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Biomedical subjects

T Ahern

Publications and source records attributed to T Ahern.

16 recordsLinked to original sources
PubMed2000

A cardiac evoked response algorithm providing threshold tracking: a North American multicenter study. Clinical Investigators of the Microny-Regency Clinical Evaluation Study.

The purpose of this study was to evaluate a pacing system using the recognition of cardiac evoked response for the automatic adjustment of pacing output. Patients were prospectively followed after primary implantation of VVIR pacemakers using AutoCapture (St. Jude Medical CRMD). Sensing and pacing thresholds, polarization signal, evoked response, and AutoCapture performance were evaluated with serial visits and 24-hour Holter monitoring. Three hundred ninety-eight patients (mean age 71 +/- 15 years) were followed for an average duration of 1 year (3 days-1.75 years) with the algorithm functional in > 90% of patients. Backup pacing in the event of exit block was confirmed in all patients. Pacing thresholds remained stable at 0.89 +/- 0.34 V with a pulse width of 0.31 ms (with chronic output autoset at 0.3 V above the actual threshold). Evoked response exhibited a small but statistically significant increase with time (8.92 mV at implant, 9.60 mV at 12 months), however, this finding did not result in any change in AutoCapture function during our follow-up period. The polarization signal remained stable with minimal variation (1.12 mV at implant, 1.18 at 12 months). No clinical adverse events were observed using the AutoCapture algorithm. In this initial experience with the AutoCapture algorithm the evoked response and polarization measurements remained adequate, allowing the system to function in the majority of patients with safe, low output pacing. High energy backup pacing provided an added safety feature over fixed output devices in cases of unexpected threshold rises. Longer follow-up is required for continued long-term validation of the algorithm.

Aged↗
PubMed1999

Efficacy and safety of ibutilide fumarate for the conversion of atrial arrhythmias after cardiac surgery.

BACKGROUND: Atrial arrhythmias occur commonly after cardiac surgery and are a cause of significant morbidity and increased hospital costs, yet there is no well-studied treatment strategy to deal with them expeditiously. The purpose of this study was to determine the efficacy and safety of ibutilide fumarate, an approved drug for the rapid conversion of atrial fibrillation and flutter, in patients after cardiac surgery. METHODS AND RESULTS: Patients with atrial fibrillation or flutter occurring 1 to 7 days after surgery and lasting 1 hour to 3 days were randomized to receive two 10-minute blinded infusions of placebo or 0.25, 0.5, or 1.0 mg of ibutilide fumarate. Treatment was considered successful if sinus rhythm was restored for any period of time by hour 1.5. A total of 302 patients were randomized, 201 with fibrillation and 101 with flutter. Treatment with ibutilide resulted in significantly higher conversion rates than placebo, and efficacy was dose related (placebo 15%; ibutilide 0.25 mg 40%, 0.5 mg 47%, and 1.0 mg 57%). Conversion rates at all doses were higher for atrial flutter than for atrial fibrillation. Mean time to conversion decreased as the dose was increased. Polymorphic ventricular tachycardia was the most serious adverse effect and occurred in 1.8% of the ibutilide-treated patients compared with 1.2% of patients who received placebo. CONCLUSIONS: Ibutilide is a useful and safe treatment alternative for the atrial arrhythmias that occur after cardiac surgery.

Adult↗
PubMed1996

Upper limit of vulnerability is a good estimator of shock strength associated with 90% probability of successful defibrillation in humans with transvenous implantable cardioverter-defibrillators.

OBJECTIVES: The goals of this study were to determine the probability of successful defibrillation at the upper limit of vulnerability and to evaluate a minimal safety margin for implantable cardioverter-defibrillator first shocks based solely on the upper limit of vulnerability. BACKGROUND: The upper limit of vulnerability is the strength at or above which ventricular fibrillation is not induced when a stimulus is delivered during the vulnerable phase of the cardiac cycle. It has been proposed as an estimate of defibrillation efficacy because it correlates with the defibrillation threshold and can be determined with a single episode of fibrillation. METHODS: We studied 40 patients prospectively at implantation of transvenous cardioverter-defibrillators. Defibrillation threshold was defined as the weakest biphasic shock that defibrillated after 10 s of ventricular fibrillation. The upper limit of vulnerability was defined as the weakest biphasic shock that did not induce ventricular fibrillation when given at 0, 20 and 40 ms before the peak of the T wave in ventricular paced rhythm at cycle length 500 ms. After determination of the upper limit of vulnerability and defibrillation threshold, patients underwent six additional fibrillation-defibrillation episodes. The strength of five of the defibrillation shocks was equal to the upper limit of vulnerability; the strength of one of the six shocks was randomly selected to be equal to the upper limit of vulnerability plus 3 J. The implantable cardioverter-defibrillator was tested at the upper limit of vulnerability plus 3 J in 28 patients. RESULTS: The defibrillation threshold was 8.8 +/- 5.0 J (mean +/- SD), and upper limit of vulnerability was 11.3 +/- 4.6 J; the defibrillation threshold and upper limit of vulnerability were highly correlated (r = 0.89, p < 0.001). The success rate for the 200 defibrillation shocks with strength equal to the upper limit of vulnerability was 90% (95% confidence intervals based on proportion of successes in 40 patients: 86% to 94%). All five defibrillation test shocks at the upper limit of vulnerability were successful in 24 patients (60%); four of five were successful in 12 patients (30%); and three of five were successful in 4 patients (10%). All 40 test shocks and 28 implantable cardioverter-defibrillator shocks with a strength equal to the upper limit of vulnerability plus 3 J were successful. CONCLUSIONS: The upper limit of vulnerability is a good estimator of the shock strength associated with 90% probability of successful defibrillation (DFT90). A strength of 3 J above the upper limit of vulnerability is a good estimate of the minimal acute safety margin for implantable cardioverter-defibrillator first shocks.

Adult↗
PubMed1996

Comparative reproducibility of defibrillation threshold and upper limit of vulnerability.

The upper limit of vulnerability (ULV) is the strength at or above which VF is not induced when a stimulus is delivered during the vulnerable phase of the cardiac cycle. Previous studies have demonstrated a statistically significant correlation between the ULV and the defibrillation threshold (DFT) in groups of patients. However, the correlation between ULV and DFT may not be close in individual patients. This imperfect correlation may be due to physiological factors or to limitations of the measurement methods. The reproducibility of either DFT or ULV has not been studied critically. The purpose of this study was to compare the reproducibility of clinically applicable methods for determination of DFT and ULV. We prospectively studied 25 patients with a transvenous implantable cardioverter defibrillator (Medtronic 7219D) at postoperative electrophysiological study. DFT was defined as the lowest energy that defibrillated after 10 seconds of VF. The ULV was defined as the lowest energy that did not induce VF with three shocks at 0, 20, and 40 ms before the peak of the T wave in ventricular paced rhythm at a cycle length of 500 ms. Both the DFT and the ULV were determined twice for biphasic pulses using a three-step, midpoint protocol. There was no significant difference between the two determinations of DFT (10.1 +/- 5.9 J vs 10.4 +/- 5.8 J), the two determinations of ULV (13.4 +/- 6.8 J vs 13.8 +/- 6.6) or the DFT-ULV Pearson correlation coefficients for each determination (0.84, P < 0.001 vs 0.75, P < 0.001). To analyze reproducibility, Lin concordance coefficients for second determination versus first determination were constructed for both ULV and DFT. This coefficient is similar to the Pearson correlation coefficient, but measures closeness to the line of identity rather than the line of regression. The Lin concordance coefficient for ULV was higher than that for DFT (0.93, 95% CI 0.85-0.97 vs 0.64, 95% CI 0.33-0.82; P < 0.01). For paired comparison of defibrillation efficacy under different experimental conditions, the sample sizes required to detect differences of 2 J, 3 J, and 4 J (80% power, P < 0.05) were 52, 24, and 15 for DFT versus 15, 8, and 6 for ULV. We conclude that a simple, clinically applicable method for determination of ULV is more reproducible than the single point DFT. Measured correlations between the ULV and single point are limited by the reproducibility of the DFT measurement.

Defibrillators, Implantable↗
PubMed1994

Underdetection of ventricular tachycardia by algorithms to enhance specificity in a tiered-therapy cardioverter-defibrillator.

OBJECTIVES: The goal of this study was to determine the incidence and clinical significance of underdetection in 125 patients treated with a tiered-therapy cardioverter-defibrillator, the Medtronic PCD. BACKGROUND: Underdetection, distinct from undersensing, is a unique, potential complication of new algorithms that enhance specificity in tiered-therapy cardioverter-defibrillators. These algorithms may delay or prevent recognition of ventricular tachycardia even though electrograms are sensed accurately and RR intervals meet the programmed interval criterion. METHODS: Underdetection was defined as delay in detection > 5 s at electrophysiologic study or symptomatic delay or detection failure at follow-up of 15 +/- 8 months. RESULTS: We identified six specific mechanisms of underdetection caused by algorithms to discriminate sustained ventricular tachycardia from sinus tachycardia, atrial fibrillation, ventricular fibrillation and nonsustained ventricular tachycardia. Underdetection caused detection delays in 13 (1.9%) of 677 induced ventricular tachyarrhythmia episodes in 12 patients (9.6%). During follow-up, underdetection occurred in 7 (9.9%) of 71 patients in whom ventricular tachycardia therapies were programmed. Failure to detect ventricular tachycardia occurred in 6 (0.6%) of 988 spontaneous ventricular tachycardia episodes in four patients (5.6%); 2 episodes required external cardioversion. After defibrillator reprogramming, underdetection did not occur. CONCLUSIONS: Algorithms to enhance specificity cause underdetection of ventricular tachycardia in a significant minority of patients with tiered-therapy cardioverter-defibrillators. Optimal programming can minimize underdetection.

Algorithms↗
PubMed1992

Incidence and timing of activity parameter changes in activity responsive pacing systems.

The incidence and timing of rate response parameter reprogramming in activity responsive pacing systems during the year after implantation was evaluated in two groups of patients: 24 patients in whom a VVI,R system was implanted (Activitrax, Medtronic, Inc.), and 21 patients in whom a DDD,R system was implanted (Synchrony, Siemens Pacesetter, Inc.). Activity parameter changes in Activitrax patients were made based on the presence of symptoms, while in Synchrony patients, changes were based on objective data obtained using a sensor indicated rate histogram with a slow and fast walk protocol. No significant difference in the incidence of activity parameter reprogramming was noted at various time intervals during the first year in Activitrax patients; in Synchrony patients a greater incidence of reprogramming changes was noted at the 1-month follow-up visit compared to later follow-up visits (P less than 0.02). Further, the incidence of changes at 1 month was greater for Synchrony compared to Activitrax patients (P less than 0.001), while no difference was detected between groups at subsequent follow-up intervals. Use of the slow and fast walk protocol, by permitting serial evaluation of sensor response, demonstrated alterations in sensor drive rates with similar levels of activity over the initial 4 to 6 postimplant weeks. This may result from postoperative changes at the pacemaker insertion site. Based on this experience, predischarge programming may not predict long-term rate response requirements. We recommend evaluation of sensor function using an exercise protocol performed at 4 to 6 postimplant weeks in all rate responsive pacing systems that utilize a piezoelectric crystal.

Aged↗
PubMed1976

An evaluation of an assay for DNA synthesis in lymphocytes with [3H]thymidine and harvesting on to glass fibre filter discs.

Different grades of glass fibre filter paper have been compared for harvesting cells labelled with tritium using a semi-automatic harvester. Grade GF/C (Whatman Ltd) was selected for routine use although GF/B was found to be useful for whole blood cultures. The factors affecting the choice of thymidine concentration and activity are discussed and 1 mjCi or [3H]thymidine (5 Ci/mMol)/0.2 ml culture is shown to give a linear relationship with total DNA synthesis for up to 22 h.

Animals↗
PubMed1976

Stimulation of lymphocytes by antigen in microplate cultures; absence of an effect of transfer factor in vitro.

The best conditions for the optimum stimulation of human leucocytes by antigens, including protein purified derivative (PPD), streptokinase-streptodornase varidase (SKSD) and tetanus toxoid have been studied in microplate cultures. The leucocytes of each donor have their own characteristic response to antigen which depends on the culturing conditions such as antigen concentrations, cell concentrations and the time of measuring the rate of DNA synthesis. Thus, no conditions provide a universal optimum for antigen stimulation in vitro. Leucocyte dialysates, i.e. potential transfer factors, have been prepared from donors, who between them are strongly positive to the antigens PPD, SKSD, tetanus toxoid, diphtheria toxoid and Keyhole limpet haemocyanin. In contrast to some previous reports these leucocyte dialysates had no effect on the thymidine incorporation by leucocytes grown in the presence of these antigens. It is suggested that the selection of optimal conditions for the response to antigen may have obscured the effect of any non-specific enhancement of reactivity b lyeucocyte dialysates.

Adult↗
PubMed1975

The control of protein synthesis during the stimulation of lymphocytes by phytohaemagglutinin. III. Poly(U) translation and the rate of polypeptide chain elongation.

1. Cell-free systems from phytohaemagglutinin-stimulated pig lymphocytes are much more active in both endogenous protein synthesis and the translation of poly(U) than those from unstimulated lymphocytes. 2. Addition of tRNA stimulates the translation of poly(U) and greatly reduces the difference between systems from stimulated and unstimulated lymphocytes. Endogenous protein synthesis is not increased by added tRNA. 3. Systems from stimulated lymphocytes have an increased capacity to form aminoacyl-tRNA with several different amino acids. This reaction is limited by the amount of tRNA present in the cell-free system. 4. The rate of polypeptide elongation is not affected by lymphocyte stimulation. The increased rate of protein synthesis must therefore be due to an increase in the frequency of initiation of the synthesis of protein molecules.

Amino Acyl-tRNA Synthetases↗
PubMedDate not supplied

The IL-6 G-174C polymorphism may be associated with ischaemic stroke in patients without a history of hypertension.

BACKGROUND: Recent data suggest that inflammatory reactions are involved in the pathogenesis of cerebral ischaemia. AIM: To investigate whether certain inflammatory genetic polymorphisms are associated with the occurrence of ischaemic stroke. METHODS: We investigated the prevalence of six polymorphisms in cytokine genes (IL-6, TNF-alpha, TNF-beta, IL-1beta, IL-10, and IL-1Ralpha) in a group of ischaemic stroke patients (n = 105) and in a control population (n = 389). We analysed the prevalence of these polymorphisms in different stroke subtypes and in relation to outcome six months post-stroke. RESULTS: There was no significant variation in cytokine gene polymorphism frequencies between control and stroke populations or for different stroke subtypes. Subgroup analysis demonstrated that the prevalence of the IL-6 -174 CC genotype was significantly lower in stroke patients without a history of hypertension compared to controls. CONCLUSION: The IL6 -174 CC genotype may be protective against stroke in those patients who have no history of hypertension. Further studies are required to verify these findings.

Adolescent↗