6.18b Pacemakers - Technique Insertion/Complications/Other

Physician's Notebooks 6  - http://physiciansnotebook.blogspot.com - See Homepage

More About Pacemakers (Update  21 Aug. 2021)

The pacemaker batteries should be Lithium-Iodine type, c.10 or more years life, easily rechargeable.

   Pacemaker leads should be a bipolar plus and minus electrode tip. The electrodes should have corticosteroid to prevent inflammatory responses in the heart. The leads should have an active fixation mechanism.

   Insulation of pacemaker wiring makes a difference. Polyurethane best. (Most important is your HMO cardiologist and her or his team.)

Pacemaker Implantation: the pacemaker leads are best inserted in an axillary (armpit vein). The subclavian (under collarbone) vein insertion done as alternative is less good because of risk of pneumothorax. For the usual A-V block, the atrium lead and ventricle lead are inserted and anchored, the former, inside the right atrial appendage and the latter inside the right ventricle apex. When pacing is done to improve severe, chronic heart failure, a left ventricular lead is also placed via right ventricle septum (Bi-ventricular pacing). 

The electricity cells (natural pacemaker) in a heart muscle produce the heartbeat impulse. Its basis is the electric tissue. One area of this tissue, the sinus (SA) node, is in the upper corner of the right atrium (RA) at the base of superior vena cava’s entrance into the RA. The normal heartbeat is initiated there and quickly spreads to left atrium. The right atrial, or main, heartbeat impulse signal takes an oblique and downward path to the AV node located in the lowest part of the septum between the Right Atrium and Left Atrium at the base of the septal flap of the tricuspid heart valve. From the AV node the signal travels just beneath the surface of the ventricles in the His (Pronounced "hiss") bundle then splits into the right and left bundle branches to the Right Ventricle and Left Ventricle. This electric conducting tissue is continuous from the sinus node in right atrium to the right and left ventricles but within this continuity there are separate potential pacemakers. The beat rate of a pacemaker is determined by the duration of the refractory period of its surrounding tissue, which is the period after initiation of a heartbeats when the tissue will not produce or transmit electricity. The shorter the refractory period the more rapid the pacing heart rate and also it follows that a natural pacemaker with the shortest refractory period will dominate the other pacemakers and set the heartbeat. Normally this is in the SA node which at rest produces a heartbeat of c.70 per minute. On the right side of the heart in the AV node the dominant beat signal from the SA node regularly blanks out the independent pacemaker located in the AV node. But, when the SA node impulse is either blocked or the node is diseased and does not produce an impulse, the AV node is freed to produce its own beat which then takes over as the dominant heartbeat. This is a protective mechanism for otherwise the animal would die from frequent heart stoppage. But the AV node in its upper portion produces a characteristically slower beat than the SA node – 40 to 60 per minute.  If the SA or AV node beat signal is blocked at the bundle of His (complete AV block) the natural pacemaker function is taken over by tissue at a lower level than the upper AV node and the characteristic heartbeat rate is around 30 bpm, barely enough for the animal in resting state. In this case implanting a pacemaker is lifesaving.

   Important ions like potassium K⁺ and calcium Ca⁺⁺ must remain in a very narrow range in the blood for a normal heartbeat rate. 

  Metabolic and Hormonal and Nervous Affects on the Heartbeat: The intrinsic heartbeat is fixed.  Normally from the SA Node the heartbeat is around 70 bpm. This is OK for a body at rest in good health. But the usual way the body increases cardiac output (the amount of blood pumped from the heart per minute) is to increase the heartbeat rate. So increase in physical activity needs increased heart rate. Also, an increase in body heat produces an increase in metabolism that needs a more rapid heart rate. The heart itself cannot do it and it relies on increased hormone from the adrenal glands (adrenaline a.k.a. epinephrine) or from sympathetic nerve endings in the heart muscles. There are also necessary mechanisms to slow the rapid heart; mostly the parasympathetic nerve endings in the heart muscle that release acetylcholine. So this is the normal state. But because of disease in the heart muscle the heartbeat may suddenly speed up as tachycardia. Whenever a natural pacemaker area of the heart muscle becomes irritated, which could be from infection, drug, physical trauma, then, a focus of rapid heartbeat may develop and if this rapid beat is faster than the normal heart pacemaker it will take control of the heartbeat. Thus, abnormal tachycardia arises up to 250 regular bpm; and even worse above 250 bpm the rate of discharge becomes too rapid for an efficient heartbeat and heart fibrillation occurs. Atrial fibrillation is tolerated but causes many symptoms. Ventricular fibrillation ends rapidly in death. 

The tachycardias are mostly treated by medications that slow the heart by blocking neurotransmitters or hormones or that act directly. When medications are ineffective for serious tachycardia or atrial fibrillation, then, an implanted pacemaker may be life saving. In these cases the pacemaker senses the rapid heart rhythm and is programmed to slow it by capturing the dominant rhythm or shocking the irritable focus and substituting its own rhythm. Pacemakers for tachycardia, however, are a small use of the technology. Most pacemakers are used for abnormally slow heartbeats (A-V blocks; bradycardia less than 60 bpm) and the major cause of slow heartbeat is the A-V block, a condition where the SA node fires off normally but its impulse gets blocked because of damage to the tissue due to coronary artery disease or as part of an old age fibrosis of heart muscle. The insertion of a pacemaker programmed to give a heartbeat of 70 bpm will correct the problem. Modern pacemakers are programmed to sense the heart rate and increase it when it falls too low and further increase or decrease depending on metabolic needs of the body for exercise or due to increased heat. This is the dual lead type of pacemaker used today, with a lead in right atrium and another in right ventricle. The pacing in A-V block must be from the ventricular lead but the atrial lead is important in sensing the atrial rate.  In cases of disease of the SA Node where there is no A-V block, all that is needed is to pace the atrium up to a normal heartbeat rate.

  Pacemaker Complications: These are mostly surgical, pneumothorax by puncturing into the lung area with the needle, and perforation of the heart wall by a lead. Best avoided by choosing a top place and experienced team.  Other complications, less frequent, are hematomas (collections of blood from blood vessel  punctures and especially extensive black and blue) usually minor and more unsightly than dangerous. Infection should be rare if antiseptic insertion technique is good and routine antibiotic  is used at time of insertion. Leads coming loose can lead to malfunctioning of pacemakers. It is least common with active fixation leads. The twiddler syndrome is due to dislodging or fracture of leads because the patient keeps poking his fingers at the pacemaker and battery which is implanted under the skin of the front chest wall. Do not touch! The pacemaker syndrome is heart failure due to asynchrony of heart chamber beats. It was seen much in the past before dual chamber pacemakers were introduced but today everyone should get a dual chamber pacemaker that can be programmed to give synchronous chamber beats.

  Pacemakers should be checked at regular 3- to 6-month intervals or immediately if anything is suspected of being wrong. The battery life is checked and usually good for 10 years. The modern dual chamber pacemaker is efficient in getting the maximum cardiac output and best heartbeat rate control check.

  Electromagnetic Interference/Pacemaker Precautions: Certain electric and magnetic signals may badly affect pacemaker. The most serious is MRI. Pacemaker users should keep away from MRI. Also electrocautery used during surgery, and radiation therapy and strong ultrasound used to break up renal and gall stones can malfunction a pacemaker. Be sure your doctors know you have a pacemaker and remind them frequently.  Potential daily life pacemaker interferences are cell phones, home WiFi internet (rare, check with your cardiologist), electronic surveillance devices found in many stores, metal detectors, electric shavers and nearness to high-voltage power lines, transformers, welding and electric motors (very close distance only). A pacemaker person should not keep a cell phone in his chest pocket or use it on his left side. If you must pass an electric surveillance or metal detector, do it quickly and do not linger in the vicinity. And, unless your pacemaker specialist reassures otherwise, keep your pacemaker at least 12 inches away from a functioning microwave oven.  End of Chapter. To read next now, click 6.19 Respiration - Air and Gas We Breath