Year : 2021 | Volume
: 22 | Issue : 2 | Page : 117--119
Perfusion index as a monitor to determine the success of a peripheral nerve block: Is the truth subjective or objective?
Sunita Sanghavi, Ghansham Biyani
Department of Anaesthesiology, Leicester General Hospital, University Hospital of Leicester NHS Trust, Leicester, UK
Dr. Ghansham Biyani
Department of Anaesthesiology, Leicester General Hospital, University Hospital of Leicester NHS trust, Leicester LE5 4PW
|How to cite this article:|
Sanghavi S, Biyani G. Perfusion index as a monitor to determine the success of a peripheral nerve block: Is the truth subjective or objective?.Indian Anaesth Forum 2021;22:117-119
|How to cite this URL:|
Sanghavi S, Biyani G. Perfusion index as a monitor to determine the success of a peripheral nerve block: Is the truth subjective or objective?. Indian Anaesth Forum [serial online] 2021 [cited 2022 Jul 5 ];22:117-119
Available from: http://www.theiaforum.org/text.asp?2021/22/2/117/326968
The popularity and practice of regional anesthesia has grown exponentially due to the advantages which are well known to anaesthetic fraternity. However, there remains many desires yet unfulfilled which includes finding a specific antagonist to local anaesthetic (LA), longer acting LA, an accurate technique for detecting intraneural injections, use of smartphones as display screens, and a perfect objective monitor to assess the success of a peripheral nerve block (PNB), among others. In this editorial we have discussed the last ambition in brief, focusing primarily on the advantages and limitations of perfusion index (PI) by reviewing the available literature so far.
We all understand the value of having a reliable test or monitor to test for the PNB, as early and accurate detection of successful block enables us in taking rapid corrective measures such as block supplementation or administration of general anesthesia (GA), thereby saving the operating room time and improving patient satisfaction. The success of a PNB is typically assessed using neurological examination which includes sensory and motor testing. However, there remain inherent limitations of this method as it is subjective, require patient's understanding and cooperation, time consuming, and cannot be performed on patients who are sedated or under GA.,
Any successful PNB results in the blockade of sympathetic nerve fibers and thereby the physiological changes such as vasodilation, increase in local blood flow, and skin temperature. To combat the limitations of conventional method stated above, several objective tests for the evaluation of block success have been developed utilizing this principle. Such tests include skin temperature, skin electrical resistance, laser doppler perfusion imager, tissue oxygen saturation, PI, etc.,,, Of these, PI has been studied more commonly in the last two decades as it is simple, rapid, noninvasive, easy to measure and interpret.
The PI is a numerical value for the ratio between pulsatile (arterial) and nonpulsatile (venous, capillary, and tissue) blood flow measured using a special pulse oximeter attached to a finger. PI is different from PI ratio which is the ratio of PI at given point of time to the baseline PI value. For instance, a “PI ratio at 10” is defined as the ratio of the PI value at 10 min to the PI value at baseline. Both PI and PI ratio are measured at different time intervals, usually at baseline before performing the PNB and at every 5 min interval for up to 30 min following the PNB. Two separate machines are required to measure the PI in both the blocked and unblocked arms for comparison. As expected, both the PI and PI ratio will be higher in the blocked limb at all time points compared with the unblocked limb due to the increase in pulsatile blood flow to the blocked limb. Hence, it is the relative change in these values over time which reliably predict the block outcome rather than the actual values.
There is enough data to suggest that PI ratio is superior in predicting the success of PNB than the absolute PI value. This is because baseline PI value show large individual variation varying from 0.1 to 10.0, and is affected by several internal and external factors like intravascular volume, elasticity of blood vessels, instrument used to measure the PI, and adjuvant added to the LA solution. These factors do not affect the PI ratio.,,
Few researchers specifically looked into the PI ratio values in patients who had partial or segmental block.,, Abdelhamid et al. looked for the ability of PI ratio to detect ulnar nerve sparing following supraclavicular brachial plexus block (BPB) using 25 ml of LA solution (equal volumes of bupivacaine 0.5% and lidocaine 2%). The authors compared the little finger readings to the index finger readings. PI ratio at the little finger was higher in patients with blocked ulnar nerve segments compared with their baseline value and compared with patients with segmental ulnar nerve sparing. Therefore, it is interesting to note that patients with failed block demonstrate minimal or no change in PI values, suggesting that the increase in PI value is directly related to the nerve blockade rather than the vasodilatory effect of LA on the blood vessels.
However, there is no definite cut off value reported in the literature above which the block is said to be 100% effective. Similarly, there is varying data reported in the literature on the accurate timing of measuring the PI and PI ratio after performing the PNB. Hence, it is difficult for us to analyse and draw any firm conclusion from the available research due to the difference in the type, volume, and concentration of LA used in these studies. For instance, Abdelnasser et al. performed ultrasound guided supraclavicular BPB in 77 patients undergoing orthopedic surgeries using 25 ml of LA solution (equal volumes of bupivacaine 0.5% and lidocaine 2%) and determined the PI and PI ratio 10 of 3.3 and 1.4 respectively as an indicator of a successful block. This is in comparison to a study by Kim et al. wherein the authors injected 25 ml of LA solution (equal volumes of ropivacaine 0.75% and lidocaine 2%) with or without the adjuvant (epinephrine 5 ug/ml of LA solution) and determined a PI and PI ratio 5 of 7.7 and 1.6 respectively as a relatively accurate predictor of a successful supraclavicular BPB. We noted the cut off value for PI ratio ranging anywhere between 1.3 and 1.7, and the timing between 5 and 15 min.
There is also lack of clinical trials evaluating the effects of adjuvants on PI. We found only one trial using epinephrine as an adjuvant to LA solution, and the authors did not find any effects of this drug on PI values despite its vasoconstrictive properties. Similarly, we found only one published study evaluating the successful placement of interscalene nerve catheter using the PI monitoring. The authors injected prilocaine 1% (20 mL) and ropivacaine 0.75% (10 mL) through the catheter. The primary outcome measurement was the difference in PI values between the anesthetized and nonanesthetized arm measured 5 min after the application of the LA through the interscalene catheter. The absolute values of PI were larger than baseline values in the blocked arm (P = 0.04 compared with baseline), while there was no change in the contralateral arm (P = 0.99 compared with baseline).
Moreover, many of the trials excluded patients with diabetes, high body mass index, peripheral vascular disease, preexisting neurological deficits or neuropathy, and patients taking β blockers. But in our view, these are the patients who would benefit the most from having a PNB and the underlying diseases may alter the degree of vasodilatation following PNB. Hence, we need more data looking into how the PI will be affected in these subsets of patients.
Any objective monitor must be specific, measurable, achievable, and realistic in achieving the stated goal. Though PI monitor delivers on most of these aspects with high sensitivity and specificity,, it has not become a gold standard and universal objective tool due to lack of robust clinical evidence and uniformity in the research studies. Moreover, many questions still remain unanswered including the validity of the monitor, the cut off value, exact time to measure PI value following the PNB, and effect of adjuvants on PI, among others. 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.
To conclude, in our opinion, we need to wait for a while to get a definite objective test for checking the success of a PNB and fulfil at least one of our long-awaited aspiration.
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