BIOEFFECTS OF POSITIVE AND NEGATIVE ACOUSTIC PRESSURES IN-VIVO

In water, the inertial collapse of a bubble is more violent after expa nsion by a negative acoustic pressure pulse than when directly compres sed by a positive pulse of equal amplitude and duration. In tissues, g as bodies may be limited in their ability to expand and, therefore, th e relatively strong effectiveness of negative pressure excursions may be tempered. To determine the relative effectiveness of positive and n egative pressure pulses in vivo, the mortality rate of Drosophila larv ae was determined as a function of exposure to microsecond length, nea rly unipolar, positive and negative pressure pulses. Air-filled trache ae in the larvae serve as biological models of small, constrained bubb les. Death from exposure to ultrasound has previously been correlated with the presence of air in the respiratory system. The degree of hemo rrhage in murine lung was also compared using positive and negative pu lses. The high sensitivity of lung to exposure to ultrasound also depe nds on its gas content. The mammalian lung is much more complex than t he respiratory system of insect larvae and, at the present time, it is not clear that acoustic cavitation is the physical mechanism for hemo rrhage. A spark from an electrohydraulic lithotripter was used to prod uce a spherically diverging positive pulse. An isolated negative pulse was generated by reflection of the lithotripter pulse from a pressure release interface. Pulse amplitudes ranging from 1 to 5 MPa were obta ined by changing the proximity of the source to the biological target. For both biological effects, the positive pulse was found to be at le ast as damaging as the negative pulse at comparable temporal peak pres sure levels. These observations may be relevant to an evaluation of th e mechanical index (MI) as an exposure parameter for tissues including lung since MI currently is defined in terms of the magnitude of the n egative pressure in the ultrasound field. (C) 1996 Acoustical Society of America.