Novel ultrasound-guided lateral approach for femoral nerve block in cats: a pilot study

Abstract

Objectives

The aim of this study was to describe the technique for a novel lateral approach to femoral nerve block in cat cadavers.

Methods

Five feline cadavers were used in our prospective cadaveric experimental study. Ultrasonography was used to identify the anatomy injection site. Both pelvic limbs were injected with a volume (0.1 ml/kg) of methylene blue in each cat. The cadavers were placed in lateral recumbency, with the limb to be blocked positioned uppermost. The transducer was held perpendicular to the most proximal area of the pelvic limb, ventral to the greater trochanter and slightly cranial to the femur. Methylene blue was injected after the localisation of the femoral nerve. Immediately after the injection, the cat was turned and the technique was repeated on the contralateral side. After performing the ‘block’ in both limbs, the area was dissected bilaterally and the success was evaluated. A positive femoral nerve staining was considered for a coverage of ⩾1 cm.

Results

A total of 10 ultrasound-guided femoral nerve injections were performed. Ninety percent (n = 9/10) of the nerves were successfully dyed.

Conclusions and relevance

The novel ultrasound-guided lateral approach may be suitable and reproducible for a successful femoral nerve blockade in cats. Further investigations about the clinical usefulness and safety of this femoral nerve block in live cats are required.

Introduction

The ventral branches of the forth to sixth lumbar spinal nerves (L4–L6) from the cranial portion of the lumbosacral plexus converge to form the femoral nerve. The femoral nerve enters the proximal pelvic limb through the femoral canal of the iliopsoas muscle. At the caudal end of the iliopsoas muscle, the femoral nerve exits the muscle and courses across the femoral triangle. At this level, the cutaneous and muscular branches of the saphenous nerve rise from the femoral nerve to innervate the sartorious muscle. The femoral triangle is delimited by the sartorius muscle cranially, the iliopsoas muscle proximally and the pectineus muscle caudally. The femoral nerve then continues distally, entering the quadriceps muscle between the vastus medialis and rectus femoris.

An ultrasonographic technique for performing nerve blocks is the preferred method for some clinicians. Benefits of the use of this technique, including a shortened procedure time, faster onset of block, painlessness, a reduction in block-related complications and a high success rate, have been proven in humans. ¹ In the veterinary literature, previous and recent studies have already described a few ultrasound (US)-guided techniques to perform the femoral nerve block in dogs.^2–7 In the clinical setting, a US-guided femoral nerve block, as a part of a locoregional technique, has been shown to provide reliable analgesia for dogs.^8,9

Limited data can be found in the literature regarding femoral nerve US blocks in feline anaesthesia. Haro et al described a feasible dorsal approach of the femoral nerve to perform a US block in cat cadavers. ¹⁰ More recently, an in vivo US technique followed by an anatomical cross-sectional study showed a reproducible ventral supra-inguinal approach for the localisation of the femoral nerve in cats. ¹¹

The purpose of this study was to describe a novel lateral US-guided technique for femoral nerve block in cat cadavers. Our hypothesis was that the lateral approach to the femoral block was feasible and reproducible in cats.

Materials and methods

A total of five adult cat cadavers with a mean weight of 3.54 kg (range 3.1–4 kg) were enrolled for the experiment. The animals died or were euthanased humanely for reasons unrelated to the present study.

The cadavers were placed in lateral recumbency, with the pelvic limb to be blocked positioned uppermost. After the technique was performed on one pelvic limb, the cat was turned and the other pelvic limb was attempted. The hair from the lateral area of each pelvic limb was clipped and the skin cleaned. A 15–16 MHz linear US transducer connected to an ultrasonography unit (SonoSite M-turbo; FUJIFILM SonoSite) with a depth setting of 33 mm was used. The transducer was held perpendicular to the most proximal area of the pelvic limb, ventral to the greater trochanter and on the femur. The orientation of the marker of the probe was cranially oriented. The transducer was moved slightly proximal or distal until a transverse view of the femoral nerve was identified (Figure 1).

Figure 1

Lateral view of the pelvic limb of a cat. Anatomical bones of the pelvic limb are indicated in the left image.

After the transverse image was determined, a 22 G 1.5 inch spinal needle (BD Medical) was inserted at an angle of <45º using an in-plane needle-insertion method from the cranial aspect of the pelvic limb towards the femoral nerve area. Then, 0.1 ml/kg of methylene blue was instilled targeting the femoral nerve. Later, the cadaver was turned to the opposite lateral recumbence and the technique was repeated on the contralateral hindlimb following the previous steps.

After the methylene blue was injected, anatomical dissections of the medial side of both pelvic limbs were conducted to evaluate the femoral nerve staining. A positive femoral nerve staining was considered with a coverage of ⩾1 cm. For this femoral study a coverage staining of <1 cm, staining in a different location and/or absence of methylene blue over the targeted nerve were considered negative results.

Results

A total of 10 US-guided femoral nerve injections were performed in five cat cadavers. The anatomical injection site was located underneath the vastus intermedius and medialis muscle, and cranial to the pectineus muscle (Figure 1). The femoral nerve was identified as a rounded-to-slightly-ovoid well-defined hypoechoic structure deep to the vastus intermedius (Figure 2). The success rate for the femoral nerve staining was 90% (n = 9/10) based on direct visualisation after anatomical dissection. An incorrect disposition of the staining was the reason for the failure in only one femoral nerve block.

Figure 2

Transversal view for the femoral nerve block in cat cadavers. (a) The femoral nerve was localised within the dotted line area and identified as a rounded-to-slightly-ovoid hypoechoic structure. (b) Methylene blue (MB) was injected (circular area). F = femur; VL = vastus lateralis muscle; VI = vastus intermedius muscle; VM = vastus medialis muscle; RF = rectus femoris muscle; P = pectineus muscle

Discussion

The present study reports a high success rate of a reproducible lateral approach for femoral nerve blockade by ultrasonography in cats.

In the clinical setting, ultrasonography has a few advantages when performing nerve blockade: (1) faster procedure times and onset of the nerve blocks; (2) in human medicine, patients reported a painless sensation vs the use of nerve stimulation; (3) a reduction in block-related complications (eg, low nerve injury incidence as visualisation of needle to nerve contact and their interaction may help, less risk for toxicity owing to the use of lower local anaesthetics doses, differentiation between intravascular and extravascular injection, and prevention of accidental puncture in non-neural structures such as lungs, liver, etc); (4) moderate-to-high success rate in human medicine (55–100%). ¹ Since 2008, US-guided methods have been successfully used in veterinary medicine to localise the femoral nerve in dogs.^{3–7,11–14} Nevertheless, only a few studies have been performed in cats.^10,11 In our study, the use of ultrasonography has been shown to be a feasible tool in identifying the femoral nerve in cats via a lateral approach.

Although ultrasonography techniques are useful, clinicians have to bear in mind a few basic equipment considerations, which could influence the success rate and tissue damage. The high-frequency linear transducer (15–16 MHz) used in the present study afforded high axial resolution of the superficial structures (up to 30–40 mm penetration). Larger sections of deeper tissue are achieved with convex low-frequency transducers, but at the expense of less axial resolution. ¹⁵ Our in-plane technique was considered a safer approach as the needle was inserted along the long axis and parallel to the ultrasonic beam to visualise the entire shaft and tip, and the needle-tip progression was continuously monitored. Fewer limitations have been found during out-of-plane techniques (eg, the inability to confirm the real-time exact location of the needle tip) compared with in-plane approaches, and therefore a decrease in success rates during performance of the nerve block has been shown.^15,16 Large (17–18 G) Hustead tip-type needles at angles of <30º inserted along the ultrasonic beam long axis showed optimal visibility during ultrasonography locoregional anaesthesia techniques. ¹⁷ Although small-gauge (22 G) spinal needles were inserted at angles <45º in our cadaveric study, no problems regarding continuous visualisation were detected during the US nerve blockade.

In our opinion, the main advantage of performing this block by a lateral approach is the simplicity of the technique. Leg position has been demonstrated to be a factor that influences the success rate when performing nerve blocks in human medicine. ¹⁸ Two studies have described an inguinal approach for femoral nerve block in dogs, where it is mandatory that the contralateral limb remains abducted with the help of an assistant.^2,3 A simpler technique using a ventral suprainguinal approach was reported in dogs positioned in dorsal recumbency and with the limb to be scanned moderately extended.^4,7,11 Recently, Tayari et al described a lateral approach for the US femoral and obturator nerve block by placing the transducer cranioventral to the iliac crest and perpendicular to the spine. ¹⁴ This technique showed a fentanyl-sparing effect and prolonged postoperative analgesia in dogs undergoing tibial plateau-levelling osteotomy surgery. ¹⁴ Despite the literature describing US femoral nerve block in dogs, only a couple of studies have been published in cats.^10,11 Mogicato et al described a technique using a ventral suprainguinal approach with the cat positioned in dorsal recumbency and with mild extension of the pelvic limb to be blocked, and determined the distance between the external iliac artery and the femoral nerve as the anatomical landmark. ¹¹ Besides, only one study in cats has been shown to have a 100% success rate of contrast medium spreading within the iliopsoas muscle using a US dorsal approach, and with the animals maintained in lateral recumbency. ¹⁰ However, the needle has to be directed in between the sixth and seventh lumbar vertebrae processes, and the clinician may have the inconvenience of hitting bone. In the present study, having the cadavers in lateral recumbency eliminated the need for an assistant to hold the contralateral limb, and it also made the needle insertion a simple in-plane technique through the vastus lateralis and rectus femoris muscles.

The in-plane technique performed in our cadaveric study had a high success rate. A few advantages of using cadavers as a learning tool for US-guided regional anaesthesia have been already described in the literature: learning the anatomy, improving hand–eye coordination, practising on an irregular surface, probe–needle alignment, providing optimal US image and echogenicity-enhancing needle visibility. ¹⁹ Beyond these benefits, an experienced clinician in US nerve blocks may achieve the blocks faster and with greater success. ¹ The in-plane needle insertion method for nerve blockade has also shown a high success rate due to the direct visualisation of the tip and the body of the needle making it easier to follow continuously, and the longer distance from skin puncture to the target nerve allowing greater needle control (due to the stiffness forces of the different tissues). ²⁰ All these discussed features were combined to perform our lateral approach to the femoral nerve block in cat cadavers, obtaining a 90% success rate.

One benefit from the high success rate of our study may be the reduction of the local anaesthesia dose in cats. Amide-linked local anaesthetics (eg, lidocaine) are metabolised in the liver by microsomal enzymes (CYP450), and the metabolites are conjugated with amino acids or glucuronide into less active and inactive metabolites during phase II reactions. Cats have shown low capacity to metabolise drugs that undergo hepatic glucuronidation during the phase II of the liver metabolism due to a lack of uridine diphosphate-glucuroninosyltransferase isoforms. ²¹ Therefore, cats are more likely to suffer from local anaesthesia toxicity.

A limitation of the study to consider is that the success of the technique has only been based on the length of the nerve stained by dye. Raymond et al concluded that there is a direct relation between the incidence of block in a fibre population and the length of nerve exposed to local anaesthesia. ²² They showed that a range of the length of the nerve exposed from 6 to 30 mm influenced the action potential and blocking-drug concentration. ²² Furthermore, a minimum length of 5–7 mm was found to be enough for blocking doses, but these doses had to be five times greater than the doses required to block fibres with longer exposure lengths. Therefore, the positive nerve blockade value based on a coverage of the nerve ⩾10 mm may be sufficient to block the femoral nerve depending on the concentration dose of the local anaesthesia.

A significant limitation is the fact that this was a cadaver study, where it is always more difficult to ascertain true patient safety. The identification of the femoral artery and vein when performing the US-guide nerve block is important to avoid vascular damage. Also, greater nerve damage should be considered when sharp needles are used, and therefore spinal needles either are not recommended or should be avoided. Although no vessels were visualised during the procedure of the femoral nerve block and spinal needles were used, no vascular damage was seen in the pelvic limbs of the cats after the dissection.

Conclusions

The described US-guided technique of a novel lateral approach may be reproducible for a successful femoral nerve blockade in cats. The high success rate, represented as a positive staining of the femoral nerve, may provide clinicians with a suitable locoregional anaesthesia technique in cats when abduction of the limb is not desired or when assistance is not available. Further in vivo investigations regarding the clinical usage, efficiency and safety of this technique are required.