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Lecture 17 - Drugs: From Molecules to Man - 16/04/18
Cardiac Arrhythmias and Antidysrhythmic
HEART IS AN ELECTRICALLY CONTROLLED PUMP:-Electrical activity starts in the SA
node, which is the primary pacemaker, and then spreads through the atria to the AV node.
From the AV node it travels down the bundles of His into the
Purkinje fibres and thence to the ventricles.
This pattern ensures that the contraction of the atria and ventricles is coordinated.
P wave - atrial depolarization
ECG - ELECTROCARDIOGRAMS:-
Compound action potential and does not correspond to the activity of individual cells.
Basis for diagnosis and treatment.
QRS complex - ventricular depolarization and atrial repolarization
T wave - ventricular repolarization
PROBLMS WITH THE ELECTRICAL SYSTEM:
MALFUNCTIONING ELECTRICAL SYSTEM LEADS TO:
- Poor pumping (inadequate supply of blood to body)
LACK OF COORDINATION OF PUMPING OR VERY FAST RATE LEADS TO:
- Turbulence (risk factor for blood clots which can lead to stroke, heart attack)
CARDIAC ARRHYTHMIAS/DYSRHYTHMIAS = ANY
DISORDER OF HEART RATE OR RHYTHM, DISRUPTION OF
THE NORMAL ELECTRICAL CONDUCTION SYSTEM OF THE
The terms arrhythmia and dysrhythmia are used interchangeably. However, there is a subtle difference. Arrhythmia means absence of rhythm whereas dysrhythmia means a disturbance.
Technically, an arrhythmia is therefore a serious subclass of dysrhythmia. Both involve disruption of the normal electrical conduction of the heart. PEOPLE AT HIGHER RISK OF DYSRHYTHMIAS:-
Coronary artery disease
Reduced oxygen supply to the cardiac muscle will depolarize cells slightly and thus make them more likely to fire inappropriate action potentials.
Heart valve disorders
Blood Chemistry disorders
Imbalance of plasma ions or hyperthyroidism
Dysrhythmias can also be caused by drugs
Beta blockers (if you give these drugs to someone with a normal cardiac rhythm, they will make it abnormal)
Some antidysrhythmic drugs
CLASSIFICATION OF DYSRHYTHMIAS: DIFFERENT DYSRHYTHMIC MECHANISMS:
DISORDERS OF IMPULSE GENERATION:
RE-ENTRY DYSRHYTHMIAS:-In normal tissue a wave of excitation tries to spread in all directions but surrounding tissue that has already been excited is refractory so the wave tends to spread in the proper direction.
In the example A, the wave has branched around the non-conducting white area and where the two branches meet up, mutual annihilation of the signals occurs, ensuring proper directional flow.
In B, however, the tissue has become damaged so that it cannot transmit signals in one direction on the right hand branch. This means that when the left hand branch signal reaches the meeting point, the signal can spread back up the right hand side. When it reaches the branch point, the tissue there
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