focus on Microscopy Microtechniques & Thermal Imaging Cameras Confirm Effectiveness of Local Anaesthetics Loic Premartin, FLIR Advanced Thermal Solutions, Tel: +33-1-6037-0100, Email:
research@flir.com
For many operations the use of local anaesthetics is preferred to general anaesthetics, since it is considered to be safer for the patient. To determine the effectiveness of local anaesthetics the patient is subjected to pin pricks. If the patient indicates a pain sensation then the local anaesthetic is considered to be ineffective. Not only is the pin prick method subjective, it is also useless if the patient is unable to communicate. Furthermore it takes approximately 30 minutes before the pin prick method can be used to reliably determine the success of a local anaesthetic. Researchers at the Erasmus University Medical Centre, Rotterdam, the Netherlands, have found thermal imaging camera to provide a new and objective tool to determine the effectiveness of the local anaesthetics.
Accurate assessment of the effectiveness of local anaesthetics (regional blocks) is of vital importance, according to Dr Ir. Sjoerd Niehof from the Anesthesiology Department of the Erasmus University Medical Centre: “Quick and accurate identification of failed blocks allows the anaesthesiologist to take appropriate action, such as administering additional anaesthetics, in an early stage.
“This will not only help to avoid unnecessary operation room delays, but will also help limiting the administration of additional anaesthetics to clinically appropriate situations. This is important because administering additional injections carries a small but definite risk of morbidity. In other words: the accurate assessment of regional blocks will help to save lives.”
Niehof compared several different methods including FLIR thermal imaging cameras: “Thermal imaging provides immediate feedback. Medical personnel can use a FLIR thermal imaging camera to objectively determine the effectiveness of the local anaesthetics. If the regional block is not effective it will clearly show in the thermal image.”
Thermal Imaging Solution
The researchers initially used a FLIR SC2000 Series thermal imaging camera with an uncooled microbolometer detector that produces thermal images with a resolution of 320 x 240 pixels.
“As a response to the local anaesthetics the blood vessels dilate, a phenomenon called vasodilatation,” explained Niehof. “This leads to an increased blood flow and subsequently to an increased skin temperature in the area affected. In our research we found that in case of a successful regional block the skin temperature rises with 4.5°C in about 20 minutes. In case of a non-effective block the maximum temperature difference was just 0.8°C. This difference in temperature increase can be detected and documented using a FLIR thermal imaging camera.”
Finding test subjects was relatively easy, according to Niehof: “We approached patients at the University Medical Centre that were to undergo surgery on the hand or forearm and asked them to participate. Thermal imaging is a non invasive method, so it is completely safe and it doesn’t cause any inconvenience for the patient, so it was easy to find patients willing to cooperate.”
This resulted in a test group of 25 patients who were subjected to regional anaesthetics (mepivacaine 1.5%). The effectiveness of the anaesthetics was determined using three methods: the pin prick test, the cold sensation test or the thermal imaging test. From the moment the anaesthetics where administered the effectiveness tests were executed every five minutes for a total duration of 30 minutes. The final check was made using a surgical forceps just before surgery.
High Specificity
In medical diagnostics the terms sensitivity and specificity are used to determine the reliability of a test method. In this case sensitivity indicates the ability of a test method to correctly identify a successful regional block and specificity is the ability of the test to correctly identify a failed block. In other words: sensitivity is the percentage of people that have a successful regional block (as confirmed surgically) which have also been successfully identified as such using the test method, while specificity is the percentage of people that have a surgically confirmed failed block which have been successfully assessed using the test method.
The thermal imaging method has the best sensitivity and specificity. It reaches its maximum sensitivity of 95% at 15 minutes and maintains that for a period of 10 minutes, after which it drops to 90%. The pin prick reaches its maximum of 80% during the same time period. The cold sensation method reaches its maximum of 60% five minutes later and maintains it for a shorter duration.
Figure 2. Two thermal images taken just before and 30 minutes after administering local anaesthetics. The rise in temperature indicates that the local anesthetic is effective.
Figure 1. This figure, taken with the FLIR SC2000 Series thermal imaging camera shows a cold pink, indicating that the ulnar is not anaesthetised.
The specificity of thermal imaging reaches 100% after ten minutes, maintaining this level until the end of the 30 minutes. Both cold sensation and the pin prick methods reach their maximum much later, when 25 minutes have passed. Their maximum specificity is respectively 95 and 90%.
INTERNATIONAL LABMATE - JANUARY/FEBRUARY 2012
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