Chapter 13 - Cardiac Arrhythmias And Their Electrocardiographic Interpretation
Lack of blood flow to the brain for more than 5 to 8 minutes
causes permanent mental impairment or even destruction of brain tissue
Simple weakening of the myocardium usually increases the heart rate because the weakened heart
does not pump blood into the arterial tree to a normal extent, and this elicits sympathetic reflexes to increase the heart rate
Characterized by the electrocardiogram as a pattern with low voltage and very irregular waves
Ventricular Fibrillation
The most serious of all cardiac arrhythmias
Ventricular Fibrillation
Drugs used to treat paroxysmal tachycardia by depressing the normal increase in sodium permeability of the cardiac muscle membrane during generation of the action potential
Quinidine and Lidocaine
Second-degree incomplete heart block is characterized by a patient with
-A P-R interval 0.25 to 0.45 second -No QRS-T waves ("Dropped" ventricular beats)
Treatment for long term LQTS
-Beta-adrenergic blocker medications -Surgical implantation of a cardiac defibrillator
A premature contraction is a contraction of the heart before the time that normal contraction would have been expected. This condition is also called
-Extrasystole -Premature Beat -Ectopic Beat
A frequent cause of atrial fibrillation is atrial enlargement resulting from
-Heart valve lesions that prevent the atria from emptying adequately into the ventricles -Ventricular failure with excess damming of blood in the atria
Circus movements due to decreased rate of conduction frequently results from these four reasons
-High blood potassium levels -Blockage of the Purkinje system -Ischemia of the muscle -Many other factors
Name four reasons for ectopic foci
-ISCHEMIA -CALCIFIED PLAQUES at different points in the heart -TOXIC IRRITATION of the A-V node, Purkinje system, or myocardium -MECHANICAL initiation of premature contractions is also frequent during cardiac catheterization
Circus movements due to a shortened refractory period commonly occurs for these two reasons
-In response to various drugs, such as epinephrine -After repetitive electrical stimulation
Three common causes of tachycardia include
-Increased Body Temperature -Sympathetic Stimulation -Toxic Conditions Of The Heart
4 conditions that can either decrease the rate of impulse conduction in A-V bundle or block the impulse
-Ischemia of the A-V node or A-V bundle fibers -Compression of the A-V bundle by scar tissue or by calcified portions of the heart -Inflammation of the A-V node or A-V bundle -Extreme stimulation of the heart by the vagus nerves
Three causes of "Circus Movements"
-Longer conduction pathway -Decreased rate of conduction -A shortened refractory period
What does it look like on an ECG when the heart undergoes a premature atrial contraction?
-Shortened P-R interval -The interval between the premature contraction and the next succeeding contraction is slightly prolonged (compensatory pause)
Premature atrial contractions occur frequently in otherwise healthy people. Indeed, they often occur in athletes whose hearts are in very healthy condition. Mild toxic conditions resulting from such factors as (5 to be specific)
-Smoking (Nicotine) -Lack of sleep -Ingestion of too much coffee -Alcoholism -Drug Use
Causes of ventricular fibrillation
-Sudden electrical shock of the heart -Ischemia of the heart muscle, of its specialized conducting system, or both.
Most patients faint a few seconds after complete block occurs because
-The brain cannot remain active for more than 4 to 7 seconds without blood supply, -Ventricular escape takes 5-30 seconds
The rapid stimulation of the heart during the early stages of ventricular fibrilation causes these two changes in the cardiac muscle itself
-The velocity of conduction through the heart muscle decreases -Refractory period in the heart muscle is shortened
During deep respiration, the heart rate increases and decreases with each respiratory cycle by as much
30 Percent
The usual lapse of time between beginning of the P wave and beginning of the QRS complex is about ______ when the heart is beating at a normal rate
0.16 Second
The voltages of the waves in the electrocardiogram in ventricular fibrillation are usually about _______ 20-30 seconds after ventricular fibrillation first begins
0.2-0.3 mV
The A-V node will not pass a second impulse for about ______ after a previous one (refractory period)
0.35 second
The voltages of the waves in the electrocardiogram in ventricular fibrillation are usually about _______ when ventricular fibrillation first begins
0.5 mV
How long does it take for a person to die from ventricular fibrillation?
1-3 Minutes
During ventricular fibrillation, the ventricular muscle contracts at as many as
30-50 small patches of the heart at a single time
The heart rate increases about ________ for each degree of Fahrenheit
10 BPM
Tachycardia is usually defined as a heart rate that is faster than
100 BPM
Instead of using 110 volts of 60-cycle alternating current applied for 0.1 second, it is also common to use
1000 volts of direct current applied for a few thousandths of a second
Beyond this temperature, the heart rate may decrease because of progressive debility of the heart muscle as a result of the fever
105 °F
When electrodes are applied directly to the two sides of the heart, fibrillation can usually be stopped using
110 volts of 60-cycle alternating current applied for 0.1 second
The heart rate increases about ________ for each degree of Celsius
18 BPM
During atrial fibrillation, the efficiency of ventricular pumping is decreased
20%-30%
Atrial flutter causes a rapid rate of contraction of the atria between
200-350 BPM
Each time A-V conduction ceases, the ventricles often do not start their own beating until after a delay of
5 to 30 seconds.
Bradycardia is usually defined as a heart rate that is slower than
60 BPM
Normal heart rate
72 BPM
First-degree incomplete heart block is characterized by a patient with (Prolonged P-R interval)
A P-R interval greater than 0.20 second
Occasionally the interval of ventricular standstill at the onset of complete block is so long that it becomes detrimental to the patient's health or even causes death. Consequently, most patients suffering from Stokes-Adams Syndrome are provided with a
A small battery-operated electrical stimulator planted beneath the skin known as an artificial pacemaker
The only means by which impulses ordinarily can pass from the atria into the ventricles is through the
A-V bundle or Bundle of His
Any circulatory reflex that stimulates the vagus nerves causes release of _________ at vegal endings of the heart
Acetylcholine
After the current has caused all heart muscle to go into a state of refractory,
All action potentials stop, and the heart remains quiescent for 3 to 5 seconds
Why does a strong high-voltage alternating electrical current passed through the ventricles for a fraction of a second stop ventricular fibrillation?
All ventricular muscle is thrown into the refractory period simultaneously.
Paroxysmal tachycardia often results from an aberrant rhythm that involves the A-V node. This usually causes
Almost normal QRS-T complexes but totally missing or obscured P waves.
Mechanical means to treat paroxysmal tachycardia
Applying pressure to the carotid sinus which elicits a vegal reflex
A frequent cause of atrial fibrillation is
Atrial Enlargement
Ventricular fibrillation often occurs without
Atrial Fibrilation
In the electrocardiogram, one can see either no P, irregular spacing of the QRS complexes with a heartbeat of 125 and 150 beats per minute.
Atrial Fibrillation
Electrical signal travels as a single large wave always in one direction around and around the atrial muscle mass
Atrial Flutter
On an electrocardiogram, the P waves are strong and are seen two to three times before a QRS-T complex can be seen.
Atrial Flutter
Looking at an ECG, you see that the BPM increases from 95 to 150. You also notice that the P Wave has become inverted. What is your diagnosis
Atrial Paroxysmal Tachycardia where the origin is somewhere away from the SA node.
In some patients with A-V block, the total block comes and goes. This condition occurs in hearts with
Borderline ischemia of the conductive system
Why is the P-Wave superimposed on the QRS-T complex when a premature beat occurs at the A-V node/bundle
Both the atria and ventricles contract at the same time since the action potential spreads in both directions.
Term that means slow heart rate
Bradycardia
At least 10 different mutations of sodium or potassium channel genes
Can cause variable degrees of Q-T prolongation have been identified.
Can occur during deep anesthesia, when many patients develop severe hypoxia because of inadequate respiration.
Cardiac Arrest
This results from cessation of all electrical control signals in the heart.
Cardiac Arrest
A technique for pumping the heart without opening the chest consists of intermittent thrusts of pressure on the chest wall along with artificial respiration
Cardiopulmonary Resuscitation (aka CPR)
An instrument that records by the height of successive spikes the duration of the interval between the successive QRS complexes in the electrocardiogram
Cardiotachometer
Even mild external pressure on the neck elicits a strong baroreceptor reflex, causing intense vagal-acetylcholine effects on the heart, including extreme bradycardia
Carotid Sinus Syndrome
In these patients, the pressure receptors (baroreceptors) in the carotid sinus region of the carotid artery walls are excessively sensitive.
Carotid Sinus Syndrome
Moderate alternating-current voltage applied directly to the ventricles
Causes Ventricular Fibrillation
When a patient loses blood and passes into a state of shock or semishock, sympathetic reflex stimulation of the heart
Causes the heart rate to increase from 150-180 BPM
When electroshock is done by applying two electrodes on the chest wall, the usual procedure is to
Charge a large electrical capacitor up to several thousand volts and then to cause the capacitor to discharge for a few thousandths of a second through the electrodes and through the heart
During a extrasystolic episode, a deficit in the number of radial pulses occurs when
Compared with the actual number of contractions of the heart.
The P wave of this beat occurred too soon in the heart cycle; the P-R interval is shortened, indicating that the ectopic origin of the beat is in the atria near the A-V node. Also, the interval between the premature contraction and the next succeeding contraction is slightly prolonged, which is called a
Compensatory Pause
The P-R interval seldom increases above 0.35 to 0.45 second because, by that time
Conduction through the A-V bundle is depressed so much that conduction stops entirely
Statistics show that people with significant numbers of PVCs (usually due to stray impulses or re-entrant signals that originate around the borders of infarcted or ischemic areas of the heart)
Higher than normal chance of developing spontaneous lethal ventricular fibrillation
Tachycardia can cause electrical alternates because when the rate of the heart is rapid, it may be
Impossible for some portions of the Purkinje system to recover from the previous refractory period quickly enough to respond during every succeeding heartbeat
Quiescent Definition
In a state or period of inactivity or dormancy
Has the appearance of a series of ventricular premature beats occurring one after another without any normal beats interspersed.
Ventricular Paroxysmal Tachycardia
-Usually does not occur unless considerable ischemic damage is present in the ventricles -Frequently initiates the lethal condition of ventricular fibrillation because of rapid repeated stimulation of the ventricular muscle
Reasons Why Ventricular Paroxysmal Tachycardia
Results mainly from "spillover" of signals from the medullary respiratory center into the adjacent vasomotor center during inspiratory and expiratory cycles of respiration
Respiratory Type Of Sinus Arrhythmia
Why does increased temperature cause an increased heart rate?
Increased temperature increases the rate of metabolism of the sinus node, which in turn directly increases its excitability and rate of rhythm
Results frequently from different types of myocarditis, caused, for example, by diphtheria or rheumatic fever. Can depress conductivity from the atria to the ventricles.
Inflammation of the A-V node or A-V bundle
Disorders of cardiac repolarization that lead to LQTS
Inherited Or Aquired
Paroxysmal tachycardia often results from an aberrant rhythm that
Involves the A-V node
Coronary insufficiency can cause ischemia of the A-V node and bundle in the same way that it can cause ischemia of the myocardium
Ischemia of the A-V node or A-V bundle fibers
In carotid sinus syndrome, the reflex that is elicited when pressure is applied to the neck can be so powerful
It stops the heart for 5-10 seconds
Vulnerable period for causing ventricular fibrillation
Just at the end of the T wave when the ventricles are coming out of refractoriness
Disorders that delay repolarization of ventricular muscle following the action potential
Long QT syndrome (LQTS)
Treatment for acute LQTS
Magnesium Sulfate
The pulse wave passing to the peripheral arteries after a premature contraction
May be so weak that it cannot be felt in the radial artery
One means for determining the severity of some heart diseases (acute rheumatic heart disease, for instance)
Measure the P-R interval.
After a few seconds, some part of the Purkinje system beyond the block begin discharging rhythmically at a rate of 15 to 40 times per minute and acting as the pacemaker of the ventricles. This is called
Ventriular Escape
In some patients, severe myocardial disease can cause permanent or semipermanent cardiac arrest, which can cause death. To treat the condition,
Rhythmical electrical impulses from an implanted electronic cardiac pacemaker have been used successfully to keep patients alive for months to years
Antiarrhythmic drug that can cause delayed repolarization (Long Q-T Intervals) when delivered in excess amounts
Quinidine
Drug that increases the refractory period and threshold for excitation of cardiac muscle, may be used to block irritable foci causing ventricular tachycardia.
Quinidine
The major reason that the long QT syndrome is of concern
Delayed repolarization of ventricular muscle increases a person's susceptibility to develop ventricular arrhythmias called torsades de pointes
Sometimes intoxication from the heart treatment drug _______ causes irritable foci that lead to ventricular tachycardia
Digitalis
Circus movements due to longer conduction pathways typically occurs in
Dilated Hearts
Increased temperature increases the rate of metabolism of the sinus node, which
Directly increases its excitability and rate of rhythm
An important feature of ventricular fibrillation that greatly lengthens the conductive pathway and helps sustain fibrillation.
Division Of Impulses
Most premature contractions result from
Ectopic Foci
Anything that can cause incomplete block (myocarditis, ischemia, digitalis toxicity) can also cause
Electrical Alternans
Impulse conduction blockage in the peripheral ventricular Purkinje system is known as
Electrical Alternans
Paroxysmal tachycardia often can be stopped by
Eliciting Vagal Reflex
In rare instances blocks impulse conduction through the A-V node. occasionally results from strong stimulation of the baroreceptors in people with carotid sinus syndrome causing a large release of acetylcholine.
Extreme stimulation of the heart by the vagus nerves
Although some people with LQTS show no major symptoms (other than the prolonged Q-T interval), others exhibit
Fainting and ventricular arrhythmias that may be precipitated by: -Physical exercise -Intense emotions such as fright or anger -Being startled by a noise
The P-R interval usually decreases in length with
Faster Heartbeat
Defined as a delay of conduction from the atria to the ventricles but not actual blockage of conduction
First-degree block
Antibiotics that can cause delayed repolarization (Long Q-T Intervals) when delivered in excess amounts
Fluoroquinolones or Erythromycin
The only way to revive the heart after 1 minute of fibrillation is
Hand pumping of the heart until enough nutrients have circulated through the coronary arteries
The congenital forms of LQTS are rare disorders caused by
Mutations of sodium or potassium channel genes
Why does weakening of the myocardium cause your heart rate to increase?
Not enough blood gets into your system. To keep your body from going into a state of shock the sympathetic system is activated increasing your heart rate.
Why is the amount of blood pumped by the atria minimal during atrial flutter?
Only one atria contracts while the other is relaxed
This means that ventricular excitability is at first in a suppressed state because the ventricles have been driven by the atria at a rate greater than their natural rate of rhythm.
Overdrive Suppression
Abnormalities in the heart that can occasionally cause rapid rhythmical discharge of impulses that spread in all directions throughout the heart. This believed to be caused most frequently by re-entrant circus movement feedback pathways that set up local repeated self-re-excitation.
Paroxysmal Tachycardia
More common are the acquired forms of LQTS that are associated with
Plasma Electrolyte Balances
T wave has an electrical potential polarity exactly opposite to that of the QRS complex
Premature Ventricular Contractions (PVC)
The QRS complex is usually considerably prolonged. The reason is that the impulse is conducted mainly through slowly conducting muscle of the ventricles rather than through the Purkinje system.
Premature Ventricular Contractions (PVC)
The impulse almost always travels in only one direction, so there is no such neutralization effect, and one entire side or end of the ventricles is depolarized ahead of the other; this causes large electrical potentials
Premature Ventricular Contractions (PVC)
Electrical Alternans can be characterized on an ECG by
QRS waves that are not uniform and appear weaker in strength.
What is the reasoning behind the compensatory pause of a premature atrial contraction.
Since contraction originated in the atrium some distance from the sinus node, and the impulse had to travel through a considerable amount of atrial muscle before it discharged the sinus node, the sinus node discharged late in the premature cycle, and this made the succeeding sinus node discharge also late in appearing.
Characterized on an ECG by a sudden cessation of P waves, with resultant standstill of the atria and a delayed QRS-T wave
Sinoatrial Block
Rare instance where the impulse from the sinus node is blocked before it enters the atrial muscle
Sinoatrial Block
The P-R interval usually increases in length with
Slower Heartbeat
Periodic fainting spells caused by third degree blockage coming and going (occurs in hearts with borderline ischemia)
Stokes-Adams Syndrome
A strong high-voltage alternating electrical current passed through the ventricles for a fraction of a second
Stops Ventricular Fibrillation
Atrial or A-V nodal paroxysmal tachycardia, both of which are called
Supraventricular Tachycardias
Usually occurs in young, otherwise healthy people, and they generally grow out of the predisposition to tachycardia after adolescence
Supraventricular Tachycardias
Term that means fast heart rate
Tachycardia
In a Premature Ventricular Contractions (PVC), why does the T wave has an electrical potential polarity exactly opposite to that of the QRS complex?
The slow conduction of the impulse through the cardiac muscle causes the muscle fibers that depolarize first also to repolarize first.
During atrial fibrillation, because of the rapid rate of the fibrillatory impulses in the atria,
The ventricle is driven at a fast heart rate, usually between 125 and 150 beats per minute.
Why is the pulse wave weak during a premature atrial heartbeat?
The ventricles do not have enough time to fill up with blood.
In Third-Degree Block, why do the P waves become dissociated from the QRS-T complex?
The ventricles spontaneously generate their own signal since impulses from the SA node cannot reach them due to complete blockage
Why is it common for athletes to have bradycardia?
Their hearts are larger and stronger which allows them to pump more blood into their arterial tree with less strokes.
What does it look like on an ECG when a premature contraction originates in the A-V node/bundle
There is no P-Wave because it is superimposed on the QRS-T complex
Characterized on the ECG by P waves which become dissociated from the QRS-T complexes
Third-Degree Block (Complete A-V Block)
The shape of the QRS complex may change over time with the onset of arrhythmia usually following a premature beat, a pause, and then another beat with a long Q-T interval, which may trigger arrhythmias, tachycardia, and in some instances ventricular fibrillation.
Torsades De Pointes
In atrial fibrillation, the interval between successive ventricular contractions varies from a
minimum of about 0.35 second to a maximum of about 0.95 second, causing a very irregular heartbeat
Unless defibrillated within 1 minute after fibrillation begins, the heart is usually too weak to be revived by defibrillation because
of the lack of nutrition from coronary blood flow
In some patients, severe myocardial disease can cause
permanent or semipermanent cardiac arrest
