You can be born with heart block (congenital) or you
can acquire it. Acquired heart block is more common.
The three types of heart block are: first degree,
second degree, and third degree. First degree is the least severe and third
degree is the most severe. This is true for both congenital and acquired heart
block.
First-Degree Heart Block
In first-degree heart block, the electrical signal
is slowed as it moves through the heart. When this occurs between the atria and
the ventricles, it appears as a slightly longer, flatter line between the P and
the R waves on the
EKG.
First-degree heart block rarely causes any symptoms.
Well-trained athletes and young people are at higher risk for first-degree
heart block caused by an overly active vagus nerve. Activity in this nerve
slows the heart rate. Some medicines, such as digitalis, also may trigger
first-degree heart block.
First-degree heart block usually doesn't require
treatment.
Second-Degree Heart Block
In this type of heart block, electrical signals
between the atria and ventricles are slowed to a large degree. Some signals
can't reach the ventricles. On an EKG, the QRS wave doesn't follow each P wave
as it normally would.
If the signal is blocked before it reaches the
ventricles, they won't contract and pump blood to the lungs and the rest of the
body.
Second-degree heart block is divided into two
different types, Mobitz type I and Mobitz type II.
Mobitz Type I
In this type (also known as Wenckebach's block), the
electrical signals are delayed more and more with each heartbeat, until the
heart skips a beat. On the EKG, the delay is shown as a line (called the PR
interval) between the P and QRS waves. The line gets longer and longer until
the QRS wave doesn't follow the next P wave.
Sometimes people with Mobitz type I feel dizzy or
have other symptoms. This type of second-degree heart block is less serious
than Mobitz type II.
The animation below shows how your heart's
electrical system works and what happens during second-degree, Mobitz type I
heart block. Click the "start" button to play the animation. Written and spoken
explanations are provided with each frame. Use the buttons in the lower right
corner to pause, restart, or replay the animation, or use the scroll bar below
the buttons to move through the frames.
The first part of this animation is the same as the
animation in the section on understanding the hearts electrical system
and EKG results. If you want to skip directly to the frames on Mobitz type I,
click the "skip intro" link above the start, pause, and replay buttons.
The first part of the animation
shows how an electrical signal moves through your heart and how an EKG records
your heart's electrical activity. The second part of the animation shows how a
pause in the hearts electrical signal can delay and momentarily block the
contraction of the ventricles.
Mobitz Type II
In this type, some of the electrical signals also
don't reach the ventricles. However, the pattern is less regular than it is in
Mobitz type I. Some signals move between the atria and ventricles normally,
while others are blocked. On an EKG, the QRS wave follows the P wave at a
normal speed. Sometimes, though, the QRS wave is missing (when a signal is
blocked).
Mobitz type II is less common than type I, but it's
usually more severe. Some people with type II need medical devices called
pacemakers
to maintain their heart rates.
Third-Degree Heart Block
In this type of heart block, none of the electrical
signals reach the ventricles. This type also is called complete heart block or
complete AV block.
When complete heart block occurs, special areas in
the ventricles may create electrical signals to cause the ventricles to
contract. This natural backup system is slow and isn't coordinated with the
contraction of the atria. On an EKG, the normal pattern is disrupted. The
P waves occur at a faster rate than the QRS waves.
Complete heart block can be fatal. It can result in
sudden
cardiac arrest and death. This type of heart block needs emergency
treatment. A temporary pacemaker may be used to keep the heart beating until
you get a permanent pacemaker.