There are several approaches to organize the research defining the minimal dose and volume of local anesthetic to perform peripheral nerve blocks. The method that Dixon suggested in 1948, and its modified version, has allowed for the definition of ED50.
Another method stated that according to the effective concentration of LA, the following concentration of anesthetic either increases or decreases by 0.1%,10 and that, therefore, the minimal effective volume of local anesthetic had not yet been defined. Alternatively, one concentration of local anesthetic may be used, and prior to the investigation, a researcher chooses several LA volumes to perform the blocks . From this, the possible ED is calculated.
Another method used to determine the minimal amount of LA for several concentrations was by sequentially reducing their volumes. The reduction in volume is defined by anesthesiologist according to the efficacy of the block with regards to the given amount of LA. The data are then listed in a table format. The necessity for further investigation is determined by different groups: the reduction in the volume; the decrease or increase in the concentration according to earlier results; and for the conveniences of everyday anesthesiology practice. This method has allowed for the definition of both the minimal effective amount of LA and the minimal effective volume of LA (while applying the proven, effective amount of LA) . This method considers the doses and volumes to be ineffective if there is failure to develop a complete block upon introduction of the whole LA volume into the target area (fascial compartment). ED50, ED80, and ED90 are not assessed. The conclusion was that an effective volume and amount of a block with LA solution should induce complete peripheral blocks. If a complete block does not develop, the amount and volume are considered to be ineffective. This method provides a good basis for recommendations as the LA amount and volume are only considered effective when they induce complete blocks of the peripheral nerve.
In the course of our research, we have stated that the minimal effective amount of lidocaine necessary for a complete block of the femoral nerve was 75 mg. Additionally, the minimal volume of local anesthetic that spreads along the whole circumference of the femoral nerve was 5 ml. Previous literature has also suggested that the minimal volume of LA solution required to spread along the whole circumference of the sciatic nerve was 5 ml . The data acquired in our research were unexpected as the sciatic nerve is larger in its diameter than the femoral nerve. This may have been accounted for by the peculiarities in the structure of the fascial planes of both nerves; by the differences in the characteristics of the surrounding tissues; and by the pressure that these tissues exert on LA in the plane of the nerve. Nonetheless, these theories warrant further investigation.
The effective amount of 75 mg of lidocaine was achieved only while applying 0.75%, 1% and 1.5% solutions. The volume acted as a limiting factor for other concentrations. Volumes of 4 ml, or less, of LA failed to spread along the whole circumference of the femoral nerve, and as a result, a complete block did not develop.
While applying a volume of 7.5 ml of the 1% lidocaine, we induced a complete block of the femoral nerve. When the volume of LA was reduced to 5 ml, the amount of lidocaine dropped to 50 mg. As had been similarly observed with the volume of 7.5 ml (56.25 mg) of the 0.75% lidocaine solution, complete motor and sensory blocks failed to develop. In both cases, the amount of 75 mg of lidocaine acted as a limiting factor. The applied volume of LA had spread along the whole circumference of the femoral nerve.
At other stages of the research (1.5%, 2%, 3%, 4%), the effectiveness of the blocks had only developed after a volume of 5 ml, or more, of the LA. Furthermore, a complete femoral block failed to develop even after the application of a volume of 4 ml, or less, of LA solution, with concentrations of 2%, 3%, and 4%, in addition to the application of lidocaine solutions that contained more than 75 mg (80, 120 and 160 mg correspondingly). The volumes of LA at 4 ml, or less, did not spread along the whole circumference of the femoral nerve. Thus, the limiting factor of the effective block was in the volume of LA, and not in its amount.
Latzke et al. [6, 15] questioned the widely accepted theory that for an effective block, a complete spread of LA along the whole circumference of the nerve was not required. Nonetheless, these authors noted that in such cases, effective blocks developed in only 50% of the scenarios.6 Furthermore, they did not assess motor blocks either .
Subsequently, data were published which suggested that incomplete spreading of LA along the whole circumference of the sciatic nerve resulted in only an incomplete sensory and motor block (even with an excessive amount of local anesthetic in the solution) . In our research, we have stated that a complete block of the femoral nerve developed only with a complete spread of LA along the whole circumference of the femoral nerve. These data support the earlier, published information [7, 15].
Moayeri et al. [14, 16] noted that the conjunctive and adipose tissue of the peripheral nerves hinders the diffusion depth of LA into the nerve and towards the axons. Additionally, the lipid layer of the epineureum serves as a deposit for LA which leads to a decrease in the amount of LA available for diffusion into the neural tissue of the peripheral nerve. The assumption was that for the same nerve (the sciatic nerve and brachial plexus), an increase in the proportion of non-neural tissue at different places resulted in an increase in the effective amount of local anesthetic [14, 16].
It was assumed that the application of small doses of LA was affected by the different thicknesses of the femoral nerve. However, in our research, statistical differences in the cross-sectional area of the femoral nerve between the groups were absent, thereby minimizing the supposed influence of the correlation between neural and non-neural tissue on the minimal effective dose of LA [14, 16, 17, 18]
With regards to the incomplete spread of LA along the whole circumference of the nerve, an increased distance between the location of LA administration, and the targeted area for diffusion in the peripheral nerve was observed. In such cases, the LA was insufficient to diffuse into all parts of the femoral nerve as “the target distance” for LA diffusion had increased (Fig. 5).
Figure 5 schematically shows the diffusion of LA during its spread along the whole circumference of the peripheral nerve, represented in (A), and its incomplete spread along the circumference of the nerve, represented in (B). On the basis of the data regarding the factors affecting the diffusion of local anesthetic published earlier [14, 16], as well as on the data indirectly supporting this literature, [17, 18] and those in current research, we assumed that in the case of (A), that a relatively even distribution of LA (c) occurs along the whole cross-section of the nerve, thereby completely blocking its action (with a sufficient amount of LA). In the case of (B) (Fig. 5), the distance for the diffusion of LA towards all sections of the nerve increases. Taking into account the barrier and lipid soluble characteristics of the conjunctive and lipid tissues of the epineurium, we assumed that the concentration, (c), of LA would be smaller in the distal nerve further along from the point of LA administration in the fascial plane. This was due to the LA failing to spread to this area (schematically referred to as “c”, with a lower frequency as indicated in Picture 5). As we have indicated in this case, effective nerve blocks do not develop, or as indicated by earlier publications, may not always develop [6, 7, 8, 15]. This warrants further investigations.