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Da sich die Stimulationsimpedanz invers zum Stimulationsstrom verhält, senken hohe Impedanzwerte bei gleichbleibenden Stimulationsimpulsen zusätzlich den Stimulationsstrom. Inwieweit der intraoperativ gemessene Impedanzwert der Ventrikelstimulation repräsentativ für den Langzeitverlauf ist, wurde bei 87 Patienten untersucht, die die VDD-Einzelelektrode UniPass 425 und den Schrittmacher Unity (Sulzer Intermedics) erhalten hatten. Es wurden die Impedanzänderungen zwischen Implantations- und 6-Monats-Zeitpunkt ermittelt. Als gleiche Impedanzwerte wurde definiert, wenn beide Meßwerte =± 100 O voneinander abwichen. Kleinere bzw. größere Impedanzwerte lagen 6 Monate nach Implantation vor, wenn sie sich > ± 100 O vom Implantationswert unterschieden. Intraoperativ betrug die Impedanz 535 ± 98 O (Spannweite: 333–811 O) und war nach 6 Monaten signifikant auf 604 ± 160 O (Spannweite: 361–1150 O) angestiegen. Der Unterschied zwischen den beiden Impedanzwerten betrug 69 ± 162 O (Spannweite: -336 bis +560 O). Gleiche Impedanzwerte lagen bei 43 (Implantation: 527 ± 75 O, 6 Monate: 531 ± 87 O), kleinere bei 11 (Implantation: 660 ± 83 O, 6 Monate: 494 ± 73 O) und größere bei 33 Patienten (Implantation: 503 ± 99 O, 6 Monate: 735 ± 168 O) vor. Im Vergleich zu den Patienten mit gleichbleibenden Impedanzwerten hatten Patienten mit niedrigeren Impedanzen bei Implantation signifikant höhere Ausgangswerte. Schlußfolgerungen: Die Stimulanzimpedanz nimmt im Laufe von 6 Monaten nach Implantation signifikant zu. Bei 51% der Patienten kommt es zu deutlichen Veränderungen der Stimulationsimpedanz. Diese lassen sich in der Tendenz, aber nicht im Einzelfall voraussagen. As pacing impedance is inversely related to pacing current, the increase of pacing impedance additionally decreases pacing current. Whether the impedance measurement at implantation predicts the outcome during follow-up, was studied in 87 patients who received the VDD-single lead UniPass 425 connected to the pacemaker Unity (Sulzer Intermedics). The impedance changes between implantation and 6 months follow-up were assessed for each patient. Similar impedance values were defined, if the two measurements were within a range = -100 to +100 O. Six-months impedance was lower or higher compared to implantation, if the difference exceeded > -100 or > +100 O. At implantation, impedance was 535 ± 98 O (range: 333–811 O) and significantly increased to 604 ± 160 O (range: 361–1150 O) after 6 months. Mean difference between the two measurements was 69 ± 162 O (range: -336 bis +560 O). Similar impedance had 43 (implantation: 527 ± 75 O, 6 months: 531 ± 87 O), lower values 11 (implantation: 660 ± 83 O, 6 months: 494 ± 73 O) and higher values 33 patients (implantation: 503 ± 99 O, 6 months: 735 ± 168 O). Compared to the patients with similar impedance patients with lower impedance had a significantly higher impedance values at implantation. Conclusions: Pacing impedance increased significantly within 6 months after implantation. Pacing impedance changed > 100 O in 51% of the patients. The long-term follow-up of pacing impedance can be predicted generally, but not for the individual patient.

The reactions of the OH radical with H2S and SH⊖are very fast (k = 1.1·1010and 5.4·109mole-1liter sec-1, respectively). The hydrated electron reacts with H2S (1.1·1010) but does not react with SH⊖. The rate constant of the reaction CNS+SH⊖CNS⊖+SH was found to be equal to 9.8·108. Species absorbing at 3800 Å (e ∼ 104mole-1liter cm-1) are formed in these reactions of H2S and SH⊖The intensity of this absorption is very low in strongly acid solutions, increases with pH, goes through a broad maximum at about pH=7, reaches a lower plateau between pH = 8.5 -10.5 and decreases in strongly alkaline solutions. At pH values below 7, the intensity is dependent on the concentrations of both SH⊖and free H2S, i. e. at constant SH⊖concentration a stronger absorption occurs if more free H2S is present. In the plateau below pH = 8.5 -10.5, the intensity is practically independent of the SH⊖concentration above 4·10-5mole/l. The decay of the 3800 A absorption after the pulse is first order with τ1/2= 5 μsec independent of the OH⊖and SH⊖concentrations in alkaline solutions.Below pH = 7, the absorption disappears by second order, the rate constant being larger at lower pH values. SH or S⊖absorb weakly in the near UV and do not contribute to the 3800 Å absorption. This absorption is attributed to the ions H2S2⊖or H2S2⊖in alkali and to H2S2⊖or higher complexes such as H3S4⊖in acid. The first order decay of 5 μsec is attributed to the spontaneous decomposition of S2H2⊖into S⊖+SH⊖. Two S⊖ions form S22⊖(2 k=2·109 mole-1liter sec-1). The complex H2S2⊖is formed in the fast reaction SH + SH⊖↔ S2H2⊖(dissociation constant about 4·10-5mole/l). The formation of an intermediate complex SHOH⊖in the reaction of OH with SH⊖is discussed. This intermediate reacts with either SH⊖to form S2H2⊖or with OH⊖(or other proton acceptors such as ΗΡΟ42Θ) to form SΘ+ H2O+OHΘ.