a b s t r a c t

Introduction: Finger injuries are commonly attended to in the emergency department, and digital nerve block is a frequently performed procedure for such injuries. This study compared the efficacy levels of the subcutaneous method according to the different injection sites.

Method: This was a simulation study for medical students who rendered medical service at the emer- gency department. One group performed subcutaneous injection of lidocaine at the volar side of the metacarpophalangeal (MCP) joint, while another group injected at the volar side of the proximal inter- phalangeal (PIP) joint. The time to anesthesia was measured at 30-s intervals. Pain at the injection site was measured using the Numeric rating scale , while the length from the fingertip to the injection site and the circumference of the injection site were measured.

Results: A total of 82 participants were included, with 41 under the MCP joint group and the rest under the PIP joint group. The mean length from the fingertip to the needling point was 3.62 +- 0.63 cm in the PIP joint group and 5.90 +- 0.65 cm in the MCP joint group, while the mean circumference of the needling point was 4.93 +- 0.51 and 5.61 +- 0.58 cm, and the mean time to anesthesia was 2.55 +- 1.11 and

3.79 +- 1.28 min (p-value < 0.001), respectively. The median value of NRS was 4 in both groups (p- value = 0.921). Length was correlated with the time to anesthesia (p-value = 0.018).

Conclusion: Injection into the PIP joint showed the same anesthetic effect as injection into the MCP joint, but this effect occurred faster in the former.

(C) 2019

Introduction

Finger injuries are commonly attended to in the emergency department. In the United States, 4.8 million patients visit the emergency department each year with hand and finger injuries [1]. Digital nerve block is a frequently performed procedure for the diagnosis and treatment of finger injuries [2]. Pain control is important in finger injuries and in emergency department cases in general. [3]. Digital nerve block is a simple and excellent method of pain control in finger injury patients [4]. When pain is controlled through digital nerve block, exploration, wound closure, irrigation, brushing, etc. become easy [5].

The most commonly used traditional method for digital nerve block is the two-injection digital nerve block method designed

* Corresponding authors at: Department of Emergency Medicine, Soonchunhyang University Bucheon Hospital, 170 Jomaru-ro, Bucheon 14584, Republic of Korea.

E-mail addresses: [email protected] (G.W. Kim), brayden0819@daum. net (S. Han).

¹ Gi Woon Kim and Sangsoo Han contributed equally to this work.

by Harris and Braun [6]. This method, however, makes the patient feel more pain because injection is done twice, making it a more invasive technique. Thus, single-injection nerve block methods were devised to take the place of the two-side injection nerve block method [7]. These methods include the transthecal digital nerve block method devised by Chui [8] and the subcutaneous digital nerve block method devised by Harbison [9]. These methods have the advantages of being much simpler and causing less pain since only a single injection is done, but they have the same efficacy level as the traditional two-injection nerve block method [10]. The sub- cutaneous method is easier to carry out than the transthecal method, and there are no significant differences in the time to anesthesia and anesthesia duration [2,11,12].

A study on digital nerve block was also performed using subcu- taneous digital nerve block. To find a more efficient method than the existing subcutaneous digital nerve block method involving injection into the metacarpophalangeal (MCP) joint crease, it was thought that injection into the proximal interphalangeal (PIP) joint crease would make the anesthesia effect occur faster due to the shorter nerve length [13]. Therefore, this study was conducted to

https://doi.org/10.1016/j.ajem.2019.04.031

0735-6757/(C) 2019

compare the anesthetization success rates and times to fingertip anesthesia, the anesthesia durations, and the Numeric Rating Scale values according to the different injection sites.

Method

This was a simulation study for medical students who had prac- ticed at the emergency department of a Tertiary university hospital in Gyeonggi-Do, South Korea. The authors have been teaching effective pain control for 3 years to medical students enrolled in the practice curriculum. It is very important for the students to safely but effectively experience the patient’s fear of and pain from the injection. The aforementioned method was decided to be used to maximize the effect of a small amount of lidocaine, and no side effects occurred in the previous 3 years. All the enrolled partici- pants were students who had been practicing for 8 months, from March to October 2018. The exclusion criteria were previous adverse reactions to lidocaine, needle phobia, injection refusal, and moving the hand before a sufficient volume is injected.

A total of 87 participants were randomly assigned a number using a random number generator from Microsoft Excel 2010, and were divided into two groups: an even-numbered group and an odd-numbered group. One emergency medical specialist per- formed all the nerve blocks. One group performed subcutaneous injection of 2% lidocaine at the MCP joint crease site of the volar side of the little finger, and another group performed subcutaneous injection of 2% lidocaine 2 cc at the PIP joint crease site of the volar side of the little finger [14]. The participants’ hands were made to assume the supine position, and their fingers were disinfected with povidone iodine solution. The injections were made in the middle parts of the MCP joint crease of the volar surface and the PIP joint crease. The needles were inserted to about 3-4 mm, and all the injections were made in the subcutaneous layer, not too shallowly or deeply (Fig. 1). The study was conducted in accordance with the provisions of the Declaration of Helsinki [15].

Success of anesthetization was defined as the absence of pain as assessed through a pinprick test, and anesthesia duration was defined as the time until pain is felt in a pinprick test after anesthe- sia was administered [16]. Both the time to anesthesia and the anesthesia duration were measured at 30-s intervals. The degree of pain at injection was measured on the NRS scale according to the injection site. The length from the fingertip to the injection site

and the circumference of the injection site were measured, and the correlation between these values and the time to anesthesia was examined. Fear ratings on the fear scale (0–10) before and after the injection in the digital nerve block were compared [17,18].

The MCP and PIP joint group data were compared using the t-test and Mann-Whitney U test. The dependent variables were analyzed through multiple linear regression. Fear ratings on the fear scale were analyzed using Wilcoxon signed rank. All the collected data were statistically analyzed using Statistical Package for the Social Sciences (SPSS) Version 25.0.

Results

A total of 87 participants were enrolled in this study. Two par- ticipants rejected the injection, and 1 participant was excluded from the study due to lidocaine allergy. Nerve block was performed in a total of 84 participants who were all successfully anesthetized, with injection into the PIP joint crease in 41 participants and into the MCP joint crease in 43 participants. Two participants moved their hands because of pain when lidocaine was injected, so these two participants were excluded from the study. Finally, 82 partic- ipants were successfully anesthetized: 49 males and 33 females (Fig. 2).

The length from the fingertip to the needling point was

3.62 +- 0.63 cm in the PIP joint crease group and 5.90 +- 0.65 cm in the MCP joint crease group. The circumference of the needling point was 4.93 +- 0.51 cm in the PIP joint crease group and

5.61 +- 0.58 cm in the MCP joint crease group. The mean time to anesthesia was 2.55 +- 1.11 min in the former and 3.79 +- 1.28 min in the latter, and the anesthesia duration time was

155.56 +- 77.98 and 166.68 +- 80.43 min (p-value < 0.001). Side effects were seen in 2 participants (both presyncope, which improved soon) (Table 1).

The relationships among the length from the fingertip to the needling point, the circumference of the needling point, and the time to anesthesia were analyzed using multiple linear regression. The length from the fingertip to the needling point was statistically correlated with the 0.321 b value (p-value = 0.018) (Table 2).

Fear ratings on the fear scale (0-10) before and after the proce- dure were measured. In the PIP joint group, it decreased from 4 to 3 points. In the MCP joint group, it decreased from 6 to 3 points. Overall, it decreased from 5 to 3 points (p-value < 0.00) (Table 3).

Fig. 1. (A) Injection at the middle of metacarpophalangeal joint. (B) Injection at the middle of proximal interphalangeal joint.

Fig. 2. Study participant flow.

Table 1

Comparison of two techniques of digital block.

method is subcutaneous volar nerve block, which is convenient and useful. It can be applied to pediatric patients who are scared

of injections [19]. Many studies have shown, however, that the

subcutaneous method has the disadvantage of not fully anesthetiz- ing the dorsal nerve in the middle to proximal phalanx [9,11]. Clin- ically, anesthetization of the distal phalanx is more important than that of the dorsal part of the middle to proximal phalanx. This is why among upper-limb injury patients, fingertip injury is the most common. This is because the hand is mostly used when doing work or other activities, and the fingertip is the part of the hand that directly touches the surrounding environment [1,14]. In this study, subcutaneous injection was performed, and anesthesia was suc- cessfully administered to all the study subjects.

	PIP joint (n = 41)	MCP joint (n = 41)	p Value
Age, years	23.41 +- 1.02	23.59 +- 1.14	0.478
Male, n (%)	25 (61.0)	24 (58.5)	0.822
Length, cm	3.62 +- 0.63	5.90 +- 0.65	<0.001
Circumstance, cm	4.93 +- 0.51	5.61 +- 0.58	<0.001
Time to anesthesia, min	2.55 +- 1.11	3.79 +- 1.28	<0.001
Duration of anesthesia,	155.56 +- 77.98	166.68 +- 80.43	0.527
min Pain scale, NRS	4 (3-6)	4 (3-5)	0.921
Side effect, n (%)	1 (2.4)	1 (2.4)	>0.999

NRS, Numeric Rating Scale; PIP, proximal interphalangeal; MCP, metacarpopha- langeal; the level of significance at p < 0.005.

Table 2

Multiple Linear regression analysis using time to anesthesia as the dependent variable.

95% CI

Variables b value Lower Upper p Value Length, cm 0.321 0.057 0.584 0.018

Circumstance, cm 0.030 –0.512 0.572 0.912

Dependent variable, time to anesthesia; Adjusted R² = 0.080; the level of signifi- cance at p < 0.005; b, regression coefficient; CI, confidence interval.

Table 3

Fear scale between pre and post-procedure.

Prior to procedure After procedure p value PIP joint 4 (3-5.5) 3 (2-4) <0.001

MCP joint 6 (4-7) 3 (2-5) <0.001

All patients 5 (3-6) 3 (2-4) <0.001

PIP, proximal interphalangeal; MCP, metacarpophalangeal; Wilcoxon signed rank analysis was done; the level of significance at p < 0.005.

Discussion

Digital nerve block is an anesthetic method commonly used in finger injuries. The two-injection digital nerve block method is tra- ditionally used for such injuries, but it has the disadvantage of needing double injections. Consequently, other nerve block meth- ods were devised, all involving a single injection [7]. It has been reported, however, that the transthecal method, a single- injection method, causes the patient discomfort when the needle passes through the tendon sheath [13]. Another single-injection

The anesthetic mechanism of lidocaine is to block the neuron conduction. Lidocaine acts on the sodium ion channel protein pre- sent in the Ranvier node, inhibiting sodium ion from entering the cell. This prevents the nerve from depolarizing and also blocks con- duction, thus preventing the patient from feeling any pain [20]. Generally, as the length of the Ranvier node or as the number of such nodes increases, the conduction velocity decreases [13]. In this study, there was a statistically significant difference in the time to anesthesia between the group where the injection was administered to the PIP joint and the group where it was adminis- tered to the MCP joint. Multiple linear regression analysis was per- formed on the circumference and length of the injection site to identify factors that affected these. As a result, the length of the fin- ger was found to be an influential factor. It is thought that the thickness of the finger and that of the nerve are not significantly related to each other; otherwise, the increase in the length of the finger increases the length of the nerve. The longer the nerve is, the greater the number of Ranvier nodes involved, which may have affected the time to anesthesia.

The lidocaine side effects that may occur after injection in local anesthetization include neurotoxicity, anaphylaxis, and vasovagal reaction [21-23]. In this study, side effects were observed in 2 patients, both of which improved after presyncope. In addition, lidocaine was used in the PIP joint at a dose of 2 cc less than that used in the MCP joint. Although there was no significant difference between the two doses, it can be expected that the side effects (e.g., neurotoxicity) that may occur with lidocaine are somewhat less likely.

Fear of needling can be reduced by directly experiencing and participating in the procedure, or handling medical instruments [21]. In this study, the fear ratings based on the fear scale before and after injection were measured. The participants’ fear of need- ling was scored from 0 (mild) to 10 (severe), with a score of 5 or higher defined as indicating a severe fear of needling [17,18]. Before injection, the middle-phalanx group showed a fear rating

of 4, and the proximal-phalanx group, 6. The fear ratings of both groups, however, were reduced to 3 after injection. This can be an important educational experience for medical students who will be able to directly follow the patient and perform the procedure.

This study has several limitations. First, it was a simulation study for students, so different results can be derived from the actual clinical setting. The participants underwent nerve block under an unaffected condition, with no digit injury. Nerve block actually performed in hospitals may have different outcomes in patients with impaired laceration, a crushing injury, or a nail injury. Second, this study had relatively few participants, and sev- eral large-scale studies are needed. In the posthoc power analysis that was performed to confirm the sample sizes, however, the sam- ple size that was used in this study was found to be >99% appropri- ate and sufficient to make the results of the study valid. To the best of the authors’ knowledge, this is the first study about the differ- ence in the efficacy level of the subcutaneous digital nerve block method according to the injection location. Further clinical studies will be needed to overcome this study’s limitations.

Conclusion

Injection into the PIP joint showed the same anesthetic effect but faster anesthetization than injection into the MCP joint. There- fore, for anesthetization of a fingertip injury, it is necessary to con- sider injection into the PIP joint instead of into the MCP joint.

Acknowledgment

This work was supported by the Soonchunhyang University Research Fund.

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