These are the circuit of an externalised artificial cardiac conduction system.
Unipolar vs bipolar pacing
A bipolar circuit has both the electrode inside the heart, with the heart muscle and intraventricular blood completing the circuit. Typically, these electrodes are only about 8mm apart, which reduces the impedance considerably. A unipolar circuit on the other hand relies on a large amount of tissue and body fluid to complete the circuit, and therefore has a much higher impedance.
For the purposes of illustrating the concept, this diagram has placed the anode and cathode at a significantly greater distance from one another than is usually expected in reality. The greater the distance, the greater the current which will be required, the more inflammatory change surrounding the wires and therefore the shorter the lifespan of the system. Additionally, having wires too far apart tends to create problems with sensing: there are more chances of interference with a greater distance. Generally the spacing between the electrodes on a bipolar lead ranges between 2.5 mm and 30 mm.
In the olden days, bipolar pacing leads were fat and rigid, and the unipolar variety of pacing was the preferred method because of convenience, ease of insertion, and because of fewer thrombotic complications. Now, the bipolar leads are quite slender, and this is no longer a problem. Which is good, because bipolar pacing offers many advantages:
uses less current
more current required
Requires two cardiac leads
Only one lead required
Less electrical interference from myopotentials
Receives interference form all the muscle in the path of the current
Less chance of diaphragmatic pacing
|Some chance of diaphragmatic pacing (and other muscles, for that matter)|
Tiny barely visible pacing spike
|Large obvious pacing spike|
In the majority of situations these days the bipolar circuit is favoured. There is much less electrical interference and substantially less current is required.
Sometimes, when one of the bipolar leads fractures or becomes encrusted with gunk, the bipolar circuit can be converted into a unipolar one.
One never knows which electrode will fracture, but it is generally known that the negative electrode gets gunked up first. Thus, we disconnect the patient lead from this gunked up electrode, and connect it to one of the praecordial ECG leads. The result is a "unipolar" circuit - there is only one pole in the myocardium, and the other pole completing the circuit is on the surface. This can sometimes rescue the situation.
The epicardial bipolar pacemaker circuit
This diagram depicts a circuit for the epicardial pacing wires.
The epicardial wires are tiny thin stainless steel cables, insulated along most of their length, which stick out through the patient's skin. Inside, they tend to be fixed to the epicardial surface with some resorbable sutures. Though the diagram above makes it look like a total of four wires come out of the skin, in practice usually each chamber is paced with a single bipolar pacing lead. The "outside" portion of the wire is connected to a straight needle which is used to pull the wires through the skin.
The wires depicted above feature a bifurcated electrode. Those needles are not left in situ- they are cut and discarded after the wires are placed (i.e. one should not expect to drag a curved needle out of the patient when removing these wires). Apart from this design, there are numerous others, including unipolar, bipolar and tetrapolar, straight electrodes, zigzag ones, looped electrodes to increase surface contact, and single-wire bipolar models which are easier to remove.
Here is a common design, after removal. Note the solid metal electrode fixation points for connecting to the external box.
The internal portion is usually a single lead.
The ideal manner of their placement is well covered by other authors, for example in the excellent review by Reade et al (2007); moreover as ICU staff we never really get to put these in, so time discussing the practical aspects of their placement would be wasted. Suffice to say there are some basic rules:
- Epicardial wires should be placed well away from thin walled areas of the heart, particularly the right atrial auricle
- The wires should not cross any coronary vessels
- The bipolar circuit should not straddle a graft or a major artery: i.e. the pacemaker current should not pass directly though the LAD. Bluntly, it is unwise to electrocute major coronary vessels. The diagram above depicts precisely this sort of placement error.
- The wires should be placed without too many loops and turns, so that they are easy to remove (i.e. the wires should be laid straight and in the direction of their exit through the skin)
- The intrathoracic wires should not be too tightly stretched, to prevent dislodgement
- Once outside the skin, the wires should be knotted around a suture which holds them to the skin- another feature which prevents dislodgement
- The leads which connect the pacing wires to the patient lead should be coiled; the coil gives ample room for error when the patient is repositioned. This way, even if they make sudden erratic movements, there will be plenty of give, and the wires wont come out of the epicardium.
The transvenous bipolar pacing wire circuit
These little pacing wires are better termed "flow directed pacing catheters", much like the Swan-Ganz. An ever better term is flussgesteurte schrittmacher-katheter.
Edwards has a nice page to advertise their product, which has some pictures.
This is not one of those pictures. It is an amateurish diagram to illustrate the position of the transvenous pacing electrodes. They are very close together- mere millimeters.
There is a considerable advantage to floating one of these things into a patient who needs pacing, when you compare it to external pacing.
Firstly, external pacing is a violent and painful process, which by any humane standard should be performed under deep sedation or anaesthesia.
Secondly, you have little choice of what you are pacing when you attach the external pads- you are merely making an educated guess when you try to stick those things "over the atrium" because in reality you have little control over what happens to the impedance in the chest- each breath, it changes.
Thirdly, the reliability of an electrode in the heart is significantly greater. It is unlikely to change position unless you tumble the patient around in a vigorous fashion.
Of course, it has similar disadvantages to any object you insert into a chamber of the heart. You could cause complications of central venous access, you could damage the tricuspid valve, and you could perforate the right ventricle.
An excellent article details this procedure very well.
I will direct the gentle reader there, and to the numerous YouTube clips of the same nature.
To summarise, the catheter is floated while attached to an ECG.
The distal electrode is attached to the right arm lead and the proximal electrode is attached to the left arm lead; the wire records Lead I. As the catheter is advanced to the right atrium, the atrial P wave and the ventricular QRS complex are about the same size; as one advances through the tricuspid annulus the QRS and T waves become more and more prominent, until one is greeted with the current of injury (which looks like ST elevation). One may also be greeted with a hearty burst of VT.
In either case, this confirms that the catheter is up against the ventricular wall.
One is then expected to confirm this position with a chest Xray, and by a successful attempt at pacing.