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Biomedical subjects

N E Fearnot

Publications and source records attributed to N E Fearnot.

30 records · Page 2Linked to original sources
PubMed1986

Trends in pacemakers which physiologically increase rate: DDD and rate responsive.

Exercise (rate) responsive pacemakers benefit patients by providing increased cardiac output when needed and permitting lower rate during rest. This paper briefly reviews trends in reported studies on rate responsive pacemakers. For patients with reliable atrial rhythms, atrial-triggered pacemakers (DDD) provide physiological ventricular rates unless complications arise. At low rates, A-V synchrony benefits patients with refractory cardiac decompensation; however, in patients with healthy myocardiums, achieving higher pacing rates is more significant than maintaining synchrony. If atrial rhythms are unreliable, an alternative sensor for determining pacing rate is indicated. Pacemakers that respond to changes in right ventricular blood temperature, respiratory rate, QT interval, body vibration (motion), and pH have been implanted in humans. Clinical evaluations have shown that increased pacing rate leads to increased exercise tolerance and cardiac output when needed, independent of the sensor type (DDD, QT, respiratory rate, etc.). The effectiveness of any sensor type to increase pacing rate appropriately requires reliable sensors that respond specifically to the need for increased pacing rate. Sensors for stroke volume, venous oxygen saturation, right atrial or ventricular pressure and catecholamines are also under preclinical investigation. The availability of a reliable, long-term sensor is a key limitation to several techniques including pH, stroke volume, oxygen saturation, pressure, and catecholamines. Sensor technology and clinical effectiveness are the keys to rate responsive pacing.

Blood↗
PubMed1984

Increasing cardiac rate by measurement of right ventricular temperature.

A pacemaker that increases pacing rate in response to exercise enables the pacemaker patient to improve his state of well-being. Possible sensors include atrial rate, Q-T interval, pH, venous oxygen saturation, respiratory rate, cardiac output, body motion, and blood temperature. To determine the efficacy of right ventricular blood temperature as an indicator of exercise, an experimental temperature-controlled pacemaker was developed and was evaluated in dogs. A resulting algorithm was implemented in a microcomputer-based VVI pacemaker which reliably detected exercise and rest from blood temperature when totally implanted. The pacemaker generated two pacing rates to provide increased cardiac output. Data is being obtained to establish feasibility in humans.

Animals↗
PubMedDate not supplied

Electrosurgical safety of guide wires during endoscopic sphincterotomy.

UNLABELLED: A study was performed to determine whether alternate site electrosurgical burns could be caused by an inadequately insulated guide wire left in place during endoscopic papillotomy to maintain cannulation of the common bile duct. The safety of four covered guide wires and one standard guide wire was evaluated for electrosurgical safety. All four covered guide wires were coated with polyurethane or polytetrafluoroethylene (Teflon), providing insulation. Leakage currents under normal, limiting case, and fault (damaged insulation) conditions were compared to safety limits. All covered guide wires met safety limits under normal conditions, one of four covered guide wires met safety limits under limiting case conditions, and none met safety limits under fault conditions. The uncovered guide wire did not meet the safety limit under any conditions. CONCLUSION: Without a well-insulated guide wire with intact coating, our measurements indicate that leaving a guide wire in place during papillotomy may result in an electrosurgical burn.

Burns, Electric↗
PubMedDate not supplied

Accuracy and precision of computer-simulated tissue temperatures in individual human intracranial tumours treated with interstitial hyperthermia.

Accurate knowledge of tissue temperature is necessary for effective delivery of clinical hyperthermia in the treatment of malignant tumours. This report compares computer-predicted versus measured intratumoral temperatures in 11 human subjects with intracranial tumours, treated with a conceptually simple 'conductive' interstitial hyperthermia system. Interstitial hyperthermia was achieved by the use of parallel arrays of implanted, electrically heated catheters. The tissue was warmed by thermal conduction and blood convection. Simulation of intratumoral temperatures was achieved by solving a modified bioheat transfer equation on a digital computer using a finite difference method. Comparison of intratumoral temperatures from simulations and measured values differed by about +/- 0.75 degrees C. Further analysis of computed temperature distributions between catheters revealed a rapidly computable relationship between the local minimum tumour temperature and nearby catheter power and temperature that accounts for effects of varying blood flow. These findings suggest that 'on-line' prediction and control of local minimum tumour temperatures are feasible with the conductive interstitial technique.

Adult↗
PubMedDate not supplied

Development of a new technique for explantation of chronic transvenous pacemaker leads: five initial case studies.

Transvenous removal of chronic pacing leads was attempted using a special locking stylet and dilator sheaths. In five initial cases, three of seven leads were infected; six of seven leads were removed. The stylet was inserted into the conductor lumen and locked into the tip to reinforce and control the lead, permitting retraction without stretching the conductor. Sheaths were advanced over the lead to detach and dilate fibrous encapsulation. In one case, force applied exceeded the locking mechanism's strength; no patient complication resulted. It is concluded that transvenous lead removal is facilitated by a locking stylet and dilation of fibrous tissue with sheaths.

Adult↗
PubMedDate not supplied

Evaluation of clearance kinetics of a portal vascular access system.

A portal vascular access system is a totally implantable system composed of a fluid reservoir with an attached catheter. The system provides easy access for the patient who needs a continuous supply of medication or repeated blood sampling. To ensure no obstruction of flow, the system must be flushed to clear the port and catheter, making it important to establish the clearance parameters. These parameters can be obtained by use of a tetrapolar impedance cell to monitor the relative impedance change using two solutions of different impedances to fill and flush the system. The resulting impedance dilution curve allows calculation of time delay, dilution time, clearance time, and clearance volume. This method and resulting data may be used to characterize a portal vascular access system and provide a basis for comparative analysis of newly introduced systems.

Catheters, Indwelling↗
PubMedDate not supplied

The effective mass concept: theoretical and quantitative analysis of the forbidden indicator phenomenon.

This paper describes the concept of effective mass, which provides a quantitative explanation for the forbidden indicator phenomenon. The forbidden indicator phenomenon pertains to the problems encountered when the concentration of an indicator used to measure volume or flow by the indicator-dilution technique approaches the concentration of the solution to which it is to be added. This phenomenon is of particular concern when applying the saline-dilution method with hypertonic saline, to measurement of cardiac output. Clinically, one would like to give as little additional salt to a patient as possible while maintaining a signal-to-noise ratio adequate to give reliable results. This paper develops an equation that permits accurate calculation of volume or flow by the indicator-dilution method, even when indicator solutions with concentrations approaching that of the original solution are used (i.e., the forbidden indicator). The validity of this equation is demonstrated experimentally using the saline-dilution method to measure flow in a hydraulic model of the circulation.

Biomedical Engineering↗