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Don't Be Puzzled by Cardiovascular Concepts

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Don't Be Puzzled by Cardiovascular Concepts patho puzzler Don’t be puzzled by cardiovascular concepts CONNIE GOWDA, RN, MSN Clinical Assistant Professor • Methodist College of Nursing • Peoria, Ill. A: Knowing the concepts of preload, after- need to be able to evaluate the effects of Q: Why are load, and cardiac output is critical to under- these medications. the concepts standing how the heart functions and how abnormal function of the heart affects your Q: What’s preload? of preload, A: afterload, and patient. This means that you must have a Preload is the degree of stretch in the ven- cardiac output thorough understanding of these concepts in tricle at the end of diastole. Hemodynamic so important? order to accurately assess and evaluate your monitoring of central venous pressure (CVP) patient’s cardiac functioning. Medications at the bedside evaluates right-sided heart are utilized to alter preload, afterload, and functioning and provides a direct measure- cardiac output to enhance the ability of the ment of preload based on volume. Normal heart to work more efficiently. So you also CVP ranges from 0 to 8 mm Hg. Understanding preload, contractility, and afterload If you think of the heart as a balloon, it will help you understand the concepts of preload, contractility, and afterload. Preload Contractility Afterload Blowing up the balloon The balloon’s stretch The knot that ties the Preload is the stretching of mus- Contractility refers to the inher- balloon cle fibers in the ventricle. This ent ability of the myocardium to Afterload refers to the pressure stretching results from blood contract normally. Contractility that the ventricular muscles volume in the ventricles at end is influenced by preload. The must generate to overcome the diastole. According to Starling’s greater the stretch, the more higher pressure in the aorta to law, the more the heart muscles forceful the contraction—or, the get the blood out of the heart. stretch during diastole, the more more air in the balloon, the Resistance is the knot on the forcefully they contract during greater the stretch and the far- end of the balloon, which the systole. Think of preload as the ther the balloon will fly when air balloon has to work against to balloon stretching as air is is allowed to expel. get the air out. blown into it: The more air, the greater the stretch. July/August 2008 Nursing made Incredibly Easy! 27 patho puzzler Conditions that affect SVR Conditions that can increase systemic My output is high when vascular resistance (SVR) include: SVR is low, and that’s the • hypothermia way I like it! • hypovolemia • stress response • syndromes of low cardiac output. Conditions that can decrease SVR include: • anaphylactic and neurogenic shock • anemia • cirrhosis • vasodilation. But what does the concept of preload output. If cardiac output is maintained, pre- really mean? Let’s start with Ernest Starling, load is adequate. who in 1914 developed the following law Now take the rubber band and stretch it (known as Starling’s law): Stretching the out only a fourth of the way. Assess the dis- myocardial fibers during diastole will tance pulled and the force at which the rub- increase the force of contraction during sys- ber band springs back. You should notice a tole. Myocardial fibers can be stretched by weak or flimsy movement. If this were the increasing ventricular diastolic volumes. heart, preload would be decreased. The degree of fiber stretch, or preload, is Lastly, take the same rubber band and determined by ventricular volume. The vol- stretch it as far as you can. The extreme ume of blood contained within the ventri- stretching may cause the rubber band to cles during diastole depends on the amount break. Similarly, the heart muscle can only of venous return. Venous return is depen- be stretched so far before it loses its ability to dent on circulating blood volume and pump effectively. For another analogy you venous tone. Increasing venous return con- can use, see Understanding preload, contractili- sequently increases venous volumes and ty, and afterload. stretches the myocardial fibers. Preload can be a compensatory mechanism when car- Q: What about afterload? diac output isn’t adequate; however, it can A: Afterload is the amount of tension that also wreak havoc on the heart’s ability to the ventricle must overcome to open the aor- pump adequately. tic valve and eject blood into the systemic It’s easy to understand preload using the circulation. Also known as systemic vascular following analogy: Take a rubber band, pull resistance (SVR), afterload reflects changes in it out halfway, and then release it. Now ask the radius of the arterioles. The arterioles are yourself: What happens to the force of the resistant vessels because they constrict or rubber band as it’s exerted by the stretch? relax. Normal SVR is 700 to 1,500 dynes/ You should feel a strong forceful movement seconds/cm-5. of the rubber band. If this were the heart, An elevation in SVR may affect the empty- blood would’ve been propelled out and into ing of the left ventricle, causing a drop in the systemic circulation, maintaining cardiac cardiac output. Conversely, if SVR decreases, 28 Nursing made Incredibly Easy! July/August 2008 patho puzzler Influences on stroke volume and cardiac output Contractile Ventricular force pressure Ventricular Diastolic size filing Wall thickness Fiber stretch Preload Contractility Afterload Stroke volume Heart rate Cardiac output the pressure that the left ventricle must exert volume returning to the right side of the in order to open the aortic valve will lessen. heart, such as diuretics and nitroglycerin. This means that the heart pumps more effi- Diuretics facilitate the excretion of sodium, ciently and cardiac output increases (see chloride, and water by the kidneys, decreas- Conditions that affect SVR). ing volume and then preload. Nitroglycerin decreases preload by means of vasodilation. Q: And what’s cardiac output? There are several drugs that can be used to A: Cardiac output is the volume of blood affect afterload, or SVR, such as angiotensin- flowing from the systemic and pulmonary converting enzyme (ACE) inhibitors and circulation per minute. The formula for sodium nitroprusside. ACE inhibitors block cardiac output is: stroke volume x heart the conversion of angiotensin I to angiotensin rate. What’s stroke volume, you ask? It’s II, which results in vasodilation. Peripheral the amount of blood ejected with each con- vasodilation causes a decrease in SVR. When traction that’s determined by three factors: afterload is decreased, cardiac output will preload, contractility, and afterload (see increase because the left ventricle meets less Influences on stroke volume and cardiac resistance and can pump blood more effi- output). And heart rate is the rate at which ciently. the heart beats in 1 minute. Normal car- In conclusion, health care professionals diac output ranges from 4 to 8 L/minute. must be aware of the concepts of preload, Here’s an example: If a patient’s stroke afterload, and cardiac output because un- volume is 70 mL with each contraction and derstanding these concepts helps promote his heart rate is 80 beats/minute, his car- comprehensive nursing care and optimize diac output is 5,600 mL/minute (or 5.6 patient outcomes. I L/minute). Learn more about it Q: Now, how do the concepts Hemodynamic Monitoring Made Incredibly Visual! Philadel- of preload and afterload phia, Pa., Lippincott Williams & Wilkins, 2007:13-15. Porth CM. Pathophysiology: Concepts of Altered Health relate to pharmacologic States, 7th edition. Philadelphia, Pa., Lippincott Williams management? & Wilkins, 2007:461-462. A: If your patient’s preload is high, several Smeltzer SC, et al. Brunner and Suddarth’s Textbook of Medical-Surgical Nursing, 11th edition. Philadelphia, Pa., medications can be utilized to decrease the Lippincott Williams & Wilkins, 2007:786-787. 30 Nursing made Incredibly Easy! July/August 2008.
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