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Use of Near Infrared Transillumination / Back Scattering Sounding (NIR-T/BSS) to assess effects of elevated intracranial pressure on width of subarachnoid space and cerebrovascular pulsation in animals.

The objective was to assess changes in the width of the subarachnoid space (SAS) and amplitude of cerebrovascular pulsation (CVP) during acute elevation of intracranial pressure (ICP) using Near Infrared Transillumination/Back Scattering Sounding (NIR-T/BSS). Changes in the width of the SAS and amplitude of CVP were observed in rabbits during experimental ICP elevation induced by: (1) quick injections of saline into the subdural space of the spinal cord, and (2) distension of a surgical catheter balloon placed intracranially in the subdural space. The amplitude of CVP was also assessed during acute elevation of blood pressure in the intracranial portion of the internal carotid artery (ICA) induced by adrenaline. Each of the injections of saline caused a transient rise in the width of the SAS and amplitude of CVP. The amplitude of the increase in CVP was dependent on changes in blood pressure in the ICA (r=-0.82, P<0.01). Distension of the intracranial balloon resulted in elimination of the respiratory oscillations in the CVP and increased its systolic-diastolic amplitude. An increase in the amplitude of CVP was evoked by adrenaline without an increase in the carotid blood flow. We demonstrated that during elevation of ICP the amplitude of CVP depends on blood pressure rather than on blood flow in large cerebral vessels. Elimination of the respiratory oscillations by a minute ("sub-critical") ICP increase may be used as an early indicator of rising ICP. The direction of changes recorded using NIR-T/BSS was consistent with changes recorded using tensometric transducers.