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INTRODUCTION

Understanding hemodynamic principles helps cardiac anesthesiologists determine the mechanisms underlying hemodynamic instability and guides treatment. Fortunately, the same basic physiologic principles apply in both the healthy patient undergoing laparoscopic cholecystectomy as well as in the patient with low ejection fraction undergoing multiple valve replacements. Unfortunately, the patient undergoing cardiac surgery is more likely to decompensate severely when faced with the hemodynamic roller coaster sometimes associated with general anesthesia induction (Figure 2–1).

Figure 2–1.

Anesthesia manipulations can often stress the heart and the patient. Although many healthy patients can tolerate these swings in blood pressure, the cardiac surgery patient may be unable to do so without developing myocardial ischemia and ventricular dysfunction. (Reproduced with permission from Wasnick JD: Handbook of Cardiac Anesthesia and Perioperative Care. Boston, MA: Butterworth Heinemann; 1998.)

HEMODYNAMIC CALCULATIONS AND INVASIVE MONITORS: WHY ARE THEY IMPORTANT AND HOW DOES ONE DETERMINE THEM?

Blood Pressure

Although the absolute definition of hypotension is somewhat clouded in the literature,1 a patient is considered hypotensive when the systolic blood pressure is reduced by 20% or more from the patient’s baseline blood pressure. Different authors set different cutoffs as to what constitutes a hypotensive patient. Although in the past a systolic pressure of less than 90 mm Hg was thought hypotensive, this value has recently been suggested as being too low. For example, the new cutoff value for hypotension has been reset to 110 mm Hg systolic in trauma patients.2 A mean arterial blood pressure less than 65 mm Hg has been associated with adverse outcomes perioperatively and should be corrected. However, at times during cardiac procedures surgeons may request relative hypotension to facilitate aortic cannulation or to control bleeding. At other times physical manipulation of the heart by the surgeons will transiently reduce blood pressure. Close communication between the surgeon and anesthesiologist is critical for effective hemodynamic management.

Each practitioner must determine for each individual patient what systemic pressures, high and low, warrant treatment. In the adult cardiac surgery patient, it is likely that any patient with a systolic blood pressure much less than 90 mm Hg would be considered in need of hemodynamic intervention of some kind depending upon the etiology of the hypotensive episode. Hypertension is also aggressively treated in the cardiac surgery patient. This chapter will examine how to approach the hypotensive cardiac patient and to apply appropriate therapy. Some of the causes of perioperative hypotension are presented in Table 2–1.

Table 2–1.Differential Diagnosis of Perioperative Hypotension
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