Effect of phasic vagal stimulation and atrial pacing site on atrioventricular conduction time.

We tested the hypothesis that the effect of phasic vagal stimulation on atrioventricular (AV) conduction time is affected by the site of atrial pacing in anesthetized dogs. We paced the right atrium at a constant cycle length from the interatrial septum (IAS), superior coronary sinus (SCS), or inferior coronary sinus (ICS) regions, and we evaluated the time-dependent effects of vagal stimulation on AV conduction at each pacing site. When we stimulated the vagi at stimulus (St)-A phases greater than 136 +/- 40 ms and less than the phase that blocked AV conduction (182 +/- 70 ms), IAS pacing prolonged A-V intervals by 8.6 +/- 8.2 ms more than ICS pacing. A change in pacing site affected the A-V intervals by up to 30 ms when we stimulated the vagus at those times that caused the A-V intervals to prolong maximally. Furthermore, the effect of atrial pacing site on A-V intervals was modulated by AV nodal recovery times during the second or third cycles after the vagal stimulus.

Automated ventricular tachyarrhythmia recognition: a combination of cycle length and nonlinear dynamics measurements.

INTRODUCTION: Cardiac monitoring devices such as external cardioverter defibrillators and ICU computerized ECG monitoring systems sense individual QRS complexes to detect and subclassify ventricular tachyarrhythmias. Many algorithms that evaluate ECG morphology mark individual QRS complexes so that specific waveform characteristics can be measured. QRS sensors miss a percentage of electrogram events, especially during fibrillatory rhythms; thus, a morphology algorithm that helps subclassify arrhythmias without marking individual electrogram events may be more robust and improve arrhythmia detection accuracy. METHODS AND RESULTS: Four nonlinear dynamics calculations were evaluated for detecting ventricular tachyarrhythmias and for subclassifying monomorphic ventricular tachycardia (MVT) and polymorphic ventricular tachycardia (PVT). Five-second epochs of normal (NML, n = 48), MVT (n = 58), and PVT (n = 75) rhythms were presented to a statistical discriminant function based on cycle length (CL), and its performance was compared to two other discriminant functions, one consisting of the nonlinear dynamics calculations and one consisting of a combination of all variables. The discriminant function based on nonlinear dynamics calculations and CL detected 100% of the ventricular tachyarrhythmias, and subclassified more (P < 0.001) MVT and PVT arrhythmias (90%) than that based on CL alone (71%). CONCLUSIONS: The nonlinear dynamics measurements used in this study significantly increased the subclassification accuracy of the CL-based discriminant function, and they were calculated from ECG signals without marking individual QRS complexes. Therefore, arrhythmia detectors that use nonlinear dynamics measurements may commit fewer classification errors due to QRS undersensing and aid therapy decisions when circumstances suggest that QRS sensing is inaccurate.

Adequacy of implantable cardioverter-defibrillator lead placement for tachyarrhythmia detection by sinus rhythm electrogram amplitude.

This study examines whether the current clinical practice of using a 5 mV minimum amplitude during normal sinus rhythm (NSR) ensures adequate detection during subsequent episodes of ventricular fibrillation (VF) at the time of the implantable cardioverter-defibrillator (ICD) threshold testing. Risk of nondetection occurs with ICDs when a substantial portion of the individual cardiac events on an electrogram goes undetected. Detection risk was assessed by 2 methods: percentage of missed cardiac events (incidence of signal dropout), and mean electrogram amplitude. During ICD implantation and testing in 27 patients utilizing 41 lead positions, 135 episodes of VF were induced and analyzed. On 64 occasions, the countershock was not successful in achieving cardioversion, and the continuing electrical activity was analyzed as a separate group of postshock waveforms. Thresholds of 1 and 2 mV were applied to each individual cardiac depolarization in a VF episode. Significant risk of nondetection was assumed when > or = 10% of individual events displayed dropout. Underdetection by signal dropout occurred in 11 of 135 preshock arrhythmia signals (8.1%) from 3 patients at a 2 mV threshold, and in 6 of 135 signals (4.4%) at a 1 mV threshold. A mean NSR amplitude > or = 5 mV was associated with significantly lower risk of nondetection during subsequent VF episodes at both 1 and 2 mV thresholds (largest p < 0.001). Similar results were observed in analysis of postshock arrhythmia signals. Further examination of signal dropout and linear regression criteria suggest that in order to eliminate the possibility of nondetection at a 1 mV threshold, minimum NSR amplitudes of 8.5 and 10.0 mV, respectively, are required.

Optical quality testing of monofocal intraocular lens implants with a 3 mm and a 4 mm aperture.

The optical quality of 81 monofocal poly(methyl methacrylate) intraocular lenses (IOLs) from eight U.S. firms was tested using a 3 mm aperture, as recommended by the American National Standards Institute Z80.7 standard for IOLs, and a 4 mm aperture. The use of the larger aperture had no effect on the measurements of refractive power and astigmatism. When examined with the 3 mm aperture, the average resolving power of the IOLs was 81% of the diffraction limit; when examined with the 4 mm aperture, the average resolving power was 67% of the diffraction limit. Use of the larger aperture would ensure that a larger area of the IOL had been examined. Incorporating the larger aperture into the ANSI minimum resolution requirement would not appreciably affect the rejection rate for currently manufactured monofocal lenses.

Survey of the optical quality of intraocular lens implants.

Intraocular lenses (IOLs) from 15 U.S. firms were tested for conformance to the requirements of the American National Standards Institute Z80.7 standard for IOLs. A total of 162 IOLs were tested for resolution, astigmatism, and accuracy of labeled power. Average resolving power was 78% of the diffraction limit, much better than the ANSI minimum requirement which is typically equivalent to 30% of the diffraction limit. This suggests that the ANSI Z80.7 requirement could be significantly tightened with little effect on current production practices. Only one IOL exhibited astigmatism in excess of 0.25 diopter. Differences between measured and labeled power in excess of 0.50 diopter were found in 22 lenses, indicating that accuracy of refractive power may be the most commonly missed optical requirement of the standard.