The perfusion index as a method of assessing epidural anaesthesia efficacy in healthy dogs

Perioperative regional anaesthesia encompasses various methods that aim to suppress nerve stimulation at the surgical site by using local anaesthetic agents. Spinal anaesthesia is the administration of a local anaesthetic into the spinal subarachnoid space, and epidural anaesthesia is the administration of a local anaesthetic into the epidural space. Block anaesthesia is the administration of a local anaesthetic near a peripheral nerve, and infiltration anaesthesia is the administration of a local anaesthetic directly at the surgical site (10).

In human medicine, local anaesthesia is usually administered under awake conditions. Therefore, the effectiveness of local anaesthesia can be assessed using sensory and motor neuron tests such as the cold-sensation test and pinprick test (8, 13). However, in veterinary medicine, to evaluate local anaesthesia effectiveness before surgery is challenging because animals need to be immobilised via general anaesthesia before local anaesthesia administration. A veterinary anaesthesiologist often judges local anaesthesia effectiveness based on pain-related clinical signs during the course of surgery. Therefore, for minimising pain to the animal, clarifying whether local anaesthesia has been successful before surgery is necessary.

Nerve blocks using local anaesthetics result in peripheral vasodilation associated with sympathetic blockade (15). The perfusion index (PI) is used to measure peripheral perfusion non-invasively and continuously to quantify the ratio of pulsatile and static blood flow by using a pulse oximeter (5). In their human medicine study reporting a significant increase in the PI after epidural anaesthesia, Ginosar et al. (4) concluded that such an increase is a useful early indicator of local anaesthesia effectiveness. As a consequence, this effect has been the target of recent attempts to establish the usefulness of PI measurements as an indicator of local anaesthesia efficacy.

In veterinary medicine, Gatson et al. (3) reported a significant increase in the PI in the hindlimb in which femoral and sciatic nerve blocks had been performed. However, another report (1) on groups of dogs that received morphine and a combination of morphine and lidocaine as epidural anaesthesia demonstrated no significant differences in the PI for each measurement time. Epidural anaesthesia, one of the most common techniques used in dogs, is ineffective in 6.8%–12% of cases, even when performed with proper techniques (6, 16). This insufficient effect may be because of the difficulty in locating the lumbosacral region and accurately administering local anaesthetics in the epidural space, especially in obese animals (7, 16).

Furthermore, local anaesthetic dosages for epidural anaesthesia in dogs are often determined by consultation of tables for volume per kg body weight; however, dosage determination needs to account for variations in skeletal structure and body mass index between animals, which may have the effect of extending or constricting the blockage regions themselves. Therefore, the purpose of this study was to assess the usefulness of the PI in determining the efficacy of epidural anaesthesia in dogs before commencing surgery.

No significant differences in the rate of PI change were observed between single- and multiple-port groups. Neither did they exist between the control and single-hole port groups or between the control and multiple-hole port groups at baseline (T₀). This situation persisted immediately after catheter placement (i.e. T₁), as well as at all time intervals after lidocaine administration (i.e. T₂–T₇). Significant variations were also not observed in each measurement over time (Table 1).

The aim of this study was to evaluate the usefulness of the PI in determining epidural anaesthesia effectiveness. We found no significant differences when lidocaine was administered into the epidural space using a single-port catheter to when it was administered using a multipleport catheter. In human medicine, the PI has been studied as a non-invasive indicator of regional anaesthesia success. However, its usefulness in veterinary medicine remains unclear. In this study, we tested single- and multiple-port epidural catheter treatments to administer a 0.2 mL/kg bolus dose of lidocaine as described by Jones (7). Based on previous reports, this agent has been widely used for experimental epidural anaesthesia and subsequent observation of PI variations in dogs. One such report examined a local anaesthetic’s area of effect when administered into the epidural space of the lumbosacral region in dogs and suggested that a dose of 0.2 mL/kg produced cutaneous sensory zone relief up to the first lumbar region (2). However, in the present study, no variation or significant difference in PI was observed when the injectant was lidocaine instead of saline.

In human medicine, individual differences in agent infiltration after epidural anaesthesia have been reported. Yokoyama et al. (18) reported that when a contrast agent was administered into the epidural space, the local anaesthetic did not infiltrate evenly and tended to spread in the vertical, horizontal and circumferential directions. However, predicting the spread of the contrast agent is difficult. In humans, unilateral nerve block is more likely to occur with a single-port catheter than with a multiple-port catheter, such a block having been reported in 78% and 22% of applications of these catheters, respectively (11). Considering the possibility that a more uniform distribution occurs near the catheter tip, we examined PI variation with multiple-port epidural catheters. However, no significant differences in PI were observed. Variations in the PI during regional anaesthesia with multiple-port epidural catheters have been reported in humans, with the PI increasing within 10 min after local anaesthetic administration and continuing to increase for at least 20 min (4). In addition, a previous report (14) compared between single- and multiple-port catheters how complications of epidural anaesthesia influenced the analgesic effect and demonstrated no significant differences between the two catheter types. However, some reports (9) suggest that single-port epidural catheters are superior for local infiltration of drug solutions. In veterinary medicine, we found no reports comparing single-with multiple-port epidural catheters in dogs. The current study found no significant difference in the evaluation of epidural anaesthesia effectiveness related to catheter type by using the PI, and this finding suggests that other evaluation methods should be used to investigate the differences in the shape of the tip of the epidural catheter.

In adult humans, regional anaesthesia is often performed in the awake state. Therefore, most reports describing the relationship between the PI and regional anaesthesia cannot be extrapolated to explain PI variation in dogs that are under general anaesthesia. Unfortunately few reports have examined the correlation between regional anaesthesia under general anaesthesia and the PI. A previous study reported an increase in the PI on administration of a caudal block under general anaesthesia in paediatric patients (17). This finding suggested that using the PI as a proxy for regional anaesthetic success could be of great utility under general anaesthetic conditions. Another report comparing the PI before and after interscalene brachial plexus block performed under awake conditions revealed a significant PI increase after regional anaesthesia (12). The same report detailed that, when general anaesthesia was subsequently switched to regional anaesthesia, the PI increased on the contralateral side instead of on the local blockade side, which indicated no significant difference in the PI between the blocked and opposite limbs (12). These findings suggested that general anaesthesia may have affected the PI. In our study, an inhalation anaesthetic was also used, which indicates that peripheral vasodilation caused by such agents may affect the PI.

In veterinary medicine, one study (3) under general anaesthesia conditions reported a significant increase in the PI on the limb that received femoral and sciatic nerve blocks using bupivacaine (i.e. a local anaesthetic agent) compared with the control limb. By contrast, another study (1) examining changes in the PI due to epidural anaesthesia (using morphine or a mixture of morphine and lidocaine) in dogs under general anaesthesia preceding knee surgery demonstrated no significant differences in the PI, either within or between treatment groups at any of the measurement times; in addition, no significant differences existed in the PI before or after skin incision or osteotomy.

The lumbosacral plexus is a possible site for epidural anaesthesia, and a wide range of blocks can be performed there, including blocks of the femoral and sciatic nerves. However, these blocks can also be performed locally and need not be through the lumbosacral plexus. Therefore, the findings of these studies suggest that the usefulness of the PI for assessing local anaesthesia success may depend on the site of local anaesthetic administration and regional anaesthesia technique. Future studies are needed to compare the usefulness of the PI using the same amount of local anaesthetic but using multiple techniques of its administration.

This study used experimental animals selected for similarity across key aspects. As such, variations in body mass, fat composition and physique were relatively minimised. Inevitably patients varying in body fat composition, weight and physique were encountered by Doyle et al. (1), there possibly being among them cases of unsuccessful epidural anaesthesia because it was administered to such physically heterogenous dogs in those clinical cases. The epidural catheter placement and epidural anaesthesia were dependable in this study, whereas anaesthesia, at least, was not in the investigation by Doyle et al. (1). A further difference between the present study and the cited one is Doyle et al. (1) having used a single puncture in their anaesthetic technique. Nonetheless, similarly to this and previous reports, we did not find significant variations in the PI under the experimental conditions.

Owing to the nature of veterinary patients, a successful assessment of regional anaesthesia must be achieved indirectly through physiological parameters, rather than via tests that require awake conditions as in human medicine. Our results suggested that using PI measurements as a method of determining the effectiveness of epidural anaesthesia using lidocaine is challenging and requires further exploration. Furthermore, other methods for local anaesthesia success assessment are necessary and further studies are needed on the effect of dosage and administration rates on regional anaesthesia.