© Acta Anæsthesiologica Belgica, 2008, 59, n° 3 Abstract : The practice of regional anaesthesia will be probably forever changed by the introduction of ultra- sonography into everyday clinical practice. The ability to now visualise directly the spread of local anaesthetic solution and its relationship with the nerve allows for immediate adjustments to needle position and/or local anaesthetic volume and spread resulting theoratically in improved block performance through faster onset, reduced local anaesthetic volumes and higher success rates. However, whether US guided blocks will ever replace neurostimulation techniques is debatable espe- cially when regional anaesthesia is performed by special- ists in the field. Key words : Ultrasound guidance ; peripheral nerve blocks ; epidural blocks. In recent years, the practice of regional anaes- thesia has been forever changed by the introduction of ultrasonography into everyday practice. The abi- lity of ultrasound (US) to visualise nerves as well as structures (vessels, pleura) to be avoided, has resul- ted in increased confidence for anaesthesiologists performing regional anaesthetic techniques. US guided nerve blocks provide real-time imaging of needle position and facilitate nerve location even when anatomical variations from traditional land- marks are present. The techniques traditionally used for regional anesthesia procedure guidance inclu- ded either subjective perceptions of arterial pulses or fascial “pops” or more objectives aims as trans arterial approaches or nerve stimulation. The ability to now visualise directly the spread of local anaes- thetic solution and its relationship with the nerve allows for immediate adjustments to needle posi- tion and/or local anaesthetic volume and spread resulting theoratically in improved block perfor- mance through faster onset, reduced local anaesthe- tic volumes and higher success rates (1, 2, 3). The recent literature reflects the exponential evolution of ultrasound guided regional blocks from descrip- tive studies demonstrating its feasability to prospec- tive RCT comparing ultrasounds to other classical techniques of guidance. Regional anaesthetic tech- niques such as the supra-clavicular block, or sub- gluteal sciatic nerve approach that were considered high risk or challenging, may now be undertaken with the knowledge that needle position relative to other structures is known. HOW : ULTRASOUND GUIDANCE – THE BASICS Ultrasonographic waves can be generated by, and converted back into electrical energy by applying an alternating current to a material with piezoelectric properties, that is to say, the ability to vibrate depending on a specific frequency and to transform the energy generated into US waves from electrical current or vice versa. The physical cha- racteristics of sound waves are described below : Wavelength : the distance travelled in one cycle in the direction of the energy propagated (units = mm), Period : the time duration of one cycle (units = seconds), Frequency : the number of cycles per second (units = Hz), Amplitude : the square root of the wave energy, Velocity : the distance travelled by the wave per unit time (units = m/s). Each medium has a unique acoustic impedance. Where two mediums interface, the difference in the acoustic impedance between both results in some waves being reflected. The greater the degree of reflection, the higher the intensity of the sound signal and this is represented a brighter ultrasonographic image also known as greater echogenicity. Hyperchoic structures appear white on screen, hypoechoic structures are dark or black. Nerves, tendons and fascia are hyperechoic, although neural structures (Acta Anaesth. Belg., 2008, 59, 147-154) How and why to use ultrasound for regional blockade X. CAPDEVILA (*), PH. BIBOULET (*), D. MORAU (*), S. MANNION (**) and O. CHOQUET (*) Xavier CAPDEVILA, M.D, Ph.D. ; Philippe BIBOULET, M.D. ; Didier MORAU, M.D., MSc. ; Stephen MANNION, M.B., M.R.C.P.I., F.C.A.R.C.S.I., M.D. ; Olivier CHOQUET, M.D. (*) Department of Anaesthesia and ICM, University Montpellier 1 and University Hospital of Montpellier, Montpellier, France. (**) Department of Anaesthesia and Intensive Care, Cork University Hospital & University College, Cork, Ireland. Correspondence address : Xavier Capdevila, Head of Department, Department of Anesthesiology and Critical Care Medicine, Lapeyronie University Hospital, Avenue du Doyen G Giraud, Montpellier, France. E-mail : x-capdevila@chu-montpellier.f