Causes of Abnormal Heart Rhythms
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Abnormal heart rhythms (arrhythmias) are sequences of
heartbeats that are irregular, too fast, too slow, or conducted via an abnormal
electrical pathway through the heart.
Heart disorders are the most common cause of an abnormal
heart rhythm.
Sometimes people are aware of abnormal heart rhythms, but
many times they feel only their consequences, such as weakness or fainting.
The diagnosis is based on electrocardiography.
Treatment involves restoring the heart to a normal rhythm
and preventing further episodes.
The heart is a muscular organ with four chambers designed to
work efficiently, reliably, and continuously over a lifetime. The muscular
walls of each chamber contract in a regulated sequence, pumping blood as
required by the body while expending as little energy as possible during each
heartbeat.
Contraction of the muscle fibers in the heart is controlled
by electricity that flows through the heart in a precise manner along distinct
pathways at a controlled speed. The electrical current that begins each heartbeat
originates in the heart’s pacemaker (called the sinus node or sinoatrial node),
located in the top of the upper right heart chamber (right atrium). The rate at
which the pacemaker discharges the electrical current determines the heart
rate. This rate is influenced by nerve impulses and by levels of certain
hormones in the bloodstream.
Conduction System
Conduction System
The heart rate is regulated automatically by the autonomic
nervous system, which consists of the sympathetic and parasympathetic
divisions. The sympathetic division increases the heart rate through a network
of nerves called the sympathetic plexus. The parasympathetic division decreases
the heart rate through a single nerve, the vagus nerve.
Heart rate is also influenced by hormones released into the
bloodstream by the sympathetic division:
Epinephrine (adrenaline)
Norepinephrine (noradrenaline)
Epinephrine and norepinephrine increase the heart rate.
Thyroid hormone, which is released into the bloodstream by the thyroid gland,
also increases the heart rate.
In an adult at rest, the normal heart rate is usually
between 60 and 100 beats per minute. However, lower rates may be normal in
young adults, particularly those who are physically fit. A person’s heart rate
varies normally in response to exercise and such stimuli as pain and anger.
Heart rhythm is considered abnormal only when the heart rate is inappropriately
fast (called tachycardia), slow (called bradycardia), or irregular or when
electrical impulses travel along abnormal pathways.
Normal electrical pathway
The electrical current from the sinoatrial node flows first
through the right atrium and then through the left atrium, causing the muscles
of these chambers to contract and blood to be pumped from the atria into the
lower heart chambers (ventricles). The electrical current then reaches the
atrioventricular node, located in the lower part of the wall between the atria
near the ventricles. The atrioventricular node provides the only electrical
connection between the atria and ventricles. Otherwise, the atria are insulated
from the ventricles by tissue that does not conduct electricity. The
atrioventricular node delays transmission of the electrical current so that the
atria can contract completely and the ventricles can fill with as much blood as
possible before the ventricles are electrically signaled to contract.
Tracing the Heart’s Electrical Pathway
The sinoatrial (sinus) node (1) initiates an electrical
impulse that flows through the right and left atria (2), making them contract.
When the electrical impulse reaches the atrioventricular node (3), it is
delayed slightly. The impulse then travels down the bundle of His (4), which
divides into the right bundle branch for the right ventricle (5) and the left
bundle branch for the left ventricle (5). The impulse then spreads through the
ventricles, making them contract.
Tracing the Heart’s Electrical Pathway
Electrical Conduction System of the Heart
electrical_conduction_system_high_blausen
After passing through the atrioventricular node, the
electrical current travels down the bundle of His, a group of fibers that
divide into a left bundle branch for the left ventricle and a right bundle
branch for the right ventricle. The electrical current then spreads in a
regulated manner over the surface of the ventricles, from the bottom up,
initiating contraction of the ventricles, which eject blood from the heart.
Causes of Abnormal Heart Rhythms
The most common cause of arrhythmias is a heart disorder,
particularly coronary artery disease, heart valve disorders, and heart failure.
Many drugs, prescription or nonprescription, including those used to treat
heart disorders, can lead to arrhythmias. Some arrhythmias are caused by
anatomic abnormalities present at birth (congenital birth defects). Age-related
changes in the heart’s electrical system make some arrhythmias more likely.
Sometimes no cause for an arrhythmia can be identified.
Fast arrhythmias
Fast arrhythmias (tachyarrhythmias) may start on their own
or be triggered by exercise, emotional stress, excessive alcohol consumption,
smoking, or use of drugs that contain stimulants, such as cold and hay fever
remedies.
An overactive thyroid gland (hyperthyroidism), producing
high levels of thyroid hormone, may cause fast arrhythmias.
Slow arrhythmias
Slow arrhythmias (bradyarrhythmias) may be triggered by
pain, hunger, fatigue, digestive disorders (such as diarrhea and vomiting), or
swallowing, which can stimulate the vagus nerve excessively. With enough
stimulation, which is rare, the vagus nerve can cause the heart to stop for a
moment. In most of these circumstances, the arrhythmia tends to resolve on its
own.
An underactive thyroid gland (hypothyroidism), producing low
levels of thyroid hormone, may cause slow arrhythmias.
Symptoms of Abnormal Heart Rhythms
Some people who have abnormal heart rhythms may be aware of
them. However, awareness of heartbeats (called palpitations) varies widely
among people. Some people can feel normal heartbeats, and most people can feel
heartbeats when they lie on their left side.
Arrhythmias have consequences that range from harmless to
life threatening. The seriousness of an arrhythmia may not be closely linked
with the severity of the symptoms it causes. Some life-threatening arrhythmias
cause no symptoms, and some otherwise inconsequential arrhythmias may cause
severe symptoms. The nature and severity of the underlying heart disorder are
often more important than the arrhythmia itself.
When arrhythmias impair the heart’s ability to pump blood,
they can cause weakness, a reduced capacity for exercise, shortness of breath,
light-headedness, dizziness, fainting (syncope), or death. Fainting occurs when
the heart is pumping so inefficiently that it can no longer maintain adequate
blood pressure. If such an arrhythmia persists, death may result. Arrhythmias
may also aggravate the symptoms of an underlying heart disorder, including
chest pain and shortness of breath. Arrhythmias that cause symptoms require
prompt attention.
Did You Know...
Some otherwise inconsequential arrhythmias may cause
troubling symptoms, while some life-threatening arrhythmias may cause no
symptoms.
Diagnosis of Abnormal Heart Rhythms
Electrocardiography
Often, a person’s description of symptoms can help doctors
make a preliminary diagnosis and determine the severity of the arrhythmia. The
most important considerations are whether the palpitations are
Fast or slow
Regular or irregular
Brief or prolonged
Another important consideration is whether the arrhythmia
causes symptoms.
Doctors also need to know whether the palpitations occur at
rest or only during strenuous or unusual activity and whether they start and
stop suddenly or gradually.
Certain diagnostic procedures are usually needed to
determine the exact nature of the arrhythmia and its cause.
Electrocardiography (ECG or EKG) is the main diagnostic
procedure for detecting arrhythmias and determining their cause. This procedure
provides a graphic representation of the electrical current producing each
heartbeat. Usually, ECG records the heart rhythm for only a very short time.
Because arrhythmias are often intermittent, a portable ECG
monitor (called a Holter monitor or an event monitor) may be used to record
heart rhythm continuously or when the wearer senses an abnormal heart rhythm
and activates the monitor. This monitor, usually worn for 24 or 48 hours, can
record sporadic arrhythmias as the person engages in normal daily activities.
During the recording period, the person also keeps a diary of symptoms and
activities, which are correlated with the arrhythmias.
To detect dangerous arrhythmias that occur very
infrequently, doctors sometimes implant a recording device under the skin below
the left collarbone (clavicle). The device can be left in place for long
periods. It electronically transmits stored recordings of abnormal heart
rhythms painlessly through the skin.
People with suspected life-threatening arrhythmias are
usually hospitalized. Their heart rhythm is continuously recorded and displayed
on a television-type monitor by the bedside or at the nursing station. Thus,
any problems can be identified promptly.
Other diagnostic procedures include
Exercise stress testing
Blood pressure measurement during exercise
Echocardiography to detect anatomic abnormalities
Electrophysiologic testing
During electrophysiologic testing, catheters with tiny
electrodes at their tip are inserted through a vein and threaded into the
heart. The electrodes are used to stimulate the heart, and the heart’s response
is monitored, so that the type of arrhythmia and the preferred treatment
options can be determined.
ECG: Reading the Waves
ECG: Reading the Waves
An electrocardiogram (ECG) represents the electrical current
moving through the heart during a heartbeat. The current's movement is divided
into parts, and each part is given an alphabetic designation in the ECG.
Each heartbeat begins with an impulse from the heart's
pacemaker (sinus or sinoatrial node). This impulse activates the upper chambers
of the heart (atria). The P wave represents activation of the atria.
Next, the electrical current flows down to the lower
chambers of the heart (ventricles). The QRS complex represents activation of
the ventricles.
The electrical current then spreads back over the ventricles
in the opposite direction. This activity is called the recovery wave, which is
represented by the T wave.
Many kinds of abnormalities can often be seen on an ECG.
They include a previous heart attack (myocardial infarction), an abnormal heart
rhythm (arrhythmia), an inadequate supply of blood and oxygen to the heart
(ischemia), and excessive thickening (hypertrophy) of the heart's muscular walls.
Certain abnormalities seen on an ECG can also suggest bulges
(aneurysms) that develop in weak areas of the heart's walls. Aneurysms may
result from a heart attack. If the rhythm is abnormal (too fast, too slow, or
irregular), the ECG may also indicate where in the heart the abnormal rhythm
starts. Such information helps doctors begin to determine the cause.
Prognosis of Abnormal Heart Rhythms
Most arrhythmias neither cause symptoms nor interfere with
the heart’s ability to pump blood. Thus, they usually pose little or no risk,
although they can cause considerable anxiety if a person becomes aware of them.
However, some arrhythmias, harmless in themselves, can lead to more serious
arrhythmias. Any arrhythmia that impairs the heart’s ability to pump blood
adequately is serious. How serious depends in part on whether the arrhythmia
originates in the sinoatrial node, in the atria, in the atrioventricular node,
or in the ventricles. Generally, arrhythmias that originate in the ventricles
are more serious than those that originate in the atria, which are more serious
than those that originate in the sinoatrial node or atrioventricular node.
However, there are many exceptions.
Treatment of Abnormal Heart Rhythms
Antiarrhythmic drugs, usually for a fast heart rate
Pacing, usually for a slow heart rate
Delivering an electric shock, usually for a fast heart rate
For people who have a harmless yet bothersome arrhythmia,
reassurance that the arrhythmia is harmless may be treatment enough. Sometimes
arrhythmias occur less often or even stop when doctors change a person’s drugs
or adjust the dosages. Avoiding alcohol, caffeine (in beverages and foods), and
smoking may also help. Avoiding strenuous exercise may help if palpitations
occur only during exercise. Sometimes people need to stop driving until doctors
can determine whether treatment is effective.
Drugs
Antiarrhythmic drugs are useful for suppressing fast
arrhythmias that cause intolerable symptoms or pose a risk. No single drug
suppresses all arrhythmias in all people. Sometimes several drugs must be tried
until the response is satisfactory. Sometimes antiarrhythmic drugs can worsen
or even cause arrhythmias. This effect is called proarrhythmia. Antiarrhythmic
drugs may also cause other side effects.
Artificial pacemakers
Artificial pacemakers are electronic devices that act in
place of the heart’s own pacemaker, the sinoatrial node. These devices are
implanted surgically under the skin, usually below the left or right
collarbone. They are connected to the heart by wires (leads) running inside a
vein. The low-energy circuitry and battery designs now in use allow these units
to function about 10 to 15 years.
Implanted Pacemaker
pacemaker_placement_high_blausen
Some people may be candidates for leadless pacemakers. With
this type of pacemaker, no wires are needed to connect the pacemaker to the
heart. Doctors make a small incision in the groin and use a catheter to insert
the pacemaker directly into the bottom right heart chamber (the right
ventricle).
New circuitry has almost completely eliminated the risk of
interference from cell phones, automobile ignition systems, radar, microwaves,
and airport security detectors. However, some equipment may interfere with
pacemakers. Examples are electrocautery devices used to stop bleeding during
surgery, diathermy (physical therapy treatments that use radiowaves to apply
heat to muscles), and sometimes magnetic resonance imaging (MRI). MRI may be
safe with certain types of pacemaker, depending on how they are constructed.
Keeping the Beat: Artificial Pacemakers
Artificial pacemakers are electronic devices that act in
place of the heart’s natural pacemaker (the sinus or sinoatrial node). They
generate electrical impulses that initiate each heartbeat. Pacemakers consist
of a battery, an impulse generator, and wires that connect the pacemaker to the
heart.
An artificial pacemaker is implanted surgically. After a
local anesthetic is used to numb the insertion site, the wires that connect the
pacemaker are usually inserted into a vein near the collarbone and threaded toward
the heart. Through a small incision, the impulse generator, which is about the
size of a silver dollar, is inserted just under the skin near the collarbone
and connected to the wires. The incision is stitched closed. Usually, the
procedure takes about 30 to 60 minutes. The person may be able to go home
shortly afterward or may briefly stay in the hospital. The battery for a
pacemaker usually lasts 10 to 15 years. Nevertheless, the battery should be
checked regularly. Battery replacement is a quick procedure.
There are different types of pacemakers. Some take over the
control of the heart rate, overriding the electrical impulses generated by the
heart. Others, called demand pacemakers, allow the heart to beat naturally
unless it skips a beat or begins to beat at an abnormal rate. Still others,
called programmable pacemakers, can do either. Some pacemakers can adjust their
rate depending on the wearer’s activity, increasing the heart rate during
exercise and decreasing it during rest.
Keeping the Beat: Artificial Pacemakers
The most common use of pacemakers is to treat slow
arrhythmias. When the heart slows below a set threshold, the artificial
pacemaker begins to produce electrical impulses. Less commonly, pacemakers are
used to treat fast arrhythmias by delivering a series of impulses to decrease
the heart rate by converting the fast arrhythmia back to normal.
Cardiac resynchronization therapy (CRT) is another use for
pacemakers. In some people with heart disorders, the four heart chambers do not
follow their normal, orderly sequence of contractions. Special pacemakers with
three leads can restore the normal sequence of contractions and improve outcome
in some people with heart failure.
Restoring normal rhythm
Sometimes an electrical shock to the heart can stop a fast
arrhythmia and restore normal rhythm. Using an electrical shock for this
purpose is called cardioversion, defibrillation, or electroversion, depending
on the type of abnormal rhythm for which it is used.
Cardioversion may be used for arrhythmias starting in the
atria (such as atrial fibrillation) or the ventricles (such as ventricular
fibrillation). However, an electrical shock cannot restart a heart that has no
electrical activity at all (asystole). The machine that delivers the shock (a
defibrillator) is used by a team of doctors and nurses, by paramedics, or by
firefighters.
Cardioversion
cardioversion_high_blausen
An implantable cardioverter-defibrillator (ICD), which is
about one half the size of a deck of cards, can be placed. Most devices are
implanted through the blood vessels just as a pacemaker is, thus eliminating
the need for open chest surgery. Another type of ICD is placed under the skin
rather than through the blood vessels.
A wearable vest-like defibrillator is sometimes used if the
person does not need an ICD for a long time.
ICDs continually monitor the rate and rhythm of the heart,
automatically detect fast arrhythmias, and deliver either a pacing treatment or
a shock to convert the arrhythmia back to a normal rhythm. Most commonly, these
devices are used in people who might otherwise die of the arrhythmia. An ICD
can also act like a pacemaker, sending electrical impulses to overcome a slow
arrhythmia. When an ICD delivers a shock, it can feel like a mild thump in the
chest. When a stronger shock is given, people may feel as if they have been
kicked.
People who have ICDs can safely be around most home
electronic devices, including microwaves, and airport security detectors.
However, some equipment with strong magnetic fields or strong electric fields
may interfere with ICDs. Examples are electrocautery devices used to stop
bleeding during surgery, diathermy (physical therapy treatments that use
radiowaves to apply heat to muscles), and sometimes MRI.
Because ICDs do not prevent arrhythmias, drugs often must be
taken as well. These devices last for about 5 to 7 years. People with an ICD
who experience a single shock from the device and who otherwise feel well
should contact their ICD clinic or specialist within the week. The device
records the person's heart rhythm, thus allowing the doctor to see why a shock
was given. People who had other symptoms, such as shortness of breath, chest
discomfort, or palpitations, immediately before or after the shock, or who had
multiple shocks may have a more serious problem. Such people should go to the
emergency department right away.
An automated external defibrillator (AED) requires only
minimal training for its use. For example, AEDs can be used by people who receive
first-aid instruction in its use. AEDs can detect the presence of an
arrhythmia, determine if a shock is advisable, and deliver the shock
automatically. They are present in many public places, such as airports, sports
arenas, hotels, and shopping malls.
Destroying abnormal tissue (ablation)
Certain types of arrhythmias can be controlled by doing
surgical and other invasive procedures. An arrhythmia due to a localized
abnormal area in the heart’s electrical system can be controlled by destroying
or removing that area (ablation).
Most often, the abnormal area is destroyed by radiofrequency
ablation (delivery of energy of a specific frequency through a catheter with a
tiny electrode at its tip that is inserted into the heart). The success of the
procedure is different for different arrhythmias ranging from 60 to 80% for
more difficult arrhythmias (atrial fibrillation, atrial tachycardia, and
ventricular tachycardia) to 90 to 95% for more responsive arrhythmias
(supraventricular tachycardias). The procedure takes several hours, and the
person is often able to go home the same day.
Ablation may also be done by freezing tissue (called
cryoablation).
Before ablation can be done, doctors do electrophysiologic
testing to identify the areas that need to be destroyed or removed.
Less commonly, the area is destroyed or removed during open
heart surgery. Sometimes surgery is needed because catheter-based ablation was
not effective. Other times, surgery is used because people are having heart
surgery for another reason, such as to replace a heart valve.
Radiofrequency Ablation
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