1 BMT414 Pacemakers Dr. Ali Saad, Biomedical Engineering Dept. College of applied medical sciences King Saud University

2 2 Cardiac Electrophysiologic Assist Devices Pacemaker Defibrillator Cardioverter These are used to treat arrhythmias: –AV block (pacemaker) –A or V fibrillation (defibrillator, cardioverter) –tachycardia (defibrillator) –bradycardia (pacemaker)

3 Short History of Pacemakers The basic approach to cardiac pacing is to supply an electrical shock to the heart, resulting in a ventricular contraction. Early pacemakers utilized skin electrodes with large surface areas or subcutaneous needle electrodes (1950’s). Electrodes placed on the surface of the heart were then introduced via an opening in the chest wall (thoracotomy). Modern pacemakers use catheter electrodes introduced into the right ventricle via the cephalic or sub-clavian vein.

4 Cardiac Conduction System

5 Cardiac depolarization

6 Representative electric activity from various regions of the heart. The bottom trace is a scalar ECG, which has a typical QRS amplitude of 1-3 mV. (© Copyright 1969 CIBA Pharmaceutical Company, Division of CIBAGEIGY Corp. Reproduced, with permission, from The Ciba Collection of Medical Illustrations, by Frank H. Netter, M. D. All rights reserved.)

7 Atrioventricular block (a) Complete heart block. Cells in the AV node are dead 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. (Adapted from Brendan Phibbs, The Human Heart, 3rd ed., St. Louis: The C. V. Mosby Company, 1975.)

8 Normal ECG followed by an ectopic beat An irritable focus, or ectopic pacemaker, within the ventricle or specialized conduction system may discharge, producing an extra beat, or extrasystole, that interrupts the normal rhythm. This extrasystole is also referred to as a premature ventricular contraction (PVC). (Adapted from Brendan Phibbs, The Human Heart, 3rd ed., St. Louis: The C. V. Mosby Company, 1975.)

9 (a) Paroxysmal tachycardia. An ectopic focus may repetitively discharge 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. (Adapted from Brendan Phibbs, The Human Heart, 3rd ed., St. Louis: The C. V. Mosby Company, 1975.)

10 (a) Atrial fibrillation. The atria stop their regular beat and begin a feeble, uncoordinated twitching. Concomitantly, low-amplitude, irregular waves appear in the ECG, as shown. This type of recording can be clearly distinguished from the very regular ECG waveform containing atrial flutter. (b) Ventricular fibrillation. Mechanically the ventricles twitch in a feeble, uncoordinated fashion with no blood being pumped from the heart. The ECG is likewise very uncoordinated, as shown (Adapted from Brendan Phibbs, The Human Heart, 3rd ed., St. Louis: The C. V. Mosby Company, 1975.)

11 Modern Pacemakers power supply timing circuit pulse output circuit lead wires & electrodes hermetically sealed stainless steel or titanium package

12 Block diagram of an asynchronous cardiac pacemaker

13 A demand-type synchronous pacemaker Electrodes serve 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.

14 Demand Synchronous Pacing (cont.) n after each stimulus, timing circuit resets, and waits a certain time interval, T (1 sec). n if amplifier detects naturally occurring R-wave during this interval, timing circuit reset again. n timing circuit keeps resetting with each naturally occurring beat as long as it occurs within T seconds of previous beat. n if no naturally occurring beat occurs after T seconds, output circuit stimulates. n useful for bradycardia (slow HR).

15 An atrial-synchronous cardiac pacemaker, which detects electric signals corresponding to the contraction of the atria and uses appropriate delays to activate a stimulus pulse to the ventricles. Figure 13.5 shows the waveforms corresponding to the voltages noted.

16 Atrial Synchronous Pacing (cont.) 120 ms 2 ms 500 ms atrial pulses v1v1 v2v2 v3v3 v4v4 t t t t triggers stimulus gate input AV node delay ventricular signal can be detected at atrium, gating insures that v. signal is not confused with an atrial signal

17 Block diagram of a rate- responsive pacemaker

18 Power Supply Lithium Iodide battery (most common): –cathode reaction: –anode reaction: –2.8V open circuit voltage –lifetime of 15 years (big improvement over earlier batteries) Experimental Sources: –transcutaneous induction –mechanical generators, based on movement in heart and large vessels. –electrochemical, using ions found in body. –plutonium

19 Piezoelectric element bonded to the inside of the pacemaker can. Body motion causes pressure fluctuations which cause the can to deflect which bends the sensor to produce a voltage. The leads from the piezoelectric sensor are connected to the pacemaker electronics. This is one possible layout for the pacemaker components.

20 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.

21 A logical diagram of relationship between rhythm disturbances and therapeutic pacing modes for selecting the proper pacing mode. From Schaldach, M. M Electrotherapy of the heart. Berlin: Springer–Verlag.

22 DDD Pacemaker combines: demand synchronous atrial synchronous ventricular synchronous n atrial sensor detects natural or stimulated atrial contraction, then triggers ventricles if no naturally occurring ventricular pulse is detected within T AV = 120 ms. n ventricular sensor detects natural or stimulated ventricular contraction, then triggers atria if no naturally occurring atrial pulse is detected within T VA = 700 ms.

23 Evolution of implantable pacemaker technology

24 Implanted pacemaker

25 Dual Chamber Pacemaker

26 Bipolar electrode configuration current pathway. Current flows from one electrode to another, the bottom electrode is in contact with cardiac muscle.

27 Bipolar Pacemaker Electrodes band electrodes stainless steel platinum titanium alloy Si rubber Electrode usually located inside heart (intratuminal), via cephalic vein. Si rubber hooks lead wire coil

28 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

29 Unipolar Pacemaker Electrodes Pt electrode Si rubber implanted on surface of heart (epicardial) reference electrode implanted away from heart

30 Two of the more commonly applied cardiac pacemaker electrodes (a) Bipolar intraluminal electrode. (b) Intramyocardial electrode.

31 Active and passive fixation mechanisms of various types for endocardial and epicardial pacing leads (From Ellenbogen, 1996).

32 Tip Electrode Si or polyurethane rubber Si rubber hooks (tines): entangle in trabeculae (net-like lining) wire coil tip electrode porous, platinized tip for steroid elution, reduces inflammation steroid

33 Cross-sectional view of a steroid-eluting intracardiac electrode (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. From Mond, H., and Stokes, K. B The electrode–tissue interface: the revolutionary role of steroid elution. PACE, 15: 95–107.

34 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 (Modified from Furman et al., 1993).

35 The current strength (I)–duration (d) curve: for canine muscle: A = atrium, V = ventricle (modified from Geddes 1984).

36 A pacemaker provides a 1 mA pulse with a duration of 1 ms so the total charge for one pulse is 1  C. The number of pulses per year at one per second is 60  60  24  365 = 31,536,000. Over the 10 year life of the pacemaker, the charge drawn from the battery is 1  C  31,536,000  10 = 315 C = 315 A  s = A  h. This is a small portion of the total battery life of 2 A  h, most of which supplies the electric circuits.

37 Block diagram of pacemaker programming and telemetry interface.

38 Timing and Output Circuits n Asynchronous: runs at a fixed pacing rate, set by technician (70-90 BPM): these are no longer used since if a stimulus is applied during the T-wave of a normal beat, can get v. fibrillation. n Synchronous: uses feedback from ECG and/or other sources to determine pacing rate ( BPM). n output circuit: n constant current pulses: 8-10 mA, ms duration n constant voltage pulses: V,  s duration

39 Lead Wires and Electrodes n Must be able to withstand constant bending due to beating of the heart (35 million beats per year). n Must be biocompatable, tissues can be very corrosive n The two above criteria are satisfied via interwound helical coils embedded in silicon or polyurethane rubber.

40 Pacemaker Placement RV trans-venous sub-clavian or cephalic vein, much less traumatic epicardial requires thoracotomy

41 Monostable Multivibrator (MSMV):  rising edge trigger input  vivi vivi vovo Gate: vivi v i = H, switch open v i = L, switch closed

42 Comparator _ + V out V+V+ V_V_

43 Monostable Multivibrator (cont) _ + R1R1 R2R2 R C V trig V out 0 V -AV-AV t0t0 VcVc + _ A > 0 V

44 Monostable Multivibrator (cont.)  when switch is open, circuit is in stable state:  at t = t 0, switch is momentarily closed: this immediately causes (momentarily)

45 Monostable Multivibrator (cont.)  when switch is open, circuit is in stable state:  at t = t 0, switch is momentarily closed: this immediately causes (momentarily)

46 Monostable Multivibrator (cont.)  diode now has a negative voltage across it, capacitor no longer clamped at 0.7 V.  capacitor begins to charge up to a negative voltage with time constant, RC  at the instant that capacitor voltage becomes more negative than, comparator output switches back to:  multivibrator is now in stable state again.  the interval during which comparator output is is called an astable state.

47 Monostable Multivibrator (cont.) 0.7V V out VcVc t t0t0 0  V 1 = diode forward bias voltage (0.7 V)

48 Triggering Circuit want to trigger monostable multivibrator on leading edge of a negative going pulse: 0 V -5 V

49 Astable Multivibrator (Square Wave Generator) _ + R1R1 R2R2 R C V out VcVc + _ no stable state

50 Astable Multivibrator (cont.) _ + R1R1 R2R2 C V out at t = t 0 : S 1 opens S 2 closes R S1S1 S2S2

51 Astable Multivibrator (cont.) n for t < t 0, assume V out = V r : n for t > t 0 : capacitor begins to charge through R with time constant RC n when capacitor voltage V c exceeds V + =  V r, V out = -V r n capacitor then begins to charge towards -V r with time constant RC n when capacitor voltage V c becomes more negative than V + = -  V r, V out = V r (stable state)

52 Astable Multivibrator (cont.) V out VcVc t t0t0 0   can be used along with MSMV for asynchronous pacing

53 Operational Amplifiers (Op Amps) _ + inputs: output: I = 0

54 Constant Current Source _ + + _ R R R R

55 Constant Current Source (cont.) _ + + _ R R R R or

56 Constant Current Source (cont.) _ + + _ R R R R node b node analysis at node b gives: or: I L is independent of load resistor R L

57 Constant Voltage Source Most modern pacemakers use constant voltage output circuit: + _ 2.8V charging C1C1 C2C2 C1C1 C2C2 R heart discharging 5.6V + _ use capacitors to increase stimulus voltage: amplitude: V pulse duration: ms + _ + _

58 Constant Voltage Source (cont.) 5.6V _ + voltage follower prevents loading by high impedance loads R R