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- Understanding hemodynamic principles helps us determine the mechanisms underlying hemodynamic instability and guides our 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 valve replacement patient is more likely to decompensate severely when faced with the hemodynamic roller coaster associated with general anesthesia induction (Figure 2–1).
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Irrespective of the nature of the anesthetic that we plan to deliver, one of the main considerations is the expected impact upon the patient's blood pressure and cardiac output. Simply, whether we are performing an interscalene block, a neuraxial technique, or a general anesthetic we know that we have the ability to seriously affect both the patient's blood pressure and their heart's pumping ability. Generally, we expect that when we deliver a large induction dose of propofol or a high-inspired concentration of an inhalational anesthetic the blood pressure will decrease.
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Of course, in otherwise healthy patients undergoing noncardiac surgery, periods of hypotension are frequently attributed to "anesthetic effects" and are routinely treated by volume administration, ephedrine or phenylephrine, and surgical stimulation. Hemodynamic calculations are rarely obtained as these events are fleeting and the underlying mechanism anticipated. Hemodynamic calculations aid clinicians in the diagnosis of hypotension and help guide treatment options when routine responses to hypotension, which are otherwise successfully employed in healthy patients, might be injudicious or harmful to the cardiac patient.
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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. While in the past 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
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Each practitioner must determine, however, for each individual patient what constitutes systemic pressures that would warrant treatment. In the adult cardiac surgery patient it is likely that any patient with a systolic blood pressure less than 80 mm Hg would be considered in need of some intervention. This chapter will examine how to approach the hypotensive cardiac patient and to apply appropriate therapy. Some of the causes of perioperative hypotension are ...