Using inotropes and vasopressors in anesthesia (Proceedings)
Arterial hypotension is a common anesthetic complication. Perfusion of vital organs depends largely on arterial blood pressure. Mean arterial blood pressure (MAP) below 60 mmHg and systolic blood pressures below 80 mmHg can result in reduced perfusion of the vital organs. Oxygen debt can ensue in morbidity and mortality. Cardiac arrest can follow a severe episode of hypotension. Assuming that the patient does not have any preexisting problem(s), the hypotension is more likely anesthetic-induced.
Signs and evidence supporting the presence of arterial hypotension in anesthetized dogs and cats should be present before treatment is initiated. Direct blood pressure measurement using a pressure transducer is the golden standard for clinical blood pressure measurement. It provides second-by-second changes in blood pressure which leads to a faster response on the part of the anesthetist. Without the direct blood pressure measurement, Doppler appears to be a helpful tool in making decision during anesthesia. Most oscillometric pressure measurements fail when the patient is moderately to severely hypotensive. Hypotension may be manifested as weak peripheral pulses. It is important to remember that a strong peripheral pulse does not guarantee a normal MAP. The arterial pulse is the difference between the systolic and diastolic pressures. A large difference between the two pressures will result in a very strong pulse and yet the MAP may be low.
During anesthesia, it may be difficult to ascertain the exact cause of arterial hypotension. General causes of arterial hypotension include one or a combination of the following: 1) reduced inflow to the heart (e.g., blood loss), 2) reduced pumping function of the heart (e.g., deep anesthesia), and 3) reduced vascular resistance (e.g., sepsis). In a healthy dog or cat presented for elective procedure, the anesthetics administered are the most cause of arterial hypotension. Some dogs and cats are very sensitive to the cardiovascular depressant effect of inhalant anesthetics. Blood loss during surgery can also result in hypotension. Preexisting conditions that can result in hypotension during anesthesia include hypovolemia, shock, cardiomyopathy, valvular heart disease, arrhythmias, hypothyroidism, hypoxemia and Addisonian crisis. Drugs, blood or blood products administered during anesthesia can cause anaphylactoid reaction. The most common manifestation of anaphylactoid reaction during anesthesia is hypotension.If the patient does not have any preexisting problem(s) and the hypotension is more likely anesthetic-induced, the vaporizer setting or infusion rate of the IV anesthetic should be reduced. IV bolus administration of crystalloid at 10 ml/kg over 10 minutes should also be done. If hypotension persists, the administration of positive inotrope or vasopressor should be done.
The positive inotropes and vasopressors that are used in anesthesia are sympathomimetics. There are three main adrenergic receptors acted upon by the sympathomimetics: beta-1, beta-2, and alpha receptors. Stimulation of the beta-1 receptors leads to increased cardiac contractility and heart rate On the other hand, beta-2 stimulation results in peripheral vasodilation. Peripheral vasoconstriction is the main effect following alpha receptor stimulation. Vasopressin does not work on adrenergic receptors. It directly produces vasoconstriction of the systemic circulation through the V1 vascular smooth muscle receptors.
The choice of positive inotrope and vasopressor during anesthesia will depend on the case and the availability of these drugs in the clinic. The specific drugs are discussed below.