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RESEARCH PRODUCT

ESRA19-0692 Which preoperative tests before regional anaesthetic procedures?

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As we know, any regional anaesthesia technique stays under failure risks. What anaesthetists have to know about the regional block before let the surgeons start after regional anaesthesia? There are few positions which should be confirmed by tests. Firstly, detect the onset of successful sensitive, motor and sympathetic block. Secondly, confirm the correct area of the regional block. Furthermore, the anaesthetist should understand if the regional block performed under deep sedation or general anaesthesia will be effective also after the surgery. What we have to remember and what we need to check for this reason? Pain is subjective phenomenon. Are we determined as dependent on the patient‘s pseudo-negative or positive response? Can we really measure the efficiency of our regional block? the actual and full control of surgical pain relief can be done only for adequate and cooperative patient. However, for using of technical devises pre-exist very limited possibilities. Monitoring of regional anaesthetic effects seems important, especially if special sensor control is not possible or not available. Several authors use technical devices for physiological effects control after the injection of local anaesthetics. the skin conductivity, temperature, peripheral flow index, brain activity, electroencephalogram, pupil diameter etc. were used. Normally, one of neurological applications is performed for confirmation of any regional blocks nociceptive effectiveness (pinprick, cold sensation, etc.). There are numbers of neurological techniques to determine nociceptive stimulus sensation loss. Successful regional anaesthesia leads to complete block which involves not only somatosensory and motor nerve fibres but also sympathetic nerve fibres. Direct local anaesthetic’s effect in the sympathetic innervation shows as loss of the sympathetic vascular tone. as a consequence, it is resulting in vasodilation and subsequent rising of microcirculation intensity and increasing of skin blood flow. In fact, the increased blood flow is changing the skin temperature in a controlled area. It means there are few benefits from measuring skin temperature. of course, thermoregulation of body, core, and peripheral temperatures affected by local anaesthetics are building a complex of homeostatic processes with complicated two-way neuronal mechanisms. Assessment of skin temperature may indeed serve to predict success of regional anaesthesia. as known, for anaesthetists exist only few effective and useful ways to determinate the effect of regional anaesthesia. the simplest one is the direct control of skin temperatures, in a good way by thermography technique. you can use also the contact thermo sensors. Another one is the direct or indirect blood flow changes control. It is possible with any laser Doppler or photoplethysmographic (PPG) technique. Skin microcirculation changes can be detected by remote photoplethysmography imaging (rPPGi) system during regional anaesthesia. Monitoring of peripheral skin blood perfusion during the regional anaesthesia is important as an appraisal for potential clinical application of the rPPGi system. the remote photoplethysmography imaging technique is a non-contact, optical and sensitive for detecting blood volume changes in the microvascular bed of tissue. the photoplethysmography imaging technique is able to assess weak blood pulsations from patient skin that can provide information about cardiovascular parameters, such as heart rate and local perfusion changes in skin tissue. The rPPGi technique requires regular video camera and light source - visible, green or near-infrared - for registering small variations of back scattered illumination. For data control use the special processing algorithms, what transforms to parametric maps that reflect spatial distribution of skin microcirculation parameters. the principle of rPPGi is based on the light absorption in the tissue. Due to the cardiac activity, the blood volume is periodically changing in tissue which leads to modulation of back scattered light intensity. Weak back scattered intensity changes can be detected by video camera, and special video processing algorithms performing visualization of the amplitude of blood pulsations in each pixel of the skin image. the main advantages of this technique is simplicity and ability of contactless monitoring of skin blood perfusion. PPG is mainly used for remote monitoring of cardiac pulse, but it also can be used for monitoring the blood circulation parameters, such as pulsate amplitude changes. Previously our group developed prototype devices and software for skin microcirculation monitoring under different physiological tests, such as occlusion, thermal provocations, liniment application and regional anaesthesia (RA) procedures. as the blood haemoglobin absorbs the light mainly in the green spectral range, it is better to use the green light for illuminating or to use green optical filters for detecting blood absorption changes from the skin. For practical clinical applications it is a good choice to use a surgical lamp as a light source, because it is widely available in most operation theatres and can be conveniently located in the room. In our research we have developed a simple and convenient rPPGi system for contactless monitoring of the effectiveness of regional anaesthesia before the surgical procedures of the human palm. The developed system has been successfully tested in clinical environment and can be used as a tool for continuous monitoring of RA. as known, a lot of measurement techniques are limited for practical use. They are mostly oriented to research, not to patient in the operation theatre. the mapping process is very tricky too. What is the relation between imaging data and pain? the dermatomes are not matching with a thermography picture. Where is the difference? There is no doubt that skin has a huge role in thermoregulation. However, remember that the increase in temperature occurs at the expense of blood flow. of course, if we can visually control the blood flow, we can better understand this mechanism. However, during anaesthesia, the skin is an important but not the only object of interest. as regards sensitivity, we are just very superficial with the skin. We should remember muscles and bones that have not less importance in nociceptive pain. If so, we need to talk not only about dermatome, but also about myotome and osteotome. Only temperature change does not reflect the full picture. the physiologists in their recommendation ask to look back at the so-called angiosomes concept. Angiosomes concept is the one that plastic surgeons, the cardiologists may talk about and use. We hope that we will be able to look deeper into the patient‘s tissues under regional anaesthesia by using photoplethysmography.