A safe MRI for people with pacemakers Most people with pacemakers and implanted defibrillators are told they are unable to have an MRI. But Johns Hopkins researchers have learned how to make MRI scans safe, even with implanted devices. Read more The heart's electrical system The heart is, in the simplest terms, a pump made up of muscle tissue. The heart's pumping action is regulated by an electrical conduction system that coordinates the contraction of the various chambers of the heart.
How does the heart beat? Click Image to Enlarge An electrical stimulus is generated by the sinus node also called the sinoatrial node, or SA node , which is a small mass of specialized tissue located in the right atrium right upper chamber of the heart. In an adult, the sinus node generates an electrical stimulus regularly for adults, 60 to times per minute under normal conditions. This electrical stimulus travels down through the conduction pathways similar to the way electricity flows through power lines from the power plant to your house and causes the heart's lower chambers to contract and pump out blood.
The right and left atria the two upper chambers of the heart are stimulated first and contract a short period of time before the right and left ventricles the two lower chambers of the heart. The electrical impulse travels from the sinus node through the atria to the atrioventricular node also called AV node , where impulses are slowed down for a very short period, then continue down the conduction pathway via the bundle of His into the ventricles.
The bundle of His divides into right and left pathways to provide electrical stimulation to the right and left ventricles. Normally at rest, as the electrical impulse moves through the heart, the heart contracts about 60 to times a minute, depending on a person's age infants normally have very high heart rates. Each contraction of the ventricles represents one heartbeat.
The atria contract a fraction of a second before the ventricles so their blood empties into the ventricles before the ventricles contract. Under some abnormal conditions, certain heart tissue is capable of starting a heartbeat, or becoming the pacemaker. Currently, sternotomy for direct placement of leads on the epicardium is rarely necessary. When disrupted by myocardial infarction or disease, cardiac electrical activity is altered causing dysrhythmia or bradycardia.
An artificial pacemaker is then programmed and implanted to provide electrical impulses and maintain a stable heart rate and cardiac output. Pacemakers vary according to lead amount, paced heart chambers, and placement approach. The simplest, a single-chamber pacemaker, contains one lead. Through the transvenous approach, the lead can pace the right atrium in patients with sinus node disease or the RV in patients with atrial fibrillation with slow ventricular response.
When pacing the RV, the lead is directed inferiorly through the right atrium, curves through the tricuspid valve, and ends in an anterior—inferior position at the RV apex, and is seen left of the spine on PA X-ray. The LAT view showing an anteriorly oriented lead confirms the lead tip is in the RV. Note that in some cases, the right ventricular outflow tract RVOT may be used as an alternate site for pacing if the RV apex is not electrophysiologically suitable.
Dual-chamber pacemakers contain two lead wires allowing pacing of the right atrium and RV simultaneously [ Figure 3 ]. These CCDs are used to coordinate electrical signals and contraction between the atrium and ventricle in heart block patients. Radiography can demonstrate the inadvertent placement of leads in unintended sites, including: All of the above have distinct radiological features.
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