The Ultimate Biomed Series
The Ultimate Biomed Series A Series by Ivan Joyner
Copyright 2011 www.bmetnow.com
Defibrillation Basics Defibs a History Defibrillation is the delivery of electrical current to the heart muscle either directly through the open chest, or indirectly through the chest wall, to terminate ventricular fibrillation and establish a normal cardiac rhythm. Through the years there have been various methods used to defibrillate patients. Mechanically the heart massage was probably most common. However in 1899 a team of scientists introduced the concept of applying large voltages across an animal’s heart and the seeds of modern defibrillation were planted. Then around 1960 direct current (dc) defibrillation introduced. In this method a large capacitor is charged to a high dc voltage and then rapidly discharged; The ancestors of modern defibrillation are born. · Interestingly it was the electric power industry that pioneered this research · Its employees were at risk of death from high voltage shocks. To help us understand the process of defibrillation we have to take a few minutes to consider the anatomy behind what is going on in the cardiac conduction system. If you remember from our ECG series the cardiac conduction system is a network of specialized tissue in the heart. Its function is to generate electrical impulses and transmit them throughout the heart causing the heart to contract (myocardial contraction) and creating a pulse. *Just as a side note former U.S. Vice President Dick Cheney has an LVAD installed and because of the pumping action, he effectively has no pulse. We’ll talk about the Left Ventricular Assist Device in another series. In very simple terms situations can occur that cause the cells of the heart to ‘misfire’ and reduce cardiac output to zero. When this electrical chaos occurs it could be referred to as fibrillation and it is not likely to correct itself. If external measures are not taken quickly to de-fibrillate, just a few minutes after the attack, tissue damage and ultimately death will occur. Medical devices designed to produce the energy to carry out this function are called defibrillators. Ventricular fibrillation is a condition that occurs when the heart muscle fibers basically get out of sync. It is one of the most serious medical emergencies facing a heart patient. · Under this condition the ventricles are unable to pump blood and unless this condition is corrected death will occur in just a few minutes. The most effective treatment for ventricular fibrillation is electric shock (defibrillation).
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What a Defib Does Just as the heart responds to the internal electric pulse from the SA node, it will also respond to an external electric pulse. If enough current is delivered to the heart chaos can be removed, defibrillation occurs and the normal pacemaker in the body can regain control. So machines that deliver the electric shock used to establish a more normal cardiac rhythm are called de-fibrillators. A defibrillator must deliver a high amount of energy to the heart safely and effectively. The energy may be delivered directly or transthoracically. · Direct delivery requires electrodes or paddles to be placed directly on the heart…Ouch! (not good for your future modeling career) · Transthoracic delivery requires large electrodes (paddles) applied to the skin in a specific position that allows as much energy as possible to be directed into the chest and ultimately to the heart. Defibrillator energy is measured in joules and old timers may call it watt-seconds. Factors that affect the energy delivery will include the · size of the paddles or electrodes · Skin to paddle conduction (that’s what the gel is for) · Transthoracic impedance (proper paddle placement helps this) The defib contains a capacitor which accepts a charge, stores the energy, and delivers it to the patient in a short controlled burst.
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The Cardioversion Component Defibrillators are also used to convert other potentially dangerous abnormalities or arrhythmias to one that is more easily managed. This process is called cardioversion. In some heart abnormalities (tachycardia) a fast QRS is present and unlike fibrillation the waveform is distinguishable. If the defib discharge is delivered during the ‘T’ wave ventricular fibrillation could occur. Because it is necessary to avoid delivering a shock during the ‘T’ wave of the ecg waveform (see ECG series) the cardioverter mode is often utilized.
In this mode additional circuitry is used to attempt to distinguish between the ‘R’ wave and the ‘T’ wave. The cardioverter will ensure that delivery of shock pulses is timed to be delivered approximately 20-30 msec after the peak of the ‘R’ wave.
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AC or DC (‘For those about to rock I salute you’ is one of the greatest rock anthems of all time). But what we’re talking about here is a/c or d/c used to deliver this energy to the heart tissue? Back in the day, a/c was used to supply a brief burst of 60 Hz at about 6 A to the patients chest with some success. The application of this shock was often enough to resynchronize the heart and cause a counter-shock. If the patient did not respond the burst was repeated until defibrillation occurred…Ouch! A/C defibrillation is no longer used. D/C defibrillation was found to be a better method for correcting fibrillation. The d/c method requires fewer repetitions and is less likely to harm the patient. DC Described There are considerations that the biomed needs to be aware of when it comes to delivery of the Defib energy. First the amount of energy is variable. The user can select the energy to be delivered from a range of available energy provided by the manufacturer. Also the method of delivery is important as well. Remember we want to minimize the potential danger to the patient and delivering a reduced level of energy while getting results is most desirable. In order to address this, defibs will deliver energy in different ways. Monophasic defibrillators will deliver energy in the way shown on the first waveform below. As you will note most of the energy is delivered above the baseline.
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To reduce the risk of damage due to potentially high peak voltages some defibrillators will use another method of energy delivery. This method is referred to as Biphasic delivery. By utilizing some additional circuitry to change the time duration of the positive and negative waveforms the energy delivered to the patient can be further controlled. Changes in the patients impedance will change the shape of the wave form as seen in wave two of our waveforms above. The biphasic waveform delivers the same amount of energy to the patient but the peak current depends on the patients’ impedance. Triphasic waveforms as seen in the third waveform may also be used. The first wave may ‘pre-condition’ the heart, the second ‘defibrillates’ and the third is a ‘healing’ post pulse.
Above are photos of common defibs. Basic Circuit The discharge circuit closely resembles a series RLC circuit. If you’re not comfortable with the analysis of such a circuit take some time and review the basics. Simply put when the switch is put into the charge position the source will allow the storage capacitor to charge and the paddles are isolated from the patient. See diagram below.
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The output of the transformer charges the storage capacitor. This stored energy is now available for defibrillation. In order to discharge the stored energy the paddles are put into place and a switch is depressed. At this time the transformer charging section will be isolated from the patient and the paddles will be in series with the charging capacitor. The energy will flow through the patient as he becomes part of the discharge circuit. Biomeds should perform testing on defibs every three to six months depending on the usage. Routine testing of the defib output, charge circuit, and battery capacity should be performed regularly. 路 The biomed should use a defib tester to verify charging and discharging of the unit at various settings. 路 Always consult the operating and service manuals for specific testing instructions.
Basic defib analyzer Below is a typical Defib testing setup
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More Technical Stuff · The typical duration of the defib pulse is 3 to 9 milliseconds at a 50 ohm load. · Charging the defib to maximum energy typically takes 4-10 seconds. · Most defibs will retain the charge for 60 seconds. Note* Stored energy differs from delivered energy. Stored energy is the amount of energy produced inside the defib. Delivered energy is the amount delivered to the patient. Delivered energy varies with the amount of impedance or resistance to the flow of energy. Well we’ve covered quite a bit about the history, basics and anatomy of defibrillation. Add this to your quiver of knowledge and continue on your quest to become the Ultimate Biomed.
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