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Biomedical subjects

R O Martin

Publications and source records attributed to R O Martin.

At least 19 recordsLinked to original sources
PubMed1986

Clinical evaluation of automatic tachycardia diagnosis by an implanted device.

Reliable discrimination between sinus tachycardia (ST) and pathologic tachycardia has been a major problem for automatic implantable antitachycardia devices. In patients whose sinus response to activity is as rapid or faster than their pathologic tachycardia (rate crossover), these unsophisticated devices deliver the programmed tachycardia response to either the pathologic or sinus tachycardia. Over a one-year period, 50 Intermedics Intertach Model 262-12 antitachycardia pulse generators were implanted to evaluate the specificity of a new group of tachycardia recognition algorithms. Patients were subjected to exercise testing and noninvasive programmed stimulation to demonstrate the efficacy of this new approach. The five recognition algorithms tested were various combinations of the following criteria: high rate (HR), sudden onset (SO), rate stability (RS), and sustained high rate (SHR). False positive rates (tachycardia response inappropriately triggered by ST) were as follows: HR (93%); HR + SO (3%); HR + RS (63%); HR + (RS or SHR) (87%); HR + HS + SO (8%). Pair-wise significance testing between HR only and HR + SO (p less than 0.001), HR + RS (p = 0.01) and HR + SO + RS (p less than 0.001), demonstrated a significant reduction in the rate of false positives through the use of the sudden onset and rate stability criteria in concert with the standard high rate criterion.

Algorithms↗
PubMed1986

Long-term performance of polyurethane pacing leads: mechanisms of design-related failures.

Environmental stress cracking has been identified as a crack propagating mechanism in polyurethane-insulated, heart pacemaker leads, which is directly related to specific lead design parameters. Lead designs imposing excessive stress on the polyurethane insulation through an interference fit between the coil and polymer have demonstrated insulation failures. Conversely, low-stress designs have shown virtually no insulation problems. The higher-stress designs have used organic solvents to facilitate coil placement during manufacturing, which may result in lowering the polymer's ability to resist the higher stress. In addition, a specific silver-containing coil wire composition has been found to galvanically corrode upon body fluid intrusion into the lead, ionizing the silver. These ions interact with the polyurethane polymer resulting in the loss of polymer strength. All polyurethane lead failures to date have been specific to high stress and/or chemical interaction. Leads using low-stress designs and nonreactive coil wire compositions continue to demonstrate a positive clinical experience.

Animals↗
PubMed1986

Comparison of the defibrillation threshold and the upper limit of ventricular vulnerability.

To examine the relationship between the defibrillation threshold and the strength of shocks that induce ventricular fibrillation during the vulnerable period, we determined the defibrillation threshold in 22 open-chest dogs using epicardial defibrillation electrodes with the cathode at the ventricular apex and the anode at the right atrium. We also determined whether there was an upper limit of shock strength that induces fibrillation in the vulnerable period by giving shocks of various energy through these same electrodes during the repolarization phase of paced rhythm. The above determinations were also made with the anode at the ventricular apex and the cathode at the right atrium in eight of the dogs and with the cathode at the ventricular apex and the anode at the left atrium in another eight of the dogs. In all dogs for all electrode configurations, there was an upper limit to the shock strength that induced ventricular fibrillation during the vulnerable period. Depending on the electrode combination, this upper limit of ventricular vulnerability either was not significantly different from or was slightly lower than the defibrillation threshold. The correlation coefficient between the two was highly significant for all three electrode configurations. These results support the hypothesis that successful defibrillation with epicardial electrodes requires a shock strength that reaches or exceeds the upper limit of ventricular vulnerability and that shocks slightly lower than the defibrillation threshold fail because they reinitiate ventricular fibrillation by stimulating portions of the myocardium during their vulnerable period.

Animals↗
PubMed1973

The uptake of gamma-aminobutyrate by organotypic cultures of chick spinal cord.

1. Explants of spinal cord from 10-day chick embryos maintained for up to 16 days in culture rapidly accumulated gamma-amino[(3)H]butyrate when incubated at 25 degrees C or 36 degrees C in a medium containing 50nm-gamma-aminobutyrate. The mechanism of the uptake process has many of the properties of an active-transport system: it is Na(+)-dependent, temperature-sensitive, inhibited by ouabain, and displays saturation kinetics. The apparent K(m) for gamma-aminobutyrate is 1.7x10(-5)m, and V(max.) is 33pmol/min per g. 2. The rate of accumulation of gamma-amino[(3)H]butyrate in cultures between the ages of 3 and 16 days was remarkably constant and was not related to the morphological maturity of the spinal-cord explants. 3. The present demonstration in spinal-cord explants of an active transport system for gamma-aminobutyrate, already established for non-cultured nervous tissue, means that nervous-tissue culture can provide a convenient model for studying uptake processes in the central nervous system.

Aminobutyrates↗
PubMed1973

Uptake of (3H)glycine and (14C)glutamate by cultures of chick spinal cord.

1. Spinal cord explants from chick embryos, grown in culture for up to 16 days, rapidly accumulated [(3)H]glycine and [(14)C]glutamate when incubated at 25 degrees C in a medium containing either 2 x 10(-10)M glycine or 4.8 x 10(-8)M glutamate.2. After 90 min incubation, a tissue/medium ratio of 60:1 and 20:1 was attained for [(14)C]glutamate and [(3)H]glycine respectively.3. The uptake systems, in addition to requiring sodium ions in the medium, were temperature sensitive, showed saturation kinetics, and were inhibited by ouabain.4. For the glutamate and glycine accumulation the K(m) value was 4.3 x 10(-5) and 4.1 x 10(-5)M respectively, indicating that a high affinity uptake process is involved.5. The rate of accumulation of both glutamate and glycine increased in cultures between the ages of 3 and 10 days thus matching their morphological development.6. In light of previous evidence, the demonstration of an active transport mechanism for both glutamate and glycine in spinal-cord-cultures that also shows a relationship with morphological maturity, suggests that these two amino acids may play a major role in spinal cord function.

Age Factors↗