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Capture. The successful depolarization and contraction of a cardiac chamber caused by the pacemaker's output pulse. One-to-one capture occurs when each pacemaker output pulse results in a contraction of the appropriate chamber.

Cardiac chambers. The heart consists of four pumping chambers. The upper chambers are called the atria (right an left) and the lower chambers are called the ventricles (right and left). The main and most important pumping chamber is the left ventricle since it pumps oxygenated blood to the body's tissues and organs. The right ventricle only pumps blood to the lungs. The atria are receiving reservoirs for blood coming back to the heart from the lungs and body tissues.

Cardiac conduction system. The route of normal flow for electrical impulses through the heart. It includes the SA node, intranodal atrial conduction tracts, interatrial conduction tract (Bachmann's bundle), AV node, bundle of His, right and left bundle branches and the Purkinje network.

Cardiac cycle. One complete heartbeat, seen on the ECG as a P wave, a QRS complex and a T wave. In a normal cardiac cycle, the atria depolarize and contract, delivering blood into the relaxed ventricles (P wave); the atria repolarize and relax while the ventricles, now filled to capacity with blood, contract and pump blood into the systemic and pulmonary circulation systems (QRS complex). After the ventricular contraction, the ventricles repolarize and relax (T wave). The cycle begins again at the end of ventricular repolarization with the onset of passive ventricular filling.

Cardiac muscle. A specially designed variety of striated muscle that makes up contractile portions of the heart. Cardiac muscle can contract indefinitely at varying rates to pump blood from the heart's chambers.

Cardiac output. The volume of blood, measured in liters, ejected by the heart per minute. Cardiac output is determined by multiplying the heart rate and the stroke volume.

Cardiac standstill. The complete absence of atrial and ventricular

Contractions in the heart. See also asystole.

Cardiac tamponade. The acute compression of the heart by fluid in the pericardial sac, preventing adequate filling and resulting in hypotension, a fall in cardiac output and possibly shock. If a pacing lead perforates the myocardium during pacemaker implantation, cardiac tamponade is a potential and serious complication.

Cardioversion. Conversion of a pathological cardiac rhythm, such as atrial fibrillation or ventricular tachycardia, to a normal sinus rhythm, usually accomplished by a cardioverter device which administers countershocks to the heart through electrodes placed on the chest wall or, more recently, through electrodes placed on or in the heart itself.

Cardioverter defibriIlator. A device for treating patients at risk of sudden factors, including how frequently the pulse generator paces and at what parameters, the circuitry and the type of pacing lead. Current drain is commonly expressed in millionths of an ampere or microamperes.

Crosstalk— In dual chamber pacemakers, the detection of the atrial output pulse by the ventricular sensing circuit, resulting in false inhibition. In the DDD mode, any ventricular sensing will inhibit the ventricular output and reset the timing.

Cycle length. A general term in pacing used for the interval between any one type of event and the next event of the same type, usually expressed in milliseconds. The pacing cycle length can be calculated by dividing 60,000 by the rate.