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Editor's Notes

• #28: Student Notes Initiation of the cardiac cycle normally begins with the SA node. A resulting wave of depolarization passes through the right and to the left atria, via Bachman’s Bundle, that stimulates atrial contraction. Following contraction of the atria, the impulse proceeds to the AV node. Impulse conduction through the AV node is slightly delayed (compared to the rest of the conduction system). This allows time for atrial contraction to complete, before ventricular contraction begins. Just below the AV node, the impulse then passes quickly through the bundle of His, the right and left bundle branches, and the Purkinje fibers, leading to contraction of the ventricles. Instructor Notes Point out the locations mentioned on this slide on the heart path model.
• #34: Student Notes A basic pacing system is made up of: Implantable pulse generator that contains: A power source—the battery within the pulse generator that generates the impulse Circuitry—controls pacemaker operations Leads—insulated wires that deliver electrical impulses from the pulse generator to the heart. Leads also transmit electrical signals from the heart to the pulse generator. Electrode—a conductor located at the end of the lead; delivers the impulse to the heart Myocardial tissue Receives electrical implulse from the lead and stimulates the heart Produces an electrical signal that the lead senses, or “sees” Instructor Notes Distribute the demo pacemakers to the class.
• #35: Student Notes Lithium-iodine is the most commonly used power source for today’s pacemakers. Microprocessors (both ROM and RAM) control sensing, output, telemetry, and diagnostic circuits. Instructor Notes
• #37: Student Notes Silicone has proven to be a reliable insulating material for more than three decades of clinical experience. However, silicone is a relatively fragile material, which can tear more easily than urethane. Therefore, the silicone layer must be relatively thick to resist tears, or shearing at implant. Silicone also has a higher coefficient of friction, and the moving of two leads through a single vein may be difficult. Medtronic’s platinum-cured silicone rubber, characterized by improved mechanical strength, has partially alleviated the issues of low tear strength and friction (Silicure). Polyurethane is a more lubricious material and slightly more durable, allowing thinner applications when compared to silicone. However, early types of urethane were subject to chemical reactions secondary to the inflammatory response leading to insulation breakdown over time. From Cardiac Pacing, K. Ellenbogen, ed., 2nd edition, 1996. PP. 77-79 Instructor Notes
• #40: The first letter refers to the chamber(s) being paced The second letter refers to the chamber(s) being sensed The third letter refers to the pacemaker’s response to a sensed event: T = TriggeredD = Dual (inhibited and triggered*) I = InhibitedO = No response *In a single chamber mode, “triggered” means that when an intrinsic event is sensed, a pace is triggered immediately thereafter. In a dual chamber mode, “triggered” means that a sensed atrial event will initiate (trigger) an A-V delay. The fourth letter denotes the pacemaker’s programmability and whether it is capable of rate response: P = Simple Programmable (rate and/or output) M = Multiprogrammable (rate, output, sensitivity, etc.) C = Communicating (pacemaker can send/receive information to/from the programmer) R = Rate Modulation O = None Note that this sequence is hierarchical. In other words, it is assumed that if a pacemaker has rate modulation capabilities, “R”, that it also can communicate, “C”. The fifth letter represents the pacemaker’s antitachycardia functions: P = PaceD = Dual (pace and shock available) S = ShockO = None
• #45: Studies have been done that demonstrate the differences in outcome, hemodynamic improvement, and quality of life assessment by using AV synchronous, or "atrial-based," pacing modes instead of VVI/R. Some of the benefits of using an atrial-based pacing mode include: AV synchrony–Clinical benefits such as increased cardiac output, augmentation of ventricular filling (especially important for the majority of the pacing population with LVD and reduced compliance from effects of aging). Providing AV synchrony minimizes valvular regurgitation, and preserves atrial electrical stability. In the Framingham Study, the development of chronic AF was associated with a doubling of overall mortality and of mortality from cardiovascular disease (Kannel, 1982) The following emphasize the importance of preventing atrial fibrillation: Patients with AF unrelated to rheumatic or prosthetic valvular disease have a risk of ischemic stroke about five times higher than those with normal sinus rhythm. AF is associated with over 75,000 cases of stroke per year. See bibliography for listing of studies cited.
• #50: Student Notes On this slide we see an atrial pacemaker in operation. Note that at the beginning of the EKG strip we see pacing at 1000 ms, or 60 bpm. Each pacing spike is closely followed by a P-wave, indicating atrial capture. Towards the second half of the strip, the patient's intrinsic heart rate exceeds 60 bpm. The pacemaker is “inhibited” by the intrinsic rate. This is also called “demand” pacing. Key P = paced S = sensed V = ventricular A = atrium VP = ventricular pace VS = ventricular sense AP = atrial pace AS = atrial sense Instructor Notes
• #51: Student Notes On this slide we see a ventricular single chamber system regularly pacing, unless it is inhibited by intrinsic activity. Instructor Notes
• #52: Student Notes Pacing in the atrium and ventricle is often described as AV sequential pacing. We’ll discuss how the pacemaker decides when to pace - the timing - in a later module. Instructor Notes