Pacing system analyzer for testing during implantation

Pacing system analyzer for testing during implantation

Pacing system analyzer is used for testing various parameters like R wave amplitude, pacing threshold and lead impedance during implantation of a permanent pacemaker. It can also deliver temporary pacing stimuli for supporting the heart rate if needed, during implant procedure. Modern pacing system analyzers have the capability of dual chamber AV sequential pacing. The PSA can display real time intracardiac electrogram sensed from the pacing lead as well as surface ECG if surface ECG leads of the analyzer are also connected. In addition to the pacing system analyzer, surface ECG is continuously monitored using a separate cardiac monitoring system during implant procedure.

Measurement of intrinsic R wave during ventricular lead implantation ensures good lead position. Instead of R wave amplitude, P wave amplitude is checked while implanting an atrial lead. A good R or P wave is a surrogate of good pacing site. If there is intrinsic conduction across the AV node, intrinsic PR interval is also measured. Measurement of R wave needs switching off the temporary pacing and can be risky if intrinsic rhythm does not come up promptly. Temporary pacing rate should be brought down gradually for the intrinsic rhythm to take over and avoid asystole.

Precaution is also needed during threshold testing to avoid prolonged asystole. Threshold is the minimum output voltage needed for pacing capture. The slew rate of intrinsic signals is also measured. Good slew rate is important for the pacemaker to detect the intrinsic signals. Pacing thresholds can be tested at different pulse widths of the pacing signal if needed. Using higher pulse widths for pacing would naturally mean more battery drain.

High voltage pacing, typically at 10 Volts and a pulse width of 2 ms is used to check for phrenic nerve stimulation. Phrenic nerve stimulation can be seen as diaphragmatic movements on fluoroscopy or felt with the palm kept on the abdomen. A good current of injury seen on the PSA monitor will indicate a good contact between the pacing lead and the myocardium. Of course, the current of injury will be more for an active fixation screw in lead, which might even cause a temporary increase in pacing threshold. Current of injury is recognized at the site of tissue injury as an increase in the duration of the intracardiac electrogram and elevation of ST segment [1].

If there is a current of injury, pacing thresholds can come down to acceptable range within 10 minutes even if it is high during initial measurement. On the other hand, if there is no current of injury during active fixation of lead, the fixation is inadequate, and the lead has to be repositioned. Current of injury can also occur with passive fixation leads, possibly because of focal damage to cell membranes due to the trauma by electrode pressure against the endocardium.

Midterm performance of active fixations leads can be predicted from the current of injury recorded at the time of lead of implantation. ST segment elevation 2 mV or more for atrial leads and 10 mV or more for ventricular leads has been recommended to improve lead performance at 6 months. In that study good lead performance was defined as P wave of 1.5 mV or more with threshold less than 1.5 V for atrial lead. In case of ventricular lead R wave amplitude of 5 mV or more and threshold less than 1 V was considered as good lead performance at 6 months [2].

References

1. Saxonhouse SJ, Conti JB, Curtis AB. Current of injury predicts adequate active lead fixation in permanent pacemaker/defibrillation leads. J Am Coll Cardiol. 2005 Feb 1;45(3):412-7. doi: 10.1016/j.jacc.2004.10.045. PMID: 15680721.
2. Haghjoo M, Mollazadeh R, Aslani A, Dastmalchi J, Mashreghi-Moghadam H, Heidari-Mokarar H, Vakili-Zarch A, Alizadeh A. Prediction of midterm performance of active-fixation leads using current of injury. Pacing Clin Electrophysiol. 2014 Feb;37(2):231-6. doi: 10.1111/pace.12262. Epub 2013 Sep 2. PMID: 23998792.