A new pacemaker, Polarity, offers a solution to an old dilemma. Using a new method to sense spontaneous heart activity, the more flexible and long-lived type of unipolar lead can now be used in the atrium as well. ”This is an important step forward, especially for children who need a pacemaker,” says the manufacturer, Pacesetter.

Normally bipolar leads are used in the atrium in order to avoid electrical interference from the patient’s chest muscles. In many cases unipolar leads are preferable, but the benefits of unipolar leads fail when faced with the risk of interference.
The new pacemaker, Polarity, can sense heart activity in the atrium using a new method: ”Combipolar” sensing. This allows unipolar leads to be used while avoiding interference.
Patients who need a pacemaker should be given leads with the longest possible service life in order to avoid the inconvenience of replacing them. The life of the pacemaker is even more important in the case of children. Moreover, bipolar leads might be too thick for the thin veins of small children. Stimulation in the atrium is important so that the atrium and ventricle will work together to provide the best possible pumping effect. This is extremely important for a child’s ability to grow normally and to be physically active.
Bipolar leads, which are thicker and more rigid, and have a more complicated design, will probably always have poorer mechanical properties than unipolar leads - which have only one spiral conductor compared with bipolar leads which have two.
A Danish study (see footnote) shows that unipolar leads last much longer than bipolar leads. When a lead ceases to function, it often has to stay where it is since it is very difficult to remove.
The disadvantage of unipolar leads is that they work like an antenna, picking up electrical signals somewhat too effectively. Instead of just registering signals that give information about the activity of the heart, a unipolar lead also absorbs signals from the chest muscles. When that happens, the pacemaker can misinterpret the information so that it mistakes muscle activity for heartbeats, and therefore does not stimulate the heart when needed.
In clinical tests, Combipolar sensing eliminated this muscle interference in over 90 percent of the patients. According to the manufacturer, this means that the mechanically superior unipolar leads can now be used in the atrium as well, without chest muscle interference.
Polarity and Combipolar are a registered trademarks.

Footnote: Appraisal of Pacemaker Lead Performance from the Danish Pacemaker Register. M. Möller and P. Arnsbo, PACE, Vol. 19, September 1996.

Caption: These printouts show the effects of interference from muscle activity (after the arrow). The upper with unipolar atrial sensing, the lower with combipolar. With combipolar sensing, the heart rhythm remains constant despite interference.

For further information, please contact:
Per Cedernaes, Director, Market Communication, Pacesetter, St. Jude Medical, Coordination Center
Tel. +32 2 774 68 16
Fax +32 2 774 69 39
E-mail: P._Cedernaes_at_SJMI@sjm.com
Avenue Ariane Laan 5
1200 Brussels, Belgium

Steve Parker, St. Jude Medical U.K. Ltd
Bow Court, Fletchworth Gate
Coventry CV56SP, UK
Tel. +44 1203 716226
Fax +44 1203 716212


Background information on pacemaker technology

A modern pacemaker is a piece of electronic equipment which monitors and stimulates a heart which would otherwise beat too slowly. The pacemaker ensures that the heart’s rhythm is adapted to the body’s activity. A dual-chamber pacemaker can ensure that the contractions of the atrium and ventricle work together for the best pumping effect.
Although the pacemaker has become a sophisticated item of electronic equipment, with a memory, programming possibilities and often a microprocessor, it is entirely dependent upon an apparently simple mechanical part: a lead. The lead consists of a connector, cable and tip. Whenever a problem arises with a pacemaker, it usually involves the lead.

Sensing the heart’s activity
The lead has two functions. One is to convey the electrical impulses, pacing, which make the heart beat. The other, sensing, is to sense spontaneous heart activity so that the pacemaker will stop stimulating when the heart beats by itself.
Sensing is based on discovering the electrical signals that arise when the heart muscles contract. The signals are monitored in a unipolar system at two monitoring points, the tip of the lead and the pacemaker itself. In a bipolar system the signals are monitored between the tip of the lead and a ring on the cable of the electrode, inside the heart.
All electrical signals that are generated between the monitoring points are registered by the pacemaker. When ordinary body muscles tense, similar signals are transmitted. This means that a pacemaker with unipolar sensing also senses signals from the chest muscles. With bipolar sensing, these signals are very weak and do not cause any problems.

A new sensing system
The new pacemaker, Polarity, has a new system for sensing in the atrium: ”Combipolar” sensing.
With Combipolar sensing, the tip of the lead in the atrium acts as one monitoring point and the lead in the ventricle is the other. This means that the signals from the chest muscles are almost as distant and insignificant as if a bipolar system were being used.
For sensing in the ventricle, interference from the chest muscles causes no problem since the signals from the ventricle’s contractions are much stronger than those from the chest muscles. Signals from the atrium, on the other hand, are too similar to those of the chest muscles for the pacemaker to be able to distinguish one from the other.
A bipolar lead could be described as two wires in one. Usually there is an inner, spiral-shaped conductor enclosed in an insulating material, normally silicon rubber which the body will accept. Outside there is another spiral-shaped conductor, also enclosed in an insulating material. A unipolar lead consists only of what is the core of the bipolar lead.
Until the middle of the 1980s, pacemakers were almost solely single-chamber pacemakers which stimulated in the ventricle only. At that time unipolar sensing worked well. But dual-chamber pacemakers offer major benefits for most patients, since they allow synchronization of the contractions in the atrium and the ventricle. At first, however, there was a problem. Signals from the chest muscles interfered seriously with sensing from the atrium. To a large extent, this interference spoiled the intended synchronization. The answer was the bipolar lead.

A shorter life
Now, in retrospect, when we compare bipolar leads with unipolar leads, we see that bipolar leads might have a much shorter service life. Many of the problems can, of course, be regarded as teething problems with a new and difficult product. At the same time, bipolar leads will always be more complex to manufacture, as well as being thicker and more rigid than unipolar leads.
The reason behind developing bipolar leads was to eliminate electrical interference. This can now be done in another way. There are cases when bipolar sensing is necessary for a temporary period, other cases when it may be preferable. However, Combipolar sensing will be an important alternative which will solve many problems, not least when it comes to pacemakers for children.
Polarity and Combipolar are a registered trademarks.

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