The heart beats throughout life and is essential to maintain life. This beating is the contraction and relaxation of the heart muscle which allows it to pump blood. The heartbeat as it is known needs to be adequate for the body’s needs. This normal heart rate varies among individuals but is usually above 60 beats per minute and less than 100 beats per minute, which s considered the normal range.
Therefore a heart rate below 60 beats per minute or above 100 beats per minute is considered abnormal in a person who is awake and not under any physical or emotional stress. However, there are instances where these abnormalities may be physiologically acceptable, particularly in professional athletes who have lower than normal heart rates or momentarily during stress which causes a higher than normal heart rate.
How does the heart beat?
The heart is a muscular pump that beats due to electric impulses. The muscles contract and relax in response to the electric impulses. These impulses are generated within the heart in the sinoatrial node (sinus node). It is also known as the natural pacemaker. These electrical impulses then need to be transmitted throughout the heart where it causes different parts of the heart to contract and relax at different times.
These impulses travel via the internodal pathways to the atrioventricular node (AV node). A delay between the impulses to the atria and ventricles of the heart ensures that blood is pushed in cyclical manner between the chambers or out 0f the heart. It is the contraction of the ventricles that are felt as the pulse in the different areas of the body. If it is too rapid then it is known as tachycardia. If it is too slow then it is known as bradycardia.
What is Tachycardia?
Tachycardia means a heart rate of 100 beats per minute (100 bpm) or more. The heart rate can be assessed by measuring the pulse at the wrist (radial), elbow (brachial), neck (carotid), thigh (femoral), knee (popliteal), ankle (posterior tibial) or foot (dorsalis pedis). Tachycardia is an abnormally high rate and is commonly seen temporarily during periods of stress, pain or strenuous activity.
Sometimes tachycardia may also be due to certain medical conditions. In these cases it is more likely to be prolonged, lasting for hours, days, months and even years. Left unattended, it will disrupt the normal heart activity and possibly lead to cardiac arrest. Most adults will have a heart rate of 65 to 85 bpm during rest or without strenuous activity.
How does tachycardia occur?
It is important to note that tachycardia in an otherwise healthy heart is not abnormal. However, it is short lived. Prolonged tachycardia is due to some abnormality or disease and it can be dangerous. There are three ways in which tachycardia may occur.
The natural pacemaker (sinus node) generates more frequent impulses which causes the heart to beat faster.. This is referred to as increased automacity (sinus tachycardia). This can happen with stimulation from the central nervous system, hormones, drugs and certain toxins.
Another way in which tachycardia can occur is where an impulse may travel through different pathways in the heart where one pathway allows the impulses to travel faster than the other.The slower traveling impulse may then pass back into the faster pathway to cause an additional beat. This is known as re-entry.
The changes in the muscle cell (depolarization and repolarization) necessary for contractions may be hampered in diseased muscle tissue. Therefore a contraction may occur prematurely. This is known as triggered activity.
Read more on electrical heart activity.
Symptoms of a Rapid Heart Rate
The following symptoms may arise due to tachycardia.
- Dizziness or fainting (extreme tachycardia)
- Chest pain or discomfort
- Shortness of breath (dyspnea)
It is not uncommon for people with persistent tachycardia to not experience any notable symptoms. In some of these cases, a person may only notice symptoms occasionally like when the abnormally high heart rate rises to extremes or when it drops to an otherwise normal level. In extreme cases, tachycardia can result in cardiac arrest may occur. This is where the heart stops or fails to pump effectively. Blood can therefore not be circulated properly and oxygen supply is hampered. Death can occur.
Causes of a Fast Pulse
- Heart disease
- Myocardial infarction (heart attack)
- Cardiac failure
- Chronic ischemic heart disease.
- Drugs – usually high doses of the following :
– Beta blockers
– Anti-arrhythmic drugs
– Antibiotics like erythromycin
– Anti-psychotic drugs
– Antimalarial drugs
- Lung diseases that affect perfusion, including infections like pneumonia and chronic obstructive pulmonary disorders (COPD)
What is Bradycardia?
Bradycardia refers to abnormally slow heart rate which is less than 60 beats per minute (BPM). It may also be referred to as a slow pulse or low pulse but should not be confused with a weak pulse. Although the normal heart rate is above 60 beats per minute, athletes who are very physically fit may have a normal heart rate under 60 beats per minute. This is not considered abnormal.
It is also not abnormal for bradycardia to occur in any person when asleep. However, the heart rate rises above 60 beats per minute upon waking. Any pulse reading that is recorded at less than 60 bpm indicates a heart that is beating too slowly. This means that the cardiac output is low – the heart is pumping out less blood than it should within a given period of time. If too low, the insufficient blood with oxygen will be distributed throughout the body.
How does bradycardia occur?
The heart rate is determined by the impulses emanating from the sinoatrial node (sinus node). These impulses travel through the internodal pathways to the atrioventricular node (AV node). Here the impulses are delayed so that the heart atria, which receive blood, do not contract at the same time as the heart ventricles, which push out blood.
It is the ventricular contraction that is felt as the pulse at various parts of the body. In bradycardia, the sinus node is either generating impulses at a slower rate (sinus bradycardia) or AV node is not passing out the impulses effectively (AV block).
Symptoms of Slow Heart Rate
A slow heart rate in athletes and when asleep or very relaxed is due to reduced sinus node impulses (sinus bradycardia). This is also known as asymptomatic sinus bradycardia because a person does not feel unwell with such a slow heart rate and it is not due to any disease process.
Pathological bradycardia may result in a number of symptoms including :
- Blurred vision
Causes of a Low Pulse Rate
The following conditions may cause sinus bradycardia or affect the AV node.
- Coronary artery disease
- Myocardial infarction (heart attack)
- Heart infections – endocarditis, myocarditis
- Drugs like beta blockers, verapamil, digoxin
- Raised intracranial pressure
- Obstructive jaundice
- Sick sinus syndrome (diseases affecting the sinus node)
- Hypersensitivity of the carotid sinus (carotid artery)
- Heart block (AV block)