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REGIONAL ANESTHESIA

Peripheral Flow Index Is a Reliable and Early Indicator of Regional Block Success

From the 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, 3015 GD, Rotterdam, The Netherlands. Address e-mail to eilishgalvin{at}hotmail.com.

	Abstract

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We investigated the usefulness of peripheral flow index (PFI) measurement using a standard pulse oximetry digit probe for early prediction of successful regional blocks. Sixty-six patients scheduled for limb surgery underwent either axillary or sciatic block using a nerve stimulator technique with mepivacaine 1.5%. PFI, which is the ratio of the pulsatile versus the nonpulsatile component of the pulse oximetry signal, was recorded from 10 min before block insertion until 30 min afterwards. PFI recordings of the unblocked limb were similarly recorded. Pinprick and cold sensation were assessed at 5-min intervals until 30 min after blockade. An increase in PFI by a factor of 1.55 at 10 min after axillary block placement (P = 0.006), and 12 min after sciatic block placement (P = 0.001) was required to predict a successful block. The sensitivity and specificity of PFI was 100% for predicting axillary block outcomes at this time. Positive predictive value was 95% and negative predictive value was 93%. For sciatic blocks, sensitivity and specificity were 90% and 100%, respectively. The calculated positive predictive value at time 12 min for sciatic blocks was 94% and negative predictive value was 92%. At 15 min after block placement, cold and pinprick sensations had the same calculated values for sensitivity and specificity at 71% and 100%, respectively, for axillary blocks. For sciatic blocks, cold sensation had a sensitivity of 77% and a specificity of 100%, whereas pinprick had a sensitivity of just 20% with a specificity of 100%. We conclude that PFI provides a simple, early, and objective assessment of the success and failure of nerve blocks.

	Introduction

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The availability of a block assessment technique with a high sensitivity and specificity may increase confidence among both staff and patients in the use of regional blocks, as well as aid operating room logistics, particularly in rapid patient turnover ambulatory units.

Traditional methods of block assessment include patient response to the sensations of cold and pinprick (1). In a previous publication, we (2) highlighted both the subjective nature and variable results obtained using such block assessment methods and demonstrated that temperature measurement using infrared thermography may be an appropriate alternative.

After successful peripheral and neuraxial blockade, local vasodilation and increased local blood flow occur as a result of blockade of sympathetic nerve fibers. Laser Doppler has been used to demonstrate the effect of epidural and sympathetic blocks on local blood flow (3,4). However, the usefulness of such blood flow changes has not been specifically investigated in relation to regional block outcome. With its current widespread availability, the potential for pulse oximetry to provide clinical measurements other than simply oxygen saturation is being exploited. One such application is that of peripheral flow index (PFI), the ratio of the pulsatile to nonpulsatile component of the pulse oximetry plethysmograph and a simple and accurate indication of changes in digital blood flow (5). To measure PFI values, a standard pulse oximeter probe with relevant monitoring software, already incorporated into some monitoring systems, is required.

The goal of this study was to determine whether PFI is a reliable and objective method for assessing the success or failure of regional anesthetic blocks at an early stage and to compare it with the currently used techniques of patient response to cold and pinprick.

	METHODS

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We conducted an observational study on 63 ASA physical status I–III adult patients, aged 18–73 yr old, who were scheduled for elective upper or lower limb surgery under regional anesthesia. The study was approved according to local ethics committee guidelines and informed consent was obtained from each patient before block placement. Exclusion criteria included patients using antihypertensive medications such as - and ß-blocking drugs, diabetes mellitus, peripheral vascular disease, neuropathy and any contraindication to the use of a regional anesthesia technique, including patient refusal, known sensitivity to local anesthetics, or skin infection at the site of needle insertion.

Before block placement, all patients had IV access established. Routine monitoring (noninvasive arterial blood pressure, electrocardiogram, and peripheral oxygen saturation) was applied. Patients were placed in a position appropriate for the insertion of the respective block as follows: axillary blocks, supine position with the arm to be anesthetized abducted to 90° and rested on a pillow; proximal sciatic nerve blocks, patient lying in a lateral position with the leg to be blocked upper most and flexed at the knee; distal sciatic nerve blocks, patient lying in a prone position with feet resting off the end of the bed allowing the distal sciatic nerve to be blocked approximately 7 cm proximal to the popliteal fossa crease. The same technique was used for each block, i.e., nerve stimulation using either 50-mm or 150-mm insulated needle and stimulator (Stimuplex® B Braun, Melsungen, Germany). Once an appropriate motor response was localized to the nerve to be blocked, with a current of 0.2-0.5 mA, either 20 mL (sciatic nerve block) or 40 mL (axillary block) of mepivacaine (AstraZeneca, Zoetermeer, Netherlands) was administered. In the case of axillary blocks, only a motor response in a nerve supplying the planned surgical site was accepted. All blocks were performed using a single injection technique.

Time zero (t = 0) was defined as the time corresponding to the end of the regional anesthesia procedure, i.e., time of removal of the insulated needle. Immediately, an assessment of patient response to pinprick and cold sensation was performed. Pinprick/cold sensory tests were repeated at 5-min intervals (t = 5, t = 10, t = 15, t = 20, t = 25, t = 30) on a dermatome, supplied by the blocked nerve, until 30 min had elapsed. Pinprick sensation was assessed using a 22-gauge needle and compared with the patient’s response to stimulation on the same dermatome of the unblocked limb. 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 same area of skin and recorded on a 2-point scale (cold/not cold) as compared with the sensation felt on the opposite limb. A pulse oximetry probe connected to a monitor (Phillips Medical Systems, Eindhoven, The Netherlands) capable of measuring PFI was placed on both the blocked and control limb for a minimum of 10 min before local anesthetic injection. For axillary blocks, the probe was placed on a digit supplied by the same nerve as supplies the planned surgical site dermatome, which was the nerve stimulated prior to local anesthetic injection. The probe remained in situ for 30 min. An identical probe was placed on the same number digit of the unblocked limb. The average PFI change was recorded at 1-min intervals from 10 min before block insertion (t = 0 minus 10) to 30 min later (t = 30) resulting in 40 individual PFI recordings.

After a minimum of 30 min, patients were transferred to the operating room. The operating surgeon assessed the operative site for pain sensation using a surgical forceps. If patients reported pain at this time, the block was described as "failed" and a supplemental block or general anesthesia was administered according to the decision of the anesthesiologist responsible for each individual patient’s care. For successful blocks, surgery proceeded as usual.

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

To calculate the sensitivity and specificity of the three methods tested (cold, pinprick and PFI), a receiver operating characteristic curve (ROC) analysis was used. The ROC is a very good indicator of the discriminating power of a diagnostic method. PFI values were taken directly from the monitoring screen. For patient response to cold, 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 for sensation and 0 for absence of sensation. For PFI values, calculations for sensitivity and specificity were made at 1-min time points. We choose "cut off" values that resulted in the highest combined sensitivity and specificity. 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 an actual 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 definite 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 area under the ROC is a measure of the accuracy of the diagnostic test. The accuracy is described using a 5-point system: 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) (6,7). More insight into the diagnostic value of assessment methods 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.

	RESULTS

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Sixty-three patients, whose demographics are shown in Table 1, scheduled to undergo upper or lower limb surgery under regional anesthetic block were included. There were no significant between-group demographic differences for sciatic and axillary groups apart from age, which was younger in the sciatic nerve block group. Arterial blood pressure, heart rate, and oxygen saturations did not differ between groups. The main types of surgery performed included tendenolysis, lesion excision, plate removal, arthroscopy, and hallux valgus correction.

PFI values recorded before block placement were averaged to give a baseline PFI value for each patient. There was no significant difference between baseline PFI values in the successful and failed block groups for both sciatic and axillary nerve blocks (Figs. 1, 2). In all patients with a successful block, PFI values increased in the blocked limb, whereas no change in PFI values occurred in the unblocked limb, indicating that external factors such as room temperature did not contribute to the increase in PFI values recorded in the blocked limb. There was no recorded statistically relevant increase in PFI values compared to baseline after a 30-min period in the failed block patients. For both block types, the earliest time at which the ROC area was 0.9 in combination with sensitivities and specificities of 90% was chosen (Fig. 3). In the successful sciatic block group, PFI values started to increase as early as 4 min after the mepivacaine was injected and achieved a statistically significant increase compared to baseline at 12 min (t = 12) (P = 0.001). At this point, PFI had increased by a factor of 1.55 and the sensitivity and specificity of PFI as a predictor of block outcome were 90% and 100%, respectively. The calculated positive predictive value at time 12 min was 94% and negative predictive value was 92%. In the axillary block group, PFI values began to increase 3 min after the mepivacaine was injected and increased by a factor of 1.55 times baseline values at 10 min (P = 0.006). At that time, both sensitivity and specificity were calculated to be 100%. Positive predictive value was 95% and negative predictive value was 93%.

View larger version (40K): [in this window] [in a new window]   Figure 1. Sciatic blocks. Average change in peripheral flow index values from baseline in blocked limb at 1-min intervals in successful and failed sciatic blocks. Values are expressed as mean ± sd unless otherwise indicated. *Statistically significant change in PFI value compared with baseline (P 0.001), area 0.9, sensitivity 90%, and specificity 90%. n = 37.

View larger version (35K): [in this window] [in a new window]   Figure 3. ROC curves of PFI values. ROC curve for sciatic blocks at 12 min. Cutpoint = 1.55, area = 0.94, sensitivity = 90% and specificity = 100%. n = 37. ROC curve for axillary blocks at 10 min. Cutpoint = 1.55, area = 1, sensitivity = 100% and specificity = 100%. n = 26.

View larger version (40K): [in this window] [in a new window]   Figure 2. Axillary blocks. Average change in peripheral flow index values from baseline in blocked limb at 1-min intervals in successful and failed axillary blocks. Values are expressed as mean ± sd unless otherwise indicated. *Statistically significant change in PFI value compared with baseline (P 0.05), area 0.9, sensitivity 90%, and specificity 90%. n = 26.

For comparative purposes, calculations for cold and pinprick were made at a time of 15 min (t = 15), which was the closest time after the statistically significant findings for PFI at 12 min and 10 min for sciatic and axillary blocks, respectively. For sciatic blocks, cold sensation revealed a sensitivity of 77% and a specificity of 100%; pinprick revealed a sensitivity of 20% and a specificity of 100%. For axillary blocks, cold and pinprick sensations had the same calculated values for sensitivity and specificity at 71% and 100%, respectively.

No patient in this study who was deemed to have a successful block, assessed using a preincision forceps, subsequently reported pain during the surgical procedure.

	DISCUSSION

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In the current study, we observed that PFI is an early and reliable predictor of regional block success or failure. The occurrence of changes in local blood flow secondary to regional anesthesia is a well recognized phenomenon and has been reported (8). However, the relative increase in blood flow that occurs after a successful block and its potential use as a predictor of the success or failure of regional techniques has not been investigated.

The results of this study show that successful blocks are associated with an increase in PFI values compared with baseline, beginning as early as 3 minutes after local anesthetic injection and reaching statistical significance at a time of 12 minutes and 10 minutes for sciatic and axillary blocks, respectively. At these times, PFI values had increased by a factor of 1.55 compared with baseline pre-block values, and in this group of patients, such an increase in PFI values indicates a successful block with high sensitivity and specificity. Significantly, patients with failed blocks demonstrated minimal or no change in PFI values, suggesting that the increase in PFI values is directly related to nerve blockade rather than serum levels of local anesthetic.

PFI measurement using a standard digital pulse oximetry probe is relatively new to clinical practice. To implement PFI as a means of block assessment, a single pulse oximetry probe placed on the limb to be blocked and a monitoring system with relevant software are required. Values are presented numerically and are easy to interpret without a requirement for specifically trained personnel. Based on this study and previously reported data (5), there is large individual variation in baseline PFI values, varying from 0.1 to 10.0. With regard to early and reliable prediction of block outcome, the important factor is not the actual PFI value itself, but the relative change in PFI value over time, after local anesthetic injection.

PFI offers several advantages over currently used block assessment techniques. First, PFI is an objective means of assessing block outcome, unlike both pinprick and cold sensation techniques, which require patients to report the precise sensation felt on application of a given stimulus. Also, patients are not subjected to the potential discomfort of pinprick and ice pack testing.

A disadvantage of regional anesthesia is the incidence of failed blocks estimated to be from 10%-20% for single injection techniques (1). This study suggests that PFI may be a simple method for identifying such failed blocks early, allowing time for alternative action such as block supplementation or conversion to general anesthesia, thus avoiding potentially costly operating room time delays and low patient and/or administration satisfaction levels. PFI may also have a useful role to play in the area of regional anesthesia research, as a reliable and objective tool in comparative studies assessing the effectiveness of various block injection techniques and local anesthetics.

Important comments on this study include the fact that additives such as epinephrine and clonidine were not added to the local anesthetic solution, and therefore we have not investigated the potential effect that such drugs might have on relative increase in PFI values. Also, patients with diabetes or neuropathic injuries were specifically excluded. Such diseases may alter the degree of vasodilatation that occurs after a successful peripheral block and thus the relative increase in PFI values. It is important to note, however, that in a previous study recording baseline PFI values in volunteers, no significant difference was found between those with or without vascular disease (diabetes, hypertension) or between those who were smokers and nonsmokers (5). Finally, as the value measured is recorded using a pulse oximetry probe, the use of this technique is currently limited to blocks that supply a digit.

In conclusion, PFI is a simple, early, objective, noninvasive technique with high specificity and sensitivity for assessing the success or failure of regional blocks as compared with conventional assessment of changes in sensation.

	Footnotes

 
Accepted for publication March 1, 2006.

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

	REFERENCES

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1. Curatolo M, Petersen-Felix S, Arendt-Nielsen L. Assessment of regional analgesia in humans: a review of methods and applications. Anesthesiology 2000;93:1517–30.[Web of Science][Medline]
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3. Valley MA, Bourke DL, Hamill MO, Raja SN. Time course of sympathetic blockade during epidural anesthesia: laser Doppler flowmetry studies of regional skin perfusion. Anesth Analg 1993;76:289–94.[Abstract]
4. Sorensen J, Bengtsson M, Malmqvist EL, et al. Laser Doppler perfusion imager (LDPI)-for the assessment of skin blood flow changes following sympathetic blocks. Acta Anaesthesiol Scand 1996;40:1145–8.[Medline]
5. Lima A, Beelen P, Bakker J. Use of a peripheral perfusion index derived from the pulse oximetry signal as a noninvasive indicator of perfusion. Crit Care Med 2002;30:1210–3.[Web of Science][Medline]
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8. Butterworth J, Ririe G, Thompson RB, et al. Differential onset of median nerve block: randomized, double-blind comparison of mepivacaine and bupivacaine in healthy volunteers. Br J Anaesth 1998;86:515–21.

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