Thermal Imaging Camera Detects Unilateral Epidural Blockade

Although epidural blockade is very effective in providing analgesia in laboring women, unilateral blockade can result in inadequate analgesia. Now, however, researchers at St. Michael’s Hospital and the University of Toronto have found that a thermal imaging camera can be used for fast, noninvasive and objective detection of temperature asymmetry in patients with unilateral epidural blockade, a step that ultimately may pave the way for adequate analgesia in these women.

Temperature Tells the Tale

Typically, the adequacy of epidural blockade is assessed by either questioning the patient or using sensorimotor testing, such as sensation to pinprick or temperature, and motor weakness. However, the level of blockade observed often differs depending on the method of assessment employed, and some of the techniques are invasive. “The problem with these assessments is that they require that the block be established before they can be performed,” Dr. Oyberman said. “That might delay identifying patients who don’t get adequate analgesia.”

Women who receive epidural blockade experience sympathectomy and lower-limb vasodilation prior to sensory block, which are typically associated with a temperature increase in the lower extremities. “One series in the literature showed that the temperature rises at about 0.6° C per minute,” she said (Anesth Analg 2006;102[4]:1247-1251).

The researchers hypothesized that a thermal imaging camera would allow for early detection of asymmetric lower-extremity temperature in women experiencing unilateral block after epidural catheter placement. “We wanted to show in this pilot study that this novel technology could be used to illustrate a relatively simple physiological phenomenon,” Dr. Oyberman said.

A total of eight patients were included in this prospective, observational cohort study; seven required an epidural top-up due to unilateral blockade (one patient had equal block and was erroneously included). Prior to the administration of the additional local anesthetic, each patient’s skin temperature was assessed over the first web space of each foot, between the first and second toes, using the FLIR i7 thermal imaging camera (FLIR Systems).

“Our patients were then questioned about which side lacked sensory block,” Dr. Oyberman reported at the 2017 annual meeting of the Canadian Anesthesiologists’ Society (abstract 285266). “Their responses were recorded along with the temperature measurements.”

The pilot study found that in all patients with unilateral block, the side of worse pain had a lower temperature (Table). Interestingly, the largest difference in temperature observed was 9.9° C and the smallest was 1.1° C.

“In every patient who had a unilateral block, the side with the sensory block invariably had a higher temperature,” Dr. Oyberman noted. “Clearly there was quite a bit of variability in terms of the temperature difference. Interestingly, the one patient who was included in error only had a temperature difference of 0.4° C.”

These preliminary results help demonstrate that thermal imaging technology may ultimately prove a valuable tool in laboring women with unilateral epidural blockade. “This was a pilot study, so future studies are needed to validate other potential uses for this technology,” she said. “Certainly we need studies that assess the sensitivity and specificity of this technology, and perhaps we’ll need to determine the minimal clinically significant temperature difference in the future as well.” Future research should also examine whether the reversibility of temperature asymmetry is associated with rectification of unilateral blockade.

Session co-moderator Anton Chau, MD, questioned whether the definition of unilateral block in the study affected its results. “Since it was patient reported, was it complete unilateral block?” Dr. Chau is a clinical assistant professor of anesthesiology, pharmacology and therapeutics at the University of British Columbia, in Vancouver. “Or was it just sacral sparing or an asymmetric block that they were experiencing?”

“If the patient reported inadequate analgesia, or if they reported that they had more pain on one side than the other, then they were approached to be included,” Dr. Oyberman replied. “Obviously there are patients that had a unilateral block but still had adequate analgesia. They simply felt that one side had a heavier block. These patients wouldn’t report having a unilateral block and therefore would not have been approached to be included.”

Session co-moderator Clarita Margarido, MD, was equally intrigued by the one patient who did not experience unilateral epidural block. “Do you have any more information on what happens with the temperature in a good, working block?” Dr. Margarido is an assistant professor of anesthesia at the University of Toronto. “What is the temperature difference in both legs when the block is working?”

“Based on human physiology, we would expect that there would be a temperature increase if we check before and after the block,” Dr. Oyberman said. “As of now, we don’t have a quantification as to how much the temperature rises. What would obviously affect that is how soon you check the temperature after you’ve given the bolus. The other part of that is the thermal camera has an error rate of 2° C, so it might not be as accurate for the first few minutes.”

Dr. Chau also saw a potential long-term benefit to the technology. “One of the questions that we always wonder about is if a patient with prolonged sensory or motor block has residual anesthetic effect or a nerve injury. I know you’re still in the early proof-of-concept stage, but I wonder what would happen if we use this technology in those patients as well.”

—Michael Vlessides