Cath Lab Digest - December 2007 - (Page 34) 34 ASK THE CLINICAL INSTRUCTOR DECEMBER 2007 Ask the Clinical Instructor A Q&A column for those new to the cath lab Questions are answered by: Todd Ginapp, EMT-P, RCIS, FSICP Todd is the Cardiology Manager for Memorial Hermann Southeast in Houston, Texas. He also teaches an online RCIS Review course for Spokane Community College, in Spokane, Washington, and regularly presents with RCIS Review Courses. Another question that I routinely get is, “Why do we give epinephrine if it is going to vasodilate the vasculature?” Many medications have the properties of BOTH alpha and beta (see Table 2). There are not any alpha receptors in the heart, so any hemodynamic changes due to alpha stimulation are not a cause of stimulation to the heart (more to follow on this). In the lungs, bronchoconstriction is bad. We treat bronchoconstriction with beta medications (specifically, beta2) to achieve bronchodilation, such as Albuterol, Isoetharine and Terbutaline. In the peripheral arteries, there are both alpha and beta receptors that are integral to our management of specific conditions. Beta medications will cause vasodilation, which is sometimes unwanted because it will decrease the blood pressure. In cases where we want help with the blood pressure by constricting the arterial vasculature, we can use neosynephrine (phenylephrine), which is a potent alpha stimulant. It has very little effect on the heart, so the resultant vasoconstriction can help create a higher blood pressure. Unfortunately, this can also increase the workload of the heart due to the increased afterload effects. Another question that I routinely get is, “Why do we give epinephrine if it is going to vasodilate the vasculature?” Many medications have the properties of BOTH alpha and beta (see Table 2). Epinephrine is one of those medications. It is PREDOMINATELY beta (to increase heart rate, force of contractions and automaticity), but also has slight alpha properties (to vasoconstrict). This is why it is a medication of choice in a cardiac arrest situation, because pure beta property, as in Isuprel, would adversely create vasodilation. This is also one reason why you do not see Isuprel in ACLS algorithms anymore. “I know that beta-blockers are supposed to slow the heart rate down, but why do the doctors use it for blood pressure?” — RCIS Review Student B eta-blockers are used for a variety of different situations. You will see them used for heart rate control, blood pressure management, long-term angina treatment and cardiomyopathy, just to name a few. This topic can generally be a 2- to 3-hour lecture just to cover the basics. I will try to address the important points in a few paragraphs. rate increases (+ chronotropy) and the automaticity of heart increases. Unfortunately, unwanted vasodilation can also occur in the periphery. To understand how beta-blockers work, we must first revisit some basic physiology and pharmacokinetics. If we recall a visit to our basic terminology, we should know that any medication that has an effect on the heart rate is considered a “chronotropic” agent (chrono = time) and any medication that affects the force of contraction is an “inotropic” agent. These are the principle properties affected by beta-blockers. Beta properties are a branch of the sympathetic nervous system, which is the cornerstone for many medications you will see given in the cath lab. The other branch that we deal with are the ‘alpha’ properties, which we will only discuss briefly. A closer look at beta-blockers Now, with beta-blockers, we are looking to ‘stop’ the results of the beta stimulation. As we saw, beta stimulation in the Table 1. Agent effects alpha Stimulation None beta Stimulation Increased rate Increased Force Increased Automaticity Vasodilation Bronchodilation Heart Arteries Lungs Vasoconstriction None. Maybe mild bronchoconstriction. What are the alpha and beta properties? In the heart, lungs and arteries, there are specific receptor sites to achieve certain actions. We are concerned with alpha and beta properties. You can see what effects these two agents have on the specific target areas in Table 1. In the heart, there are only beta receptors (beta1, to be specific). Only beta medications can increase the heart rate through this stimulation. That’s why if we give Isuprel, the heart Table 2. Many medications have the properties of BOTH alpha and beta Phenylephrine (Neosynphrine) = pure alpha Norepinephrine (Levophed) = predominately alpha, slight beta Epinephrine = predominately beta, slight alpha Isoproterenol (Isuprel) = pure beta
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