Retroconduction Selective Recognition in Wide-Dipole Floating Atrial Sensing

Effective discrimination of retrogradely conducted P waves would allow distinguishing sinus tachycardia from supraventricular tachycardias due to A V or nodal reentry, and would prevent pacemaker-mediated tachycardia in AV sequential pacing. This might be especially relevant in VDD implants, where retroconduction could be induced by escape ventricular stimulation. In order to analyze the respective waveform properties, anterograde and retrograde atrial signals were recorded by a wide floating electrode dipole, on the implantation of a permanent single-pass lead for VDD pacing. Generally, bipolar recording did not allow reliable discrimination, while the signal nature could be readily diagnosed from the main features of the unipolar atrial electrograms. The unipolar waveform recorded under sinus rhythm in high right atrium, close to the superior vena cava opening (proximal EGM), started with a negative deflection in 88% of the patients. In 7% of the patients, the first deflection of the signal was positive in some cardiac cycles only, and, on the average, the amplitude of the positive phase was not higher than 5% of the signal peak-to-peak amplitude. Conversely, under retroconduction, the starting deflection attained higher positive values in 98% of the patients, being stably over 15% of the peak-to-peak amplitude in 86% of the cases. Furthermore, in 69% of the cases, the lag time between the onset of the negative deflection of proximal and distal (mid-low atrium) unipolar EGM changed unambiguously when retroconduction occurred, exceeding the range of variation observed in each patient during sinus activity. The combined evaluation of unipolar EGM shape and lag time allowed specific retroconduction recognition in 95% of the patients. We suggest that this approach may yield useful information for the discrimination of retrograde atrial signals, provided that the recording dipole is sufficiently long and the proximal electrode is properly positioned in the high right atrium.     

Synthesis and conformational studies of peptides encompassing the carboxy-terminal helix of thermolysin

The 21-residue fragment Tyr-Gly-Ser-Thr-Ser-Gln-Glu-Val-Ala-Ser-Val-Lys-Gln-Ala-Phe-Asp-Ala-Val-Gly-Val-Lys, corresponding to sequence 296–316 of thermolysin and thus encompassing the COOH-termi-nal helical segment 301–312 of the native protein, was synthesized by solid-phase methods and purified to homogeneity by reverse-phase high performance liquid chromatography. The peptide 296–316 was then cleaved with trypsin at Lys³⁰⁷ and Staphylococcus aureus V8 protease at Glu³⁰², producing the additional fragments 296–307, 308–316, 296–302, and 303–316. All these peptides, when dissolved in aqueous solution at neutral pH, are essentially structureless, as determined by circular dichroism (CD) measurements in the far-ultraviolet region. On the other hand, fragment 296–316, as well as some of its proteolytic fragments, acquires significant helical conformation when dissolved in aqueous trifluoroethanol or ethanol. In general, the peptides mostly encompassing the helical segment 301–312 in the native thermolysin show helical conformation in aqueous alcohol. In particular, quantitative analysis of CD data indicated that fragment 296–316 attains in 90% aqueous trifluoroethanol the same percentage (～58%) of helical secondary structure of the corresponding chain segment in native thermolysin. These results indicate that peptide 296–316 and its subfragments are unable to fold into a stable native-like structure in aqueous solution, in agreement with predicted location and stabilities of isolated subdomains of the COOH-terminal domain of thermolysin based on buried surface area calculations of the molecule     

Inadvertent Transarterial Pacemaker Insertion: An Unusual Complication

We describe an unusual complication of pacemaker treatment in a patient who died after a replacement operation. In a difficult situation in which a functioning pacemaker was highly desirable and in which most of the available veins had already been used, the pacemaker electrode was inserted, by mistake, through a small artery. This was not detected by fluoroscopy during surgery. The postoperative X-ray examination seemed to indicate that the electrode tip was located in the coronary sinus, but the subsequent autopsy revealed it to be located in the left ventricle.