Pacers use a 5-letter code: first 3 letters most important

Transcription

1 PACEMAKERS 2

2 Pacemakers: Nomenclature Pacers use a 5-letter code: first 3 letters most important t First Letter: Chamber Paced A= Atrium V= Ventricle D= Dual (A+V) 2nd Letter: Chamber Sensed A= Atrium V= Ventricle D= Dual (A+V) O= None 3rd Letter: Response after Sensing: I = Pacing Inhibited T= Pacing Triggered D= Dual (I+T) O= None 4th Letter: Programmability P = Rate & Output M = Multiprogramable C = Communicating R = Rate adaptive O = None 5th Letter: Arrhythmia Control P = pacing S= shock D= Dual (P+S) O = None

3 Pacemaker Terminology Rate: the heart rate at which the pacemaker will pace. Standby rate: the lowest rate at which the pacemaker will pace. Capture: depolarization i and resultant contraction of the myocardium in response to a pacemaker generated electrical stimulus. Sensing: Dependent on the amplitude, slew rate and signal frequency, it describes the pacemakers ability to recognize a native electrical l signal.

4 Pacemaker Terminology Sensitivity: the minimum intracardiac signaling required by the pacemaker to initiate a pacemaker response. Threshold: minimum quantity, of either amplitude (milliamperes, volts) pulse duration (milliseconds), charge (coulombs) or energy (Joules) produced d by the pacemaker that persistently produces an action potential and myocardial contraction.

5 Pacemaker Terminology.. Mode: indicates pacemaker capabilities: fixed rate or demand. Fixed rate: pacemaker is one that fires at a specific preset rate, regardless of the patient's own heart rate. Demand: that is, only when the patient's heart rate falls below a preset value. Pulse interval: the total time of the AV and VA intervals. number of pulses per minute. Hysteresis: an intentional prolonged pulse interval in order to allow the generation of a spontaneous-intrinsic electrical depolarization event.

6 Pacemaker Terminology Atrioventricular (AV) interval (ie AV delay): described for dual-chamber pacemakers. The equivalent to a native PR interval. Represents the time (msec) between an atrial event and a paced ventricular event. Time that the pacemaker discerns whether or not to pace dependant upon sensing a native R wave. Allows the ventricle time to fill following an atrial contraction. Ventriculoatrial (VA) interval: described for dual- chamber pacemakers. Represents the time (msec) between a ventricular event and a paced atrial event.

7 Ventricular Synchronous Demand Pacemaker Hz, Centered at 30 Hz Detection sensitivity :1 2 mv Cardiac Signal Range: 1-30 mv Pulse Generator: functions Pacing and Sensing T 1 : limits pulse delivery rate in presence of EMI and prevents retriggering of astable MV Free running MV provides fixed rate mode with an interval of T 2 via o/p circuit O/P pulses of length T 3 synchronous with i/p signals that falls outside of sensing refractory period T1 are delivered to stimulating electrodes

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9 Commercial pacemakers Use Defibrillation ill protection ti circuit it One diode or 2 back-to-back placed diodes (symmetrical) Symmetrical diodes minimize high-level, high-frequency, pulsed EMI artifacts raise noise detection threshold Output of pacemaker Constant current 8-10 ma with 1.0 to 1.2ms Constant Voltage 5V with ms Pulse rates bpm Refractory period: ms Unipolar or Bipolar electrodes are used Unipolar Bipolar

10 Programmable pacemaker External Internal External Pacemaker Pulsating Electromagnet Rate and width adjustable Reed Switch Heart Pulse Generator Methods of transmitting information Magnetic Radio-frequency waves Acoustic-ultrasonic pressure waves

11 Programmable pacemaker Main Requirement: immune to accidental programming Information to be sent is coded pacemaker decodes after receiving Consists of 3 systems System 1 Main timing function System 2 Sensing and signal discriminating function System 3 Programmable control

12 System 1 Timing Control Circuit Rate Limiter Output Circuit Vario function Battery test Reed Switch System 3 Crystal System 2 Data validate Width Rate RP HYST Amplitude Programmable Control Circuit Sensitivity Stimulate Electrode Sense Comparator Amplifier R.F. Filter Indifferent Electrode

13 Detection System 2 Signal from electrode High frequency filtered by RF filter Selectively amplified depending on sensitivity level Resultant signal compared with preset value at comparator Abs. Magnitude greater than preset enable an i/p signal g g p /p g to be fed to timing control circuit Signals below preset value ignore

14 Operation System 1 Normal Operating Conditions i Timing control circuit periodically triggers o/p circuit to send stimulation pulses of programmed pulse width and amplitude Period b/w 2 pulses checked by rate limiter Any component failure rate is limited to <180 bpm If inhibit signal from system 2 timing controls checks with programmed refractory period Within refractory period ignore Outside refractory period 0 hysteresis - Reset timing control inhibit o/p circuit Hysteresis reset timing control with period of escape before next stimulation = programmed basic level + hysteresis period

15 Programming System 3 Reed switch receives programming signals Feeds these signals to data validate Data validate checks if reed switch was closed for a minimum period of 300 ms Checks for speed of arrival of signals and executes code validation check When all checks are satisfied, new code is stored in programmable control circuit Parameters programmed: rate, pulse width and amplitude, sensitivity, refractory period and hysteresis Measurements taken at 37C with 500 resistance Program contained in 20-bit command code in addition to parameters contains, mode of operation, identification and check codes

16 Rete responsive Pacemakers When SA node is diseased HR cannot be increased in response to metabolic demands Synchronous pacemakers cannot replicate functions of heart during stressful activities like exercise. Sensor ph, Respiratory rate, Vibration/ motion Blood temperature QT interval Controller circuit Control Algorithm Pulse generator Lead Wire and electrode system

17 Power sources Mercury Batteries Used by first American Pacemaker Mercury zinc oxide with 1200mAh Produces 1.35 V Biological Power Sources Galvanic Cells using body fluids Failure due to dendritic mercury growth, zinc oxide migration, leaky separators and corroded wells Eventually become permanently electrically isolated Nuclear Batteries Lithium-iodine Cells

18 Nuclear Batteries Used Plutonium 238 with half-life of 87 years Energy liberated by decay of 1g of Pu238 with a power density of 0.56 W/g is 780kWh With 1% efficiency for only pulse power 20 mg of With 1% efficiency, for only pulse power, 20 mg of Pu will be required.

19 Lithium-iodine iodine Cells Most commonly used because it offers long battery life It is solid-state device consisting of metallic Lithium anode and molecular iodine bonded in complex form o an organic carrier as cathode 2Li + I 2 = 2LiI + e - Lithium has highest electrochemical equivalent of alkali metal - Most energetic anode material Ideal for use in high energy density batteries Develops 2.8 V which can be stepped up to 5 V

20 Cardiac pacemaker electrodes (a) Bipolar intraluminal electrode. (b) Intramyocardial electrode (formerly used).

21 Active and passive fixation mechanisms of various types for endocardial and epicardial pacing leads

22 Unipolar and bipolar implementations of both J-shaped and nonpreshaped leads. All models have distal cathode. Bipolar designs typically have a ring anode proximal mm on the lead.

23 Pacing voltage loss at the myocardium electrode interface is reduced by implementing a porous as opposed to relatively smooth tipped electrode. Decrease in voltage loss is largely contributed to by decreased electrode polarization associated with increased active surface area.

24 Electrode body Porous, platinum coated titanium tip Silicon rubber plug (impregnated with DSP steroid) 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.