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REGIONAL ANESTHESIA

Thermographic Temperature Measurement Compared with Pinprick and Cold Sensation in Predicting the Effectiveness of Regional Blocks

Department of Anesthesiology, Erasmus University Medical Center, Rotterdam, The Netherlands

Address correspondence and reprint requests to Eilish Galvin, MB, FCARCSI, Department of Anesthesiology, Erasmus University Medical Center Rotterdam, P.O. Box 2040, 3015 GD Rotterdam, The Netherlands. Address e-mail to eilishgalvin{at}hotmail.com.

	Abstract

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We designed this study to evaluate the usefulness of thermographic temperature measurement with an infrared camera, compared with patient response to cold and pinprick, as a means of assessing the success or failure of axillary blockades. Axillary blocks were performed on 25 patients undergoing surgery on the hand or forearm using a nerve stimulator technique with mepivacaine 1.5%. Pinprick and cold sensation were assessed on the operative site at 5-min intervals for 30 min. A thermographic image of the operative limb was recorded at similar time intervals. Thermographic images of the unblocked limb were taken before block placement and at 30 min. Temperature values at the operative site and unblocked limb were calculated from the thermographic images. Results revealed that thermography had higher combined values for sensitivity, specificity, and positive and negative predictive values than both cold and pinprick at all time intervals, with statistically significant differences at 15 min (thermography versus cold, P = 0.006; thermography versus pinprick, P = 0.026) and 30 min (thermography versus cold, P = 0.038; thermography versus pinprick, P = 0.040). For thermography as a method of block assessment, an optimal time of 15 min after mepivacaine local anesthetic injection gives the highest combined values for predicting a successful block (P = 0.004). We conclude that thermography provides an early and objective assessment of the success and failure of axillary regional blockades.

	Introduction

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Various assessment methods are used to determine the adequacy of perineural block techniques, such as the patient’s response to the sensations of cold and pinprick (1,2). A review of regional techniques by Curatolo et al. (3) stated that no standard method of applying these assessment techniques appears to exist. Moreover, sensitivity, specificity, and predictive values of such sensory tests in relation to surgical stimuli have not been investigated but may provide useful information for daily clinical practice (3).

A successful block occurs when a local anesthetic blocks both sensory and sympathetic nerve fibers (4). Sensory A () fibers are assessed using tests such as response to cold (4,5) and pinprick sensation (6). Blockade of small unmyelinated sympathetic nerve fibers with local anesthetics causes vasodilatation, an increase in blood flow and an increase in local temperature. It is not yet known whether the extent of the temperature change can be used to predict successful and failed regional blocks. Thermography is a technique whereby temperature can be accurately measured over a large area of skin surface at a specific time (7). The goal of this study was to determine whether thermography can be used as an early and reliable method of assessing the success or failure of axillary regional anesthesia blocks and to compare it with the currently used techniques of patient response to cold and pinprick.

	Methods

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We performed an observational study on 25 ASA physical status I–II adult patients, from 22- to 75-yr-old, who were scheduled for elective hand surgery under axillary plexus anesthesia. The study was approved by the institutional ethics committee and informed consent was obtained from each patient before axillary block placement. Exclusion criteria included patient refusal, sensitivity to local anesthetics, anticoagulation, and skin infection at the site of needle insertion. Patients who were using analgesic medications including opioids and nonsteroidal antiinflammatory drugs or had evidence of a peripheral neuropathy were not included. Any patient who requested sedation during block insertion was excluded.

Before insertion of the axillary block, all patients had IV access secured. Routine monitoring i.e., noninvasive arterial blood pressure, electrocardiogram, and oxygen saturation was applied. Patients were placed in a supine position with the arm to be anesthetized abducted to 90° and rested on a pillow. Blocks were performed by various anesthesiologists, all using the same technique of nerve stimulation with a 50-mm insulated needle and stimulator (Stimuplex®; B Braun, Melsungen, Germany). Once an appropriate motor response was localized to a nerve that supplied the area on which surgery was planned with a current of 0.2–0.5mA, 40 mL of mepivacaine 1.5% local anesthetic solution (AstraZeneca, Zoetermeer, Netherlands) was administered over a period of 2 min. A single injection technique was used on all patients.

Time zero (t = 0) was defined as the time corresponding to the end of the regional anesthesia procedure; i.e., the time of removal of the insulated needle from the skin. A thermographic image was taken of the operative limb at t = 0, immediately followed by an assessment of patient response to pinprick and cold sensation. Both a thermographic photo and pinprick/cold sensory tests were repeated at 5-min intervals (t = 5, t = 10, t = 15, t = 20, t = 25, t = 30) on the operative dermatome(s) until 30 min post completion of the axillary blockade (total = 7 recordings). At each time point, a thermographic image was made before pinprick and cold sensation assessment. Pinprick sensation was assessed using a 22-gauge needle and compared with the patient’s response to similar stimulation on the same dermatome of the control limb. Pinprick response was recorded on a 2-point scale (sensation or no sensation/numb). Cold sensation was assessed by applying a gel pack cooled to 4°C to the operative dermatome and was recorded on a 2-point scale (cold/not cold) as compared with the sensation felt on the opposite limb. A thermographic photo was taken of the unblocked limb, before block insertion, and at t = 30 to calculate limb temperature changes arising from factors other than the block, such as such as limb immobility and exposure to environmental temperature.

The investigator who recorded the thermographic photos was not present during the nerve localization with the nerve stimulator. Cold and pinprick sensation were assessed by the same anesthesiologist for each patient. Thermography was performed using a computer-assisted infrared thermography camera (ThermaCAM SC2000; Flir Systems, Sweden). The spectral range is 7.5 to 13 mm and the built-in digital video has 320 x 240 pixels (total 76800 pixels). Data were obtained through a high-speed (50 Hz) analysis and recording system (Thermacam Researcher 2001® HS, Berchem, Belgium) coupled with a desktop PC. Thermograms were stored on a hard disk (14-bit resolution) until the images were processed and evaluated with analytical software (Thermacam Researcher 2001 HS).

The thermographic camera produces a matrix of temperature values. These temperature values are each represented by a pixel in the thermographic image. For further analysis a frequency table is calculated. Temperature classes are created consisting of temperatures with an interval of 0.1°C. The emissive factor of the skin is 0.98, which means that the heat radiated by the skin is almost entirely dependent on the temperature of the skin itself and not on the heat reflected onto the skin from the surroundings. The analytical software uses the mean of the pixels composing the targeted area within the image to calculate the temperature. The target area was marked by applying a 1 cm circular piece of silver paper to the operative dermatome(s), which ensured that temperature measurements were made at the same site on each consecutive image. The calculations were done as follows; 1. Dermatome(s) supplying the operative site was/were selected. 2. Average temperature for the dermatome(s) was calculated at 5-min time intervals from time zero (t = 0) to 30 min later (t = 30). This resulted in 7 individual temperatures. 3. Temperature difference between each time interval was then calculated; e.g., the temperature change from t = 0 to t = 5 was calculated by t = 5 minus t = 0. 4. Results were plotted in a graph as temperature difference in the successful group against time and temperature difference in the failed group against time.

A minimum of 30 min after block placement, patients were transferred to the operating room. The operating surgeon assessed the operative site for pain sensation using a surgical forceps. If patients reported the sensation of pain at this time, the block was described as unsuccessful and either a supplemental regional block or general anesthesia was administered by the anesthesiologist responsible for each individual patient’s care. If the block was successful, surgery proceeded as usual.

The aim of this study was to obtain a significant sensitivity and specificity for thermography as an axillary block assessment method. As no useful data could be found in the literature to perform a power analysis, we conducted a pilot study. Seven patients were included in the pilot study and underwent the protocol as described in Methods. The results obtained from the pilot study were used to perform a power analysis. The failure rate for axillary blockades is reported in the literature to be from 10%–15%. The average mean temperature increase in the successful blocks at 15 min was 4.5°C ± 2.0°C. For the failed blocks the average increase in temperature at 15 min was 1.5°C ± 0.87°C. These numbers were used to perform a power analysis, using average temperature increase of dermatomes involved as the primary outcome. To obtain a power of 95% at 15 min, with = 0.05, 20 patients are required. A total of 25 patients were included in this current study.

Patient characteristics on age, height, weight, and body mass index between the successful and failed groups where compared using a Student’s t-test. Between-group comparisons for ASA distribution, male-female distribution, nerve involved, operation site involved, and operation type were analyzed with ² test for independence.

To calculate the sensitivity and specificity of the three methods tested (cold pack, pinprick, and thermography), a receiver operating characteristic curve (ROC) analysis was performed. For patient response to cold pack, the sensation of cold was assigned a value of 1 and the absence of cold sensation was assigned a value of 0. For patient response to pinprick the same method was applied, 1 = sensation and 0 = absence of sensation. Temperature was calculated according to the method described in Methods. The coordinates of the graph are defined by calculating the sensitivity and specificity at different values of the diagnostic test, so-called "cutoff points." Sensitivity is the ratio of the number of patients in whom a block assessment method predicted a successful block over the total number of patients with a surgically successful block. Specificity is the ratio of the number of patients in whom a block assessment method predicted a failed block over the number of patients with a surgically failed block. This results in a graph of the true positive rate against the false positive rate for the different possible cutoff points in a given diagnostic test. The accuracy is described using a 5-point system based on the area under the curve: excellent (area of 1–0.9), good (area of 0.9–0.8), fair (area 0.8–0.7), poor (area of 0.7–0.6), and fail (area of 0.6–0.5) (8,9). More insight into the diagnostic value of thermography is gained when the positive and negative predictive values are calculated. The positive predictive value is the proportion of patients with positive test results who are correctly diagnosed. The negative predictive value is the proportion of patients with negative test results who are correctly diagnosed. A summary is given in Table 1 explaining the mathematical equations used in the calculation of the diagnostic values.

	Results

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Twenty-five patients, whose demographics are shown in Table 2, scheduled to undergo hand or wrist surgery under axillary plexus blockade were included. There were no significant between group differences for successful and failed groups, apart from weight, which was heavier in the failed block group (P = 0.0423). The overall block success rate was 80%.

At time zero (t = 0) there was no significant difference in baseline limb temperatures in the successful and failed block groups (Fig. 1). After 5 min the temperature change in the successful group started to increase rapidly, achieving a maximum temperature change from baseline of +4.5°C after 20 min. In the failed group a slight temperature increase began after 10 min, but the maximum temperature increase was only 0.8°C at 20 min.

View larger version (23K): [in this window] [in a new window]   Figure 1. Average temperature change in involved dermatomes at 5-min intervals in successful and failed blocks.

Data on sensitivity and specificity over time for the 3 different types of block assessment methods (cold, pinprick, and thermography) are shown in Table 3 and Figure 2. Up to time 5 min, cold and thermography demonstrate similar sensitivity as a block assessment technique. However, at 10 min the sensitivity of cold sensation decreases compared with that of thermography, which achieves a sensitivity of 95% at time 15 min and maintains a sensitivity in excess of 90% until 30 min. The sensitivity of pinprick as an assessment method is initially low at 20% but by 15 min reaches a maximum of 80%.

View larger version (20K): [in this window] [in a new window]   Figure 2. A, Sensitivity of cold, pinprick and thermography methods for the assessment of successful and failed axillary plexus blocks. B, Specificity of cold, pinprick and thermography methods for the assessment of successful and failed axillary plexus blocks.

The specificity of cold and thermography block assessment methods is initially comparable, but at 10 min, thermography achieves a specificity of 100%, maintaining this level until 30 min. The specificity calculated for pinprick is inconsistent ranging from a high level of 85% at time 5 min and decreasing to 55% at time 15 min.

Positive predictive values are higher for thermography at all time intervals compared with cold and pinprick, achieving a value of 100% at 10 min, which is maintained until the end of the study period. Negative predictive values are more similar for the assessment methods; thermography achieved a negative predictive value of 98 at 10 min, increased to 99 at 15 min, and maintained these values to the end of the study period. Cold had a maximal negative predictive value of 93 at 20 min, which was less at 30 min. Similarly, pinprick, which achieved a maximal value of 99 at 25 min, decreased to 93 at 30 min (P < 0.05) (Table 3)

Statistically comparing the area under the ROC curve for each assessment method (Table 4) revealed a significant difference in the accuracy values between thermography and cold at 10 min (P = 0.035), 15 min (P = 0.006), and 30 min (P = 0.038). Difference in accuracy values between thermography and pinprick were statistically significant at 15 (P = 0.026) and 30 (P = 0.040) min. Comparison between cold and pinprick was not significant at the different time intervals.

For thermography as a method of block assessment, the ROC curve at 15 min (t = 15) gives the highest combined values of sensitivity and specificity to predict a successful block (P = 0.004). Highest combined values for cold pack and pinprick were calculated to be at 25 min; P = 0.019 and P = 0.023, respectively.

	Discussion

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In the current study we observed that thermography is an early and accurate predictor of axillary block success or failure. The occurrence of a temperature increase secondary to regional anesthesia blockade is a well recognized phenomenon and has been previously reported (10). However, the sensitivity and specificity of temperature measurement as a predictor of successful and failed blocks has not been studied.

Our findings demonstrate that a successful axillary block is associated with an increase in skin temperature in the anesthetized dermatomes. A failed block is not associated with a temperature increase and is confirmed by patients reporting pain on application of a forceps to skin, a test used routinely in clinical practice as the final check of a block before surgical incision. Statistical analysis using ROC provides a means of combining calculations of specificity, sensitivity, and positive and negative predictive values, allowing more complete analysis of a block assessment method. The highest combined values for the use of thermography as a predictor of regional block outcome were achieved 15 minutes after the local anesthetic was injected. A period of 15 minutes correlates well with the known onset time of mepivacaine, which is up to 20 minutes for peripheral nerve blockades (11).

An important advantage of thermography in the assessment of regional blockades is that it is completely objective. No patient input is required, unlike currently used methods such as pinprick and cold sensation, which are subjective and depend on the patient’s ability to interpret the stimulus applied. Also, such techniques may be influenced by patient anxiety levels and prior administration of sedating and analgesic drugs. As thermography is an objective method, sedating drugs may be used without affecting the usefulness of the technique. Thermography is likely to be a useful option in younger patients or patients with communication difficulties.

This objectivity of thermography may represent a significant advantage over the "swelling illusion" technique recently described by Paqueron et al. (12), whereby patients report a sensation of swelling in a successfully anesthetized limb. The subjective nature of the assessment and the fact that the anesthetized limb is not always perceived as uniformly swollen, with swelling being more vivid in the distal parts of the limb, limits its application (12). In contrast, thermography may be used on any part of the body as a measure of temperature change and, based on the results of this study, is an accurate predictor of block outcome.

According to our results, thermographic temperature measurement predicts a successful axillary block with a sensitivity of 95% and a positive predictive value of 100%. For unsuccessful blocks, thermography predicts with a specificity of 100% and a negative predictive value of 99%. In this current group of patients, predictive values for cold and pinprick concerning both successful and failed blocks are less than previously reported (12). The higher values reported in that study may have been influenced by the fact that 53 patients were used to calculate results on 201 nerves. This means that for the same patient both successful and failed nerve blocks were registered, which may have biased results. Also, not all dermatomes from which results were calculated were eventually operated on as an absolute confirmation of a successful nerve blockade.

For the purposes of this study, patients with neuropathies in the limb to be anesthetized were excluded. Such neuropathies may interfere with a patient’s ability to perceive pinprick and cold sensation. Such patients may also have altered sympathetic function, the extent to which this might alter vasodilatation and subsequent increase in temperature observed after successful blockade is not known. Thermography has, however, been shown to be a reliable tool in the diagnosis and assessment of patients with chronic pain, many of whom have documented neuropathies (13). Other factors which may potentially influence the temperature changes in a limb after blockade include the patient’s use of caffeine and tobacco products.

A system of color coding of the thermographic photos according to temperature allows photos to be quickly and easily interpreted (14). The higher the temperature of a dermatome, the brighter the color it displays on the thermographic photo; conversely darker areas represent lower temperatures (Fig. 3). A color to temperature scale allows accurate interpretation of the increase in temperature. No complex mathematical calculations or formulas are necessary. In clinical practice, the camera may be used in isolation without the aid of a computer. The anesthesiologist may directly visualize the anesthetized limb through the camera as required and observe the temperature changes immediately. The computer software is only required for purposes of photo storage and detailed interpretation and comparison, as in a research situation.

View larger version (71K): [in this window] [in a new window]   Figure 3. A thermographic photo taken 15 min after an axillary block was performed. The area supplied by the ulnar nerve is dark in color, representing a lower temperature than the remainder of the hand, indicating that the ulnar nerve is not anesthetized, while medial and radial nerves are successfully anesthetized. The failed ulnar nerve block was confirmed when the patient reported pain on application of a surgical forceps to the fifth digit.

Thermography may be useful in clinical anesthetic practice, where limited financial resources require maximal use of operating room time. Precise temperature measurement using thermography may allow anesthesiologists to quickly and accurately identify failed blocks. Thus, appropriate action such as supplemental block administration may be taken at an early stage, avoiding unnecessary operating room time delays. Indeed, thermography may represent a means of limiting such additional blocks to clinically appropriate situations because the administration of additional injections carries a small, but definite, risk of morbidity (15). As regards cost issues, the previously expensive purchase of infrared thermographic cameras is no longer a restrictive issue. Growing demand for such devices in both health and nonhealth care settings has led to large reductions in purchasing costs and portable, hand-held, user-friendly models may now be purchased for a fraction of previously quoted prices.

Further studies are necessary to establish the specificity, sensitivity, and predictive values of thermography in assessing other types of regional blocks and other local anesthetics. Future research on the reliability of performing local temperature measurements with a simple laser point thermometer is also warranted. Importantly, the costs associated with implementing the routine use of thermography in regional blockade assessment need to be evaluated in the contexts of operating room time and patient satisfaction.

We conclude that thermography is an early, objective, noninvasive technique with high specificity, sensitivity, positive and negative predictive values for assessing the success or failure of axillary blockades.

	Footnotes

 
Accepted for publication September 6, 2005.

Supported, in part, by the department of Anesthesiology, Erasmus University Medical Center Rotterdam, Rotterdam.

	References

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This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a colleague Similar articles in this journal Similar articles in Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Permissions Citing Articles Citing Articles via HighWire Citing Articles via Web of Science (9) Citing Articles via Google Scholar Google Scholar Articles by Galvin, E. M. Articles by Verbrugge, S. J. C. Search for Related Content PubMed PubMed Citation Articles by Galvin, E. M. Articles by Verbrugge, S. J. C.