Cardiac Arrythmias Flashcards
Tachycardia
Fast heart rate defined as more than 100 beats per minute in an adult.
Causes:
1. Increased body temperature
- Stimulation of the heart by the sympathetic nerves
- Toxic Conditions of the heart.
Fever causes tachycardia because increased temperature increases the rate of metabolism of the sinus node,which inceases its excitability and rate of rhythm.
Bradycardia
Slow heart rate defined as less that 60 beats per minute.
Bradycardia can occur from two reasons:
- In Atletes, The atlhetes heart is often large and considerably stonger than that of a normal person allowing the atlethes heart to pump a large stroke volume output per beat even during periods of rest.
- Vagal stimulation causes Bradycardia-Any circulatory reflex stimulates the vagus nerve cause release of Ach at vagal endings in the heart thus giving a parasympathetic effect-Carotid Sinus Syndrome-Baroreceptors-carotid sinus causing a barorecptor effect on the heart.
Sinus Arrhythmia
Can result from many circularory conditions that alter the of strenghts of the sympathetic and parasympathetic nerve signals to the heart sinus node.
The respiratory type of sinus arryhtmia results from spillover of signals from the medullary respiratory centre into the adjacent vasomotoor centre during inspiration and expiration.
The spillover siganls cause an alternae increase in and decrease in the number of impulses transmitted thoguh the sympathetic and vagus nerve to the the heart
Sinoatrial Block
In rare instances impulses from the sinus node is blocked before entering the atrial muscle.
Sudden ceasation of P-waves with resultant standstil of the atria.
The ventricles pick up a new rhythm with the impulse usually originating spontaneously in the AV node so that the ventricular QRS-T complex is slowed but not otherwise altered.
Atrioventricular Block
The only way in which impulses can pass from the atria into the ventricles is through the AV bundle/Bundle of His
Conditions which result in this bundle or block the impulse entirely are:
I.I.C.E
I-ischamia of the AV node or bundle fibres-Coronary insufficiency
I-inflamation of the AV node/AV bundle-depresses conductance and can be as a result of myocarditis that are caused for example: Diptheria and Rheumatic fever
C-ompression of the AV node-by scar tissue/calcfified portions of the heart
E-extreme stimulation of the heart by the vagus nerves-Carotid Sinus syndrome
The different types of Atrioventricular Heart Block
There are three types of blocks.
1st Degree AV Block
2nd Degree AV Block: Wenckback and Mobitz
3rd Degree AV Block
- First Degree AV Block/Prolonged P-R/P-Q Interval
The usual lapse of time between the beginning of the p wave and the beginning of the QRS complex is about 0.16 when the heart is normal
PR interval increases in length with a fast heartbeat and decreases with a slow heartbeat.
When PR interval increases to greater than 0.20 seconds the pt is said to have first-degree incomplete heart block
Defined as a delay of conduction from the atria to the ventricles but not an actual blockage of conductance.
Acute Rhuematic Heart disease can be measured using the P-R interval
Second Degree Block
There are two types of Second Degree Blocks:
- Wenckeback Periodicity
- Morbitz
There are some dropped beats,There will be an atrial p-wave but no QRS-T wave.
Wenckeback Periodicity
Characterized by progressive prolongation of the PR interval until a ventricular beat is dropped and is then followed by resetting of the PR and repeating of the abnormal cycle
Almost always caused by abnormality of the AV node.
Morbitz
There is usually a fixed number of nonconducted p waves for every qrs-complex
It is represented in a ratio.
Caused by an abnormality of the bundle of his-purkinje system and may require implantation of a pacemaker to prevent progression to complete heart block and cardiac arrest.
Complete A-V Block/Third Degree Block
When the condition that causes poor conduction in the AV node/ bundle becomes severe,complete block of the impulse from the atria into the ventricles occurs.
Ventricles spontaneously establish their own signal usually originating in the AV node or distal to the block.
Therefore the p-waves become dissociated from the QRS-T complexes
Stokes-Adam Syndrome-Ventricular Escape
The total block comes andd goes,impulses are conducted from the atria into the ventricles for a period of time and then are suddenly no longer conducted.
The duration of the block may last for minutes,hours ,days before conduction returns.
This condition occurs in hearts with borderline ischemia of the conductive system
The peridic fainting spells are know as the stokes-Adams Syndrome
Artificial Pacemakes may be needed.
Incomplete Intraventricular Block-Electrical Alternans
Most of the factors which cause AV-Block can also block impulse conduction in the peripheral ventricular Purkinje system
Electrical Alternans results from partial intraventricular block every other heartbeat
Conditions which depress the heart such as Ischemia,Myocarditis or digitalis toxicity can cause incomplete intraventricula block
Premature Contractions
A premature contraction is a contraction of the heart before the time that the normal contraction would have been expected.
This condition is know as Extrasystole,Premature beat or Ectopic Beat.
What are the causes of Premature Contractions
Possible causes of Ectopic Foci are:
- Local areas of ischemia
- Small calcified plaques at different points in the heart.
- Toxic irritation of the AV node,Purkinje system or myocardium caused by infection ,drugs, nicotine or caffeine.
Mechanical initiation of premature contractions are also frequent during cardiac catheterization,large numbers of premature contractions often occur when the catheter enters the right ventricle and presses against the endocardium
