Hyperthermic teratogenicity, thermal dose and diagnostic ultrasound during pregnancy: implications of new standards on tissue heating.

Hyperthermia is a recognized teratogen in mammalian laboratory animals and is a suspected teratogen for humans. The purpose of this synopsis is to reanalyse existing data on hyperthermia-induced teratogenic effects in experimental mammalian systems in terms of a thermal dose (temperature:time) concept, and then to illustrate the utility of this concept to human situations involving potential thermal increments to post-implantation embryos and foetuses. For example, the threshold temperature elevation for hyperthermia-induced teratogenic effects in experimental mammals is estimated (but not rigorously tested) to be approximately 1.5 degrees C above core values for exposures of long duration, possibly with a thermal dose of approximately 5 min duration or more at 4 degrees C. This level of tissue temperature increment is within the capability of some modern diagnostic ultrasound (DUS) devices sold within the USA and abroad. Epidemiological studies have not indicated any hazard from the use of DUS, but such studies are limited in sensitivity and were conducted with DUS devices whose acoustic outputs were relatively low compared to those presently available. After a regulatory change that allowed for substantially increased acoustic outputs, modern DUS devices were mandated to provide the user with on-screen information (the Thermal Index, or 'TI') about ultrasound-induced temperature increments in the target tissue. The TI is generally accurate to within a factor of 2, but the factor may be as high as 6 in certain obstetric settings. Thus, informed use of and attention to the TI is strongly advised, with this admonition gaining increased emphasis if the present regulations regarding allowable acoustic outputs of DUS devices were to be further relaxed or eliminated.