Reduction of pacing output coupling capacitance for sensing the evoked response

Sensing of the intracardiac evoked response (ER) after a pacing stimulus ha s been used in implantable pacemakers for automatic verification of capture , Reliable detection of ER is hampered by large residual afterpotentials as sociated with pacing stimuli. This led to the development of various techno logical solutions, like the use of triphasic pacing pulses and low polarizi ng electrode systems. This study investigated the effect of reducing the co upling capacitance (CC) in the pacemaker output circuitry on the magnitude of afterpotential, and the ability to automate detection of ventricular evo ked response. A CC of 2.2 muF and four different blanking and recharge time settings were clinically tested to evaluate its impact on sensing of the v entricular ER and pacing threshold. Using an automatic step-down threshold algorithm, 54 consecutive patients, aged 70 +/- 10 years with acutely (n = 27) or chronically (n = 27) implanted ventricular pacing leads were enrolle d for measurement testing. Routine measurements, using a standard pacing sy stem analyzer (PSA), were (mean SD) impedance 569 +/- 155 Omega, R wave amp litude baseline to peak 9.8 +/- 3.7 mV and threshold 0.9 +/- 0.7 V at 0.4-m s pulse width. This new capture verification scheme, based on a CC of 2.2 m uF and recharge/blanking timing setting of 10/12 ms, was successful in 52 p atients which is equivalent to a success rate of 96%. In a subgroup of 26 p atients implanted with bipolar ventricular leads (10 chronic, 16 acute), da ta were collected in unipolar (UP) and bipolar (BP) pace/sense configuratio ns. Also, ER signals were recorded with two different band-pass filters: a wider band (WB) of 6-250 Hz and a conventional narrow band (NB) of 20-100Hz , WB sensing from UP lead configuration yielded statistically significant l arger signal to artifact ratios (SAR) than the other settings (P < 0.01). A dedicated unipolar ER sensing configuration using a small output capacitor and a wider band-pass filter enables adequate automatic capture verificati on, without any restrictions on pacing lead models or pacing/sensing config urations.