Rate Control in Atrial Fibrillation
In the treatment of atrial fibrillation, the control of the heart
rate is very important. The heart rate in atrial fibrillation is
determined by the number of impulses that goes from the upper
chambers, the atria, through to the lower chambers, the ventricles.
These electrical impulses travel from the upper chamber to the lower
chamber through the AV node. This is like a staircase that connects
the top floor of the building to the bottom floor of the building.
The AV node acts as an effective filter in blocking most of the beats
that go from the top chamber to the bottom chamber.
There are a number of reasons to control the heart rate during
atrial fibrillation. In many patients, controlling the heart rate
markedly improves their symptoms.
These symptoms may include:
- shortness of breath,
- dizziness,
- a racing
heart sensation,
- a pounding sensation, as if their heart
is going to jump out of their chest, or
- fatigue.
Determining the rate during atrial fibrillation, therefore, is an
important first step in treating atrial fibrillation. By assessing
the heart rate in atrial fibrillation, one can determine whether it is
necessary for medications to be used to control the heart rate further.
There are a number of different forms of electrocardiographic
monitoring or recording that can be used to determine whether the
heart rate control in atrial fibrillation is adequate. This may
include a standard routine echocardiogram (ECG, EKG) that may be
performed in a doctor’s office, in the hospital, or in an emergency
department. Alternately, the EKG recordings may be obtained using a
longer term monitor. These monitors may include a 24-hour recording
using a device that the patient carries with him or herself. This is
often called a Holter monitor. Other forms of monitoring include a
longer term monitor that lasts several weeks. This form of monitor is
able to capture the heart rate over a more extended period of time.
The appropriate heart rate for the patient will depend on the
patient’s heart condition, their needs at a particular period of time,
and their overall medical and cardiac status. Therefore it is not
possible to select a particular heart rate for all individuals. As a
rule of thumb, however, it is a common target that we will try to
achieve a heart rate of approximately 60 to 80 beats per minute while
at rest. In some cases, we will need to achieve a lower heart rate
such as 50 beats per minute, or possibly a higher heart rate between
80 and 90 beats per minute. For example, in some cases, patients may
be treated for various kinds of heart conditions such as artery
problems with the heart, the heart rate may need to be slower. On the
other hand, some individuals in the time of acute illness may need to
have a faster heart rate such as 80 beats per minute.
During any exertion, even mild exertion, many patients with atrial
fibrillation have an exaggerated heart rate increase. For example,
instead of their heart rate going from 60 or 80 to 100 beats per
minute with a mild walk, their heart rate might jump up to 120, 130 or
140 beats per minute. The goal of treatment therefore may also be to
prevent this increase, or to prevent a large increase in heart rate
with exertion.
Methods of controlling the heart rate, and slowing it, include
medications that have specific effects on the AV node, the staircase
that we have described between the upper and lower heart chambers. The
groups of medications used to control the rate fit into three
categories. These categories include beta-blockers, calcium channel
blockers, and digoxin.
Beta blockers and calcium channel blockers are used for other
conditions, such as high blood pressure. Beta blockers in particular
may also be used in certain heart conditions which may be described as
heart failure. Examples of beta blockers include metoprolol,
atenolol, and carvedilol. A list of a variety of beta blockers listed
in table #1. Beta blockers should be used with caution in patients
with asthma. Both beta blockers and calcium channel blockers may also
slow the heart rate in normal rhythm, and therefore need to be used
with caution in order to prevent the heart rate from being too slow.
Some patients with atrial fibrillation have a tendency to have slow
rates in normal rhythm and relatively rapid rates in atrial
fibrillation. This is because the heart’s own pacemaker, the sinus
node, is responsible for the rate in normal rhythm, and the sinus node
may have a tendency to be slow. On the other hand, the heart rate in
atrial fibrillation is largely dependent upon the properties of the AV
node, which is the connector between the upper and lower chambers. In
these same individuals, it is possible that the AV nodal function may
be quite intact, and therefore lead to a relatively rapid rate. Some
patients require a pacemaker to be implanted, so that their heart
rate, particularly in normal rhythm, does not get too slow, when
medications are used to decrease the rate during atrial fibrillation.
Digoxin is a medication that also may be used to control the heart
rate in atrial fibrillation. There are some medications that elevate
the digoxin level. The digoxin level may be assessed with a simple
blood test, and can be used to determine if the dose is appropriate,
in addition to following and assessing the patient’s heart rate.
Medications such as amiodarone may increase the digoxin level. Digoxin
is removed from the body by the kidneys, and therefore patients with
kidney impairment or decreased kidney function will generally need a
lower digoxin dose. Significantly elevated digoxin levels may lead to
symptoms such as nausea, vomiting and yellow vision. In addition,
there may be other beats, such as ventricular premature beats, or
other fast rhythms that can occur as a result of a high digoxin level.
Assessing rate control is usually performed by examining the heart
rate at rest with an electrocardiogram, the longer term monitoring as
discussed above, and the assessment of symptoms. If patients continue
to have a significant palpitation, or other symptoms, and their heart
rate remains elevated, additional medications are often prescribed.
In very selected patients, it may be extremely difficult to control
the heart rate. This can lead to severe symptoms that require repeated
visits to the emergency department. For such patients, it may be
necessary to consider alternatives to medications.