Pacing ventricular thresholds

Effects on pacemaker thresholds Flecainide increases endocardial pacing thresholds and may suppress ventricular escape rhythms. These effects are reversible. Use with caution in patients with permanent pacemakers or temporary... 

In healthy volunteers, tocainide produced a slight depression in His-Purkinje conduction as well as a slightly delayed enhancement of A-V node conduction during atrial pacing. No significant alterations in heart rate, right ventricular ERP or the excitation thresholds of atrial or ventricular muscle were observed in these subjects. 

Low doses (100-200 mg/d) of amiodarone are effective in maintaining normal sinus rhythm in patients with atrial fibrillation. The drug is effective in the prevention of recurrent ventricular tachycardia. It is not associated with an increase in mortality in patients with coronary artery disease or heart failure. In many centers, the implanted cardioverter-defibrillator (ICD) has succeeded drug therapy as the primary treatment modality for ventricular tachycardia, but amiodarone may be used for ventricular tachycardia as adjuvant therapy to decrease the frequency of uncomfortable cardioverter-defibrillator discharges. The drug increases the pacing and defibrillation threshold and these devices require retesting after a maintenance dose has been achieved. 

DelBufalo AGA, Schlaepfer J, Fromer M et al. Acute and long-term ventricular stimulation thresholds with a new, Iridium oxide-coated electrode. PACE 1993 16 1240-1244. 

Fig. 4.91 A. Biventricular thres hold testing the left side of the strip show biventricular capture as the threshold is decreased intermittent capture of either the RV or LV occurs as indicated by a change in morphology. B. With continued decrease in output total loss of ventricular occurs, (from Belott PH Implantation Techniqnes for Cardiac resynchronization Therapy Barold SS, Mugica J Fifth Decade of Cardiac Pacing. Armonk NY Futura. 2004 pp 22)...

Capture and sensing thresholds should be tested once the pacing catheter is placed. The pulse generator rate is set approximately 10-20 pulses/min above the patient s native ventricular rate (if present) to determine capture thresholds, while the output is gradually deaeased until capture is lost. Alternatively, the ouq>ut can be slowly increased until consistent ventricular capture is observed (Fig. 7.5). Tanporary pulse generators designed for temporary pacing deliver an electrical... 

Fig. 7.5 Threshold testing. The first three stimuli are delivered with 1 mA output. The third pacing stimulus (arrow) does not result in ventricular depolarization. The output is increased slightly to 1.25 mA and consistent capture is observed.

Rub N, Schweitzer O, Mewis C, Kettering K, KuehUcamp V. Addition of a defibrillation electrode in the low right atrium to a right ventricular lead does not reduce ventricular defibiillation thresholds. Pacing Clin Electrophysiol 2004 27 346-51. 

In approximately 50% of cases, antiarrhythmic drugs are required in patients with ICDs to reduce the frequency of recurrent ventricular arrhythmias (199). Antiarrhythmic drugs may also be necessary to suppress atrial arrhythmias, which may interfere with proper detection and lead to inappropriate shocks (200,201). In both situations, drugs must be used judiciously because of potential drug-device interactions. Drugs can (a) slow ventricular tachycardia below the programmed rate cutoff, (b) increase energy threshold to defibril-late, (c) have an effect on pace termination of ventricular tachycardia, (d) be proarrhythmic, and (e) cause bradycardia and AV block. The process of... 

Khastgir T, Lattuca J, Aarons D, et al. Ventricular pacing threshold and time to capture postdefibrillation in patients undergoing implantable cardioverter-defibrillator implantation. Pacing Clin Electrophysiol 1991 14 768-772. 

Fig. 18.16 Posteroanterior (A) and lateral (B) chest radiographs of a dual-chamber pacing system. Both the atrial and the ventricular leads have inadequate redundancy, which may result in poor pacing thresholds or lead dislodgment.

Fig. 19.20 Lateral chest x-ray from a patient with a dual-unipolar DDD pacing system. The course of the ventricular lead is bizarre and attributed to a very pronounced pectus excavatum chest wall deformity. Capture thresholds were excellent, the paced QRS had a LBBB pattern, and a transesophageal echo confirmed that the lead was in the right ventricle.

Ventricular stimulation by NIPS is done using the same protocols that are employed for standard invasive electrophysiologic testing (46). If ventricular tachycardia is induced, the effectiveness of ATP protocols is assessed. At the end of the study the ability of the ICD to detect and treat ventricular fibrillation can be evaluated. Ventricular fibrillation can usually be induced by ultrarapid ventricular pacing (30-50 stimuli/s) or an appropriately timed low-energy shock on the T wave. The defibrillation threshold can be evaluated using the methods outlined in Chap. 8. 

---