Rhythm analysis and arrhythmias
Analyzing an ECG tracing
Performing ECGs is within the medical assistant’s scope of practice, and when delegated by the supervising provider, analyzing the rhythm strip is too. Some medical assistants work primarily as ECG technicians, where analyzing a tracing is a core part of the job.
The ECG rhythm strip (lead II view) is evaluated from left to right. The following should be assessed:
- Rate: Count the number of QRS complexes in a six-second strip and multiply by ten to estimate the beats per minute.
- Rhythm: Is it regular or irregular? An irregular rhythm on an ECG tracing will have time differences between cardiac cycles. With a regular rhythm, each cardiac cycle occurs the same length of time apart.
- Appearance of the segments, waves, and intervals.
Identifying an abnormal rhythm
When performing an ECG, the medical assistant must identify if the patient has an arrhythmia that requires immediate care by the provider. If the medical assistant identifies a life-threatening arrhythmia, it is important to keep the patient hooked up to the ECG machine and to get the provider immediately.
Sinus arrhythmias
A sinus rhythm is considered normal. The electrical activity begins in the SA node and goes through the rest of the conduction system. Atrial and ventricular depolarizations occur. With sinus arrhythmias, the electrical pathway is normal. The rate or rhythm of the heartbeat is altered. The alteration may result from the SA node firing too slowly or too quickly.
- Sinus bradycardia: The adult heart rate is below 60 beats per minute. This is a normal finding in well-conditioned athletes. It is abnormal in other individuals.
- Sinus tachycardia: The adult heart rate is above 100 beats per minute. This is normal in a person doing aerobic exercise. It is abnormal in a resting individual.
Atrial arrhythmias
Atrial arrhythmias occur when there is a problem with the SA node starting the impulse. They can also occur due to a conduction problem in the atria.
- Premature atrial contractions (PACs): Occur when the atria contract sooner than they should. The P wave can be abnormally shaped, or an extra P wave can be seen. PACs can be seen in people who smoke or consume large amounts of caffeine. An occasional PAC is not abnormal. More than six PACs in a minute is considered abnormal.
- Atrial flutter: Occurs when the atria contract faster than the ventricles (up to 300 beats per minute). They become out of sync with the ventricles. Extra P waves are seen with regular QRS complexes. Atrial flutter is most often caused by coronary heart disease, hypertension, or stimulant use (alcohol, caffeine, cocaine). Atrial flutter is reversed with medication to slow the heart or with cardioversion (electrical shock).
Heart block
A heart block occurs when there is a disruption or slowing of the electrical impulse through the heart. Heart block can be congenital or acquired. Heart disease, surgery, or medications can cause acquired heart block. There are three types of heart block, with third-degree heart block being the most severe:
- First-degree heart block: The impulse slows as it moves from the atria to the ventricles. This creates a longer PR interval (more than 0.20 second). First-degree heart block may not cause symptoms. It may not require treatment.
- Second-degree heart block: The impulse slows or is blocked as it moves into the ventricles. When blocked, there is no QRS complex after the P wave, and the ventricles do not contract. When the impulse slows, the PR interval is longer. The more serious form (Mobitz type II) usually requires a pacemaker to help maintain the heart rate; the milder form (Mobitz type I) often does not. Pacemakers are discussed in a later section of this chapter.
- Third-degree heart block: The impulse does not reach the ventricles. As a backup system, special ventricular cells create an impulse that causes the ventricles to contract. On the ECG tracing, the P waves occur at a normal rate but faster than the QRS complexes, and the QRS complex is not coordinated with the P wave. This is a life-threatening arrhythmia and requires emergency treatment and a pacemaker.
Ventricular arrhythmias
Unlike heart blocks, these arrhythmias arise in the ventricles themselves rather than from a delay in the conduction pathway:
- Ventricular tachycardia (V-tach): Occurs when the ventricles beat at a rapid rate (up to 250 beats per minute). It may be seen with multiple PVCs in a row. It may be a short run of fast beats or may last longer than 30 seconds. V-tach is a life-threatening condition. If it is not reversed with drugs or cardioversion, it can become ventricular fibrillation.
- Ventricular fibrillation (V-fib): Occurs when the ventricles quiver uncontrollably. They are essentially ineffective at pumping any blood. The patient has no pulse, is not breathing, and is unresponsive. This is the most critical, life-threatening arrhythmia. Cardioversion with a defibrillator is necessary to restore normal function of the electrical conduction system.
- Asystole: Results in the absence of a heartbeat. A flat line appears on the tracing.
Implantable loop recorders continuously monitor the rhythm for up to about 3 years, catching infrequent arrhythmias that a standard ECG might miss.