17/03/30 PACEMAKERS Dr. AB Khalaf What are pacemakers? • A pacemaker is an electronic device used to treat patients who have symptoms caused by abnormally slow heartbeats. • A pacemaker is capable of keeping track of the patient's heartbeats. If the patient's heart is beating too slowly, the pacemaker will generate electrical signals similar to the heart's natural signals, causing the heart to beat faster. • The purpose of the pacemaker is to maintain heartbeats so that adequate oxygen and nutrients are delivered through the blood to the organs of the body. • A pacemaker is a man-made device that instructs the heart to beat. What is a natural pacemaker? • The heart's "natural" pacemaker is called the sinoatrial (SA) node or sinus node. It's a small mass of specialized cells in the top of the heart's right atrium (upper chamber). It makes the electrical impulses that cause your heart to beat. • A chamber of the heart contracts when an electrical impulse moves across it. For the heart to beat properly, the signal must travel down a specific path to reach the ventricles, the heart's lower (pumping) chambers. • The natural pacemaker may be defecLve, causing the heartbeat to be too fast, too slow or irregular. The heart's electrical pathways also may be blocked. 1 17/03/30 What’s an arLficial pacemaker? An "ar6ficial pacemaker" is a small, baNery-operated device that helps the heart beat in a regular rhythm. Some are permanent (internal) and some are temporary (external). They can replace a defecLve natural pacemaker or blocked pathway. • A pacemaker uses baNeries to send electrical impulses to the heart to help it pump properly. An electrode is placed next to the heart wall and small electrical charges travel through the wire to the heart. • Most pacemakers are demand pacemakers. They have a sensing device. It turns the signal off when the heartbeat is above a certain level. It turns the signal back on when the heartbeat is too slow. Who needs a pacemaker? • • • • • A pacemaker fixes irregular heartbeats. It is used to correct various types of arrhythmias: Too Slow Heartbeat ("Bradycardia") Too Rapid Heartbeat ("Tachycardia") ErraLc Heartbeat – Ventricular FibrillaLon or – Atrial FibrillaLon Hindered Regular Internal Pacemaker ("Heart Block" of the sinoatrial node) What are the causes of slow heart rate? Abnormally slow heart rates (bradycardias) can result from diseases affecLng the SA node, the conducLon Lssues, and the AV node. • Sick sinus syndrome is a disease wherein the SA node cannot generate signals frequently enough to maintain adequate heart rate. • Heart blocks are condiLons where diseases (such as heart aNacks) or degeneraLon (due to processes such as aging) of the AV node and/or the conducLon Lssues impair the transmission of signals from the SA node to the heart muscles. • The most common cause of bradycardia is degeneraLon of the conducLon system that occurs as part of the aging process. • 2 17/03/30 How can an abnormally slow heart rate be increased? • Temporary pacemakers are usually used first, especially if the abnormally slow heart rate is believed to be transient (lasLng only days) and caused by condiLons that are reversible or correctable. Temporary pacemakers are easily disconnected if the heart rate returns to normal. • Permanent pacemakers are necessary when the slow heart rate becomes chronic (lasLng more than a few days) or is believed to be irreversible. What is the design of permanent pacemakers? • A permanent pacemaker has two parts; the pacemaker chamber and the lead(s). The pacemaker chamber contains a Lming device for se\ng the pacing rate, a circuitry that detects electrical signals from the heart, and a baNery. • In some paLents who need permanent pacemakers for abnormally slow heart rates, fluctuaLons of the heart rate can occur. The pacemaker is capable of "listening" to the natural electrical signals from the heart. When the heart is beaLng normally, the pacemaker does nothing. When the heart stops beaLng or beats too slowly, the pacemaker takes over generaLng electrical signals for the heart at a frequency set by the doctor. • While temporary pacemakers are housed in chambers outside of the body and can use external electrical power sources, permanent pacemakers are implanted inside the body and therefore need their own baNeries. Most of the modern permanent pacemaker chambers are small, weighing less than 30 grams. What is the design of permanent pacemakers? • The pacemaker lead is inserted through a vein in the chest into the heart. The Lp of the lead is placed in contact with the inner wall of the right atrium or the right ventricle, while the other end of the lead is connected to the pacemaker chamber. These leads are safe, and generally do not injure or cause infecLons in the veins or the heart. • These small, lightweight devices are comfortable to wear inside the body and are barely visible. The small baNeries inside the pacemaker chambers are also durable. Most implanted pacemakers have baNeries that will last 7-10 years before needing to be replaced. 3 17/03/30 What is the design of permanent pacemakers? • The conducLng wire(s) that carry electrical signals between the heart and the pacemaker is usually made of plaLnum. These wires are insulated with silicone or polyurethane. The insulted wires are called leads. Some pacemakers have only one lead, and are called single-chamber pacemakers. Others have two leads and are called dual- chamber pacemakers. • The pacemaker lead is inserted through a vein in the chest into the heart. The Lp of the lead is placed in contact with the inner wall of the right atrium or the right ventricle, while the other end of the lead is connected to the pacemaker chamber. These leads are safe, and generally do not injure or cause infecLons in the veins or the heart. POWER SUPPLY Pacemakers have special baNeries with excepLonally long lives-reliable for at least seven years. They are made of the following: • • • • Mercury Zinc Lithium Nuclear Rechargeable Nickel-sodium Types of pacemakers? Pacemakers work when they need to or all of the Lme. There are 3 different kinds of pacemakers: • Permanent: A permanent pacemaker is also called a long-term pacemaker. An incision (cut) is made in your neck or chest. The leads are guided through a vein into your heart, and aNached to the generator. The generator is placed in a pocket under your skin. You will have this type of pacemaker forever. • Transcutaneous (trans-q-TA-nee-us): This pacemaker is also called an external pacemaker. It is used in emergency situaLons when your heart is beaLng too slowly. When this happens, your caregiver needs to speed up your heart rate quickly. Small sLcky patches are applied to your chest, and leads are aNached to these patches. At the end of the leads is a pacemaker with dials on it. 4 17/03/30 Types of pacemakers? • These dials allow your caregiver to immediately adjust the se\ngs of the pacemaker. This type of pacemaker is used for a short Lme. Ager a Lme, you may need to get a permanent pacemaker, or you may not need a pacemaker anymore. • Trasvenous (trans-V-nus): A transvenous pacemaker is also called a temporary pacemaker. The leads for the pacemaker are guided into your heart through an incision in your neck or chest. With a transvenous pacemaker, the generator is worn outside your body in a small pouch. You will carry this pouch with you everywhere. • Your caregiver will decide which pacemaker is right for you. Two types of pacemaker lead wires? • Endocardial- Via vein to right atrium. • Myocardial-directly to the heart muscle. Pacemakers charge a capacitor from a baNery 5 17/03/30 Then, periodically discharge the capacitor into the heart And, recharge the capacitor • Current is convenLonally shown to flow from posiLve to negaLve. • Actual flow is movement of electrons (negaLve charge). • For safety and to conserve energy, pacemakers sLmulate with negaLve pulses CARDIAC PACEMAKERS • Asynchronous device is free-running – Produces uniform sLmulaLon regardless of cardiac acLvity (i.e. fixed heart-rate) – Block diagram (right) shows components of asynchronous pacemaker » Power supply – provides energy » Oscillator – controls pulse rate » Pulse output – produces sLmuli » Lead wires – conduct sLmuli » Electrodes – transmit sLmuli to the Lssue – The simplest form of the pacemaker; not common any longer (what you will build) Power Supply Oscillator Pulse Output Circuit Lead Wires Electrodes 6 17/03/30 Pacemaker: Output Signal Pacemaker: Leads • Important characterisLcs of the leads – Good conductor – Mechanically strong and reliable • Must withstand effects of moLon due to beaLng of heart and movement of body – Good electrical insulaLon • Current designs – Interwound helical coil of spring-wire alloy molded in a silicone-rubber or polyurethane cylinder – Coil minimizes mechanical stresses – MulLple strands prevent loss of sLmulaLon in event of failure of one wire – Sog coaLng provides flexibility, electrical insulaLon and biological compaLbility Pacemaker Design 7 17/03/30 Pacemaker: Leads • Important characterisLcs of the leads – Good conductor – Mechanically strong and reliable • Must withstand effects of moLon due to beaLng of heart and movement of body – Good electrical insulaLon • Current designs – Interwound helical coil of spring-wire alloy molded in a silicone-rubber or polyurethane cylinder – Coil minimizes mechanical stresses – MulLple strands prevent loss of sLmulaLon in event of failure of one wire – Sog coaLng provides flexibility, electrical insulaLon and biological compaLbility PACEMAKER: LEADS 8 17/03/30 Pacemaker: Electrodes • Unipolar vs. Bipolar Pacemakers – Unipolar: • Single electrode in contact with the heart • NegaLve-going pulses are conducted • A large indifferent electrode is located elsewhere in the body to complete the circuit – Bipolar: • Two electrodes in contact with the heart • SLmuli are applied across these electrodes • SLmulus parameters (i.e. voltage/current, duraLon) are consistent for both Pacemaker: Electrodes • Important characterisLcs of electrodes – Mechanically durable – Material cannot: • • • • Dissolve in Lssue Irritate the Lssue Undergo electrolyLc reacLon due to sLmulaLon React biologically – Good Interface with leads • Current designs – PlaLnum, plaLnum alloys, and other specialized alloys are used 9 17/03/30 Pacemaker: Electrodes Pacemaker: Sensing Electrodes • Unipolar and bipolar electrodes are also used as sensing electrodes • Used in conjuncLon with advanced pacemaker technologies 10 17/03/30 Pacemaker: Packaging • Housing for the components must be compaLble and well tolerated by the body • Needs to provide protecLon to circuit components to ensure reliable operaLon • Size and weight must be considered • Common designs consist of hermeLcally sealed Ltanium or stainless steel Advanced Pacemakers • Synchronous Pacemakers – Used for intermiNent sLmulaLon as opposed to conLnuous sLmulaLon as in asynchronous pacemakers • Rate-Responsive Pacemakers – Used for variable rates of pacing as needed based on changes in physiological demand Synchronous Pacemakers • Prevents possible deleterious outcomes of conLnuous pacing (i.e. tachycardia, fibrillaLon) – Minimizes compeLLon between normal pacing • Two general types of synchronous pacemakers – Demand pacemakers – Atrial-synchronous pacemakers 11 17/03/30 Demand Pacemakers • Consists of asynchronous components and feedback loop • Timing circuit runs at a fixed rate (60 to 80 bpm) • Ager each sLmulus, Lming circuit is reset • If natural beats occur between sLmuli, Lming circuit is reset • Normal cardiac rhythms prevent pacemaker sLmulaLon Timing Circuit Output Circuit Reset Circuit Amp Electrodes Atrial-Synchronous Pacemaker Atrial Electrode • SA node firing triggers the pacemaker • Delays are used to simulate natural delay from SA to AV node (120ms)i and to create a refractory period (500ms) • Output circuit controls ventricular contracLon • Combining the demand pacemaker with this design allows the device to let natural SA node firing to control the cardiac acLvity Amp Monostable Multivibrator 500ms Delay Output Circuit Gate Monostable Multivibrator 120ms Delay Monostable Multivibrator 2ms Delay Ventricular Electrode Rate-Responsive Pacing Sensor • Replicates cardiac funcLon in a physiologically intact individual • Sensor is used to convert physiological variable to an electrical signal that serves as an input • Controller circuit changes heart rate based on sensor signal (demand-type pacing can be implemented here) Control Algorithm Controller Circuit Pulse Generator Lead Wires/ Electrodes 12 17/03/30 Ventricular Inhibited or R Wave Blocked Pacemaker • Used for paLents who generally have sinus rhythm with occasional heart block. • The circuitry detects spontaneous R wave potenLals at the electrodes & the pacemaker provides sLmulus to heart ager preset asystole. • In case of ventricular acLvity. The R wave does not trigger the output circuit of the pacemaker but blocks the output circuit & no sLmulaLon impulse is given to the heart Commercial Examples Commercial Examples • Typical size and shape of the implantable pacemaker • Upper porLon is used for interfacing with the leads 13 17/03/30 What outside electrical sources can interfere with the pacemaker? • Carry a wallet I.D. card with you. Equipment used by doctors and denLsts can affect your pacemaker, so tell them you have one. How are pacemakers implanted? Pacemaker systems are ogen implanted under local anesthesia in a cardiac catheterizaLon laboratory. ImplantaLon of a pacemaker is considered a minor procedure. • PaLents are typically awake or only mildly sedated during the procedure. A local anestheLc is injected under the skin over the area where the pacemaker will be implanted, usually in the right or leg upper chest near the collarbone. The numbing injecLon keeps the paLent from feeling pain when a small incision is made over the same area to create a small space. The pacemaker lead is then inserted into a vein located in the upper chest near the collarbone. The lead is placed in the right atrium or the right ventricle with the visual guidance of x-rays. How are pacemakers implanted? The Lp of the lead is then aNached to the inner surface of the heart by small Lnes or with a small screw. If there is more than one lead, the process is repeated. Because there are no nerve endings inside the blood vessels and the heart, the paLent usually does not feel the placement of the lead(s). The other end of the lead(s) is then connected to the pacemaker chamber, which is then placed under the skin. Closing the incision with sutures completes the process. The procedure usually takes about an hour. PaLents are discharged the same or the next day if there are no complicaLons. PaLents are given anLbioLcs to prevent possible infecLon, and sent home with pain medicaLons to alleviate post-surgical pain at the incision site. 14 17/03/30 What outside electrical sources can interfere with the pacemaker? • • • • MagneLc resonance imaging (MRI scan) is a diagnosLc test for studying the brain, the joints, the spine, the liver, and other organs. The strong magneLc field from the MRI scan can interfere with pacemakers. PaLents with pacemakers should not undergo MRI scanning. Digital cellular phones can interfere with pacemakers. Therefore, the cellular phone should be held on the ear opposite from the side of the pacemaker. Do not carry the phone in the pocket near the chest. Theg detector gates in certain stores can generate signals that interfere with the pacemaker. The metal detector gates at airports can also interfere with the pacemaker. Heavy-duty electrical powered equipment such as arc welders, a running car engine, and certain electrically powered surgical tools can also cause disturbances with pacemakers. 15 17/03/30 Reference • www.medicalonline.com • www.londoncardiac.ca 16
