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Microshock refers to the risk that patients undergoing medical procedures involving externally protruding intracardiac electrical conductors, such as external pacemaker electrodes, or saline filled catheters, could suffer an electric shock causing ventricular fibrillation (VF) due to currents entering the body via these parts. [1]
It is important to note that microshock (or micro-shock) are not IEV [2] defined terms and are not used in any international standard.
"Micro-shock" is an otherwise imperceptible electric current applied directly, or in very close proximity, to the heart muscle of sufficient strength, frequency, and duration to cause disruption of normal cardiac function.
Note: It can be safely assumed (and it usually is) that micro-shock is only possible during certain medical procedures as the electric current needs to be focused directly into the heart by some conductor inserted by invasive means for some desired medical outcome (for example Cardiac Catheterisation).
Micro-shock, if it occurs, is not always lethal. “Micro-electrocution” is the term that should be used whenever a micro-shock causes death.
“Macro-shock” is when a much larger current is passed through the body, usually via a skin to skin pathway, but more generally the current is not applied directly through the heart muscle. The current in macro-shock events can vary widely from being imperceptible to being extremely destructive of tissue. (see Macroshock)
“Electric Shock” is usually referring to macro-shock. (see Electric Shock)
“Electrocution” is usually referring to a macro-shock that has caused prolonged or severe disruption of normal cardiac function - ultimately leading to death. (see Electrocution)
Microschock requires direct electrical connection to the heart muscle and is normally illustrated using a diagram such as Figure 1 (from TGE).
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In this scenario the patient has inadvertently contacted both a source of current (it does not have to be AC, as shown) as well as a common return pathway during an invasive cardiac medical procedure. If the current flowing is below the threshold of perception, or the patient is sedated, or anaesthetized, there may be no pain or reflex response of either arm. If the current flow continues for sufficient time, at sufficient strength, the patient may die. Because of the low current and lack of patient response, this death may be unexpected, and without any obvious cause. In practice, however, this has never been proven to have happened. [3]
To a novice, however, this scenario looks incredibly dangerous, and it is therefore worth examining in some detail.
Firstly, let's follow the path of the electric current. There is a generic source of current. This source can be either large or small, as only a small voltage is required to drive the low current for micro-shock. Such sources might be, a wall socket, a faulty item of equipment, an inappropriate item of equipment, a poorly designed item of equipment, or an item of equipment designed to deliver current into the body. Our patient has unfortunately been contacted to one such electrical source and current is dispersing through their right arm and upper torso, to eventually converge on a catheter (as labelled – but it could be a lead or wire) that is placed into their heart. This concentration of current flow at the heart muscle is the danger from micro-shock. If the catheter is conductive (from end to end) insulated in its passage through the body, the current may follow the catheter, emerging through the skin into some other energized item of equipment. For the circuit as shown to be complete, the loop needs to also be connected to the same ground/low-potential as the equipment. Finally, the hazardous circuit is complete – current can flow and if it continues the patient is in mortal danger. Again, while this is theoretically possible this has never been proven to have actually happened. [3] Proving this type of event after the fact is difficult in autopsy, as the cause of the fatal ventricular fibrillation would be unknown and the death appears to be idiopathic.
So, why has this situation not arisen? Generally, electrically conductive connections made in or around a patient's heart will be those of medical-grade electrical equipment. In countries that have a regulatory environment (example: such as much of Europe and North America) micro-shock is contemplated and the surgical equipment is regulated to prevent micro-shock. These medical-grade products are generally constructed to strict standards that limit the allowable currents flowing via such connections (applied parts). This decreases the risk to the patient and increases the margin of safety.
There has never been a documented case of microshock. A U.S. Senate inquiry in the early 1970s, sparked by exaggerated reports of thousands of U.S. hospital patients dying of microshock, heard expert testimony about the effect. A review of the evidence in the early 2000s found that not a single case had been reported in the 30 years since the Senate inquiry. [4] Regular checks of the FDA's MAUDE database also show no evidence of this risk being manifest, before or since the review.
Based on studies with dogs by Prof Leslie Geddes in the middle of last century, it is theorised that a current as low as 10 μA (microampere) directly through the heart, may send a human patient directly into ventricular fibrillation. Of course, the exact outcome is dependent on the duration of the current, the exact position of contact, the frequency of current oscillation, and the timing of the shock with the heart's rhythm e.g. R on T phenomenon. It is feared that such a small current may be introduced unwittingly, and unobserved, creating a very perilous situation for the patient. To guard against this slim theoretical possibility then, modern medical devices include a range of protective measures to limit current in cardiac-connected circuits to the assumed safe levels of below 10 μA (microampere). These measures include isolated patient connections, high impedance connections and current limiting circuits. Despite the in-built protections, and lack of observed incidents, microshock continues to be a concern to many practitioners of the fields of Biomedical and Clinical Engineering.
Despite the evidence of decades of absence of reports, in any condition where electrical conductors are run into the body in proximity of the heart (i.e. cardiac catheterizations) precautions are still taken to ensure hazardous current is not introduced through these conductors and it is still regarded as a high-risk activity.
An artificial cardiac pacemaker is a medical device, currently always implanted, that generates electrical pulses delivered by electrodes to one or more of the chambers of the heart, the upper atria or lower ventricles. Each pulse causes the targeted chamber(s) to contract and pump blood, thus regulating the function of the electrical conduction system of the heart.
Cardioversion is a medical procedure by which an abnormally fast heart rate (tachycardia) or other cardiac arrhythmia is converted to a normal rhythm using electricity or drugs. Synchronized electrical cardioversion uses a therapeutic dose of electric current to the heart at a specific moment in the cardiac cycle, restoring the activity of the electrical conduction system of the heart. Pharmacologic cardioversion, also called chemical cardioversion, uses antiarrhythmia medication instead of an electrical shock.
Defibrillation is a treatment for life-threatening cardiac arrhythmias, specifically ventricular fibrillation (V-Fib) and non-perfusing ventricular tachycardia (V-Tach). A defibrillator delivers a dose of electric current to the heart. Although not fully understood, this process depolarizes a large amount of the heart muscle, ending the arrhythmia. Subsequently, the body's natural pacemaker in the sinoatrial node of the heart is able to re-establish normal sinus rhythm. A heart which is in asystole (flatline) cannot be restarted by a defibrillator; it would be treated only by cardiopulmonary resuscitation (CPR) and medication, and then by cardioversion or defibrillation if it converts into a shockable rhythm.
Tachycardia, also called tachyarrhythmia, is a heart rate that exceeds the normal resting rate. In general, a resting heart rate over 100 beats per minute is accepted as tachycardia in adults. Heart rates above the resting rate may be normal or abnormal.
Ventricular fibrillation is an abnormal heart rhythm in which the ventricles of the heart quiver. It is due to disorganized electrical activity. Ventricular fibrillation results in cardiac arrest with loss of consciousness and no pulse. This is followed by sudden cardiac death in the absence of treatment. Ventricular fibrillation is initially found in about 10% of people with cardiac arrest.
An electrical injury, or electrical shock is damage sustained to the skin or internal organs on direct contact with an electric current.
Asystole is the absence of ventricular contractions in the context of a lethal heart arrhythmia. Asystole is the most serious form of cardiac arrest and is usually irreversible. Also referred to as cardiac flatline, asystole is the state of total cessation of electrical activity from the heart, which means no tissue contraction from the heart muscle and therefore no blood flow to the rest of the body.
Wolff–Parkinson–White syndrome (WPWS) is a disorder due to a specific type of problem with the electrical system of the heart involving an accessory pathway able to conduct electrical current between the atria and the ventricles, thus bypassing the atrioventricular node. About 60% of people with the electrical problem developed symptoms, which may include an abnormally fast heartbeat, palpitations, shortness of breath, lightheadedness, or syncope. Rarely, cardiac arrest may occur. The most common type of irregular heartbeat that occurs is known as paroxysmal supraventricular tachycardia.
Atrial flutter (AFL) is a common abnormal heart rhythm that starts in the atrial chambers of the heart. When it first occurs, it is usually associated with a fast heart rate and is classified as a type of supraventricular tachycardia. Atrial flutter is characterized by a sudden-onset (usually) regular abnormal heart rhythm on an electrocardiogram (ECG) in which the heart rate is fast. Symptoms may include a feeling of the heart beating too fast, too hard, or skipping beats, chest discomfort, difficulty breathing, a feeling as if one's stomach has dropped, a feeling of being light-headed, or loss of consciousness.
Ventricular tachycardia is a cardiovascular disorder in which fast heart rate occurs in the ventricles of the heart. Although a few seconds of VT may not result in permanent problems, longer periods are dangerous; and multiple episodes over a short period of time are referred to as an electrical storm. Short periods may occur without symptoms, or present with lightheadedness, palpitations, or chest pain. Ventricular tachycardia may result in ventricular fibrillation (VF) and turn into cardiac arrest. This conversion of the VT into VF is called the degeneration of the VT. It is found initially in about 7% of people in cardiac arrest.
Appliance classes specify measures to prevent dangerous contact voltages on unenergized parts, such as the metallic casing, of an electronic device. In the electrical appliance manufacturing industry, the following appliance classes are defined in IEC 61140 and used to differentiate between the protective-earth connection requirements of devices.
Catheter ablation is a procedure that uses radio-frequency energy or other sources to terminate or modify a faulty electrical pathway from sections of the heart of those who are prone to developing cardiac arrhythmias such as atrial fibrillation, atrial flutter and Wolff-Parkinson-White syndrome. If not controlled, such arrhythmias increase the risk of ventricular fibrillation and sudden cardiac arrest. The ablation procedure can be classified by energy source: radiofrequency ablation and cryoablation.
Transcutaneous pacing (TCP), also called external pacing, is a temporary means of pacing a patient's heart during a medical emergency. It should not be confused with defibrillation using a manual or automatic defibrillator, though some newer defibrillators can do both, and pads and an electrical stimulus to the heart are used in transcutaneous pacing and defibrillation. Transcutaneous pacing is accomplished by delivering pulses of electric current through the patient's chest, which stimulates the heart to contract.
William Bennet Kouwenhoven, also known as the "Father of Cardiopulmonary Resuscitation," is famous for his contributions to the development of the closed-chest cardiac massage and his invention of the cardiac defibrillator. After obtaining his doctorate degree in engineering from the Karlsruhe Technische Hochschule in Germany, Kouwenhoven began his career as the dean at the Johns Hopkins University in Baltimore. Kouwenhoven focused his research mainly on improving and saving lives of patients through the application of electricity. With the help and cooperation of the Johns Hopkins School of Medicine's Department of Surgery and an Edison Electric Institute grant, Kouwenhoven was able to develop a closed-chest defibrillator. For his contributions to the field of medical science, he became the first ever recipient of an honorary degree conferred by the Johns Hopkins School of Medicine. Two years before his death, Kouwenhoven was also awarded the Albert Lasker Award for Clinical Medical Research.
Michel Haïssaguerre is a French cardiologist and electrophysiologist. His investigations have been the basis for development of new markers and therapies for atrial and ventricular fibrillation.
Macroshock (mak´ro-shok″) is a medical term for the effects of body exposed to electrical current, which can lead to severe injury or death by electrocution. It is used most often in the medical field, but is also commonly used in the fields of electrophysiology and bioengineering. Definitions of the term are inconsistent; there are three most commonly accepted definitions. Depending upon the medical text used, a macroshock is either:
A cardiac electrophysiology study is a minimally invasive procedure using catheters introduced through a vein or artery to record electrical activity from within the heart. This electrical activity is recorded when the heart is in a normal rhythm to assess the conduction system of the heart and to look for additional electrical connections, and during any abnormal heart rhythms that can be induced. EP studies are used to investigate the cause, location of origin, and best treatment for various abnormal heart rhythms, and are often followed by a catheter ablation during the same procedure.
Arrhythmias, also known as cardiac arrhythmias, heart arrhythmias, or dysrhythmias, are irregularities in the heartbeat, including when it is too fast or too slow. A resting heart rate that is too fast – above 100 beats per minute in adults – is called tachycardia, and a resting heart rate that is too slow – below 60 beats per minute – is called bradycardia. Some types of arrhythmias have no symptoms. Symptoms, when present, may include palpitations or feeling a pause between heartbeats. In more serious cases, there may be lightheadedness, passing out, shortness of breath, chest pain, or decreased level of consciousness. While most cases of arrhythmia are not serious, some predispose a person to complications such as stroke or heart failure. Others may result in sudden death.
Topera, Inc. is a cardiac arrhythmia mapping company for targeting catheter ablation company launched in San Diego, California and specializes in mapping electrical signals of the heart. Topera's headquarters are located in Palo Alto, California. The company uses 3D analysis and mapping to detect the sources of atrial fibrillation, atrial flutter, and atrial tachycardia and ventricular tachycardia to identify targets for catheter ablation.
Bruce B. Lerman is a cardiologist. He is the Hilda Altschul Master Professor of Medicine at Weill Cornell Medical College, and was chief of the Division of Cardiology and director of the Cardiac Electrophysiology Laboratory at Weill Cornell Medicine and the New York Presbyterian Hospital.