Devices
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Pacemakers
A pacemaker monitors the electrical impulses in the heart. When needed, it sends small electrical impulses to the heart muscle to maintain a normal heart rate.
A pacemaker rhythm can easily be recognized on the ECG. It shows pacemaker spikes: vertical signals that represent the electrical activity of the pacemaker. Usually these spikes are more visible in unipolar than in bipolar pacing (see below).
Patients
In general, pacemaker implantation is indicated in patients with a documented irreversible bradycardia which causes symptoms. Symptoms associated with bradycardia can be lightheadedness, palpitations, dyspnea, angina and syncope. The causal relation between the abnormal heart rhythm and symptoms is crucial for the decision to implant a pacemaker.
Implantation
Pacemaker implantation is performed under local anaesthesia in a sterile operating room or catheterization laboratory. Implantation starts with inserting the pacemaker lead(s), most often into the left or right cephalic vein. Fluoroscopy is used to ensure the appropriate location in the atrium or ventricle. Leads are often positioned in the auricle of the right atrium and/or the right ventricular apex.
| Wet van Ohm: |
|---|
| U = I x R |
| U = Voltage |
| I = Current |
| R = Impedance |
Device measements
- Stimulation threshold (in mV@ms): The stimulation threshold is the minimum stimulus intensity (in mV) and duration (in ms) necessary to capture the heart.
- Sensitivity (in mV): The degree that the pacing system “sees” or senses signals, controlled by the sensitivity setting which is graduated in mV.
- Impedance (in Ω): The total pacing impedance is the sum of all resistance to the flow of the electrical impulses.
- Battery: Device longevity is estimated based on the remaining battery voltage and battery impedance (and past history of percent pacing in case battery status is checked at follow up visits).
If these tests are within limits, the pacemaker leads are properly positioned. Thereafter, the pacemaker pulse generator is implanted subcutaneously or subpectorally under the left or right clavicle.
Programming
Pacemaker codes
Pacemakers can be categorized according to the NASPE coding system that usually consists of 3-5 letters.
| I | II | III | IV | V |
|---|---|---|---|---|
| Chamber(s) paced | Chamber(s) sensed | Response to sensing | Rate modulation | Multisite pacing |
| O = none
A = atrium V = ventricle D = dual (A+V) |
O = none
A = atrium V = ventricle D = dual (A+V) |
O = none
T = triggered I = inhibited D = dual (I+T) |
O = none
R = rate adaptive |
O = none
A = atrium V = ventricle D = dual (A+V) |
Explanations of table:
- III:
Triggered: A sensed event triggers a pacemaker output pulse Inhibited: Detection of physiological heart activitity will inhibit an electrical pacemaker impulse Dual: A pacemaker with dual response to sensing will inhibit a pacemaker output pulse if it senses an intrinsic event in that same chamber, but it will trigger a pacemaker output pulse in the ventricle if it senses an intrinsic event in the atrium (after a programmed atrioventricular interval)
- IV:
Rate modulation: In some patients, rate adaptive pacing is programmed “on” to ensure that when patients exercise increases, the pacemaker ensures that the heart rate increases to provide additional cardiac output. There are many ways to sense physiological exercise, including motion sensors and ventilation sensors.
Commonly used pacemakers
- AAI: The atria are paced, when the intrinsic atrial rhythm falls below the pacemaker's threshold.
- VVI: The ventricles are paced, when the intrinsic ventricular rhythm falls below the pacemaker's threshold.
- VDD: The pacemaker senses atrial and ventricular events, but can only pace the ventricle. This type of pacemaker is used in patients with a reliable sinus node, but with an AV-block.
- DDD: The pacemaker records both atrial and ventricular rates and can pace either chamber when needed.
Uni- and bipolar sensing and stimulation
Sensing and stimulation of the myocardium demands a closed electrical circuit. A pacemaker can sense and stimulate in a unipolar and bipolar fashion:
- Unipolar: Pacemaker circuit between 1 electrode at the distal end of lead and the pacemaker pulse generator (large circuit ± 40-60 cm).
- Advantage: large pacemaker spikes (easier interpretation pacemaker ECG)
- Disadvantage: extracardiac stimulation (pectoral muscle), sensing of extracardiac signals (such as ventricular depolarisations in the atrial sensing channel (far field R waves) or non-physiological noise)
- Bipolar: Pacemaker circuit between 2 electrodes at the distal end of the lead (small circuit ± 10-15 mm)
- Advantage: more reliable sensing
- Disadvantage: small pacemaker spikes (difficult interpretation pacemaker ECG)
Follow up
The first 6-8 weeks after implantation, patients are advised not to over-stretch their arm on the same side as the pacemaker (such as golf, swimming etc.) to allow time for the lead to mature.
Control visits for pacemakers are usually every 6 months. During this visit several electrical parameters are measured: battery status, stimulation thresholds and impedance.
Complications
- Undersensing: An intrinsic depolarization that is present, is not sensed by the pacemaker. This can be due to:
- Inappropriately programmed sensitivity
- Lead dislodgement
- Lead failure, such as lead fracture or insulation failure
- Lead maturation: the amplitude may abruptly decline during the first week after implantion, but these values return to the implantation values after about 6-8 weeks as the lead “matures”
- Oversensing: The pacemakers senses signals on the marker channel that do not correspond to the ECG pattern. This can be physiologic (e.g. ventricular pulse or myopotentials) or non-physiologic (e.g. lead fracture or if the lead is loose from the pacemaker pulse generator or outside interference such as TENS therapy or surgical diathermy).
ICD
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