Transcript
Chapter 12: Determinants and Assessment of Cardiac Output
Chapter Summary
•The cardiopulmonary system consists of two interdependent circuits: the pulmonary circuit (a low-pressure system), which facilitates pulmonary gas exchange, and the systemic circuit (a high-pressure system), which facilitates gas exchange at the tissue level. Blood vessels are composed of three wall layers: the tunica intima, tunica media, and tunica externa.
The arterioles are resistance vessels that control blood flow and blood pressure through vasoconstriction and vasodilation. The medium and small veins are capacitance vessels that can hold large volumes of blood.
Capillary beds are composed of one layer, the tunica intima, which is made up of endothelial cells. Gas exchange and the shifting of nutrients and metabolic waste products occur at the capillary beds.The heart is composed of a double pump system (right and left heart) with two sets of paired chambers, the atria and the ventricles. The right heart is a low-pressure system that accepts oxygen-poor venous blood from the periphery and pumps it into the lungs. The left heart is a high-pressure system that accepts oxygen-enriched blood from the lungs and pumps it throughout the body.
The heart wall is composed of three layers: the endocardium, the myocardium, and the epicardium.
The heart has two sets of valves that maintain normal forward blood flow through the heart: the atrioventricular (AV) valves and the semilunar valves. Valves open and close based on unequal pressures on either side of the valve throughout the cardiac cycle.
The cardiac cycle consists of heart muscle activities that occur throughout one heartbeat. There are two phases, systole and diastole. All four heart chambers experience both phases. Systole refers to muscle contraction, in which blood is ejected from one area of the heart to the next, and diastole refers to muscle relaxation, whereby the chamber fills with blood.
There are four main coronary arteries, which lie on the outside surface of the heart: the left main coronary artery (LMCA), left anterior descending artery (LAD), left circumflex artery (LCX), and right coronary artery (RCA). These arteries receive oxygen-enriched blood before any other organ or tissue because of their anatomic location.
Cardiac output (CO) is the amount of blood pumped by the heart each minute. It is the product of stroke volume (SV) and heart rate (HR). Both SV and HR can be modified to ensure that CO is adequate to meet the body’s needs.
The three components of stroke volume are preload, contractility, and afterload. Preload is the volume of blood the heart has to pump; contractility is the ability of the heart to function as a pump; and afterload is the resistance the heart meets when it pumps out its volume.
The Frank–Starling Law of the heart states that an increase in preload results in an increase in stroke volume to a certain point. Once that optimal preload is exceeded, more preload results in a decrease in stroke volume because the myocardial fibers are stretched excessively and the heart cannot effectively pump out its volume.
Blood pressure is the amount of pressure exerted against blood vessel walls by circulating blood as it is pumped throughout the body. Arterial blood pressure is a function of peripheral resistance (PR) and cardiac output (CO). Peripheral resistance consists of afterload, particularly arteriolar vasoconstriction or vasodilation. Arterial blood pressure is regulated by three regulatory systems: the RAAS, the kidneys, and the autonomic nervous system. Venous pressure is influenced by four factors: systemic filling pressure, venous muscle pump, venous peripheral resistance, and right atrial pressure.
A focused nursing assessment begins with the nursing history to identify cardiovascular risk factors, chest pain, abnormal heart rhythms, or changes in mental state. A focused cardiovascular assessment may reveal signs of impaired CO. Certain lab values are important as indicators of myocardial and fluid status, including CK-MB, troponin, CRP, and BNP.
Several cardiovascular diagnostic procedures are used to assess the components of cardiac function. Imaging techniques include radiographs and MRIs. Exercise ECG evaluates heart muscle and coronary perfusion during exercise to identify myocardial ischemia. Echocardiogram is an ultrasound technique used to assess cardiac and valve function and to estimate ejection fraction. Cardiac catheterization is used to evaluate coronary tissue perfusion. EPS focuses on diagnosing problems in the cardiac conduction system.
