Unipolar leads

The conventional limb leads record potentials between two apices of the triangle. Lead 1 records potentials between the right arm and left arm, lead 2 records potentials between the right arm and left leg and lead 3 records potentials between the left arm and left leg. These are known as bipolar leads. In addition there are three unipolar leads attached to the limbs, which record potentials between the limb and a reference zero. 

The 12-lead ECG consists of three bipolar and nine unipolar leads, six of which are placed directly on the chest wall. Damage to the heart muscle results in characteristic ECG changes. 

Sodi Pallares D, Rodriguez H. Morphology of the unipolar leads recorded at the septal surface its application to diagnosis of left bundle branch block complicated by myocardial infarction. Am Heart J 1952 43 27. 

Transvenous lead polarity simply indicates the number of electrodes in contact with the heart. A unipolar lead has only one electrode, the cathode, on his tip. In this configuration, the PM case serves as the anode of the circuit (Fig. 1.1). A bipolar lead has two electrodes at the distal end separated by a short distance. The tip electrode is the cathode, and the proximal ring electrode serves as the anode (Fig. 1.2). In this configuration, the PM case is not involved in the electrical circuit. The distance between tip and ring electrode varies with manufacturer and design. 

Fig 1.10 Elema-Schonander unipolar lead with conductor made of stainless steel bands, (from [1], Chap. 9, p. 1) (courtesy of Medtronic)... 

Unipolar leads obviously have only one conductor coil, whereas bipolar leads require two -one for the proximal and the other for the tip electrode. Bipolar leads may have a coaxial, a co-radi-al or parallel wound, and a parallel or multiluminal design. Coaxial bipolar leads, standard in treating... 

The connector electrically binds the lead to the PM. Many types of connectors have been developed in the past, and this created considerable confusion among physicians and, in some cases, a potential hazard to the patient. Historically, unipolar leads were 5 or 6 mm in diameter, and bipolar leads required a bifurcated design with similar 5- or 6-mm sizes. Only after some years a bipolar, in-line design with a 3.2-mm diameter was developed. 

Fig 10 2 Preprocedural fluoroscopic posteroanterior view of patient referred to our center for lead extraction. Note the free-floating, fractured, old unipolar lead solid arrow) with the proximal end in the atrial region as a result of a previous, unsuccessful attempt of removal. The temporal pacing lead is placed in the right ventricle dashed arrow)... 

Because this is an unsolvable problem, the ideal unipolar lead does not exist. Nonideal solutions are to use one electrode at some remote point (e.g., at a limb or an earlobe). Another solution is to add the voltages picked up by more than one electrode, such as two limb electrodes (augmented leads) or three limb electrodes (Wilson central terminal). 

Fig. 1.6 Diagram demonstrating differences between unipolar and bipolar lead systems. Note the bipolar lead has both poles anode and cathode on the lead, whereas the unipolar lead has only the cathode. The anode lies on the can of the pulse generator.

The pacing lead conductor is composed of wire that conducts the electrical current from the pulse generator to the stimulating electrode and the sensed cardiac signals (intrinsic or evoked) from the electrode(s) to the sensing amplifier of the pulse generator. Unipolar leads require one conductor, whilst bipolar... 

Fig. 1.22 MultifilaT helically coil conductors. Above Unipolar. In this example, there are five single uninsulated strands of tightly coiled wire around a central core though which the stylet passes. The coils are covered with insulating tubing. Middle Bipolar coaxial design, which is currently the most common conductor arrangement There are two multifilar helical coils surrounded by two insulating tubes. The inner is the cathode and the outer surrounding conductor is die anode. This round design is of intermediate size and thicker than a standard unipolar lead. (Permission for use Medtronic.)...

Fig. 1.23 Three examples of fractured unipolar leads occurring at points of stress. A In the neck where the lead enters the external jugular vein. There is a sharp bend in the lead at the venous entry site and both limbs of the lead move differently with neck movement. B Within the right atrium. The lead is encased in a short endocardial tunnel, where it makes contact with the wall as it turns to enter the right ventricle. Again both limbs move differently with cardiac contractions. C At the lead connector site. There is a sharp bend in the lead after it emerges from the connector.

There is an ongoing debate with respect to the safety and efficacy of blind subclavian puncture. Furman has demonstrated remarkable efficiency of the cutdown approach for dual-chambered pacing, particularly with unipolar leads. The cutdown technique was less successful for bipolar leads via a single... 

In some cases, a buddy wire approach (Fig. 5.11) with two 0.014in. guide wires can be used to facihtate lead dehvery in acutely angled vessels. Alternatively, an acutely angled lead (such as the Medtronic Model 2187 unipolar lead) might be able to be manipulated into the proximal portion of the CS branch. 

Lead selection Steroid-eluting bipolar leads are nearly always preferable for pacemakers, and are now available, but are bulky due to their bifurcation and take up some room on the heart surface, and so may not be feasible in small hearts. Unipolar leads work fairly well, but problans such as local muscle stimulation and far-field sensing of myopotentials may be seen. The use of steroid-eluting leads has decreased the incidence of late exit bltx k in patients following implantation (28,29,38). They have a button configuration and are sewn to the epicardial surface. 

Also available are the so-called stab-on unipolar epimycKardial leads, which have a small barb that allows fixation to the mycKardium, as well as the so-called screw-in unipolar leads, which have a corkscrew-Uke fixation mechanism and are fixed to the heart by 2-3 turns. The former lead is best suited to the atrium, and to the ventricle in very small hearts, while the latter cmmot be used in the atrium and should be used with extreme caution in smaller hearts because of the depth to which they penetrate. These leads may be obtained with platinized electrodes, which may minimize subsequent threshold rise. 

Fig. 18.6 Posteroanterior radiograph (A) and close-up view (B) from a patient with intermittent failure to pace. Comparison of the upper and lower pins reveals that the lower of the two unipolar leads is not completely advanced. This difference is more evident on the close-up view. By convention, the lower of the two leads in the connector block is the ventricular lead, so that this patient must have had intermittent or permanent ventricular failure to output. An unrelated ohservation (arrowhead on 6 A) is the shallow positioning of the atrial lead, i.e., the J is much wider than 90°. (From Hayes DL. Pacemaker radiography. In Furman S, Hayes DL, Holmes DR Jr, editors. A practice of cardiac pacing, third edition. Mount Kisco [NY] Futura Publishing, 1993 361 100. Used with permission of Mayo Foundation for Medical Education and Research.)...

The unipolar leads are leads with only one pole. After the creation of the bipolar leads the unipolar leads were created to detect the heart s electrical activity from anywhere on the body, called VF, VR and VL. They work by taking an average from between any two of the bipolar leads (Fig. 2.5). 

MCLj (modified chest lead) and MCL (similar to the unipolar leads Vj and Vg of the 12-lead ECG). 

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