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    • br Historical overview Before the development of

    2019-06-19


    Historical overview Before the development of screw-in leads, several types of non-screw pacing leads were designed to passively attach to the Alisol B 23-acetate muscle with tips of different shapes, including straight, fringed, finned, or tined. However, a high incidence of dislodgement was a major concern, and it currently remains a routine complication. In particular, for atrial pacing, the right atrial appendage is the only feasible fixation position with non-screw leads. Thus, the surgeon has no alternative, even when the pacing threshold is unacceptably high. Often, the result is to abandon atrial pacing. This problem may be resolved with the introduction of the screw-in lead. The screw-in lead provides high stability, and re-fixation is possible at any site when the electrical parameters at the initial position are unacceptable. Additionally, the hemodynamic importance of the atrial contribution was recently recognized. This has emphasized the importance of atrioventricular synchronous pacing [10]. Thus, stable atrial pacing has become indispensable, and the use of the screw-in lead has spread rapidly. Furthermore, recent studies have demonstrated that right ventricular apical pacing can prolong the QRS width, which results in a deterioration of cardiac function due to increased inter-ventricular dyssynchrony [7]. Those results led to the recommendation that right ventricular septal pacing should be preferred because it preserves a narrow QRS, and thus, apical pacing is currently avoided. Septal pacing is only possible with the screw-in lead. Thus, indications for the screw-in lead have increased, and it has become an essential tool for both atrial and ventricular pacing practice.
    Classification of the screw-in lead Currently, screw-in leads are available in two types of designs. One is the retractable screw-in lead and the other is the Sweet Tip™ (Fig. 1A and B). The name “Sweet Tip™” is a trade mark of Boston Scientific Corp., Natick, MA, USA (Sweet Tip™ Rx). In this review, a fixed helix type of lead coating with mannitol is described as Sweet Tip™. The retractable type features the ability to store the helix within an insulating tube at the tip. Then, by applying torque to the end of the connector several times, the helix protrudes from the tip, and it can be screwed into cardiac tissue. The Sweet Tip™ comprises a helix covered with mannitol [5]. After insertion, the mannitol melts within 3–5min at body temperature; thus, the helix is exposed once it is correctly placed in the heart, just before fixation. Once the helix is in contact with the target, it can be fixed by turning the lead body clockwise several times to screw it into the cardiac tissue. Before the development of the Sweet Tip™ lead, only an “exposed” helix type lead was available. Its use was associated with the risks of entrapment in the venous system when en route to the target area or perforations before reaching the right atrium. To avoid these complications, the mannitol coating was developed. The current Sweet Tip™ lead is available in two helix lengths, 1.5mm and 1.6mm. Both the retractable and Sweet Tip™ leads are designed with a bipolar configuration to include a steroid eluting system. In addition, the lead insulation is available in both silicon and polyurethane. More details on the differences between lead types are presented elsewhere. Before the implantation procedure, surgeons should be aware of the lead characteristics; the combination of tip design, insulation materials, and helix length confer differences in stiffness and maneuverability. It is important that the operator understands both the electrical and mechanical characteristics of the lead to avoid the occurrence of adverse complications, including cardiac perforation, during the implantation procedure.
    Retractable or Sweet Tip™?
    Electrical characteristics of the Sweet Tip™ screw-in lead It is common that the pacing threshold increases mildly because of tissue injury, immediately after a screw-in lead is fixed on the atrium or ventricle, i.e., during the acute phase of fixation. However, if the measured threshold value is exceedingly high, the lead position should be changed immediately, without hesitation. In our experience, in the former cases, the thresholds improve after several minutes; therefore, we recommend waiting for approximately 5m. In these cases, long-term results are also favorable. Thus, frequent changes in lead position are not always necessary [11].