The use of epidural puncture pressure profile to identify the epidural space in cats
MeSH keywords:cats, epidural space
Ιn cats, the identification of the epidural space is challenging, because of its small size. The objective of this prospective clinical study was to investigate the presence of distinct epidural puncture pressure profiles that confirm the correct needle placement in the epidural space in cats.
Twenty-two female adult cats were used in the study, scheduled for ovariohysterectomy. Mean bodyweight was 3.37 kg (ranging from 2.5 to 4.5 kg). After lumbosacral epidural puncture, the epidural needle was connected to a pressure transducer and then to a computer, where the epidural puncture pressure profile was recorded. Then, local anaesthetic was administered through the epidural needle. Correct placement of the needle was evaluated by “lack of resistance to injection of saline” technique.
In 20 out of 22 animals (91%), epidural anaesthesia was proved to be effective. A pressure drop was recorded in 13 cats, in all of which epidural anaesthesia was successful. In the remaining 9 cats, the pressure drop was not clear, with many artefacts, while in two of them the epidural anaesthesia was not successful.
The presence of characteristic epidural puncture pressure profiles could confirm the correct needle placement in the epidural space and may be used in a clinical setting in cats.
In recent years, epidural anaesthesia is increasingly used in companion animals (Fowler et al. 2003), as it is a simple and effective technique of local anaesthesia. In cats, identification of the epidural space (EDS) may present specific challenges because of their small size. Additionally, the spinal cord in cats terminates at the seventh lumbar vertebra and the meninges extend to the sacral region, resulting in a greater risk of complications, such as unintended dural puncture, compared with other species (Campoy et al. 2015).
In practice, various techniques are used to confirm the proper placement of the epidural needle, with the most common being the “hanging drop” and the “lack of resistance to injection of saline or air (LOR)” techniques (Campoy et al. 2015). A new technique that has recently been developed in dogs, ruminants and horses (Iff et al. 2007; Iff, Mosing, et al. 2009; Iff, Franz, et al. 2009; Iff & Moens 2010) is the recording and assessment of a characteristic pressure drop as the needle enters the EDS.
The purpose of this study was to investigate whether the epidural pressure profile could be used to confirm correct needle placement in the EDS in cats. As it has been found in other species (Iff, Mosing, et al. 2009; Iff, Franz, et al. 2009), most of the times the pressure inside the EDS is negative (i.e. less than the atmospheric pressure), so it was anticipated that once the needle penetrated the EDS, a drop of the pressure would be recorded.
Materials and methods
Twenty-two European domestic shorthaired female cats aged 1-3.5 years old entered the study. Median body weight was 3.5 kg (range 2.5 to 4.5 kg). All cats were stray animals and were scheduled for elective ovariohysterectomy. Moreover, they were considered healthy based on the physical examination (status ASA I-II). The study was approved by the Institutional Ethics Committee on Animal Research.
Sixteen cats were premedicated with an alpha-2 agonist (dexmedetomidine, medetomidine, xylazine) in combination with an opioid (butorphanol, morphine). Anaesthesia was induced in 20 cats with propofol (Propofol, Fresenius Kabi, Greece) at 1-5 mg kg-1 intravenously (IV), while in the other two cats with ketamine (Imalgene, Merial, France) at 10 mg kg-1 intramuscularly (IM) and midazolame (Dormixal, Demo, Greece) at 0.5 mg kg-1 IM. After endotracheal intubation, anaesthesia was maintained with isoflurane in oxygen, using a T-piece system. Meloxicam (Metacam, Boehringer Ingelheim, Germany) was administered IV (0.1 mg kg-1) for pre-emptive analgesia. Heart and respiratory rate and rhythm were monitored throughout surgery.
The cats were placed in sternal recumbency with the hind limbs positioned cranially. After aseptic preparation of the lumbosacral area, a standard technique of epidural anaesthesia was performed (Campoy et al. 2015). All the injections were carried out by the same clinician. A 22G and 1.5-inch epidural needle (Spinocath, B. Braun, Germany) was entered in the lumbosacral intervertebral space. Once the needle penetrated the skin, the stylet was removed, and the needle was connected via a tube and a three-way tap, filled with normal saline to a single-use pressure transducer. The transducer and cannula were flushed before starting the puncture. The transducer was positioned and zeroed at the level of the transverse process of the seventh lumbar vertebra and was connected, via an analogue–to-digital converter (Pressure Monitoring system Buzzer-II; Michael Roehrich, Austria), to a computer, where the epidural pressure waveforms were displayed and recorded. Then, the needle was further advanced until it was believed to have entered the EDS. At this point, the tube and the device were disconnected from the needle. Aspiration and lack of resistance to injection of saline with a low-resistance syringe were also performed, before the administration of 2% lidocaine (Xylocaine, AstraZeneca, France), at a dose of 2-4 mg kg-1 (Lee et al. 2004). Finally, successful anaesthesia was evaluated clinically by subjective assessment of the anal sphincter tone and the reaction of the animal to the painful stimuli intraoperatively. Particularly, when the respiratory and the heart rate showed a 10-15% increase, epidural anaesthesia was considered to be unsuccessful and fentanyl (Fentanyl, Janssen, Belgium) at 2 μg kg-1 IV was administered.
A total of twenty-two attempts for epidural anaesthesia were made, out of which 20 were successful (91%). Clear pressure drop, which is a negative pressure waveform, was recorded in 13 cats when the needle was introduced into the EDS (Figure 1). All these cats had a successful epidural anaesthesia clinically, as indicated by the relaxed anal sphincter and the stable heart and respiratory rate intraoperatively. The remaining 9 cats did not show any negative waveform. However, in the 7/9 cats the epidural anaesthesia was successful. On the other hand, in 2/9 cats the epidural anaesthesia was unsuccessful and in these cases the absence of negative waveform was compatible with unsuccessful epidural anaesthesia.
In the present study, all the cats included were considered healthy animals that underwent elective ovariohysterectomy. In all of them, the epidural puncture was performed by the same investigator and the correct needle placement was confirmed with the use of lack of resistance to injection technique, while the success of the epidural anaesthesia was evaluated by intraoperative clinical assessment.
In our study, while the needle was entering the EDS, a sudden pressure drop in the epidural puncture profile would indicate that the tip of the needle is inside the EDS. Consequently, the presence of negative pressure waveform in these cats was in line with successful epidural anaesthesia. The success rate of epidural anaesthesia in humans relies on the anaesthetist’s level of training and experience, as well as the patient’s position and the quality of the anatomical landmarks (de Oliveira Filho et al. 2002). In our study, it was 91%, while in dogs, an 88% success rate has been recorded (Iff & Moens 2010).
LOR test is often used in identification of the EDS in animals (Campoy et al. 2015), even though it has been described as “subjective” and “operator dependent” technique in human medicine (Riley & Carvalho 2007). Epidural pressure waveforms have been used successfully in identifying the lumbar EDS in humans and dogs (Ghia Jn et al. 2001; Iff et al. 2007). In dogs, the recordings of the epidural pressure waveforms showed a sensitivity of 89% and specificity of 100% (Iff & Moens 2010). However, in cattle and goats, the usefulness of this technique in confirming the correct needle placement is limited (Iff, Mosing, et al. 2009; Iff, Franz, et al. 2009).
In our study, it is possible that the small size of the cat and the smaller EDS compared to that of other species, may have resulted in the presence of smaller differences between subcutaneous and epidural pressures in the lumbosacral region. The latter may cause a pressure drop during the insertion of the needle in the EDS, which cannot be recorded by the device. Another explanation is that the repeated attempts for finding the EDS may have resulted in blocking of the tip of the needle by tissue and, consequently, in inability of pressure recording. This is particularly possible to happen in cats, because of their small size and the small diameter of epidural needle used. Furthermore, the repeated attempts may have caused loss of the negative epidural pressure or equilibration of the low epidural pressure with the subcutaneous pressure.
In summary, the presence of pressure waves could be used to confirm the correct needle placement in the EDS, but it is important to mention that further research is needed because of the small sample size in this study.
Conflict of interest
The authors declare no conflicts of interest.
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