McKenna's Pharmacology for Nursing, 2e

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P A R T 8  Drugs acting on the cardiovascular system

heart failure and cases of hypotension. The supply is most frequently altered, however, when the coronary vessels become narrowed and unresponsive to stimuli to dilate and deliver more blood. This happens in athero­ sclerosis or coronary artery disease. The end result of this narrowing can be total blockage of a coronary artery, leading to hypoxia and eventual death of the cells that depend on that vessel for oxygen. This is called a myocardial infarction (MI), and it is one of the leading causes of death in Australia and New Zealand. Systemic arterial pressure The contraction of the left ventricle, which sends blood surging out into the aorta, creates a pressure that con­ tinues to force blood into all of the branches of the aorta. This pressure against arterial walls is greatest during systole (cardiac contraction) and falls to its lowest level during diastole. Measurement of both the systolic and the diastolic pressure indicates both the pumping pressure of the ventricle and the generalised pressure in the system, or the pressure the ventricle has to overcome to pump blood out of the heart. Hypotension The pressure of the blood in the arteries needs to remain relatively high to ensure that blood is delivered to every cell in the body and to keep the blood flowing from high-pressure to low-pressure areas. The pressure can fall dramatically—termed hypotension—from loss of blood volume or from failure of the heart muscle to pump effectively. Severe hypotension can progress to shock and even death as cells are cut off from their Constant, excessive high blood pressure—called hyper­ tension—can damage the fragile inner lining of blood vessels and cause a disruption of blood flow to the tissues. It also puts a tremendous strain on the heart muscle, increasing myocardial oxygen consumption and putting the heart muscle at risk. Hypertension can be caused by neurostimulation of the blood vessels that causes them to constrict, subsequently raising pressure, or by increased volume in the system. In most cases, the cause of hypertension is not known and drug therapy to correct it is aimed at changing one or more of the normal reflexes that control vascular resistance or the force of cardiac muscle contraction. Cardiac: Hypertension Vasomotor tone The smooth muscles in the walls of the arteries receive constant input from nerve fibres of the sympathetic nervous system. These impulses work to dilate the oxygen supply. Hypertension

vessels if more blood flow is needed in an area; to constrict vessels if increased pressure is needed in the system; and to maintain muscle tone so that the vessel remains patent and responsive. The coordination of these impulses is regulated through the medulla in an area called the cardiovascu­ lar centre. If increased pressure is needed, this centre increases sympathetic flow to the vessels. If pressure rises too high, this is sensed by baroreceptors or pres­ sure receptors and the sympathetic flow is decreased. Chapter 43 discusses the drugs that are used to influence the stimulation of vessels to alter blood pressure. Renin–angiotensin–aldosterone system Another determinant of blood pressure is the renin– angiotensin–aldosterone system. This system is activ­ ated when blood flow to the kidneys is decreased. Cells in the kidney release an enzyme called renin. Renin is transported to the liver, where it converts angio­ tensinogen (produced in the liver) to angiotensin I. Angiotensin I travels to the lungs, where it is converted by angiotensin-converting enzyme (ACE) to angiotensin II. Angiotensin II travels through the body and reacts with angiotensin II receptor sites on blood vessels to cause a severe vasoconstriction. This increases blood pressure and should increase blood flow to the kidneys to decrease the release of renin. Angiotensin II also causes the release of aldosterone from the adrenal cortex, which causes retention of sodium and water, leading to the release of antidiuretic hormone (ADH) to retain water and increase blood volume. Increasing blood volume increases blood flow to the kidney. This system works constantly, whenever a position change alters flow to the kidney or blood volume or pressure changes, to help maintain the blood pressure within a range that ensures perfusion (delivery of blood to all of the tissues) (Figure 42.9). Venous pressure Blood in the veins also exerts a pressure that may some­ times rise above normal. This can happen if the heart is not pumping effectively and is unable to pump out all of the blood that is trying to return to it. This results in a backup or congestion of blood waiting to enter the heart. Pressure rises in the right atrium and then in the veins that are trying to return blood to the heart as they encounter resistance. The venous system begins to back up or become congested with blood. Heart failure If the heart muscle fails to do its job of effectively pumping blood through the system, blood backs up and the system becomes congested. This is called heart failure (HF). The rise in venous pressure that results from this backup of blood increases the hydrostatic