Setting of Relatively Low Energy Outputs May Permit Implantation of a Nonthoracotomy Automatic Cardioverter Defibrillator System When High Energy Outputs Prove Ineffective

At intraoperative testing of defibrillation thresholds during implantation of internal Cardioverter defibrillators, standard step-down approaches of energy outputs are used. If relatively high energy outputs are not successful at defibrillating the heart, the electrodes are frequently reconfigured. When attempting implantation of a nonthoracotomy lead system, high defibrillation thresholds may warrant opening of the chest cavity to place one or more epicardial electrodes. A case is presented where a nonthoracotomy system was able to be implanted using relatively low energy outputs which were reproducibly successful at terminating ventricular fibrillation when higher energy outputs were unsuccessful. Mechanisms for this phenomenon and alternate recommendations for defibrillation testing are presented.     

Effect of Varying Atrial Sensitivity, AV Interval, and Detection Algorithm on Automatic Mode Switching

Automatic mode switching (AMS) is absolutely dependent on atrial tachyarrhythmia detection. The effects of programming several features that could influence tachyarrhythmia detection were assessed in 18 patients (six women; mean age 64 years) with pacemakers having AMS capability. The atrial electrogram amplitude in sinus rhythm at implant (SR-EGM), last measured atrial sensing threshold prior to tachycardia (A-SENS), and atrial sensing threshold for effective AMS during atrial tachyarrhythmia (AMS-SENS) were obtained. Additionally, ten patients had AV intervals increased from 60 to 200 ms, while seven patients had detection algorithms made more stringent from 5 beats at 150 beats/min to 11 beats at 200 beats/min to assess their effects on AMS efficacy. Results: Sensitivities: Mean SR-EGM = 3.55 mV; mean A-SENS = 2.06 mV; and mean AMS-SENS = 1.46 mV. Fourteen patients developed atrial fibrillation and four atrial flutter. Thirteen of 14 patients who developed atrial fibrillation sensed adequately at ≥ 1.0 mV in normal sinus rhythm (NSR), but only six patients had effective AMS at these settings in atrial fibrillation. Three of four patients who developed atrial flutter had effective AMS at ≥ 2.0 mV. AV Interval: AMS was effective in eight of ten patients at AV intervals up to 200 ms. One patient lost AMS at an AV interval of 120 ms. Algorithm: In two of seven patients, AMS was not effective if the detection algorithm was more stringent than five beats at 150 beats/min. Conclusions: (1) In atrial fibrillation, effective AMS requires more sensitive atrial settings than in NSR: (2) AV intervals as short as 120 ms can interfere with AMS function; and (3) More stringent detection algorithms may be inappropriate for effective AMS function.     

Mitral Stenosis, Sinus Venosus Atrial Septal Defect, and Partial Anomalous Pulmonary Venous Return

We describe a 42-year-old man with rheumatic mitral stenosis, sinus venosus atrial septal defect, and anomalous drainage of the right upper pulmonary vein to the superior vena cava. Transthoracic echocardiography (TTE) failed to identify the atrial septal defect and the partial anomalous pulmonary venous return. Transesophageal echocardiography (TEE), using a multiplane probe, was useful in delineating the abnormalities. To our knowledge, this is the first reported patient with rheumatic mitral stenosis and sinus venosus defect.     

Idiosyncratic Pacing with a Commercial Pacing System Analyzer

An idiosyncratic response of the Medtronic AV Pacing System Analyzers (models 5311 [unipolar] and 5311B [bipolar]) during high output AAI pacing is reported. Using the bipolar analyzer with unipolar cables and a 2734 adaptor, ventricular capture was noted during high output AAI pacing when the ground was displaced from the patient. Using the unipolar analyzer with bipolar cables also resulted in ventricular capture with high output AAI pacing. Implanting physicians should be aware of this phenomenon to avoid false conclusions.     

STUDIES ON HYDROGEN BONDS: Part IV. Proposed Working Criteria for Assessing Qualitative Strength of Hydrogen Bonds*

The data obtained in the earlier parts of this series for the donor and acceptor end parameters of N-H. O and O-H. O hydrogen bonds have been utilised to obtain a qualitative working criterion to classify the hydrogen bonds into three categories: “very good” (VG), “moderately good” (MG) and weak (W). The general distribution curves for all the four parameters are found to be nearly of the Gaussian type. Assuming that the VG hydrogen bonds lie between 0 and ± la, MG hydrogen bonds between ± 1s? and ± 2s?, W hydrogen bonds beyond ± 2s? (where s? is the standard deviation), suitable cut-off limits for classifying the hydrogen bonds in the three categories have been derived. These limits are used to get VG and MG ranges for the four parameters 1 and θ (at the donor end) and ± and ± (at the acceptor end). The qualitative strength of a hydrogen bond is decided by the cumulative application of the criteria to all the four parameters. The criterion has been further applied to some practical examples in conformational studies such as α-helix and can be used for obtaining suitable location of hydrogen atoms to form good hydrogen bonds. An empirical approach to the energy of hydrogen bonds in the three categories has also been presented.     

Sinus Node Recovery After 25 Years of Atrial Flutter

PALMA, E.C., et al. : Sinus Node Recovery After 25 Years of Atrial Flutter. This case report demonstrates that the sinus node can recover relatively quickly even after being suppressed by atrial flutter for 25 years, and that a permanent pacemaker may not always be necessary in all patients with sinus arrest after a successful atrial flutter ablation.     

Cumulative Hazard Analysis of J-Wire Fracture in the Accufix Series of Atrial Permanent Pacemaker Leads

To permit a more complete analysis of J-wire fracture in the Accufix series of atrial permanent pacemaker leads, the time to occurrence of all known fractures and injuries has been redefined relative to the duration of risk exposure, that is, according to the interval of time between implant and occurrence of the event. This redefinition permits application of a cumulative hazards model to the data, which previously has not been explored. Predictors of J-wire fracture can be tested using this method. This also permits parametric curve-fitting for determination of linearity or constancy of risk of events over time. Results: Among 2,063 Multicenter Study (MCS) leads analyzed, 381 fractures of the J-wire were identified. Stratified analysis based on cumulative hazard curves identified a more open shape of the J-wire as predictive of fracture, which supports the results previously reported based on logistic regression analysis. Fitting a Weibull curve to the cumulative hazard of J-wire fracture gives a shape parameter equal to 0.85. This value indicates that the instantaneous hazard of J-wire fracture decreased over time from implant. Conclusions: (1) The cumulative hazard function can be used to examine predictors of J-wire fracture and preliminary findings support the previously identified predictor of J shape; (2) Based on these analyses, the rate of J-wire fracture appears to decrease slightly as time from implant increases.     

Review of the Evidence Regarding the Carcinogenicity of Hexavalent Chromium in Drinking Water

Recent analyses have revealed that 38% of municipal sources of drinking water in California have detectable levels of hexavalent chromium. This observation provided new impetus to characterize the carcinogenic risk associated with oral exposure to hexavalent chromium in drinking water. Notwithstanding the well-characterized increases in cancer associated with inhalation exposure to this chemical, the marked reduction of hexavalent chromium to trivalent chromium in the stomach suggests that exposure to hexavalent chromium in drinking water may not pose a carcinogenic risk. A reevaluation of studies that investigated the toxicokinetics, the genotoxicity, and the mechanism of carcinogenicity of hexavalent chromium, as well as the available human and animal cancer studies, was undertaken to determine if there is evidence that exposure to this chemical in drinking water may pose a carcinogenic risk. Mechanistic studies suggest the potential for a carcinogenic response if hexavalent chromium enters cells. Both toxicokinetic and genotoxicity studies indicate that a portion of an orally administered dose of hexavalent chromium is absorbed and gets into cells of several tissues, causing DNA damage. The only lifetime oral study of hexavalent chromium in animals conducted thus far yielded a statistically significant increase in stomach tumors compared to controls. Also, in a limited-term cancer study, co-exposure to hexavalent chromium in drinking water and ultraviolet light produced skin tumors in mice. The only available cancer study of humans exposed to hexavalent chromium in drinking water revealed a statistically significant increase in stomach tumors. Moreover, a meta-analysis of occupational studies also revealed a statistically significant increase in stomach cancers. The increases in stomach tumors in both human and animal studies, along with the toxicokinetic, genotoxic, and mechanistic data, suggest that oral exposure to this agent appears to pose a carcinogenic risk.     

THE UTILIZATION OF THE METHYL GROUP OF METHIONINE IN THE BIOLOGICAL SYNTHESIS OF CHOLINE AND CREATINE

The transfer of methyl groups from methionine to choline and creatine has been demonstrated by the isolation of deuteriocholine and deuteriocreatine from the tissues, and of deutriocreatinine from the urine, or rats fed methionine containg a deuteriomethyl group. It was found from the deuterium ocntents of the isolated compounds that at the end of a given experimental period, each of the isolated compounds had derived approximately the same percentage of methyl groups from the deuteriomethionine of the diet. All the deuterium in the isolated choline was shown to be in the methyl grops. The concentraion of deuterium in the methyl gorups of the choline and creatine from the tissues as well as of the urinary creatinine rose to 85 per cent of that of the deuterio-methionine fed over a period of 14 weeks, strongly indicating that there was no other precursor of methyl groups in the diet employed.
The transfer of methyl groups from choline to creatime has been demonstrated by the isolation of deuteriocreatime from the tissues and of deuteriocreattnine from the urine of rats maintained on a diet containing deuteriocholine and homocystine.
Thus direct proof has been afforded to substantiate the hypothesis previously presentd that methionine may be a precursor of choline in so far as the methyl groups are concerned. The significance of these findings with regard to the prevention of fatty infiltraton of the liver and to the prevention of hemorrhagic kidneys resulting from a choline-deficient diet has been pointed out. The data support the hypothesis that the body is incapable of generationg methyl groups for certain methlations and that methyl groups must be supplied in the diet in a biologically labile from such as occurs in methionine and choline.