Tachycardia refers to an abnormally fast resting heart rate - usually at least 100 beats per minute.
The threshold of a normal heart rate (pulse) is generally based on the person's age. Tachycardia can be dangerous; depending on how hard the heart has to work.
In general, the adult resting heart beats between 60 and 100 times per minute (some doctors place the healthy limit at 90, so some of them may diagnose tachycardia at slightly lower than 100 beats per minute). When an individual has tachycardia the upper or lower chambers of the heart beat significantly faster - sometimes this happens to both chambers.
When the heart beats too rapidly, it pumps less efficiently and blood flow to the rest of the body, including the heart itself is reduced. The higher-than-normal heartbeat means there is an increase in demand for oxygen by the myocardium (heart muscle) - if this persists it can lead to myocardial infarction (heart attack), caused by the dying off of oxygen-starved myocardial cells.
Some patients with tachycardia may have no symptoms or complications. Tachycardia significantly increases the risk of stroke, sudden cardiac arrest or death.
Our heart rates are controlled by electrical signals which are sent across heart tissues. When the heart produces rapid electrical signals tachycardia occurs.
Causes of tachycardia
Tachycardia is generally caused by a disruption in the normal electrical impulses that control our heart's pumping action rhythm - the rate at which our heart pumps. The following situations, conditions and illnesses are possible causes:
- A reaction to certain medications
- Congenital (present at birth) electrical pathway abnormalities in the heart
- Congenital abnormalities of the heart
- Consuming too much alcohol
- Consumption of cocaine and some other recreational drugs
- Electrolyte imbalance
- Heart disease which has resulted in poor blood supply and damage to heart tissues, including coronary artery disease (atherosclerosis), heart valve disease, heart failure, heart muscle disease (cardiomyopathy), tumors, or infections.
- Hyperthyroidism (overactive thyroid gland)
- Certain lung diseases
Sometimes the medical team may not identify the exact cause of the tachycardia.
Atria, ventricles and the electrical circuitry of the heart
The human heart consists of four chambers:
- Atria - the two upper chambers; a left atrium and a right atrium.
- Ventricles - the two lower chambers; a left ventricle and a right ventricle.
The heart has a natural pacemaker called the sinus node; it is located in the right atrium. The sinus node produces electrical impulses; each one triggers an individual heartbeat.
The electrical impulses leave the sinus node and go across the atria, making the atria muscles contract. This atria muscle contraction pushes blood into the ventricles.
The electrical impulses continue to the atrioventricular node (AV node), a cluster of cells. The AV node slows down the electrical signals, and then sends them on to the ventricles. By delaying the electrical signals the AV node is able to give the ventricles time to fill with blood first. When the ventricle muscles receive the electrical signals they contract, pumping blood either to the lungs or the rest of the body.
When there is a problem with the electrical signals resulting in a faster-than-normal heartbeat, the patient has tachycardia. The most common types of tachycardia include:
When the two upper chambers - the atria - contract at an excessively high rate, and in an irregular way, the patient has atrial fibrillation. During atrial fibrillation the contractions of the two upper chambers of the heart are not synchronized with the contractions of the two lower chambers, causing rapid and irregular heartbeats. Atrial fibrillation is caused by chaotic electrical impulses in the atria; the AV node is bombarded with chaotic signals. An atrial fibrillation episode may last from a few hours to several days. Sometimes the episode does not go away without treatment. Most atrial fibrillation patients have some heart abnormality related to the condition.
The atria beats rapidly, but regularly. It is caused by a circuit problem within the atria. The contractions of the atria are weak because of the rapid heartbeat. There is a rapid and sometimes irregular ventricular rate, caused by rapid signals entering the AV node. An atrial flutter episode may last a few hours or some days. Sometimes it may not go away until treated. Atrial flutter is sometimes a complication of surgery, but it also can be caused by various forms of heart disease. Patients with atrial flutter commonly experience atrial fibrillation too.
Supraventricular tachycardias (SVTs)
Any tachycardic (accelerated) heart rhythm originating above the ventricular tissue. The abnormal circuitry in the heart if usually congenital (present at birth) and creates a loop of overlapping signals. An SVT episode may last from a few seconds to several hours. In one SVT form the AV node splits the electrical signal in two, with one signal going to the ventricles while the other goes back to the atria. There may also be an extra electrical pathway from the atria to the ventricles, effectively bypassing the AV node and resulting in a signal going down one pathway and up the other.
Abnormal electrical signals in the ventricles result in a rapid heart rate. The speed of the heart beat does not allow the ventricles to fill and contract properly, resulting in poor blood supply to the body. This type of tachycardia is frequently a life threatening condition and is treated as a medical emergency. Ventricular tachycardia is linked to heart muscle damage from a previous heart attack or cardiomyopathy (disease of the heart muscle).
The ventricles quiver in an ineffective way, resulting in poor blood supply to the body. If normal heart rhythm is not restored rapidly, blood circulation will cease and the patient will die. Patients with an underlying heart condition, or those who have been struck by lightning causing serious trauma may experience ventricular fibrillation.