The normal conduction system of the heart is comprised of the sinoatrial (SA) node located at the junction of the right atrium and the superior vena cava. This area contains the pacing cells that initiate the contraction of the heart. The SA node is considered to be the main pacer of the heart rate. The atrioventricular (AV) node is located in the interventricular septum and receives the impulse and transmits it on to the Bundle of His, which extends down through the ventricular septum and merges with the Purkinje fibers in the lower portion of the ventricles. Figure 13.1 shows an anatomical drawing of the human heart.
Figure 13.1 Anatomical drawing of the heart.
Heart block is a condition in which the conduction system of the heart fails to conduct impulses normally. Heart block can occur as a result of structural changes in the conduction system, such as tumors, myocardial infarctions, coronary artery disease, infections of the heart, or toxic effects of drugs such as digoxin. First-degree AV block occurs when the SA node continues to function normally, but transmission of the impulse fails. Because of the conduction dysfunction and ventricular depolarization, the heart beats irregularly. These clients are usually asymptomatic and all impulses eventually reach the ventricles. Second-degree heart block is a block in which impulses reach the ventricles, but others do not. In third-degree heart block or complete heart block, none of the sinus impulses reach the ventricle. This results in erratic heart rates where the sinus node and the atrioventricular nodes are beating independently. The result of this type of heart block can be hypotension, seizures, cerebral ischemia, or cardiac arrest. Detection of a heart block is made by assessing the electrocardiogram. See Figure 13.2 for a graph depicting a normal electrocardiogram.
Figure 13.2 A normal electrocardiogram.
The P wave as shown in the graph is the SA node firing, the QRS complex is the contraction phase of the heart, and the T wave is the repolarization of the heart.
Toxicity to Medications
Toxicity to medications, such as Digoxin, can be associated with heart block. Clients taking Digitalis should be taught to check their pulse rate and to return to the physician for regular evaluation of their Digitalis level. The therapeutic level for Digoxin is 0.5–2.0 ng/ml. If the client's blood level of Digoxin exceeds 2.0 ng/ml, the client is considered to be toxic. Clients with Digoxin toxicity often complain of nausea, vomiting, and seeing halos around lights. The nurse should teach the client to check his heart rate prior to taking Digoxin. A resting pulse rate of less than 60 bpm in the adult client should alert the nurse to the possibility of toxicity. Treatment for Digoxin toxicity includes checking the potassium level because hypokalemia can contribute to Digoxin toxicity. The physician often will order potassium be given IV or orally and that the Digoxin be held until serum levels return to normal. Other medications, such as Isuprel or Atropine, and Digibind, are frequently ordered to increase the heart rate.
Malfunction of the Conduction System
Because a malfunction of the conduction system of the heart is the most common cause for heart block, a pacing mechanism is frequently implanted to facilitate conduction. Pacemakers can be permanent or temporary and categorized as demand or set. A demand pacemaker initiates an impulse if the client's heart rate fails below the prescribed beats per minute. A set pacemaker overrides the heart's own conduction system and delivers an impulse at the rate set by the physician. Frequently, pacemakers are also combined with an internal defibrillation device.
Permanent Pacemakers/Internal Defibrillators: What the Client Should Know
Clients with internal defibrillators or pacemakers should be taught to avoid direct contact with electrical equipment. Clients should be instructed to
- Wear a medic alert stating that a pacemaker/internal defibrillator is implanted. Identification will alert the healthcare worker so that alterations in care can be made.
- Take the pulse for 1 full minute and report the rate to the physician.
- Avoid applying pressure over the pacemaker/internal defibrillator. Pressure on the defibrillator or pacemaker can interfere with the electrical leads.
- Inform the dentist of the presence of a pacemaker/internal defibrillor because electrical devices are often used in dentistry.
- Avoid having a magnetic resonance imaging (MRI). Magnetic resonance interferes with the electrical impulse of the implant.
- Avoid close contact with electrical appliances, electrical or gasoline engines, transmitter towers, antitheft devices, metal detectors, and welding equipment because they can interfere with the electrical conduction of the device.
- Be careful when using microwaves. Microwaves are generally safe for use, but the client should be taught to stand approximately 5 feet away from the device while cooking.
- Report fever, redness, swelling, or soreness at the implantation site.
- If a vibration or beeping tone is noted coming from the internal defibrillator, immediately move away from any electromagnetic source. Stand clear from other people because shock can affect anyone touching the client during defibrillation.
- Report dizziness, fainting, weakness, blackouts, or a rapid pulse rate. The client will most likely be told not to drive a car for several months after the internal defibrillator is inserted to evaluate any dysrhythmias.
- Report persistent hiccupping because this can indicate misfiring of the pacemaker/internal defibrillator.
- Do not lift the left arm higher than shoulder level for approximately two weeks since this may increase the chances of displacement of the leads.