Tachycardia

Tachycardia is a medical term for a heart rate that is faster than the normal resting rate. The word comes from Greek, combining "tachys," meaning quick, and "kardia," meaning heart. In adults, a resting heart rate exceeding 100 beats per minute (bpm) is generally accepted as tachycardia. While a fast heart rate can be a normal response to physical exercise, it can also indicate abnormal electrical problems within the heart. When the heart beats too rapidly, it may pump blood less efficiently. This can reduce blood flow to the rest of the body and even to the heart muscle itself. This increased demand for oxygen can lead to a condition called rate-related ischemia.

Doctors use a tool called an electrocardiogram, or ECG, to classify different types of tachycardia. They look specifically at the QRS complex, which is a part of the electrical signal recorded by the ECG. Tachycardias are divided into two main categories: narrow complex and wide complex. A narrow complex has a duration of 0.1 seconds or less. Narrow complex tachycardias, also called supraventricular tachycardias, usually originate in the atria, the upper chambers of the heart. Wide complex tachycardias typically originate in the ventricles, which are the lower chambers.

There are many specific types of narrow complex rhythms. Sinus tachycardia is one of the most common and starts in the sino-atrial (SA) node near the superior vena cava. Atrial fibrillation is another common type, often appearing as an irregular rhythm. Atrial flutter and atrial tachycardia are also distinct types. Other forms include AV nodal reentrant tachycardia (AVNRT), which is the most common reentrant tachycardia, and junctional tachycardia, which originates in the AV junction. Some types, like AV reentrant tachycardia (AVRT), require an accessory pathway to maintain the rhythm. This can involve orthodromic conduction, which results in a narrow complex, or antidromic conduction, which can appear as a wide complex.

Wide complex tachycardias include ventricular tachycardia (VT), which originates in the ventricles. VT is often a regular rhythm with a rate between 120 and 250 bpm. A serious subvariant of VT is called torsades de pointes, which means "twisting of the points" due to its appearance on an ECG. If VT persists, it can lead to ventricular fibrillation, which is extremely dangerous. Other wide complex rhythms might include supraventricular tachycardia with aberrancy, where a narrow complex rhythm is combined with a conduction system problem. Pacemaker-mediated tachycardia can also present in this way.

Many different factors can trigger tachycardia. Some are simple physiological responses, like fever, dehydration, or anxiety. The body uses feedback mechanisms to maintain blood pressure; for example, reflex tachycardia occurs when blood pressure drops. Physical or psychological stress can increase sympathetic nervous system stimulation. This triggers the release of hormones like epinephrine, also known as adrenaline. Certain medical conditions, such as hyperthyroidism, anemia, or pheochromocytoma, can also cause a rapid heart rate. Even metabolic issues, like McArdle's disease, can cause an "inappropriate" sinus tachycardia during exercise because the muscles cannot create energy efficiently.

Because heart rates vary by age, doctors use specific cutoff values to diagnose tachycardia. For a newborn only 1 to 2 days old, the threshold is over 159 bpm. For infants aged 3 to 5 months, it is over 186 bpm. Children aged 8 to 11 years have a cutoff of 130 bpm. For teenagers and adults, the standard cutoff is 100 bpm. Doctors must always consider these numbers within the patient's full clinical picture. They must also watch for complications like fainting or thrombosis. Thrombosis, or blood clots within vessels, can occur due to turbulence caused by rapid blood flow.

Managing tachycardia depends on whether a patient is stable or unstable. An unstable patient is one whose organ functions are affected or who is at risk of cardiac arrest. For unstable patients with narrow complex tachycardia, doctors may use intravenous adenosine. For other unstable types, immediate cardioversion is recommended. A stable patient is one where the fast heart rate is not an immediate threat. For stable patients, doctors might use vagal maneuvers, which are physical actions to slow the heart. If the cause is chronic, the tachycardia may return unless the underlying problem is corrected.