysis. Changes on the EKG can be correlated with changes in heart rate, blood pressure, and oxygenation. Blood pressure can be monitored with a noninvasive cuff. Arterial lines should be considered in anticipation of large fluid shifts, multiple blood draws, or poor BP cuff readings as often occurs in obese patients. Pulse oximeter and end-tidal CO2 monitoring are used in all cases. Though permissive hypercapnea is often considered safe, in hypertensive patients increased CO2 can be detrimental. Hypercapnea causes sympathetic nervous system stimulation increasing heart rate and blood pressure and increases pulmonary hypertension when present.
Urinary catheters are used in many cases to monitor urine output and, in addition to laboratory tests, help evaluate for renal hypoperfusion and subsequent perioperative renal failure.
Temperature monitoring and normothermia are necessary particularly to decrease risk of shivering in postoperative period. Shivering increases oxygen consumption.
In normotensive patients, cerebral blood flow is autoregulated at a constant rate between MAP of 50–150 mmHg (Figure 75-1). Above and below these levels blood flow is pressure-dependent increasing the risk of hypoperfusion if low or intracranial hemorrhage if high. In hypertensive patients, the cerebral pressure–blood flow curve shifts right. Thus, hypertensive patients have increased risk of cerebral hypoperfusion and ischemia at a relatively normal MAP.
Cerebral perfusion curve. Cerebral perfusion pressure = mean arterial pressure – intracranial pressure. ICP is normally less than 10 mmHg; thus, cerebral perfusion pressure is primarily dependent on the mean arterial pressure.