Biomedical subjects

K H Joyner

Publications and source records attributed to K H Joyner.

10 recordsLinked to original sources

PubMed2000

Nonlinear cell response to strong electric fields.

The response of living cells to externally applied electric fields is of widespread interest. In particular, the intensification of electric fields across cell membranes is believed to be responsible, through membrane rupture and reversible membrane breakdown processes, for certain types of tissue damage in electrical trauma cases which cannot be attributed to Joule heating. Large elongated cells such as skeletal muscle fibres are particularly vulnerable to such damage. Previous theoretical studies of field intensification across cell membranes in such cells have assumed the membrane current to be linear in the applied field (Ohmic membrane conductivity) and were limited to sinusoidal applied fields. In this paper, we investigate a simple model of a long cylindrical cell, corresponding to nerve or skeletal muscle cells. Employing the electroquasistatic approximation, a system of coupled first-order differential equations for the membrane electric field is derived which incorporates arbitrary time dependence in the external field and nonlinear membrane response (non-Ohmic conductivity). The behaviour of this model is investigated for a variety of applied fields in both the linear and highly nonlinear regimes. We find that peak membrane fields predicted by the nonlinear model are approximately twice as intense, for low-frequency electrical trauma conditions, as those of the linear theory.

Algorithms↗

PubMed1996

The safety of digital mobile cellular telephones with minute ventilation rate adaptive pacemakers.

In vitro tests suggest that rate adaptive pacemakers using changes in transthoracic impedance to vary pacing rate may be affected by digital mobile telephones. Electromagnetic fields generated by digital mobile telephones (Global System for Mobile [GSM]) represent a potential source of electromagnetic interference (EMI) for the Telectronics META rate adaptive pacemakers, which use transthoracic impedance as a sensor to determine changes in minute ventilation. Sixteen implanted Telectronics META pulse generators were exposed to 25-W simulated GSM transmissions (900-MHz carrier pulsed at 2, 8, and 217 Hz with a pulse width of 0.6 ms) and the antenna of a 2-W digital mobile telephone (900-MHz, 217-Hz pulse, 0.6-ms pulse width). The 12 dual and four single chamber devices were programmed to maximum sensitivity and assessed in unipolar and bipolar settings and rate adaptive and nonrate adaptive modes. In all cases of EMI, testing was repeated at lower, more routinely set bipolar sensitivity levels. At maximum sensitivity, 11 of 16 devices displayed no evidence of EMI. Brief ventricular triggering occurred in 2, a brief pause in 1, a combination of both in 1, and a brief episode of pacemaker-mediated tachycardia in 1. With pulse generators programmed to more routine sensitivities, only one device displayed rare single beat ventricular triggering. No changes in minute ventilation rate adaptive pacing were observed. At maximum unipolar sensitivities, the META series of rate adaptive pacemakers are resistant to clinically important EMI from digital mobile telephones. Set at routine sensitivities, these devices perform reliably in the presence of digital mobile telephones.

Cardiography, Impedance↗

PubMed1995

Specific absorption rate levels measured in a phantom head exposed to radio frequency transmissions from analog hand-held mobile phones.

Electric fields (E-fields) induced within a phantom head from exposure to three different advanced mobile phone system (AMPS) hand-held telephones were measured using an implantable E-field probe. Measurements were taken in the eye nearest the phone and along a lateral scan through the brain from its centre to the side nearest the phone. During measurement, the phones were positioned alongside the phantom head as in typical use and were configured to transmit at maximum power (600 mW nominal). The specific absorption rate (SAR) was calculated from the in situ E-field measurements, which varied significantly between phone models and antenna configuration. The SARs induced in the eye ranged from 0.007 to 0.21 W/kg. Metal-framed spectacles enhanced SAR levels in the eye by 9-29%. In the brain, maximum levels were recorded at the measurement point closest to the phone and ranged from 0.12 to 0.83 W/kg. These SARs are below peak spatial limits recommended in the U.S. and Australian national standards [IEEE Standards Coordinating Committee 28 (1991): C95.1-1991 and Standards Australia (1990): AS2772.1-1990] and the IRPA guidelines for safe exposure to radio frequency (RF) electromagnetic fields [IRPA (1988): Health Phys 54:115-123]. Furthermore, a detailed thermal analysis of the eye indicated only a 0.022 degrees C maximum steady-state temperature rise in the eye from a uniform SAR loading of 0.21 W/kg. A more approximate thermal analysis in the brain also indicated only a small maximum temperature rise of 0.034 degrees C for a local SAR loading of 0.83 W/kg.

Absorption↗

PubMed1994

Mobile telephones interfere with medical electrical equipment.

Electromagnetic interference in medical electrical equipment has not been a serious problem in recent years even with the proliferation of analogue mobile phones and two-way handheld radios. With the introduction of GSM digital mobile phones into Australia we have conducted measurements and found that, within 2m, the electric fields from digital mobile phones can exceed the immunity level of 7 V/m recommended by the US Food and Drug Administration (FDA) for medical electrical equipment. Current analogue mobile phones were shown to produce electric fields that exceed the 7 V/m level only at relatively shorter distances. In another test, both analogue and digital mobile phones were operated close to a range of typical medical electrical equipment. It was found that existing equipment generally meets the FDA standard, but digital mobile phones caused a variety of artefacts and alarm conditions. This problem must be addressed by the medical engineering profession; in the meantime, nursing and other staff should be educated to recognise these problems and restrictions must be placed upon the use of mobile phones in hospitals.

Electromagnetic Fields↗

PubMed1992

Estimates of absorption of radiofrequency radiation by the embryo and fetus during pregnancy.

This paper reports that the specific absorption rate induced in the embryo or fetus can exceed that recommended for the general public when the mother is exposed to radiofrequency radiation at the occupational limits. This result applies to two-tiered radiofrequency radiation standards where a factor of 5 differentiates occupational and nonoccupational exposure limits. Using simple axisymmetric geometries for the pregnant worker, and assuming plane wave exposures, a finite element method provides estimates of prenatal exposure. Various layered shapes are used to model skin, fat, uterus, blood, embryonic, and fetal tissues. Applying current exposure limits given by IRPA, ANSI, and SAA, the results indicate that overexposures to the embryo or fetus can occur from early pregnancy at 80-100 MHz, and in late pregnancy across the range 300-1500 MHz.

Embryo, Mammalian↗

PubMed1991

Implanted medical devices in workers exposed to radio-frequency radiation.

This paper discusses the management of radio-frequency radiation workers who have implanted medical devices which may be adversely affected by such radiation fields. The implants include orthopedic devices, cardiac pacemakers, and cochlear implants, but exclude dental work. The effect of radio-frequency radiation on the devices may be to increase heat load and/or produce signal interference. The mechanics of interaction are outlined and protocols for managing cases are described. The implications for safety standards are discussed.

Cochlear Implants↗

PubMed1989

An automated dosimetry system for microwave and thermal exposure of biological samples in vitro.

A waveguide exposure system with automated sample temperature measurement is described. This system provides on-line determination of the temperature profile over time of biological samples in vitro. It allows automated computation of the specific absorption rate determined from heating/cooling curves, uses minimally-perturbing thermometry, is biocompatible and can be used for measurements of both microwave and conventional heating.

Electronic Data Processing↗

PubMed1989

Microwave cataract and litigation: a case study.

This paper gives details of a recent court case conducted in Australia concerning a compensation claim made by a radar technician for subcapsular posterior cataracts allegedly caused by exposure to microwave radiation. A discussion of some relevant material, including the likely invalidation of certain in vivo ocular exposures due to the use of a metallic cannula and the relevance of in vitro exposures of rat lenses, are presented. The key findings of the court are also summarized. The ruling of the court was that it was probable that the level of microwave exposure did not cause, or accelerate the development of, the cataracts or contribute to doing so. Consequently, the claim was dismissed.

Animals↗

PubMed1986

Exposure survey of civilian airport radar workers in Australia.

An exposure survey of civilian airport radar workers within Australia has been carried out over a two year period beginning late 1983. This paper describes the survey equipment, measurement procedures, radar characteristics and survey results. It was found that, unless working on open waveguide and/or components utilizing waveguide slots, or within transmitter cabinets when high voltage arcing was occurring, personnel were, in general, not exposed to levels of radiation exceeding the specified limits in the recently published Australian radiofrequency exposure standard. Recommended changes to work procedures as a result of the survey are also discussed.

Air Pollution↗

PubMed1986

Exposure survey of operators of radiofrequency dielectric heaters in Australia.

A survey of industrial radiofrequency (RF) dielectric heaters within Australia was carried out during 1982 and 1983. The heaters operated within the frequency range 11-50 MHz and had available RF powers between 0.5 and 70 kW. The electric and magnetic field strengths in the vicinity of the operators of 101 heaters were surveyed. It was found that 39% of the heaters surveyed exposed the operators to levels greater than one but less than 10 times the recommended Australian limits. A further 23% of all heaters surveyed exposed the operators to levels equal to or greater than 10 times the recommended Australian limits.

Data Collection↗