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Welcome Chris Stamper
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Technical Field Engineer
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The Fundamentals of Cardiac Devices Module 2
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Objectives
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After completing this introduction to Cardiac Devices, you will be able to: • Identify the various cardiac device systems
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• Identify the basic indications for implantation for each device
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• Recognize each device type on a chest x-ray
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Cardiac Devices • Designed to:
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– Restore or maintain a rhythm and rate sufficient to meet metabolic needs
• Device operation
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• The patient
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– Provide diagnostic information about
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Cardiac Devices • Pacemakers or Implantable Pulse Generators (IPG) – Provide various diagnostics
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– Single and dual chamber
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– Provide a rate to support metabolic needs
– About 8-10 years longevity
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– Some of the newer pacemakers include therapies which pace-terminate AT/AF
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Cardiac Devices
• Implantable Cardioverter Defibrillators (ICDs)
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– Restore sinus rhythm in the presence of tachycardia • Defibrillate
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• Cardiovert • Anti-Tachy Pace (ATP)
– Provide a rate to support metabolic needs
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• Includes single or dual chamber pacing
– Provide various diagnostics
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– About 6-9 years longevity
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– There are also devices designed to terminate AF via cardioversion/ATP in addition to standard ICD therapies 6
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Cardiac Devices • Cardiac Resynchronization Therapy (CRT) – Restore ventricular synchrony
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– IPG with CRT, or an IPG + ICD with CRT
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• Uses a specially designed lead placed usually on the posterior-lateral wall of the LV via the Coronary Sinus circulation • LV epicardial lead placement is an option
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• Provides RV and LV synchronous pacing
– May restore rhythms in presence of lethal tachycardia
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• CRT pacing +ICD (High-Power CRT)
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– Provides a rate to support metabolic needs • CRT pacing only (Low-Power CRT)
– Provides various diagnostics 7
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Cardiac Devices
– Provides rate-based monitoring • Fast rates
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• Slow rates
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• Implantable Loop Recorders (ILR)
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– Provides EGM during patient triggered events
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Indications
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Pacemakers
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• The AHA and ACC have defined the indications for pacing by the underlying arrhythmia • Very detailed, but to simplify: – Typical diagnoses:
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– Symptomatic bradycardia refractory to any treatment
• Sinus Node Disease (SND), or Sick Sinus Syndrome
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• Complete Heart Block
• Chronotropic Incompetence
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• Vaso-vagal syncope
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• Carotid sinus hypersensitivity
– Usually excludes “low grade” blocks (Mobitz I and 1st degree) 9
Indications
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Defibrillators
• Primary vs. Secondary Prevention for SCA
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– Primary
• Patients who have experienced a previous SCA or ventricular arrhythmia
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• Studies such as AVID1, CIDS2, CASH3 support the use of ICDs in this population
– Secondary
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• Patients who have not previously experienced SCA/VA, but are at risk
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• Studies, such as MADIT II4 and SCD-HeFT5, have demonstrated the use of ICDs in these patients
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Indications
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Defibrillators (cont.) Well defined by Heart Rhythm Society (HRS) and include:
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Spontaneous sustained VT with structural heart disease
Syncope of undetermined origin with: –
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Sustained VT or VF induced during EP
Nonsustained VT with:
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•
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Due to VT or VF, not transient or reversible
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Coronary disease or prior MI and LV Dysfunction
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Inducible VF or sustained VT (non-suppressible by antiarrhythmic drugs)
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Cardiac Arrest
Spontaneous sustained VT –
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Not amenable to other treatments
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Indications •
ICD Class I Recommendation •
LVEF ≤ 30 – 40%
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NYHA class II or III
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• Non-ischemic patients
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• Patients at least 40 days post-MI
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Defibrillators (cont.)
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LVEF ≤ 30 – 35%
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NYHA class II or III
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• Patients at risk of SCA due to genetic disorders Long QT syndrome
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Brugada syndrome
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Hypertrophic cardiomyopathy (HCM) Arrhythmogenic right ventricular dysplasia (ARVD)
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Note: This list includes the current major indications for an ICD 12
Indications
• On optimal medical therapy • QRS > 120 ms wide
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• NYHA Class III or IV heart failure
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Cardiac Resynchronization Therapy
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– Or Echo evidence of ventricular dyssynchrony
• Ejection Fraction of < 30%
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• Who are candidates for an ICD with CRT, and who for an IPG with CRT?
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– Most get an ICD with CRT (also called “High-Power CRT”) because of the risk of SCD in patients with LV dysfunction
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Indications
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Loop Recorders
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• Transient, infrequent but recurrent syncope
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Recognizing Systems Pacemakers
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RA Lead in Appendage
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Dual Chamber Pacemaker
RV Lead at the Apex 15
Recognizing Systems
Right Atrial Lead
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Right Ventricular Lead
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Approximate position outlined
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ICD or High Power
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With RV and SVC coils
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Recognizing Systems
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CRT Low-Power
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Right Atrial Lead
Left Ventricular Lead
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Placed on the surface of the LV via the Coronary Sinus
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Right Ventricular Lead
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Recognizing Systems
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CRT High-Power
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Right Atrial Lead
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Placed on the surface of the LV via the Coronary Sinus
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Left Ventricular Lead
Right Ventricular Lead
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Note the 2 high voltage defib coils Surface ECG leads
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Pacing Therapy
– Delivers low energy electrical pulses when rate falls below programmed limit
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• Low energy pulses capture and depolarize the heart muscle causing it to contract
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• Senses underlying heart rate
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– Dual Chamber IPGs provide A-V synchrony
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ICD Pacing Therapy • Atrium and Ventricle – Pacing
• Ventricle
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Atrial Lead
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– Antitachycardia pacing (ATP)
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– Sensing
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Ventricular Lead
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Cardioversion and Defibrillation are delivered in biphasic waves
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High-Voltage Therapy
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For example: First, from the SVC coil and the Can to the RV coil, and then reverse.
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The device must detect – charge – confirm – and deliver the shock.
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Fast, accurate detection, and fast charge times are critical
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ICD Therapy
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• The ICD lead is designed to carry both high voltage and pacing therapies – ATP – Cardioversion
Brady and Anti-Tachy pacing (ATP)
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– Defibrillation
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– Brady
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• Video of CRT in Action
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• Video of Dyssynchronous Heart
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CRT Therapy
(Click above to play video)
(Click above to play video)
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By pacing from both the right and left ventricles, CRT may improve EF and reduce patient symptoms.
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Implant Evolution Pacemakers—Yesterday
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• First Implants in early 1960s
• About 2 year longevity • About 200 cc
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• Single Chamber, non programmable
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• Abdominal implants, sternotomy for epicardial leads
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Implant Evolution Pacemakers—Today
• 6-7 F transvenous lead placement • < 30 cc
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• 8-10 years longevity
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– Outpatient/overnight stay
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• Pectoral implants
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• Dual chamber, multiprogrammable
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• Advanced diagnostics and trending information
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Implant Evolution ICD Past: ~10-12 years ago ICDs • Required major surgery
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– Abdominal implants
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– Median sternotomy to suture defib patches on heart – Length of hospital stay > 1 week
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• Nonprogrammable • High-energy shock only • Indicated for patients who survived cardiac arrest - twice • 1 ½ year longevity • 209 cc • < 1,000 implants/year 26
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Implant Evolution ICDs Today
• Similar to a pacemaker implant
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• Transvenous, single incision – Pectoral implant
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– Overnight stay
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• Local anesthesia, conscious sedation • Programmable therapy options • Single, dual and triple chamber (CRT) • Up to 9 years longevity • About 35 cc • > ~100,000 implants/year 27
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Status Check • Given the following patient – EF 20% – History includes
• Hyperlipidemia
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• Tobacco 2 packs/day
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– 67 YO male patient, had an anterior MI 2 years ago
• CAD
– Medications
• Lipitor (Statin)
• Coreg (ARB)
• Aldosterone (ADH)
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• Vasopril (ACE Inhibitor)
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– C/O
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• Atenolol (Beta Blocker)
• Severe shortness of breath at rest, fatigue, unable to perform ADL, vertigo, rare syncope 28
Status Check
LBB
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NSR
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• Patient’s ECG:
What device system (if any) is the patient likely to get? Click for Answer
High-Power CRT Wide QRS, EF < 30% on max medical therapy and is S/P MI. = ↑ risk of SCD
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Status Check • Could this be the previous patient’s chest x-ray?
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– Is it CRT?
Click for Answer
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– Is it High-Power?
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Yes, we see an RV lead with RV and SVC coils, an atrial pacing lead, and an LV pacing lead
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Status Check • Consider the following patient – Medical history includes
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– Elderly female patient
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• Ex-smoker – quit 10 years ago • Mild exertional angina
– Cardiac cath shows mild disease right coronary artery
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• 3 adult children
– Medications
• Aspirin 81 mg. 1/day
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– C/O
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• Nitroglycerine PRN
• Fatigue, unable to perform normal activities 31
• Patient’s stress test indicates: – Stopped after 4 minutes for fatigue
What device system would you recommend?
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NSR, Rate 57
Click for Answer
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– ECG immediately after stress test:
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Status Check
Likely a pacemaker, as the patient has chronotropic incompetence – her heart rate does not increase with exercise. 32
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Basic Concepts—Electricity and Pacemakers Module 3
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Objectives Upon completion you will be able to:
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• Describe the relationship between voltage, current, and resistance
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• Describe the clinical significance of alterations in voltage, current, and resistance
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Characteristics of an electrical circuit:
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Including a pacemaker circuit
• Voltage
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• Current
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• Impedance
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Voltage
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• Voltage is the force, or “push,” that causes electrons to move through a circuit • In a pacing system, voltage is:
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– Measured in volts (V) – Represented by the letter “V”
– Provided by the pacemaker battery
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– Often referred to as amplitude or pulse amplitude
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Current • The flow of electrons in a completed circuit
– Measured in milliamps (mA)
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– Represented by the letter “I”
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• In a pacing system, current is:
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– Determined by the amount of electrons that move through a circuit
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• The opposition to current flow
– Measured in ohms (Ω)
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– Represented by the letter “R”
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• In a pacing system, impedance is:
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Impedance
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– The measurement of the sum of all resistance to the flow of current
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Voltage, Current, and Impedance are Interdependent
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• The interrelationship of the three components is analogous to the flow of water through a hose – Voltage represents the force with which . . .
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– Current (water) is delivered through . . .
– A hose, where each component represents the total impedance: • The nozzle, representing the electrode
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• The tubing, representing the lead wire
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Voltage, Current, and Impedance
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Recap
• Voltage: The force moving the current (V)
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– In pacemakers it is a function of the battery chemistry
• Current: The actual continuing volume of flow of electricity (I)
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– This flow of electrons causes the myocardial cells to depolarize (to “beat”)
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• Impedance: The sum of all resistance to current flow (R or Ω or sometimes Z)
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– Impedance is a function of the characteristics of the conductor (wire), the electrode (tip), and the myocardium
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Voltage and Current Flow
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Electrical Analogies
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Water pressure in system is analogous to voltage – providing the force to move the current
Spigot (voltage) turned up, lots of water flows (high current drain)
Spigot (voltage) turned low, little flow (low current drain)
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Resistance and Current Flow
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Electrical Analogies
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• Normal resistance – friction caused by the hose and nozzle
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• Low resistance – leaks in the hose reduce the resistance
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More water discharges, but is all of it going to the nozzle?
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• High resistance – a knot results in low total current flow
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V
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• I=V/R
R
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• V=IXR
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• Describes the relationship between voltage, current, and resistance
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Ohm’s Law
• R=V/I
V = I X R V
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= R
V I = R
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Ohm’s law tells us:
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1. If the impedance remains constant, and the voltage decreases, the current decreases
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2. If the voltage is constant, and the impedance decreases, the current increases
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So What?
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Status Check
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What happens to current if the voltage is reduced but the impedance is unchanged?
• Reduce the voltage to 2.5 V
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• Start with:
– Voltage = 5 V
– Impedance = 500 Ω – Current = 10 mA
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• Solve for Current (I): – I = V/R
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– I = 5 V ÷ 500 Ω = 0.010 Amps
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– Current is 10 mA
– Impedance = 500 Ω
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– Voltage = 5 V
– Current = ?
• Is the current increased/ decreased or unchanged? – I = V/R – V = 2.5 V ÷ 500 Ω = 0.005 Amps or 5 mA
• The current is reduced 45
Status Check
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What happens to current if the impedance is reduced but the voltage is unchanged? • Reduce impedance to 250 Ω
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• Start with: – Voltage = 5 V
– Voltage = 5 V
– Current = 10 mA
– Impedance = 250 Ω
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– Impedance = 500 Ω
– I = V/R
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• Solve for Current (I):
– Current = ?
• Is the current increased/ decreased or unchanged? – I = V/R
– Current is 10 mA
– V = 2.5 V ÷ 250 Ω =
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– I = 5 V ÷ 500 Ω = 0.010 Amps
0.02 Amps or 20 mA
• The current is increased 46
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Other terms
• Anode: A positively charged electrode
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– Examples:
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– For example, the electrode on the tip of a pacing lead
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• Cathode: A negatively charged electrode
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The “ring” electrode on a bipolar lead
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The IPG case on a unipolar system
Anode
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– More on this later (see: Pacemaker Basics)
Cathode
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Battery Basics
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So where does the current come from?
– A negative electrode (anode)
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• A battery produces electricity as a result of a chemical reaction. In its simplest form, a battery consists of:
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– An electrolyte, (which conducts ions)
Positive terminal
– A separator, (also an ion conductor) and
Cathode
Anode
Separator
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– A positive electrode (cathode)
Negative terminal
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Applying Electrical Concepts to Pacemakers Module 4
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Objectives
• Upon completion you will be able to:
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– Recognize a high impedance condition – Recognize a low impedance condition
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– Recognize capture threshold
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– Determine which sensitivity value is more (or less) sensitive
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Electrical Information • Why is this electrical information relevant?
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• A pacemaker is implanted to:
– Provide a heart rate to meet metabolic needs
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• In order to pace the heart, it must capture the myocardium • In order to pace the heart, it must know when to pace, i.e., it must be able to sense
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• A pacemaker requires an intact electrical circuit
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Ohm’s Law
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Relevance to Pacemaker Patients
• High impedance conditions reduce battery current drain
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– Can increase pacemaker battery longevity – Why?
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• R = V/I If “R” increases and “V” remains the same, then “I” must decrease
• Low impedance conditions increase battery current drain – Why?
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– Can decrease pacemaker battery longevity
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• R = V/I If “R” decreases and “V” remains the same, then “I” must increase
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The Effect of Lead Performance on Myocardial Capture
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What would you expect to happen if a lead was partially fractured? Impedance (or Resistance) would rise
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Current would decrease and battery energy conserved
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- but -
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Could you guarantee that enough current (I) can flow through this fractured lead so that each time the pacemaker fired the myocardium would beat?
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High Impedance Conditions
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A Fractured Conductor
– Current flow from the battery may be too low to be effective
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• Impedance values may exceed 3,000 Ω
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• A fractured wire can cause Impedance values to rise
Increased resistance
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Lead wire fracture
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Other reason for high impedance: Lead not seated properly in pacemaker header. 54
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Lead Impedance Values Change as a Result of: • Wire fractures
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• Insulation breaks
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Typically, normal impedance reading values range from 300 to 1,000 Ω
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– Some leads are high impedance by design. These leads will normally show impedance reading values greater than 1,000 ohms • Medtronic High Impedance leads are:
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– CapSure® Z
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– CapSure® Z Novus
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Low Impedance Conditions
– Body fluids, which have a low resistance, or
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– Another lead wire (in a bipolar lead)
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• Insulation breaks expose the lead wire to the following
• Insulation break that exposes a conductor causes the following
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– Impedance values to fall
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– Current to drain through the insulation break into the body, or into the other wire
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– Potential for loss of capture – More rapid battery depletion
Current will follow the path of LEAST resistance
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Capture Threshold
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• The minimum electrical stimulus needed to consistently capture the heart outside of the heart’s own refractory period
Non-Capture
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Capture
Ventricular pacemaker 60 ppm 57
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Effect of Lead Design on Capture • Lead maturation
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– May gradually raise threshold
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– Fibrotic “capsule” develops around the electrode following lead implantation
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– Usually no measurable effect on impedance
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– Exhibit little to no acute stimulation threshold peaking
Porous, platinized tip for steroid elution
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• Steroid eluting leads reduce the inflammatory process
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Steroid Eluting Leads
Silicone rubber plug containing steroid
Tines for stable fixation
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– Leads maintain low chronic thresholds
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Effect of Steroid on Stimulation Thresholds
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Smooth Metal Electrode
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Textured Metal Electrode
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Volts
5
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1
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Steroid-Eluting Electrode
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10 11 12
Implant Time (Weeks) Pulse Width = 0.5 msec
References: Pacing Reference Guide, Bakken Education Center, 1995, UC199601047aEN. Cardiac Pacing, 2nd Edition, Edited by Kenneth A. Ellenbogen. 1996. 60
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Myocardial Capture • Capture is a function of:
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– Amplitude—the strength of the impulse expressed in volts • The amplitude of the impulse must be large enough to cause depolarization (i.e., to “capture” the heart)
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• The amplitude of the impulse must be sufficient to provide an appropriate pacing safety margin
– Pulse width—the duration of the current flow expressed in milliseconds
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• The pulse width must be long enough for depolarization to disperse to the surrounding tissue
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Comparison
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5.0 Volt Amplitude at Different Pulse Widths
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Amplitude 5.0 V
0.25 ms
1.0 ms
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0.5 ms
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– Any combination of pulse width and voltage, on or above the curve, will result in capture
Volts
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• The strength-duration curve illustrates the relationship of amplitude and pulse width
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The Strength-Duration Curve
Chronaxie
Capture
1.0
Rheobase
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No Capture 0.5
1.0
1.5
Pulse Width
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X
Programmed Output
1.5
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– Thresholds may differ in acute or chronic pacing systems
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• By accurately determining capture threshold, we can assure adequate safety margins because:
Stimulation Threshold (Volts)
Clinical Utility of the Strength-Duration Curve
– Thresholds fluctuate slightly daily
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– Thresholds can change due to metabolic conditions or medications
1.0
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0.5
1.0
Duration Pulse Width (ms)
1.5
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Programming Outputs
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• Primary goal: Ensure patient safety and appropriate device performance • Secondary goal: Extend the service life of the battery
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– Typically program amplitude to < 2.5 V, but always maintain adequate safety margins • A common output value might be 2.0 V at 0.4 ms
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– Amplitude values greater than the cell capacity of the pacemaker battery (usually about 2.8 V) require a voltage multiplier, resulting in markedly decreased battery longevity
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Pacemaker Sensing • Refers to the ability of the pacemaker to “see” signals
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– Expressed in millivolts (mV)
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• The millivolts (mV) refers to the size of the signal the pacemaker is able to “see”
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0.5 mV signal
2.0 mV signal
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Sensitivity
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The Value Programmed into the IPG
5.0 mV 2.5 mV
1.25 mV
Time
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Sensitivity
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The Value Programmed into the IPG
2.5 mV 1.25 mV
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5 mV sensitivity
5.0 mV
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Time At this value the pacemaker will not see the 3.0 mV signal 68
Sensitivity
But what about this?
5.0 mV 2.5 mV
1.25 mV
Time
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1.25 mV Sensitivity
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The Value Programmed into the IPG
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At this value, the pacemaker can see both the 3.0 mV and the 1.30 mV signal. So, is “more sensitive” better, because the pacemaker sees smaller signals? 69
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Sensing Amplifiers/Filters • Accurate sensing requires that extraneous signals are filtered out – Because whatever a pacemaker senses is by definition a P- or an R-wave
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– Sensing amplifiers use filters that allow appropriate sensing of P- and Rwaves, and reject inappropriate signals
• Unwanted signals most commonly sensed are: – T-waves (which the pacemaker defines as an R-wave)
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– Far-field events (R-waves sensed by the atrial channel, which the pacemaker thinks are P-waves)
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– Skeletal muscle myopotentials (e.g., from the pectoral muscle, which the pacemaker may think are either P- or R-waves)
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– Signals from the pacemaker (e.g., a ventricular pacing spike sensed on the atrial channel “crosstalk”)
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• Affected by:
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– Pacemaker circuit (lead) integrity
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Sensing Accuracy
• Insulation break • Wire fracture
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– The characteristics of the electrode
– Electrode placement within the heart
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– The sensing amplifiers of the pacemaker – Lead polarity (unipolar vs. bipolar)
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– The electrophysiological properties of the myocardium
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– EMI – Electromagnetic Interference
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Lead Conductor Coil Integrity
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Affect on Sensing
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• Undersensing occurs when the cardiac signal is unable to get back to the pacemaker – Intrinsic signals cannot cross the wire fracture
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• Oversensing occurs when the severed ends of the wire intermittently make contact
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– Creates signals interpreted by the pacemaker as P- or R-waves
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Lead Insulation Integrity
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Affect on Sensing
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• Undersensing occurs when inner and outer conductor coils are in continuous contact – Signals from intrinsic beats are reduced at the sense amplifier, and amplitude no longer meets the programmed sensing value
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• Oversensing occurs when inner and outer conductor coils make intermittent contact
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– Signals are incorrectly interpreted as P- or R-waves
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• Where is the sensing circuit?
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Unipolar Pacemaker
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Click for Answer
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Anode
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Lead tip to can This can produce a large potential difference (signal) because the cathode and anode are far apart
_ Cathode
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Click for Answer
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• Where is the sensing circuit?
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Bipolar Pacemaker
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Lead tip to ring on the lead This usually produces a smaller potential difference due to the short inter-electrode distance • But, electrical signals from outside the heart (such as myopotentials) are less likely to be sensed Anode and Cathode 75
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Cardiac Conduction and Device Sensing
By now we should be familiar with the surface ECG and its relationship to cardiac conduction. But, how does this relate to pacemaker sensing?
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Sense
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Vectors and Gradients
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2.5 mV
Click for More
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The wave of depolarization produced by normal conduction creates a gradient across the cathode and anode. This changing polarity creates the signal.
Once this signal exceeds the programmed sensitivity – it is sensed by the device.
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Sense
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Changing the Vector
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2.5 mV
Click for More
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A PVC occurs, which is conducted abnormally. Since the vector relative to the lead has changed, what effect might this have on sensing?
In this case, the wave of depolarization strikes the anode and cathode almost simultaneously. This will create a smaller gradient and thus, a smaller signal. 78
Putting It All Together – But, do not compromise patient safety!
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• Appropriate output programming can improve device longevity
– Steroid eluting leads
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• Lead design can improve device longevity via
• Can help keep chronic pacing thresholds low by reducing inflammation and scarring
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– High Impedance leads
• Medtronic CapSure Z and Medtronic CapSure Z Novus • Designed so electrode Ω is high, but V low so current (I) is low as well, reducing battery drain
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• Control of manufacturing
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– Batteries, circuit boards, capacitors, etc., specific to needs, can lead to improved efficiencies and lowered static current drain
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– Highly reliable lead design
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Putting It All Together • Pacemaker Longevity is:
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– A function of programmed parameters (rate, output, % time pacing) – A function of useful battery capacity
• Static current drain • Circuit efficiency
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• Output Impedance
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– A function of
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• The lower the programmed sensitivity the MORE sensitive the device
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– Lead integrity also affects sensing
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• Determine the threshold amplitude 0.75 V
1.00 V
0.05 V
Click for Answer
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1.25 V
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Status Check
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Capture threshold = lowest value with consist capture
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This is at 1.25 V
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Status Check
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• Which of these pacemakers is more sensitive?
Pacemaker A
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Programmed Sensitivity 0.5 mV
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OR
Pacemaker B
Programmed Sensitivity 2.5 mV
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Click for Answer
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Pacemaker A is able to “see” signals as small as 0.5 mV. Thus, it is more sensitive. 82
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Status Check
Click for Answer
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• A pacemaker lead must flex and move as the heart beats. On average, how many times does a heart beat in 1 year?
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35 MILLION times. It is not a simple task to design a lead that is small, reliable, and lasts a lifetime.
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Status Check
Click for Answer
Lead Fracture: • High Impedance • Possible failure to capture myocardium
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Do you notice anything on this x-ray?
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Status Check • Which value is out of range?
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• What could have caused this?
Mode: DDDR
USR: 130 ppm
Click for Answer
Insulation failure
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Lower: Rate 60 ppm
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Pacemaker Interrogation Report
UTR: 130 ppm
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What would you expect?
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Atrial Lead Impedance: 475 Ohms
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Ventricular Lead Impedance: 195 Ohms
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