Thermal thresholds for teratogenicity, reproduction, and development.
ABSTRACT The human embryo and foetus may be especially vulnerable to chemical and physical insults during defined stages of development. In particular, the scheduled processes of cell proliferation, cell migration, cell differentiation, and apoptosis that occur at different times for different organ structures can be susceptible to elevated temperatures. With limited ability to regulate temperature on its own, the developing embryo and foetus is entirely dependent upon the mother's thermoregulatory capacity. As a general rule, maternal core body temperature increases of ∼2°C above normal for extended periods of time, 2-2.5°C above normal for 0.5-1 h, or ≥4°C above normal for 15 min have resulted in developmental abnormalities in animal models. Significant differences in thermoregulation and thermoneutral ambient temperatures make direct extrapolation of animal data to humans challenging, and the above temperatures may or may not be reasonable threshold predictions for adverse developmental effects in humans. Corresponding specific absorption rate (SAR) values that would be necessary to cause such temperature elevations in a healthy adult female would be in the range of ≥15 W/kg (whole body average or WBA), with ∼4 W/kg required to increase core temperature 1°C. However, smaller levels of thermal stress in the mother that are asymptomatic might theoretically result in increased shunting of blood volume to the periphery as a heat dissipation mechanism. This could conceivably result in altered placental and umbilical blood perfusion and reduce heat exchange with the foetus. It is difficult to predict the magnitude and threshold for such an effect, as many factors are involved in the thermoregulatory response. However, a very conservative estimate of 1.5 W/kg WBA (1/10th the threshold to protect against measurable temperature increases) would seem sufficient to protect against any significant reduction in blood flow to the embryo or foetus in the pregnant mother. This is more than three times above the current WBA limit for occupational exposure (0.4 W/kg) as outlined in both IEEE C95.1-2005 and ICNIRP-1998 international safety standards for radiofrequency (RF) exposures. With regard to local RF exposure directly to the embryo or foetus, significant absorption by the mother as well as heat dissipation due to conductive and convective exchange would offer significant protection. However, a theoretical 1-W/kg exposure averaged over the entire 28-day embryo, or averaged over a 1-g volume in the foetus, should not elevate temperature more than 0.2°C. Because of safety standards, exposures to the foetus this great would not be attainable with the usual RF sources. Foetal exposures to ultrasound are limited by the US Food and Drug Administration (FDA) to a maximum spatial peak temporal average intensity of 720 mW/cm(2). Routine ultrasound scanning typically occurs at lower values and temperature elevations are negligible. However, some higher power Doppler ultrasound devices under some conditions are capable of raising foetal temperature several degrees and their use in examinations of the foetus should be minimised.
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ABSTRACT: A transcutaneous ultrasonic Doppler technique for measurement of aortic blood-flow velocities has been developed and compared to more established techniques in order to evaluate its potential usefulness. It is possible by this method to quantitate blood velocity in both the ascending aorta and the aortic arch with ease and reliability. Ultrasonic access to the aorta from the suprasternal notch proved adequate in more than 90 per cent of the normal subjects examined. If further clinical trials prove as encouraging, this technique may be of significant value for patient monitoring and cardiac diagnosis.American Heart Journal 06/1975; 89(5):605-12. · 4.50 Impact Factor
- The Lancet 10/1978; 2(8090):634. · 39.06 Impact Factor
Article: Fever versus hyperthermia.[show abstract] [hide abstract]
ABSTRACT: A variety of conditions that result in the elevation of body temperature are described and discussed. These hyperthermias are divided into four categories; fever, exercise hyperthermia, hyperthermias due to inadequate means of heat dissipation, and hyperthermias resulting from pathological or pharmacological impairments of thermoregulatory mechanisms. A comparison of the physical and physiological characteristics of these hyperthermias is presented and distinctions are drawn on the basis of these characteristics. Fever is shown to differ markedly from all other forms of hyperthermias. Specifically, the elevation in body temperature encountered during fever is a regulated rise that is defended by fully functional thermoregulatory mechanisms; the thermopreferendum is also elevated in fever, particularly at the onset or "chill phase"; and aspirin-like drugs can intervene in febrile hyperthermia and will return body temperature to its normal level. No other forms of hyperthermia possess these characteristics and thus only fever can be attributed to an upward displacement of the "set point" for body temperature regulation. Furthermore, in attempting to control rises in body temperature, it is apparent that aspirin is effective only in fever, while whole-body cooling is all but ineffective. In all other forms of hyperthermia, whole-body cooling is the only effective treatment.Federation proceedings 02/1979; 38(1):39-43.