Endocardial ventricular pacing leads

Pacing electrodes (5-7 French) are passed through a central venous valved sheath inserted through the subclavian, the femoral, or preferably the right internal jugular vein. The left subclavian system is best avoided if permanent pacemaker insertion is likely. The wire tip is positioned in the right ventricular apex (pointing anteriorly and inferiorly, left and lateral to the spine, and near the cardiac border) in a stable position and with a gentle curve. Advancing the wire by approximately 3 cm once the wire tip is in position improves stability.

Pacing threshold (the minimum output current producing consistent ventricular electrical capture) and sensitivity (the minumum intrinsic R-wave voltage which can be sensed by the pacing box) are assessed sequentially. Sensitivity is set at 3 mV, the rate is set at 20 beats/min above the intrinsic rate, the output is set at 5 mA, and the pacing box is placed in demand mode. Consistent pacing of the heart should be seen on the monitor. Output is steadily reduced until the pacing threshold, i.e. failure to capture, is reached. Ideally, this should be 1 mA. The pacing current is then set at two to three times the threshold. A rapidly rising threshold or a threshold above 6 mA are indications for wire repositioning (using a new wire if the patient is pacemaker dependent). The sensing threshold is now tested, with the box in demand mode, and the rate is set at 20 beats/min below the intrinsic rate. The indicator system of the box should detect 'sensing' with each native beat. The sensing level is steadily reduced from 3 mV until sensing failure occurs. Sensitivity should be as close to 1 mV as possible. Setting the threshold at very low levels may cause failure to sense, while too high a threshold permits detection of artifactual electrical activity such as voluntary muscle movement.

Pacing electrodes may be positioned using fluoroscopic guidance, electrocardiographic guidance, or blind insertion.

Fluoroscopic guidance

Stiff bipolar wires, which can be 'shaped' by hand into a gentle curve before introduction, are generally used. The use of balloon flotation wires is faster, more reliable, and associated with lower morbidity. If placement is difficult, the wire is advanced along the lateral wall of the atrium until the tip catches on the tricuspid valve annulus, advanced to produce a loop of wire in the right ventricle, and then rotated anticlockwise causing the tip to flip inferiorly and leftwards into the right ventricular apex.

Electrocardiographic guidance

ECG limb leads are connected, and the negative-pole electrode is attached to a chest (V) lead. The P-wave of the internal ECG complex will be inverted in the high right atrium, biphasic in the mid-atrium, and upright lower down. The P-wave amplitude normally exceeds that of the QRS complex, with a reduction compared with that of the QRS complex as the right ventricle is entered. All voltages decrease if the inferior vena cava is entered, and all increase upon entry into the coronary sinus.

Blind insertion

This technique has a low risk of perforation and a high success rate. A 1- to 2-ml inflatable balloon is located proximal to the bipolar tip electrodes. With surface ECG monitoring, the pacing wire is connected to the pacing box, the rate is set at 20 beats/min above the intrinsic rate, and a high output is chosen (below 10 mV when immediate capture is essential for lifesaving). The wire is quickly measured against the patient in order to determine the approximate distance from the insertion point to the tip of the right ventricle. Once through the sheath, the balloon is inflated and the wire is advanced smoothly. The lead tip should sweep naturally into and lodge in the right ventricular apex. When ventricular (or, more rarely, accidental atrial) capture is seen on the monitor, the balloon is deflated and the catheter is advanced a further 2 cm. Prior wire 'shaping' (as described above), cooling (to stiffen), or altered patient position (left side up, head-down tilt) may aid placement. In significant tricuspid regurgitation, balloon deflation with blind passage may be needed.

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