Is atrial sensing of ventricular far-field signals important in single-lead VDD pacing?

UNLABELLED: In single-lead VDD pacing the atrial sensitivity frequently is programmed to sensitive values. Atrial sensing of ventricular far-field signals should be reduced by differential atrial sensing. The aim of the study was to evaluate the effectiveness of this approach. METHODS: The study included 10 patients with a single-lead VDD pacemaker (Thera 8948, Lead 5032). The atrial sensitivity was set to its most sensitive value of 0.18 mV and the telemetered intraatrial EGM was continuously recorded. After atrial tracked ventricular pacing, VVI pacing was performed with pacing rates from 100 to 160 beats/min in steps of 10 beats/min and up to 165 beats/min. The peak-to-peak amplitudes of P waves (P) and ventricular far-field signals (VFFS) were measured from the recordings. The ratio P/VFFS that defines the atrial signal-to-noise ratio was calculated, and the time from stimulus to maximum of the far-field signals amplitude (Tmax) was measured. RESULTS: P measured 0.98 +/- 0.76 mV. A VFFS was visible in the atrial channel in all patients with an amplitude of 0.45 +/- 0.25 mV (range 0.01-1.0 mV), independent of the pacing rate. The ratio P/VFFS was 3.9 +/- 4.2 (range 0.9-21.0). Tmax measured 99.4 +/- 15.2 ms during sinus rhythm. A rate dependent shortening of Tmax to 92.7 +/- 11.2 ms at 140 beats/min was observed (P = 0.001). At rates above 140 beats/min no further shortening occurred. CONCLUSION: Ventricular far-field signals are measurable in the atrial channel of VDD systems and may reach considerable amplitudes, which are not rate dependent. Although differential sensing provides favorable P waves to ventricular far-field signal ratios, refractory periods are needed to avoid far-field sensing. The rate dependent shortening of the ventricular signal can be detected in the atrial channel in VDD pacing.

Limitations of the algorithm to reconfirm ventricular fibrillation in a third-generation implantable cardioverter defibrillator.

We report on inadequate shocks delivered by an ICD for VT converting spontaneously to AF with slow ventricular response. This represents a correct behavior of the reconfirmation algorithm, as the detection of intervals longer than the fibrillation detection interval and shorter than the bradycardia escape interval is required to inhibit the shock.

Rotational and translational water diffusion in the hemoglobin hydration shell: dielectric and proton nuclear relaxation measurements.

The dynamic properties of water in the hydration shell of hemoglobin have been studied by means of dielectric permittivity measurements and nuclear magnetic resonance spectroscopy. The temperature behavior of the complex permittivity of hemoglobin solutions has been measured at 3.02, 3.98, 8.59, and 10.80 GHz. At a temperature of 298 K the average rotational correlation time tau of water within a hydration shell of 0.5-nm thickness is determined from the activation parameters to be 68 +/- 10 ps, which is 8-fold the corresponding value of bulk water. Solvent proton magnetic relaxation induced by electron-nuclear dipole interaction between hemoglobin bound nitroxide spin labels and water protons is used to determine the translational diffusion coefficient D(T) of the hydration water. The temperature dependent relaxation behavior for Lamor frequencies between 3 and 90 MHz yields an average value D(298K) = (5 +/- 2) x 10(-10)m2 s-1, which is about one-fifth of the corresponding value of bulk water. The decrease of the water mobility in the hydration shell compared to the bulk is mainly due to an enhanced activation enthalpy.