Devices: Difference between revisions

==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.  

==Patients==

===Device measurements===

====Stimulation threshold (in mV@ms):====  

The stimulation threshold is the minimum stimulus intensity (in mV) and duration (in ms) necessary to capture the heart.

The degree that the pacing system “sees” or senses signals, controlled by the sensitivity setting which is graduated in mV.

The total pacing impedance is the sum of all resistance to the flow of the electrical impulses.  

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.

===Pacemaker codes===

Pacemakers can be categorized according to the NASPE coding system that usually consists of 3-5 letters.

|-

!I

===Explanations of table:===

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.  

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)

==Complications==

===Undersensing:===  

An intrinsic depolarization that is present, is not sensed by the pacemaker. This can be due to:

===Oversensing:===  

===Non-capture:===  

The electrical impulse is not followed by the myocardium.

This can be due to:

==Implantable cardioverter defibrillators (ICD)==

An ICD is a device that monitors heart rhythms. If it senses dangerous rhythms, it delivers shocks or anti-tachypacing (ATP) therapy. Many ICDs record the heart's electrical patterns when there is an abnormal heartbeat.

Patients do not meet the evidence based ICD implantation criteria if they have (1) a myocardial infarction within 40 days before ICD implantation; (2) newly diagnosed heart failure at the time of ICD implantation without prior therapy; (3) NYHA class IV symptoms of congestive heart failure.

ICDs are implanted under local anaesthesia in a sterile operating room or catheterization laboratory. Transvenous ICD implantation starts, like pacemaker implantation, with inserting the ICD leads in 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.  

===Arrhythmia detection zones===

The ICD diagnoses rhythm disorders by counting intervals on the intracardiac electrogram. This is a rate-based detection scheme that can be adjusted to meet the individual patient’s needs by programming. The ICD counts the current interval as one value and then average of the current interval and the preceding intervals. If these intervals fall into the same category, the event is binned in that category. If both events are tachycardia of fibrillation, but not in the same category, the interval is binned in the higher category.  

The arrhythmia detection zone is the category in which a predefined therapy will be given:

*'''Monitor zone (e.g. 160-180 bpm):''' All events in this zone will be recorded in the ICD and can be seen during follow up visits, however no therapy is given.

*'''Fast VT zone (e.g. 180-240 bpm):''' All events in this zone will be recorded in the ICD and can be seen during follow up visits. Therapy is given or not if the arrhythmia satisfies several criteria, which are programmed as well, such as SVT/VT discriminators.

*'''VF zone (e.g. >240 bpm):''' All events in this zone will be recorded in the ICD and can be seen during follow up visits and therapy is given immediately.

==Cardiac resynchronisation therapy (CRT)==

CRT-pacemaker (CRT-P) is a biventricular pacemaker with leads in both ventricles to ensure synchronized contraction. A CRT-defibrillator (CRT-D) is an ICD with biventricular pacing option.

It appears that atrio-ventricular and intraventricular conduction delays further aggrevates left ventricular (LV) dysfunction in patients with underlying cardiomyopathies. Notably, left bundle branch block (LBBB) alters the sequence of LV contraction, causing wall segments to contract early or late. Dyssynchrony seems to represent a pathophysiological process that directly depresses ventricular function, causing LV remodelling and congestive heart failure and as a consequence causes a higher risk of morbidity and mortality. Atrio-biventricular pacing (CRT) for patients with symptomatic heart failure and intra- or interventricular conduction disturbances has proved beneficial.  

*Right atrial lead

*Right ventricular lead

===Programming, follow up and complications===

Programming should specifically aim at ensuring atrial-synchronous permanent biventricular pacing, by performing AV-interval optimization (echocardiography guided or using invasive haemodynamic measurments) and performing ventricular-ventricle (VV) interval optimization. Further programming, follow up and complications are similar to pacemakers and ICDs (see above).