Page 43 - Volume 8, Issue 4 - Winter 2012

treat resistant hypertension. Among
the modalities for denervation is
radiofrequency ablation via a catheter
inserted into the renal artery. Outward
from the lumen, the radiofrequency
energy passes through (and damages)
three concentric layers of the arterial
wall: the tunica intima (consisting of
endothelium and the internal elastic
membrane), the tunica media (consist-
ing of smooth muscle tissue), and the
tunica externa or adventitia, a connec-
tive tissue sheath containing nerve
3
fibers. Catheter based ultrasonic abla-
tion can focus the ultrasonic energy, avoiding high intensities within proxi- mal tissues, and instead ablating the
4
Over the past several decades, numerous experiments of the effects of ultrasound on nerves were per- formed in many laboratories with ex vivo preparations, including non- mammalian nerves. Some experi- ments were performed in vivo, and some clinically. Variations in firing rate, compound action potential, and temporary and permanent inhibition
1,5
response of nerve activity to ultrasonic dose is illustrated in Fig. 2. The hori- zontal dose axis intercepts the vertical activity axis at the normal activity level exhibited by a nerve before insonifica- tion. The dashed lower horizontal line represents a cessation of nerve activity. The vertical activity axis intersects the horizontal dose axis at the threshold of responsiveness. Here, nerve activity can mean enhanced firing rate, direct stimulus of firing, amplitude of the compound action potential (related to the number of fibers recruited), or entrainment to the stimulus.
As acoustic dose rises beyond the threshold, the nerve is stimulated. Nerve activity increases with dose. Eventually a peak level of stimulation is reached. Beyond that, increasing dose leads to a lower stimulus. A cross-over level is reached at which there is no apparent effect on the nerve. Beyond that, the nerve is reversibly inhibited.
renal nerve branches in the adventitia.
Nerves exhibit a spectrum of responses to varying ultrasonic dose
Fig. 1. Sensitivity of the receptive fields in fingertips to incident ultrasound. The threshold intensities required for human sensations of touch, heat, and pain in fingertips increase exponentially as a function of ultrasound fre- quency. This illustrates the wide range of just a small part of the body (the fingertips) to a wide range of just a
1
single acoustic parameter (intensity). After Gavrilov.
were observed.
A conceptual graph of an idealized
Fig. 2. A conceptual graph of an idealized response of nerve activity to ultrasonic dose. The horizontal dose axis intercepts the vertical activity axis at the normal activity level exhibited by a nerve before insonification. The dashed lower horizontal line represents a cessation of nerve activity. The vertical activity axis intersects the hori- zontal dose axis at the threshold of responsiveness. As acoustic dose rises beyond the threshold, the nerve is stim- ulated. Nerve activity increases with dose. Eventually a peak level of stimulation is reached. Beyond that, increas- ing dose leads to a lower stimulus. A cross-over level is reached at which there is no apparent effect on the nerve. Beyond that, the nerve is reversibly inhibited. At very high doses, irreversible nerve damage and, eventually, com-
plete ablation occur. The neurolytic mechanism is thermal coagulation, but the low-dose mechanisms are
unknown and are labeled here as a unified “second” mechanism. This graph is based on Vaitekunas.
6
Ultrasonic Bioeffects on Peripheral Nerves 39