Áp lực chỉ đủ để hấp thụ các mạch máu bên dưới kết quả trong một khối dẫn truyền thần kinh trong 15-45 phút. Ở áp suất vòng bít là 150 mm Hg, mất cảm giác và tê liệt phát triển ở mức tương tự như khi một áp lực của 300 mm Hg được sử dụng. Điều này cho thấy rằng thiếu máu cục bộ chứ không phải là áp lực cơ khí là nguyên nhân cơ bản | The Tourniquet Manual @ @ @ Pressure just sufficient to occlude the underlying blood vessels results in a block of nerve conduction in 15-45 minutes. At a cuff pressure of 150 mm Hg sensory loss and paralysis develop at the same rate as when a pressure of 300 mm Hg is used. This indicates that ischaemia rather than mechanical pressure is the underlying cause of such conduction block which is rapidly reversible and physiological. When the cuff is inflated to a higher pressure there is a risk of mechanical damage to the nerve fibres resulting in a longer-lasting conduction block - a local demyelinating block which has been called tourniquet paralysis .33 34 The underlying force seems to be the pressure gradient within the nerve between its compressed and uncompressed portions the displacements being away from the region of high pressure towards the uncompressed region beyond the edge of the cuff tourniquet. The biological basis of localised conduction blocks induced by direct pressure has been analysed extensively in a series of experimental These experiments were carried out on baboons with a tourniquet cuff pressure of about 1000 mm Hg for 90-180 minutes. When single teased fibres were examined within a few hours or days they showed a specific morphological phenomenon under each border zone of the compressed segment the nodes of Ranvier had been displaced along each fibre so that the paranodal myelin was stretched on one side of the node and invagi-nated on the other. The whole picture is strongly reminiscent of an intussusception as it occurs in the bowel. The underlying force seemed to be the pressure gradient within the nerve between its compressed and uncompressed portions. In each case the displacement was away from the region of high pressure towards the uncompressed region beyond the edge of the cuff Figure . The result was localised degenerative changes of the damaged myelin paranodal demyelination . Only large myelinated fibres were .
