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Cardiac Arrhythmia

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anthony deldin

on 17 April 2013

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Transcript of Cardiac Arrhythmia

Anthony Deldin Mechanisms of Cardiac Arrhythmias Your heart pumps nearly 5 quarts of blood through your body every minute.
Even while you are sitting still, your heart beats (expands and contracts) 60 to 100 times each minute.
These heartbeats are triggered by electrical impulses that begin in your heart's natural pacemaker, called the sinoatrial node (SA node). Arrhythmias Occur when the electrical impulses in your heart that coordinate your heartbeats don't work properly, causing your heart to beat too fast, too slow or irregularly.
May feel like a fluttering or racing heart
often harmless
Some arrhythmias may cause bothersome, sometimes even life-threatening, signs and symptoms.
Categories of Arrhythmias Ventricular
Heart’s two lower chambers (ventricles)
Atrial or Supraventricular
Heart’s upper chambers (atria)
Defined by speed of heart beats
HR < 60 BPM
HR > 100 BPM
Most serious
Fast, uncoordinated beats
Heart block
SA node’s electrical signal cannot travel to the heart’s lower chambers
What causes an arrhythmia? Congenital arrhythmia
Some medical conditions
types of heart disease
high blood pressure
hemochromatosis (iron build-up in the body)
OTC cough and cold medicines
Symptoms Often no signs or symptoms
Routine exams may detect arrhythmias
Signs and symptoms do not always mean there's a problem
Noticeable symptoms may include:
A fluttering in your chest
A racing heartbeat (tachycardia)
A slow heartbeat (bradycardia)
Chest pain
Shortness of breath
Fainting (syncope) or near fainting
The heart beats too fast: Tachycardia (also called tachyarrhythmia) There are two major mechanisms for inducing tachycardias:
focal activity
re-entrant activity
Focal activity means that an abnormal site is generating impulses.
This site is often called “ectopic”, which really means outside the normal location (i.e. the sinus node).
The site of ectopic activity can be located anywhere in the heart, in the right or left atria, in the right or left ventricle, in the AV-node etc.
A second mechanism is called re-entry.
In re-entry, the impulse turns around in a loop or a circuit. This is also called a circus movement arrhythmia.
Mechanisms of Focal Arrhythmias: There are several mechanisms that may induce focal arrhythmias:
a. Enhanced Diastolic Depolarization

b. Triggered Activity
early after depolarization
delayed after depolarization
Mechanisms of Re-entrant Arrhythmias: Types of Re-entrant Arrhythmias: In the previous examples, there was only one circuit revolving either around a ‘hole’ (= anatomical re-entry) or around its own refractory tail (= functional re-entry).
But what happens if there is more than one impulse revolving around in the heart?
This is shown in the next simulation where 5-6 circus movements are simultaneously taking place.
Obviously, in this situation, there is no longer any rhythm in the heart. Normal rhythm has now been replaced by chaos. This chaos is called ‘fibrillation’ Fibrillation The heart beats too slow: Bradycardia Sick Sinus Syndrome
In this disease, the sinus node is not functioning properly and action potentials are either not initiated or they have difficulty propagating into the right atrium.
The major problem is fibrosis of the sinus node area, mainly due to old age.
Therapy: implantation of an artificial pacemaker that will stimulate the heart if the sinus node stops.
The ECG looks normal (normal P, QRS, T waves and normal PQ and ST intervals). The heart just beats too slowly. Enhanced Diastolic Depolarization: This occurs in the SA-node, usually by excessive sympathetic activity. Because the depolarization is faster, the potential will reach threshold quicker and produce action potentials at a faster rate. An example of this mechanism is sinus tachycardia. Triggered activity: Early After Depolarization Sometimes, during the plateau phase of the action potential, a spontaneous depolarization may occur. This is often the case when there is too much calcium in the cell. These depolarizations may reach threshold and induce, too soon, a new action potential. Triggered activity: Delayed After Depolarization This is similar to the early after depolarizations but these occur after full repolarization has taken place, hence their name, delayed (or late) after depolarizations Re-entry literally means ‘to re-enter’ or ‘to come back’. In this case, it means that the action potential in the heart ‘comes back’ or re-excites itself. It can only do that if it turns around and ‘bite in its own tail’, just like the fox in Firefox. Because the path of the impulse is no longer straight but propagates in a circle, this type of re-entry is often also called ‘circus movement’ arrhythmia.
Note that if the impulse runs in a circle, it can only re-excite the cells in front of the advancing wave if those cells have recovered from the previous excitation. In other words, the refractory period of the cells now becomes very important.
If you remember your cellular electrophysiology, the refractory period consists of two parts:
the absolute refractory period
the relative refractory period If the impulse propagates in a straight line, the absolute refractory period follows immediately after the depolarization. The relative refractory period occurs after the absolute refractory period. But in a circle, the depolarization may bite in its own tail, which may consist of relative refractory tissue. In fact, the circle cannot become ‘smaller’ than the (total) refractory period! In general, there are two types of re-entrant arrhythmias
anatomical re-entry
functional re-entry The anatomical re-entry occurs around an obstacle, such as dead tissue, for example caused by an old infarct.
In the following movie, I have simulated the initiation of such an anatomical re-entry. I first start with a normal focal activity: In the next movie, I have introduced a ‘hole’. In this situation, two impulses will propagate around the block, collide against each other and thereby stop this strange propagation. But, suppose that one of the two impulses was blocked at an earlier stage? Then the other impulse can continue to propagate around this hole. In fact, it will continue forever! This is an arrhythmia! Because the impulse is running around an obstacle, this is called an anatomical re-entry or an anatomical circus movement. And if there is no hole at all? Yes, you can still induce re-entry. But there is now no hole around which the impulse propagates. It now actually propagates around its own refractory tail! This is called ‘functional re-entry’. Thank You
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