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E-letters: Electronic letters published:
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Real time visualization of ultrasound-guided retrobulbar blockade: an imaging study
- Anthony P Rubin (17 January 2009)
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Real-time visualization of ultrasound-guided retrobulbar blockade: an imaging study
- Cédric Luyet, Urs Eichenberger, Christoph Tappeiner and Robert Greif (10 January 2009)
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Real-time visualization of ultrasound-guided retrobulbar blockade: an imaging study
- Steven Gayer, Professor Chandra Kumar & Howard Palte (5 December 2008)
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Anthony P Rubin
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Real time visualization of ultrasound-guided retrobulbar blockade: an imaging study I was interested in this study showing needle position and spread of solution in retrobulbar block (1). This well conducted study adds to our knowledge although the authors acknowledge that their cadaver results would have to be confirmed in humans. I am concerned about one aspect of their article and that is the suggestion that in a retrobulbar block, the aim is to place the injection near to the optic nerve. In fact the aim is quite the opposite, ie to place the needle within the retrobulbar (intraconal) space as far away from the optic nerve as possible. It is for this reason that we advocate short needles (not longer than 2.5cm) and term the technique a “shallow” retrobulbar block. Once the needle has entered the cone, it has gone far enough, and the solution will spread throughout the space. Although slightly larger volumes (3-5 ml) may be used, safety is increased by being far away from the optic nerve and avoiding the closely packed structures at the apex of the orbit. The orbital apex is relatively vascular, increasing the chance of a significant retrobulbar haemorrhage. Proximity to the optic nerve can only increase the chance of damage to the nerve and also lead to an increased incidence of intradural cuff injections with central spread of local anaesthetic. While the use of a 5 cm needle may be acceptable in a research study in cadavers it cannot be advocated in real patients. Dr Anthony Rubin Consultant Anaesthetist Wellington Eye Centre London 1. Luyet C, Eichenberger U, Moriggi B, Remonda L, Greif R. Real time visualization of ultrasound-guided retrobulbar blockade: an imaging study. Br J Anaesth 2008; 101: 855 Conflict of Interest:None declared |
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Cédric Luyet Department of Anaesthesiology and Pain Therapy, University Hospital of Bern, Switzerland, Urs Eichenberger, Christoph Tappeiner and Robert Greif
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We thank Dr. Gayer, Dr. Palte and Prof. Kumar for this important note in their e-letter about the technical side of using ultrasound for regional anaesthesia in eye surgery. We are aware of the potential risk of ultrasound waves, especially for sensitive structures like neuroretinal tissue. Our cadaver study was intended to prove the concept of a new technique without danger to patients. We investigated the ultrasound- guided correct placement of the needle and the proper spread of the applied drug. The bioeffects of ultrasound were not in the scope of our investigation. The ultrasound device used for our study on cadavers did not respect the mentioned FDA limits, especially not the limits for the Mechanical Index which was 0,4 and therefore higher than the allowed 0,23. Regardless of which energies were applied to the eyes, it did not make any difference as long as the subjects were cadavers. We agree that the use of ultrasound for eye block anaesthesia should improve the safety rather than represent a potential risk to the eye tissues. Therefore only orbital-rated transducers for in vivo sonography according to FDA recommendations should be used for clinical studies and daily routine practice. The lower output energy of these transducers does not impair the detectability of intraorbital structures or needles used for the eye block – and the method described in our study is also valuable with other small curved array transducers. Companies providing ultrasound equipment will produce suitable transducers in the near future which will be in accordance with the regulations mentioned in the e-letter. However we are sorry that we could not find a peer reviewed article describing ophthalmic ultrasound under controlled conditions or a case series documenting clinical practice. Cédric Luyet, MD., Department of Anaesthesiology and Pain Therapy, University Hospital and University of Bern, Bern, Switzerland Urs Eichenberger, MD. Department of Anaesthesiology and Pain Therapy, University Hospital and University of Bern, Bern, Switzerland Christoph Tappeiner, MD, Department of Ophthalmology, University Hospital and University of Bern, Bern, Switzerland Robert Greif, MD., MME, Department of Anaesthesiology and Pain Therapy, University Hospital and University of Bern, Bern, Switzerland Conflict of Interest:None declared |
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Steven Gayer, Chief, Anaesthesia Services Bascom Palmer Eye Institute Associate Professor of Anaesthesia, University of Miami School of Medicine, Professor Chandra Kumar & Howard Palte
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We read with interest the journal’s article by Luyet, Eichenberger, Moriggl, Remonda & Greif (1,2). The authors’ elegantly designed study using CAT scan imaging to localize ultrasound-guided contrast dye injected into cadaveric specimens confirms the utility of this tool. However, having already introduced the use of real-time bedside ophthalmic ultrasound into our clinical practice, and published and lectured on the subject internationally, we must emphasize important theoretical caveats. The bioeffects of ultrasound on the eye must be considered prior to encouraging translational application and clinical use of sonography for ophthalmic anaesthesia. Ultrasonic oscillating waves are essentially pulsed energy. They create both thermal and mechanical perturbations in tissue and can induce tissue heating and acoustic pressure. Two indices, Thermal Index (TI) and Mechanical Index (MI) are denotive of heat and mechanical agitation that may be generated by a particular ultrasonic transducer (3). Recognizing the special nature of orbital tissue, the Food and Drug Administration and Health Canada permit an MI of ≤ 1.9 for all target tissues, with the exception of the eye (≤0.23)(4,5). Additionally, they recommend equipment have a maximal achievable TI of 6.0; however, the limit for the eye is 1.0. The British Medical Ultrasound Society concurs that TI be no greater than 1.0.6 Most commercially marketed ultrasound transducers do not comply with these recommendations. Bedside ultrasound may indeed have application in improving the safety profile of ophthalmic regional anaesthetic blocks, principally perforation or penetration complications associated with needle-based techniques. Clinicians should be cognisant of differences between probes and ensure that they employ appropriate orbital-rated transducers. Steven Gayer MD, MBA Chief, Anaesthesia Services Bascom Palmer Eye Institute Associate Professor of Anaesthesia & Ophthalmology University of Miami Miller School of Medicine Miami, Florida, USA Howard Palte MBChB FCA(SA) Anesthetist, Bascom Palmer Eye Institute Assistant Professor of Anaesthesia University of Miami Miller School of Medicine Miami, Florida, USA Professor Chandra Kumar Consultant Anaesthetist The James Cook University Hospital Middlesbrough TS4 3BW, UK 1. Luyet C, Eichenberger U, Moriggl B, Remonda L, Greif R: Real-time visualization of ultrasound-guided retrobulbar blockade: an imaging study. Br J Anaesth 2008; 101:855-9 2. Gayer S, Kumar CM: Ultrasound-guided orbital blocks. In: Harmon, Frizelle, Sandhu, Colreavy, and Griffin, eds. Perioperative Diagnostic and Interventional Ultrasound. Saunders, Elsevier. Philadelphia, Pennsylvania. 2008; 123-130 3. Abbott JG: Rationale and derivation of MI and TI- A review. Ultrasound Med & Biol 1999; 25:431-41 4. Phillips R, Harris G: Information for manufacturers seeking marketing clearance of diagnostic ultrasound systems and transducers. www.fda.gov/cdrh/ode/guidance/560.pdf Document issued on September 9, 2008. Accessed on December 2, 2008. Pages 23, 31, 53, 64 5. Health Canada- Guidelines for the safe use of diagnostic ultrasound – Recommendations. www.hc-sc.gc.ca/ewh-semt/pubs/radiation/01hecs- secs255/rec-eng.php Document modified on January 1, 2008. Accessed on December 2, 2008. 6. British Medical Ultrasound Society- Guidelines for the safe use of diagnostic ultrasound equipment. www.bmus.org/ultras-safety/us- safety03.asp. Accessed on December 2, 2008. Conflict of Interest:None declared |
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