Ventricular fibrillation
Encyclopedia
Ventricular fibrillation (V-fib or VF) is a condition in which there is uncoordinated contraction of the cardiac muscle
of the ventricle
s in the heart
, making them quiver rather than contract properly. Ventricular fibrillation is a medical emergency and most commonly identified arrythmia in cardiac arrest patients. While there is activity, it is undetectable by palpation (feeling) at major pulse points of the carotid and femoral arteries especially by the lay person. Such an arrhythmia is only confirmed by electrocardiography. Ventricular fibrillation is a medical emergency
that requires prompt Basic Life Support
interventions because should the arrhythmia continue for more than a few seconds, it will likely degenerate further into asystole
("flatline"). The condition results in cardiogenic shock
, cessation of effective blood circulation, and sudden cardiac death (SCD)
will result in a matter of minutes. If however the patient is revived after a sufficient period (at room temperature, roughly 5 minutes) of cerebral hypoxia
, the patient could sustain irreversible brain damage and possibly be left brain dead (death often occurs if normal sinus rhythm
is not restored within 90 seconds of the onset of VF, especially if it has degenerated further into asystole).
and sudden cardiac death
. The ventricular muscle twitches randomly, rather than contracting in a coordinated fashion (from the apex of the heart to the outflow of the ventricles), and so the ventricle
s fail to pump blood into the arteries
and into systemic circulation
.
Ventricular fibrillation is a sudden lethal arrhythmia responsible for many deaths in the Western world, mostly brought on by ischemic heart disease. Despite much work, the underlying nature of fibrillation is not completely understood. Most episodes of fibrillation occur in diseased hearts, but others occur in so-called normal hearts. Much work still has to be done to elucidate the mechanisms of ventricular fibrillation.
myocardium can be hyperirritable myocardial cells. These may then act as pacemakers. The ventricles are then being stimulated by more than one pacemaker
. Scar and dying tissue is inexcitable, but around these areas usually lies a penumbra of hypoxic tissue that is excitable. Ventricular excitability may generate re-entry arrhythmias.
It is interesting to note that most cardiac myocardial cells with an associated increased propensity to arrhythmia development have an associated loss of membrane potential
. That is, the maximum diastolic potential is less negative and therefore exists closer to the threshold potential
. Cellular depolarisation can be due to a raised external concentration of potassium
ions K+, a decreased intracellular concentration of sodium
ions Na+, increased permeability to Na+, or a decreased permeability to K+. The ionic basic automaticity is the net gain of an intracellular positive charge during diastole in the presence of a voltage-dependent channel activated by potentials negative to –50 to –60 mV.
Myocardial cells are exposed to different environments. Normal cells may be exposed to hyperkalaemia; abnormal cells may be perfused by normal environment. For example, with a healed myocardial infarction, abnormal cells can be exposed to an abnormal environment such as with a myocardial infarction with myocardial ischaemia. In conditions such as myocardial ischaemia, possible mechanism of arrhythmia generation include the resulting decreased internal K+ concentration, the increased external K+ concentration, norepinephrine release and acidosis. When myocardial cell are exposed to hyperkaliemia, the maximum diastolic potential is depolarized as a result of the alteration of Ik1 potassium current, whose intensity and direction is strictly dependant on intracellular and extracellular potassium concentrations. With Ik1 suppressed, an hyperpolarizing effect is lost and therefore there can be activation of funny current
even in myocardial cells (which is normally suppressed by the hyperpolarizing effect of coexisting potassium currents). This can lead to the instauration of automaticity in ischemic tissue.
ventricle. They were both able to show that, if a ring of excitable tissue was stimulated at a single point, the subsequent waves of depolarisation would pass around the ring. The waves eventually meet and cancel each other out, but, if an area of transient block occurred with a refractory period that blocked one wavefront and subsequently allowed the other to proceed retrogradely over the other path, then a self-sustaining circus movement phenomenon would result. For this to happen, however, it is necessary that there be some form of non-uniformity. In practice, this may be an area of ischaemic or infarcted myocardium, or underlying scar tissue
.
It is possible to think of the advancing wave of depolarisation as a dipole with a head and a tail. The length of the refractory period and the time taken for the dipole to travel a certain distance—the propagation velocity—will determine whether such a circumstance will arise for re-entry to occur. Factors that promote re-entry would include a slow-propagation velocity, a short refractory period with a sufficient size of ring of conduction tissue. These would enable a dipole to reach an area that had been refractory and is now able to be depolarised with continuation of the wavefront
.
In clinical practice, therefore, factors that would lead to the right conditions to favour such re-entry mechanisms include increased heart size through hypertrophy
or dilatation, drugs which alter the length of the refractory period and areas of cardiac disease. Therefore, the substrate of ventricular fibrillation is transient or permanent conduction block. Block due either to areas of damaged or refractory tissue leads to areas of myocardium for initiation and perpetuation of fibrillation through the phenomenon of re-entry.
ic deflections continuously change in shape, magnitude and direction".
Ventricular fibrillation most commonly occurs within disease
d hearts, and, in the vast majority of cases, is a manifestation of underlying ischemic heart disease. Ventricular fibrillation is also seen in those with cardiomyopathy
, myocarditis
, and other heart pathologies. In addition, it is seen with electrolyte disturbances and overdoses of cardiotoxic drugs. It is also notable that ventricular fibrillation occurs where there is no discernible heart pathology or other evident cause, the so-called idiopathic ventricular fibrillation.
Idiopathic ventricular fibrillation occurs with a reputed incidence of approximately 1% of all cases of out-of-hospital arrest, as well as 3%-9% of the cases of ventricular fibrillation unrelated to myocardial infarction
, and 14% of all ventricular fibrillation resuscitations in patients under the age of 40. It follows then that, on the basis of the fact that ventricular fibrillation itself is common, idiopathic ventricular fibrillation accounts for an appreciable mortality. Recently-described syndromes such as the Brugada Syndrome
may give clues to the underlying mechanism of ventricular arrhythmias. In the Brugada syndrome, changes may be found in the resting ECG with evidence of right bundle branch block
(RBBB) and ST elevation in the chest leads V1-V3, with an underlying propensity to sudden cardiac death.
The relevance of this is that theories of the underlying pathophysiology and electrophysiology must account for the occurrence of fibrillation in the apparent "healthy" heart. It is evident that there are mechanisms at work that we do not fully appreciate and understand. Investigators are exploring new techniques of detecting and understanding the underlying mechanisms of sudden cardiac death in these patients without pathological evidence of underlying heart disease.
Familial conditions that predispose individuals to developing ventricular fibrillation and sudden cardiac death are often the result of gene mutations that affect cellular transmembrane ion channels. For example, in Brugada Syndrome, sodium channels are affected. In certain forms of long QT syndrome, the potassium inward rectifier channel is affected.
. These are depolarising oscillations in the membrane voltage induced by preceding action potentials. These can occur before or after full repolarisation of the fiber and as such are termed either early (EADs) or delayed afterdepolarisations (DADs). All afterdepolarisations may not reach threshold potential, but, if they do, they can trigger another afterdepolarisation, and thus self-perpetuate.
of rotors of fibrillation. Analysis of the fibrillation waveform is performed using a mathematical technique known as Fourier analysis.
The distribution of frequency and power of a waveform can be expressed as a power spectrum in which the contribution of different waveform frequencies to the waveform under analysis is measured. This can be expressed as either the dominant or peak frequency, i.e., the frequency with the greatest power or the median frequency, which divides the spectrum in two halves.
Frequency analysis has many other uses in medicine and in cardiology, including analysis of heart rate variability and assessment of cardiac function, as well as in imaging and acoustics.
from a defibrillator. Although a defibrillator is designed to correct the problem, and its effects can be dramatic, it is not always successful.
In patients at high risk of ventricular fibrillation, the use of an
implantable cardioverter defibrillator
has been shown to be beneficial.
can be delivered at the onset of VF for a small chance to regain cardiac function. However, research has shown that the precordial thump releases no more than 30 joule
s of energy. This is far less than the 200–360 J typically used to bring about normal sinus rhythm. Consequently, in the hospital setting, this treatment is not used.
or lidocaine
can help, but, unlike atrial fibrillation
, ventricular fibrillation rarely reverses spontaneously in large adult mammals. Drug therapy with antiarrhythmic agents in ventricular fibrillation does not replace defibrillation
and is not the first priority, but is sometimes needed in cases where initial defibrillation attempts are not successful.
is the leading cause of death in the industrialised world
. It exacts a significant mortality with approximately 70,000 to 90,000 sudden cardiac deaths each year in the United Kingdom
, and survival rates are only 2%. The majority of these deaths are due to ventricular fibrillation secondary to myocardial infarction
, or "heart attack". During ventricular fibrillation, cardiac output
drops to zero, and, unless remedied promptly, death usually ensues within minutes.
of ancient Egypt
. The extract recorded 3500 years ago may even date from as far back as 3500 BC. It states: "When the heart is diseased, its work is imperfectly performed: the vessels proceeding from the heart become inactive, so that you cannot feel them … if the heart trembles, has little power and sinks, the disease is advanced and death is near." A book authored by Jo Miles suggests that it may even go back farther. Tests done on frozen remains found in the Himalayas seemed fairly conclusive that the first known case of ventricular fibrillation dates back to at least 2500 BC.
Whether this is a description of ventricular fibrillation is debatable. The next recorded description occurs 3000 years later and is recorded by Vesalius
, who described the appearance of "worm-like" movements of the heart in animals prior to death.
The significance and clinical importance of these observations and descriptions possibly of ventricular fibrillation were not recognised until John Erichsen in 1842 described ventricular fibrillation following the ligation
of a coronary artery (Erichsen JE 1842). Subsequent to this in 1850, fibrillation was described by Ludwig and Hoffa when they demonstrated the provocation of ventricular fibrillation in an animal by applying a "Faradic
" (electrical) current to the heart.
In 1874, Edmé Félix Alfred Vulpian
coined the term mouvement fibrillaire, a term that he seems to have used to describe both atrial and ventricular fibrillation. John A. MacWilliam, a physiologist who had trained under Ludwig and who subsequently became Professor of Physiology at the University of Aberdeen
, gave an accurate description of the arrhythmia in 1887. This definition still holds today, and is interesting in the fact that his studies and description predate the use of electrocardiography. His description is as follows: "The ventricular muscle is thrown into a state of irregular arrhythmic contraction, whilst there is a great fall in the arterial blood pressure, the ventricles become dilated with blood as the rapid quivering movement of their walls is insufficient to expel their contents; the muscular action partakes of the nature of a rapid incoordinate twitching of the muscular tissue … The cardiac pump is thrown out of gear, and the last of its vital energy is dissipated in the violent and the prolonged turmoil of fruitless activity in the ventricular walls." MacWilliam spent many years working on ventricular fibrillation and was one of the first to show that ventricular fibrillation could be terminated by a series of induction shocks through the heart.
The first electrocardiogram
recording of ventricular fibrillation was by August Hoffman in a paper published in 1912. At this time, two other researchers, Mines and Garrey, working separately, produced work demonstrating the phenomenon of circus movement and re-entry as possible substrates for the generation of arrhythmias. This work was also accompanied by Lewis, who performed further outstanding work into the concept of "circus movement."
Later milestones include the work by Kerr
and Bender
in 1922, who produced an electrocardiogram showing ventricular tachycardia
evolving into ventricular fibrillation. The re-entry mechanism was also advocated by DeBoer, who showed that ventricular fibrillation could be induced in late systole with a single shock to a frog heart. The concept of "R on T ectopics" was further brought out by Katz in 1928. This was called the “vulnerable period” by Wiggers and Wegria in 1940, who brought to attention the concept of the danger of premature ventricular beats
occurring on a T wave
.
Another definition of VF was produced by Wiggers in 1940. He described ventricular fibrillation as "an incoordinate type of contraction which, despite a high metabolic rate of the myocardium, produces no useful beats. As a result, the arterial pressure falls abruptly to very low levels, and death results within six to eight minutes from anemia
of the brain
and spinal cord
".
Spontaneous conversion of ventricular fibrillation to a more benign rhythm is rare in all but small animals. Defibrillation
is the process that converts ventricular fibrillation to a more benign rhythm. This is usually by application of an electric shock
to the myocardium and is discussed in detail in the relevant article.
Cardiac muscle
Cardiac muscle is a type of involuntary striated muscle found in the walls and histologic foundation of the heart, specifically the myocardium. Cardiac muscle is one of three major types of muscle, the others being skeletal and smooth muscle...
of the ventricle
Ventricle (heart)
In the heart, a ventricle is one of two large chambers that collect and expel blood received from an atrium towards the peripheral beds within the body and lungs. The Atria primes the Pump...
s in the heart
Heart
The heart is a myogenic muscular organ found in all animals with a circulatory system , that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...
, making them quiver rather than contract properly. Ventricular fibrillation is a medical emergency and most commonly identified arrythmia in cardiac arrest patients. While there is activity, it is undetectable by palpation (feeling) at major pulse points of the carotid and femoral arteries especially by the lay person. Such an arrhythmia is only confirmed by electrocardiography. Ventricular fibrillation is a medical emergency
Medical emergency
A medical emergency is an injury or illness that is acute and poses an immediate risk to a person's life or long term health. These emergencies may require assistance from another person, who should ideally be suitably qualified to do so, although some of these emergencies can be dealt with by the...
that requires prompt Basic Life Support
Basic life support
Basic life support is the level of medical care which is used for patients with life-threatening illnesses or injuries until the patient can be given full medical care at a hospital. It can be provided by trained medical personnel, including emergency medical technicians, paramedics, and by...
interventions because should the arrhythmia continue for more than a few seconds, it will likely degenerate further into asystole
Asystole
In medicine, asystole is a state of no cardiac electrical activity, hence no contractions of the myocardium and no cardiac output or blood flow...
("flatline"). The condition results in cardiogenic shock
Cardiogenic shock
Cardiogenic shock is based upon an inadequate circulation of blood due to primary failure of the ventricles of the heart to function effectively....
, cessation of effective blood circulation, and sudden cardiac death (SCD)
Sudden Cardiac Death
Sudden cardiac death is natural death from cardiac causes, heralded by abrupt loss of consciousness within one hour of the onset of acute symptoms. Other forms of sudden death may be noncardiac in origin...
will result in a matter of minutes. If however the patient is revived after a sufficient period (at room temperature, roughly 5 minutes) of cerebral hypoxia
Hypoxia (medical)
Hypoxia, or hypoxiation, is a pathological condition in which the body as a whole or a region of the body is deprived of adequate oxygen supply. Variations in arterial oxygen concentrations can be part of the normal physiology, for example, during strenuous physical exercise...
, the patient could sustain irreversible brain damage and possibly be left brain dead (death often occurs if normal sinus rhythm
Sinus rhythm
In medicine, sinus rhythm is the normal beating of the heart, as measured by an electrocardiogram . It has certain generic features that serve as hallmarks for comparison with normal ECGs.- ECG structure :...
is not restored within 90 seconds of the onset of VF, especially if it has degenerated further into asystole).
Signs and symptoms
Ventricular fibrillation is a cause of cardiac arrestCardiac arrest
Cardiac arrest, is the cessation of normal circulation of the blood due to failure of the heart to contract effectively...
and sudden cardiac death
Sudden Cardiac Death
Sudden cardiac death is natural death from cardiac causes, heralded by abrupt loss of consciousness within one hour of the onset of acute symptoms. Other forms of sudden death may be noncardiac in origin...
. The ventricular muscle twitches randomly, rather than contracting in a coordinated fashion (from the apex of the heart to the outflow of the ventricles), and so the ventricle
Ventricle (heart)
In the heart, a ventricle is one of two large chambers that collect and expel blood received from an atrium towards the peripheral beds within the body and lungs. The Atria primes the Pump...
s fail to pump blood into the arteries
Artery
Arteries are blood vessels that carry blood away from the heart. This blood is normally oxygenated, exceptions made for the pulmonary and umbilical arteries....
and into systemic circulation
Systemic circulation
Systemic circulation is the part of the cardiovascular system which carries oxygenated blood away from the heart to the body, and returns deoxygenated blood back to the heart. This physiologic theory of circulation was first described by William Harvey...
.
Ventricular fibrillation is a sudden lethal arrhythmia responsible for many deaths in the Western world, mostly brought on by ischemic heart disease. Despite much work, the underlying nature of fibrillation is not completely understood. Most episodes of fibrillation occur in diseased hearts, but others occur in so-called normal hearts. Much work still has to be done to elucidate the mechanisms of ventricular fibrillation.
Abnormal automaticity
Automaticity is a measure of the propensity of a fiber to initiate an impulse spontaneously. The product of a hypoxicHypoxia (medical)
Hypoxia, or hypoxiation, is a pathological condition in which the body as a whole or a region of the body is deprived of adequate oxygen supply. Variations in arterial oxygen concentrations can be part of the normal physiology, for example, during strenuous physical exercise...
myocardium can be hyperirritable myocardial cells. These may then act as pacemakers. The ventricles are then being stimulated by more than one pacemaker
Pacemaker
An artificial pacemaker is a medical device that uses electrical impulses to regulate the beating of the heart.Pacemaker may also refer to:-Medicine:...
. Scar and dying tissue is inexcitable, but around these areas usually lies a penumbra of hypoxic tissue that is excitable. Ventricular excitability may generate re-entry arrhythmias.
It is interesting to note that most cardiac myocardial cells with an associated increased propensity to arrhythmia development have an associated loss of membrane potential
Membrane potential
Membrane potential is the difference in electrical potential between the interior and exterior of a biological cell. All animal cells are surrounded by a plasma membrane composed of a lipid bilayer with a variety of types of proteins embedded in it...
. That is, the maximum diastolic potential is less negative and therefore exists closer to the threshold potential
Threshold potential
The threshold potential is the membrane potential to which a membrane must be depolarized to initiate an action potential.It often can be between −40 and -55 mV, but it can vary based upon several factors...
. Cellular depolarisation can be due to a raised external concentration of potassium
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
ions K+, a decreased intracellular concentration of sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
ions Na+, increased permeability to Na+, or a decreased permeability to K+. The ionic basic automaticity is the net gain of an intracellular positive charge during diastole in the presence of a voltage-dependent channel activated by potentials negative to –50 to –60 mV.
Myocardial cells are exposed to different environments. Normal cells may be exposed to hyperkalaemia; abnormal cells may be perfused by normal environment. For example, with a healed myocardial infarction, abnormal cells can be exposed to an abnormal environment such as with a myocardial infarction with myocardial ischaemia. In conditions such as myocardial ischaemia, possible mechanism of arrhythmia generation include the resulting decreased internal K+ concentration, the increased external K+ concentration, norepinephrine release and acidosis. When myocardial cell are exposed to hyperkaliemia, the maximum diastolic potential is depolarized as a result of the alteration of Ik1 potassium current, whose intensity and direction is strictly dependant on intracellular and extracellular potassium concentrations. With Ik1 suppressed, an hyperpolarizing effect is lost and therefore there can be activation of funny current
Funny current
Funny current refers to a specific current in the heart....
even in myocardial cells (which is normally suppressed by the hyperpolarizing effect of coexisting potassium currents). This can lead to the instauration of automaticity in ischemic tissue.
Re-entry
The role of re-entry or circus motion was demonstrated separately by Mines and Garrey. Mines created a ring of excitable tissue by cutting the atria out of the ray fish. Garrey cut out a similar ring from the turtleTurtle
Turtles are reptiles of the order Testudines , characterised by a special bony or cartilaginous shell developed from their ribs that acts as a shield...
ventricle. They were both able to show that, if a ring of excitable tissue was stimulated at a single point, the subsequent waves of depolarisation would pass around the ring. The waves eventually meet and cancel each other out, but, if an area of transient block occurred with a refractory period that blocked one wavefront and subsequently allowed the other to proceed retrogradely over the other path, then a self-sustaining circus movement phenomenon would result. For this to happen, however, it is necessary that there be some form of non-uniformity. In practice, this may be an area of ischaemic or infarcted myocardium, or underlying scar tissue
Myocardial scarring
Myocardial scarring is fibrous tissue that replaces normal tissue destroyed by injury or disease pertaining to the muscular tissue of the heart....
.
It is possible to think of the advancing wave of depolarisation as a dipole with a head and a tail. The length of the refractory period and the time taken for the dipole to travel a certain distance—the propagation velocity—will determine whether such a circumstance will arise for re-entry to occur. Factors that promote re-entry would include a slow-propagation velocity, a short refractory period with a sufficient size of ring of conduction tissue. These would enable a dipole to reach an area that had been refractory and is now able to be depolarised with continuation of the wavefront
Wavefront
In physics, a wavefront is the locus of points having the same phase. Since infrared, optical, x-ray and gamma-ray frequencies are so high, the temporal component of electromagnetic waves is usually ignored at these wavelengths, and it is only the phase of the spatial oscillation that is described...
.
In clinical practice, therefore, factors that would lead to the right conditions to favour such re-entry mechanisms include increased heart size through hypertrophy
Hypertrophy
Hypertrophy is the increase in the volume of an organ or tissue due to the enlargement of its component cells. It should be distinguished from hyperplasia, in which the cells remain approximately the same size but increase in number...
or dilatation, drugs which alter the length of the refractory period and areas of cardiac disease. Therefore, the substrate of ventricular fibrillation is transient or permanent conduction block. Block due either to areas of damaged or refractory tissue leads to areas of myocardium for initiation and perpetuation of fibrillation through the phenomenon of re-entry.
Pathophysiology
Ventricular fibrillation has been described as "chaotic asynchronous fractionated activity of the heart" (Moe et al. 1964). A more complete definition is that ventricular fibrillation is a "turbulent, disorganized electrical activity of the heart in such a way that the recorded electrocardiographElectrocardiogram
Electrocardiography is a transthoracic interpretation of the electrical activity of the heart over a period of time, as detected by electrodes attached to the outer surface of the skin and recorded by a device external to the body...
ic deflections continuously change in shape, magnitude and direction".
Ventricular fibrillation most commonly occurs within disease
Disease
A disease is an abnormal condition affecting the body of an organism. It is often construed to be a medical condition associated with specific symptoms and signs. It may be caused by external factors, such as infectious disease, or it may be caused by internal dysfunctions, such as autoimmune...
d hearts, and, in the vast majority of cases, is a manifestation of underlying ischemic heart disease. Ventricular fibrillation is also seen in those with cardiomyopathy
Cardiomyopathy
Cardiomyopathy, which literally means "heart muscle disease," is the deterioration of the function of the myocardium for any reason. People with cardiomyopathy are often at risk of arrhythmia or sudden cardiac death or both. Cardiomyopathy can often go undetected, making it especially dangerous to...
, myocarditis
Myocarditis
Myocarditis is inflammation of heart muscle . It resembles a heart attack but coronary arteries are not blocked.Myocarditis is most often due to infection by common viruses, such as parvovirus B19, less commonly non-viral pathogens such as Borrelia burgdorferi or Trypanosoma cruzi, or as a...
, and other heart pathologies. In addition, it is seen with electrolyte disturbances and overdoses of cardiotoxic drugs. It is also notable that ventricular fibrillation occurs where there is no discernible heart pathology or other evident cause, the so-called idiopathic ventricular fibrillation.
Idiopathic ventricular fibrillation occurs with a reputed incidence of approximately 1% of all cases of out-of-hospital arrest, as well as 3%-9% of the cases of ventricular fibrillation unrelated to myocardial infarction
Myocardial infarction
Myocardial infarction or acute myocardial infarction , commonly known as a heart attack, results from the interruption of blood supply to a part of the heart, causing heart cells to die...
, and 14% of all ventricular fibrillation resuscitations in patients under the age of 40. It follows then that, on the basis of the fact that ventricular fibrillation itself is common, idiopathic ventricular fibrillation accounts for an appreciable mortality. Recently-described syndromes such as the Brugada Syndrome
Brugada syndrome
The Brugada syndrome is a genetic disease that is characterised by abnormal electrocardiogram findings and an increased risk of sudden cardiac death. It is named by the Spanish cardiologists Pedro Brugada and Josep Brugada...
may give clues to the underlying mechanism of ventricular arrhythmias. In the Brugada syndrome, changes may be found in the resting ECG with evidence of right bundle branch block
Right bundle branch block
A right bundle branch block is a defect in the heart's electrical conduction system.During a right bundle branch block, the right ventricle is not directly activated by impulses travelling through the right bundle branch. The left ventricle however, is still normally activated by the left bundle...
(RBBB) and ST elevation in the chest leads V1-V3, with an underlying propensity to sudden cardiac death.
The relevance of this is that theories of the underlying pathophysiology and electrophysiology must account for the occurrence of fibrillation in the apparent "healthy" heart. It is evident that there are mechanisms at work that we do not fully appreciate and understand. Investigators are exploring new techniques of detecting and understanding the underlying mechanisms of sudden cardiac death in these patients without pathological evidence of underlying heart disease.
Familial conditions that predispose individuals to developing ventricular fibrillation and sudden cardiac death are often the result of gene mutations that affect cellular transmembrane ion channels. For example, in Brugada Syndrome, sodium channels are affected. In certain forms of long QT syndrome, the potassium inward rectifier channel is affected.
Triggered activity
Triggered activity can occur due to the presence of afterdepolarisationsAfterdepolarization
Afterdepolarizations are abnormal depolarizations of cardiac myocytes that interrupt phase 2, phase 3, or phase 4 of the cardiac action potential in the electrical conduction system of the heart...
. These are depolarising oscillations in the membrane voltage induced by preceding action potentials. These can occur before or after full repolarisation of the fiber and as such are termed either early (EADs) or delayed afterdepolarisations (DADs). All afterdepolarisations may not reach threshold potential, but, if they do, they can trigger another afterdepolarisation, and thus self-perpetuate.
Characteristics of the ventricular fibrillation waveform
Ventricular fibrillation can be described in terms of its electrocardiographic waveform appearance. All waveforms can be described in terms of certain features, such as amplitude and frequency. Researchers have looked at the frequency of the ventricular fibrillation waveform to see if it helps to elucidate the underlying mechanism of the arrhythmia or holds any clinically useful information. More recently, Gray has suggested an underlying mechanism for the frequency of the waveform that has puzzled investigators as possibly being a manifestation of the Doppler effectDoppler effect
The Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842 in Prague, is the change in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from...
of rotors of fibrillation. Analysis of the fibrillation waveform is performed using a mathematical technique known as Fourier analysis.
Power spectrum
Frequency analysis has many other uses in medicine and in cardiology, including analysis of heart rate variability and assessment of cardiac function, as well as in imaging and acoustics.
Electric defibrillator
The condition can often be reversed by the electric discharge of direct currentDirect current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
from a defibrillator. Although a defibrillator is designed to correct the problem, and its effects can be dramatic, it is not always successful.
Implantable electric defibrillator
In patients at high risk of ventricular fibrillation, the use of an
implantable cardioverter defibrillator
Implantable cardioverter-defibrillator
An implantable cardioverter-defibrillator is a small battery-powered electrical impulse generator which is implanted in patients who are at risk of sudden cardiac death due to ventricular fibrillation and ventricular tachycardia. The device is programmed to detect cardiac arrhythmia and correct it...
has been shown to be beneficial.
Precordial thump
If no defibrillator is available, a precordial thumpPrecordial thump
The precordial thump is a medical procedure that may used by healthcare professionals, to respond to ventricular fibrillation or ventricular tachycardia under certain conditions. The procedure is outside the scope of first-aid treatment and requires, at minimum, training in advanced cardiac life...
can be delivered at the onset of VF for a small chance to regain cardiac function. However, research has shown that the precordial thump releases no more than 30 joule
Joule
The joule ; symbol J) is a derived unit of energy or work in the International System of Units. It is equal to the energy expended in applying a force of one newton through a distance of one metre , or in passing an electric current of one ampere through a resistance of one ohm for one second...
s of energy. This is far less than the 200–360 J typically used to bring about normal sinus rhythm. Consequently, in the hospital setting, this treatment is not used.
Antiarrhythmic agents
Antiarrhythmic agents like amiodaroneAmiodarone
Amiodarone is an antiarrhythmic agent used for various types of tachyarrhythmias , both ventricular and supraventricular arrhythmias. Discovered in 1961, it was not approved for use in the United States until 1985...
or lidocaine
Lidocaine
Lidocaine , Xylocaine, or lignocaine is a common local anesthetic and antiarrhythmic drug. Lidocaine is used topically to relieve itching, burning and pain from skin inflammations, injected as a dental anesthetic or as a local anesthetic for minor surgery.- History :Lidocaine, the first amino...
can help, but, unlike atrial fibrillation
Atrial fibrillation
Atrial fibrillation is the most common cardiac arrhythmia . It is a common cause of irregular heart beat, identified clinically by taking a pulse. Chaotic electrical activity in the two upper chambers of the heart result in the muscle fibrillating , instead of achieving coordinated contraction...
, ventricular fibrillation rarely reverses spontaneously in large adult mammals. Drug therapy with antiarrhythmic agents in ventricular fibrillation does not replace defibrillation
Defibrillation
Defibrillation is a common treatment for life-threatening cardiac arrhythmias, ventricular fibrillation and pulseless ventricular tachycardia. Defibrillation consists of delivering a therapeutic dose of electrical energy to the affected heart with a device called a defibrillator...
and is not the first priority, but is sometimes needed in cases where initial defibrillation attempts are not successful.
Epidemiology
Sudden cardiac arrestCardiac arrest
Cardiac arrest, is the cessation of normal circulation of the blood due to failure of the heart to contract effectively...
is the leading cause of death in the industrialised world
World
World is a common name for the whole of human civilization, specifically human experience, history, or the human condition in general, worldwide, i.e. anywhere on Earth....
. It exacts a significant mortality with approximately 70,000 to 90,000 sudden cardiac deaths each year in the United Kingdom
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
, and survival rates are only 2%. The majority of these deaths are due to ventricular fibrillation secondary to myocardial infarction
Myocardial infarction
Myocardial infarction or acute myocardial infarction , commonly known as a heart attack, results from the interruption of blood supply to a part of the heart, causing heart cells to die...
, or "heart attack". During ventricular fibrillation, cardiac output
Cardiac output
Cardiac output is the volume of blood being pumped by the heart, in particular by a left or right ventricle in the time interval of one minute. CO may be measured in many ways, for example dm3/min...
drops to zero, and, unless remedied promptly, death usually ensues within minutes.
History of knowledge of ventricular fibrillation
Lyman Brewer suggests that the first recorded account of ventricular fibrillation dates as far back as 1500 BC, and can be found in the Ebers papyrusEbers papyrus
The Ebers Papyrus, also known as Papyrus Ebers, is an Egyptian medical papyrus dating to circa 1550 BC. Among the oldest and most important medical papyri of ancient Egypt, it was purchased at Luxor, in the winter of 1873–74 by Georg Ebers...
of ancient Egypt
Ancient Egypt
Ancient Egypt was an ancient civilization of Northeastern Africa, concentrated along the lower reaches of the Nile River in what is now the modern country of Egypt. Egyptian civilization coalesced around 3150 BC with the political unification of Upper and Lower Egypt under the first pharaoh...
. The extract recorded 3500 years ago may even date from as far back as 3500 BC. It states: "When the heart is diseased, its work is imperfectly performed: the vessels proceeding from the heart become inactive, so that you cannot feel them … if the heart trembles, has little power and sinks, the disease is advanced and death is near." A book authored by Jo Miles suggests that it may even go back farther. Tests done on frozen remains found in the Himalayas seemed fairly conclusive that the first known case of ventricular fibrillation dates back to at least 2500 BC.
Whether this is a description of ventricular fibrillation is debatable. The next recorded description occurs 3000 years later and is recorded by Vesalius
Vesalius
Andreas Vesalius was a Flemish anatomist, physician, and author of one of the most influential books on human anatomy, De humani corporis fabrica . Vesalius is often referred to as the founder of modern human anatomy. Vesalius is the Latinized form of Andries van Wesel...
, who described the appearance of "worm-like" movements of the heart in animals prior to death.
The significance and clinical importance of these observations and descriptions possibly of ventricular fibrillation were not recognised until John Erichsen in 1842 described ventricular fibrillation following the ligation
Ligation
Ligation may refer to:* In molecular biology, the covalent linking of two ends of DNA molecules using DNA ligase* In medicine, the making of a ligature * Chemical ligation, the production of peptides from amino acids...
of a coronary artery (Erichsen JE 1842). Subsequent to this in 1850, fibrillation was described by Ludwig and Hoffa when they demonstrated the provocation of ventricular fibrillation in an animal by applying a "Faradic
Michael Faraday
Michael Faraday, FRS was an English chemist and physicist who contributed to the fields of electromagnetism and electrochemistry....
" (electrical) current to the heart.
In 1874, Edmé Félix Alfred Vulpian
Alfred Vulpian
Edmé Félix Alfred Vulpian was a French physician and neurologist. He was the co-discoverer of Vulpian-Bernard spinal muscular atrophy and the Vulpian-Heidenhain-Sherrington phenomenon....
coined the term mouvement fibrillaire, a term that he seems to have used to describe both atrial and ventricular fibrillation. John A. MacWilliam, a physiologist who had trained under Ludwig and who subsequently became Professor of Physiology at the University of Aberdeen
University of Aberdeen
The University of Aberdeen, an ancient university founded in 1495, in Aberdeen, Scotland, is a British university. It is the third oldest university in Scotland, and the fifth oldest in the United Kingdom and wider English-speaking world...
, gave an accurate description of the arrhythmia in 1887. This definition still holds today, and is interesting in the fact that his studies and description predate the use of electrocardiography. His description is as follows: "The ventricular muscle is thrown into a state of irregular arrhythmic contraction, whilst there is a great fall in the arterial blood pressure, the ventricles become dilated with blood as the rapid quivering movement of their walls is insufficient to expel their contents; the muscular action partakes of the nature of a rapid incoordinate twitching of the muscular tissue … The cardiac pump is thrown out of gear, and the last of its vital energy is dissipated in the violent and the prolonged turmoil of fruitless activity in the ventricular walls." MacWilliam spent many years working on ventricular fibrillation and was one of the first to show that ventricular fibrillation could be terminated by a series of induction shocks through the heart.
The first electrocardiogram
Electrocardiogram
Electrocardiography is a transthoracic interpretation of the electrical activity of the heart over a period of time, as detected by electrodes attached to the outer surface of the skin and recorded by a device external to the body...
recording of ventricular fibrillation was by August Hoffman in a paper published in 1912. At this time, two other researchers, Mines and Garrey, working separately, produced work demonstrating the phenomenon of circus movement and re-entry as possible substrates for the generation of arrhythmias. This work was also accompanied by Lewis, who performed further outstanding work into the concept of "circus movement."
Later milestones include the work by Kerr
Kerr
-Places:*Kerr Township, Champaign County, Illinois*Kerr, Montana, A US census-designated place*Kerr, Ohio, USA, an unincorporated community*Kerr County, Texas, USA-Buildings and institutions:*Kerr Hall, on the campus of Ryerson University, Toronto, Canada...
and Bender
Bender
- Places:* Bender, Moldova, also known as Bendery or Tighina* Bender, California, a former settlement in Fresno County, California* Bender Bayla District, a district of Bari, Somalia- Fiction :...
in 1922, who produced an electrocardiogram showing ventricular tachycardia
Ventricular tachycardia
Ventricular tachycardia is a tachycardia, or fast heart rhythm, that originates in one of the ventricles of the heart...
evolving into ventricular fibrillation. The re-entry mechanism was also advocated by DeBoer, who showed that ventricular fibrillation could be induced in late systole with a single shock to a frog heart. The concept of "R on T ectopics" was further brought out by Katz in 1928. This was called the “vulnerable period” by Wiggers and Wegria in 1940, who brought to attention the concept of the danger of premature ventricular beats
Premature ventricular contraction
A premature ventricular contraction , also known as a premature ventricular complex, ventricular premature contraction , ventricular premature beat , or extrasystole, is a relatively common event where the heartbeat is initiated by the heart ventricles rather than by the sinoatrial node, the...
occurring on a T wave
T wave
In electrocardiography, the T wave represents the repolarization of the ventricles. The interval from the beginning of the QRS complex to the apex of the T wave is referred to as the absolute refractory period. The last half of the T wave is referred to as the relative refractory period...
.
Another definition of VF was produced by Wiggers in 1940. He described ventricular fibrillation as "an incoordinate type of contraction which, despite a high metabolic rate of the myocardium, produces no useful beats. As a result, the arterial pressure falls abruptly to very low levels, and death results within six to eight minutes from anemia
Anemia
Anemia is a decrease in number of red blood cells or less than the normal quantity of hemoglobin in the blood. However, it can include decreased oxygen-binding ability of each hemoglobin molecule due to deformity or lack in numerical development as in some other types of hemoglobin...
of the brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
and spinal cord
Spinal cord
The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain . The brain and spinal cord together make up the central nervous system...
".
Spontaneous conversion of ventricular fibrillation to a more benign rhythm is rare in all but small animals. Defibrillation
Defibrillation
Defibrillation is a common treatment for life-threatening cardiac arrhythmias, ventricular fibrillation and pulseless ventricular tachycardia. Defibrillation consists of delivering a therapeutic dose of electrical energy to the affected heart with a device called a defibrillator...
is the process that converts ventricular fibrillation to a more benign rhythm. This is usually by application of an electric shock
Electric shock
Electric Shock of a body with any source of electricity that causes a sufficient current through the skin, muscles or hair. Typically, the expression is used to denote an unwanted exposure to electricity, hence the effects are considered undesirable....
to the myocardium and is discussed in detail in the relevant article.
External links
- Interactive models and information on ventricular fibrillation and other arrhythmias
- Research on Primary Ventricular Fibrillation during Acute Myocardial Infarction
- When a child's heart stops, onset time of abnormal rhythms is crucial