McKenna's Pharmacology for Nursing, 2e

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P A R T 8  Drugs acting on the cardiovascular system

demands for oxygen. A faster-than-normal heart rate—usually anything faster than 100 beats/min in an adult—with a normal-appearing ECG pattern is called sinus tachycardia. If sinus tachycardia becomes too fast, it can lead to a decreased time for cardiac filling and a decrease in cardiac output. Many activities or conditions can cause a sinus tachycardia, such as exercise, fear or stress. The underlying physical condition of the person will determine whether this fast heart rate is problem­ atic. Sinus bradycardia is a slower-than-normal heart rate (usually less than 60 beats/min) with a normal- appearing ECG pattern. Sinus bradycardia allows increased time for ventricular filling and an increased cardiac output. This is often seen with athletes who have a slow heart rate. In other people, this rate might be too slow to adequately perfuse all of the tissues. Supraventricular arrhythmias Arrhythmias that originate above the ventricles but not in the SA node are called supraventricular arrhythmias. These arrhythmias feature an abnormally-shaped P wave because the site of origin is not the sinus node. However, they show normal QRS complexes because the ventricles are still conducting impulses normally. Supraventricular arrhythmias include the following: • Premature atrial contractions (PACs) , which reflect an ectopic focus (a shift in the pacemaker of the heart from the SA node to some other site) in the atria that is generating an impulse out of the normal rhythm. • Paroxysmal atrial tachycardia (PAT) , sporadically occurring runs of rapid heart rate originating in the atria. • Atrial flutter , characterised by sawtooth-shaped P waves reflecting a single ectopic focus that is generating a regular, fast atrial depolarisation. • Atrial fibrillation , with irregular P waves representing many ectopic foci firing in an uncoordinated manner through the atria. With atrial flutter, often one of every two or one of every three impulses is transmitted to the ventricles. The person may have a 2:1 or 3:1 ratio of P waves to QRS complexes. The ventricles beat faster than normal, losing some efficiency. With atrial fibrillation, so many impulses are bombarding the AV node that an unpre­ dictable number of impulses are transmitted to the ventricles. The ventricles are stimulated to beat in a fast, irregular and often inefficient manner. Atrioventricular block Atrioventricular block, also called heart block, reflects a slowing or lack of conduction at the AV node. This can occur because of structural damage, hypoxia or injury to the heart muscle. First-degree heart block, in which all of the impulses from the SA node arrive in the ventricles but after a longer-than-normal period,

is characterised by a lengthening of the P–R interval beyond the normal 0.16 to 0.20 seconds. Each P wave is followed by a QRS complex. In second-degree heart block, some of the impulses are lost and do not get through, resulting in a slow rate of ventricular contraction. With this arrhythmia, a QRS complex may follow one, two, three or four P waves. In third- degree heart block, or complete heart block, no impulses from the SA node get through to the ventricles, and the much slower ventricular automaticity takes over. The waveform shows a total dissociation of P waves from QRS complexes and T waves. Because the P waves can come at any time, the P–R interval is not constant. The QRS complexes appear at a very slow rate and may not be sufficient to meet the body’s needs. Ventricular arrhythmias Impulses that originate below the AV node originate from ectopic foci that do not use the normal conduc­ tion pathways. The QRS complexes appear wide and prolonged, and the T waves are inverted, reflecting the slower conduction across cardiac tissue that is not part of the rapid conduction system. Premature ventricu­ lar contractions (PVCs) can arise from a single ectopic focus in the ventricles, with all of them having the same shape, or from many ectopic foci, which produces PVCs with different shapes. Runs or bursts of PVCs from many different foci are more ominous because they can reflect extensive damage or hypoxia in the myocardium. Runs of several PVCs at a rapid rate are called ven­ tricular tachycardia. Ventricular fibrillation is seen as a bizarre, irregular, distorted wave. It is potentially fatal because it reflects a lack of any coordinated stimulation of the ventricles. The ventricles’ inability to contract in a coordinated fashion results in no blood being pumped to the body or the brain. Thus there is a total loss of cardiac output. ■■ The normal ECG waveform is made up of five main waves: the P wave, which is formed as impulses originating in the SA node or pacemaker pass through the atrial tissues; the QRS complex, which represents depolarisation of the bundle of His (Q) and the ventricles (RS); and the T wave, which represents repolarisation of the ventricles. ■■ A person with a normal ECG pattern and a heart rate within the normal range for that person’s age group is said to be in normal sinus rhythm. ■■ When the generation of impulses is altered, the result is known as an arrhythmia (or dysrhythmia) that can upset the normal balance in the cardiovascular system. A decrease in cardiac output, which affects all of the cells of the body, follows. KEY POINTS