Dysfunction in the Specialized Conducting System Leads to Abnormalities in Cardiac Rhythm (Arrhythmias)
Cardiac arrhythmias result either from problems with the formation of action potentials or from problems with the propagation (conduction) of action potentials. One example of a problem with action potential formation has already been mentioned: sinus arrest, in which the SA node completely fails to form action potentials.
In a patient with sinus arrest, the auxiliary pacemaker function of the AV node keeps the heart beating, although at an extremely low rate. Complete cessation of the SA node is the extreme case of the condition called sick sinus syndrome. In its more common and less extreme form, sick sinus syndrome is characterized by sluggish depolarization of the SA node pacemaker cells. Patients exhibit an abnormally slow heart rate at rest (bradycardia) and an insufficient increase in heart rate during exercise. Specifically, in sick sinus syndrome, the intrinsic sinus rate is abnormally low.Even though the problem in sick sinus syndrome is intrinsic to the sinus itself, one treatment strategy is to administer a drug that blocks parasympathetic action on the heart (a cholinergic muscarinic antagonist, such as atropine). Table 19-3 illustrates the logic behind this treatment. In a normal, healthy large dog, the intrinsic rate of the heart is 140 beats/min. However, the heart rate at rest is about 90 beats/min because high parasympathetic tone normally slows the SA node pacemaker to a rate below its intrinsic rate. A drug that blocks parasympathetic effects on the heart would return the heart rate to 140 beats/min. A dog with a sick sinus has a low intrinsic heart rate, perhaps 80 beats/min. Parasympathetic tone makes the resting heart rate even lower, approximately 30 beats/min. A drug that
Table 19-3
Treatment of Sick Sinus Syndrome by Blocking Parasympathetic Effects on Heart Rate with a Cholinergic MuscarinicAntagonist
| Heart rate | Normal dog (beats/min) | Dog with sick sinus syndrome (beats/min) |
| Intrinsic rate | 140 | 80 |
| Resting rate (with | 90 | 30 |
| parasympathetic | ||
| tone) | ||
| Rate after atropine | 140 | 80 |
blocks parasympathetic effects restores the heart rate to its intrinsic level, 80 beats/min. Therefore a dog with sick sinus syndrome treated with atropine has a heart rate that closely matches the rate of a normal resting dog.
Another possible therapeutic approach is to increase the heart rate by administering a drug that mimics the action of sympathetic nerves, specifically, a β-adrenergic agonist (e.g., isoproterenol). Enough isoproterenol would be given to increase the resting rate from 30 to 80 beats/min.
If drug treatment of sick sinus syndrome is ineffective, an alternative way to increase the heart rate is through the use of an artificial cardiac pacemaker. Such a device periodically applies electric shocks to the heart, which depolarize cardiac muscle to threshold. Shocks applied to the atria initiate atrial action potentials. If the AV node is functioning normally, these atrial action potentials are conducted to the ventricles, and the ventricles also contract. For temporary or emergency treatment, the pacemaker electrodes can be inserted intravenously (e.g., via the jugular vein) and advanced into the right atrial chamber. For long-term treatment, a battery-powered electrical stimulator can be surgically implanted under the patient’s skin and attached to electrodes that are either inserted into one of the heart’s chambers or attached to the outside surface of the heart.