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Unformatted text preview: Pacemakers & defibrillators-I
Department of Mechanical Engineering AUB Representative electric activity from various regions of the heart. The bottom trace is a scalar Representative The ECG, which has a typical QRS amplitude of 1-3 mV. ECG, Blood flows from the right atrium to Blood right ventricle to lungs to left atrium to left ventricle to the body.
To head and arms From head and arms Aorta Right lung Left lung Left Atrium Right coronary artery (3) Right Atrium (2) (1) Right Ventricle (4) Left Ventricle To trunk and legs Left coronary artery From trunk and legs (1) (2) (3) (4) Tricuspid valve Pulmonary valve Mitral valve Aortic valve Aorta Left atrium HRA Right atrium HBE Right ventricle Left ventricle III HRA HBE P A H V Catheters Catheters within the atria and ventricles provide access to specific Catheters ific locations from which to measure electrical activity. The A–H interval is a locations interval relatively accurate measure of conduction through the AV node. The H relatively he wave is a result of the His bundle activation. III: lead three of the wave activation. surface ECG; HRA: electrogram from the upper right atrium; HBE: electrogram from electrogram from the His bundle near the top of right ventricle. electrogram Hierarchical display of influential factors on Hierarchical overall cardiac performance Intrinsic adaptations Extrinsic adaptations Preload Afterload Chemical factors Neural factors Contraction frequency Ionic Ca++ Na+ K+ Gaseous O2 CO2 Hormonal Glucagon Thyroid hormone Adrenal cortical steroids Sympathetic Parasympathetic Central & peripheral interactions Change in blood [O 2 ] & [CO 2 ] Epinephrine Norepinephrine Cardiac performance Atrioventricular block (a) Complete heart block. Cells in the AV node are dead Atrioventricular and activity cannot pass from atria to ventricles. Atria and ventricles beat independently, ventricles being driven by an ectopic (other-than-normal) pacemaker. (B) AV block wherein the node is diseased (examples include rheumatic heart disease and viral infections of the heart). Although each wave from the atria reaches the ventricles, the AV nodal delay is greatly increased. This is first-degree heart block. heart Normal ECG followed by an ectopic beat An irritable focus, or ectopic ectopic beat ectopic pacemaker, within the ventricle or specialized conduction system may within discharge, producing an extra beat, or extrasystole, that interrupts the extrasystole that normal rhythm. This extrasystole is also referred to as a premature extrasystole is ventricular contraction (PVC). ventricular (a) Paroxysmal tachycardia. An ectopic focus may repetitively discharge (a) at a rapid regular rate for minutes, hours, or even days. (b) Atrial flutter. The atria begin a very rapid, perfectly regular "flapping" movement, beating at rates of 200 to 300 beats/min. (a) Atrial fibrillation. The atria stop their regular beat and begin a feeble, Atrial ble, uncoordinated twitching. Concomitantly, low-amplitude, irregular waves uncoordinated amplitude, appear in the ECG, as shown. This type of recording can be clearly appear ly distinguished from the very regular ECG waveform containing atrial atrial flutter. (b) Ventricular fibrillation. Mechanically the ventricles twitch in a fibrillation. feeble, uncoordinated fashion with no blood being pumped from the heart. feeble, heart. The ECG is likewise very uncoordinated, as shown The Back Front Position of transcutaneous electrodes. Circular electrode (the cathode) is placed on the anterior of thorax. The rectangular electrode (the anode) can be placed on the anterior or posterior of the thorax Pulse generator +
Atrial ball tip cathode Ventricular distal cathode
Temporary transvenous pacing. The leads are passed through veins transvenous pacing. under fluoroscopy. under A practical pacemaker (Medtronic inc., 2001) Cardiac pacemaker electrodes (a) Bipolar intraluminal intraluminal electrode. (b) Intramyocardial electrode (formerly used). electrode. Intramyocardial Active and passive fixation mechanisms of various types for endocardial and epicardial pacing leads endocardial and epicardial Atrial J-shaped Atrial J-shaped Ventricular Unipo lar Ventricular Bipo lar Unipolar and bipolar implementations of both JUnipolar shaped and nonpreshaped leads. All models have shaped nonpreshaped leads. distal cathode. Bipolar designs typically have a ring anode proximal 10–15 mm on the lead. anode 30 Pacing voltage loss (Vrms, dB) 25 20 15 10 5 0 1 10 100 1000 10,000 Frequency of stimulation, f (Hz) Porous electrode surface Smooth electrode surface Pacing voltage loss at the myocardium–electrode interface is reduced Pacing electrode by implementing a porous as opposed to relatively smooth tipped electrode. Decrease in voltage loss is largely contributed to by electrode. decreased electrode polarization associated with increased active surface area. surface Electrode body Po rous, platinum coated titanium tip Silicon rubber plug (impregnated with DSP steroid) Cross-sectional view of a steroid-eluting intracardiac electrode Cross (Medtronic CapSure® electrode, model 4003). Note silicone rubber plug with impregnated steroid DSP. Steroid elutes through the porous tip into surrounding tissue, thus reducing inflammation. 700 600 500 % Change of threshold 400 300 200 100 Acute threshold 0 1 2 3 4 2 3 4 5 6 7 8 Steroid electrode Chronic threshold Solid electrode Weeks Months Threshold evolution after implantation. (a) Once an electrode is placed against or within sensitive tissue, local reaction causes enlargement of its surface area as the virtual electrode is formed. As chronicity is reached, the virtual electrode is smaller than early after implant and the threshold decreases and is stabilized. (b) Steroid-eluting electrodes have produced a distinct reduction in stimulation threshold acutely and chronically. Sensing characteristics have also improved. This figure compares similar solid tips, without steroid and steroid-eluting electrodes. The increase in stimulation threshold for the steroid electrode early after implant is much reduced and the long-term stable threshold for both is characteristic Pace Generator Lead Electrode Unipolar electrode configuration current pathway. Only the cathode electrode is in contact with myocardium with unipolar stimulation, the other (anode) electrode often is the case of the pulse generator, which is some distance from the heart Lead wire Electrode Bipolar electrode configuration current pathway. Current flows from one electrode to another, the bottom electrode is in contact with cardiac muscle. Cell membrane Cathode -20 mV -90 mV current flow Anode + +20 mV -90 mV Stimulation of a cell with the anode and the cathode. The cell becomes depolarized when the potential drop across the cell membrane is approximately –70 mV. The cathodal stimulation of magnitude –20 mV causes the potential drop across the membrane to reach the depolarization threshold. However, anodal stimulation causes a hyperpolarization of the cell, forcing the cell membrane potential opposite the desired direction. If the anodal stimulus is increased to a large enough value the cell will eventually fire, but the anodal stimulus magnitude required for depolarization can be from two to three times as much as the cathodal value. mA 100 Q = constant V: Ventricle A: Atrium 10 Stimulating Current 1 A V 0.1 I = constant 0.01 0.1 10 1 Pulse Width 100 ms The current strength (I)–duration (d) curve: for canine muscle: A = atrium, V = ventricle (a) (b) L o g C u r r e n t S t i m u l a t o r Cell Membrane Rm Cm rheobase Log Pulse Duration chronaxie time (a) A simplified version of a pacemaker stimulating a load. In this case, case, the load is simply a cell membrane. (b) The strength–duration curve the duration associated with the circuit in part (a). While this specific curve relates to associated ). the depolarization of a single cell membrane, the strength–duration curve the duration for stimulation of the heart has the same shape and characteristics. The for rheobase is the minimal current that will cause the membrane to is depolarize. The chronaxie time is the pulse duration required to cause chronaxie time depolarization when the stimulating current magnitude is twice the depolarization he rheobase. rheobase Sensing unit Lead Power source Control unit Pulse generator The pacemaker senses from the heart and applies pulses to the heart Block diagram of an asynchronous cardiac pacemaker cardiac A demand-type synchronous pacemaker Electrodes serve demand Electrodes as a means of both applying the stimulus pulse and detecting the electric signal from spontaneously occurring ventricular contractions that are used to inhibit the pacemaker's timing circuit. inhibit An atrial-synchronous cardiac pacemaker, atrial synchronous which detects electric signals corresponding to the contraction of the atria and uses appropriate delays to activate a stimulus pulse to the ventricles. pulse Block diagram of a rateBlock responsive pacemaker The VAI is defined as the time between a paced or sensed ventricular The event to the next consecutive atrial event. It can be derived by subtracting the AVI from the LRI LRI AVI VAI Start P-wave sensed State 0 Initialize refractory period timers R-wave sensed Atrium sensed State 1 R-wave sensed P-wave sensed Ventricle sensed State 2 Atrial refractory period exceeded Ventricular refractory P-wave period exceeded sensed R-wave sensed Atrial refractory Ventricle period exceeded paced State 3 Ventricular refractory period exceeded Atrium paced State 4 A simplified state-machine pacemaker. The processor simplified changes state in response to elapsed intervals and sensed signals. Each node in the diagram represents a state, and every state has its unique set of rules to steer the processor from one state to another based on the inputs The three-letter pacemaker coding system was recommended by ICHD in 1974 and became the first widely adopted pacemaker code. It was simple and easy to use and it only contained three letters. The first letter designates the chamber(s) paced: ventricle (V), atrium (A), or both (D for double). The second letter designates the chamber(s) sensed. The third letter designates the mode of response(s): T = triggered, I = inhibited, D = double, O = none. The code was revised in 1981 to accommodate new functionalities of pacemakers. ...
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