INTRODUCTION
There are many devices designed to help identify the epidural space[1–3] but the loss of resistance (LOR) is still the most common technique for administering epidural anaesthesia.[4] However, LOR involves the use of a subjective feeling felt only by the operator, and this tactile sensation can be indistinguishable from non-epidural LOR, including interspinous ligaments, intermuscular planes, paravertebral muscles, and cysts in interspinous ligaments.[5,6] Trainees can easily create a false LOR by pushing too hard during the process of needle advancement. For the supervisor, it is also hard to know whether the trainees feel true LOR when visually supervising the whole process.
In clinical practice, we noticed that while testing LOR for air using a B Braun air syringe Perifix® (B Braun, Melsungen, Germany), the air can be pushed into the epidural space with low resistance and this air often flows back into the syringe spontaneously and pushes the plunger back [Supplemental Video File 1]. We termed this the air-refill phenomenon. In Hea-Jo Yoon’s study, the incidence of an audible air leak after removing the syringe was 35.9%, which showed a positive predictive value of up to 98.6% for successful labour epidural analgesia.[7] This suggests that if the air can be pushed in with low resistance and can also leak out freely, it indicates true LOR. Whenever air was seen refilling the syringe, we were more confident that it was true LOR.
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Since 2016, we have asked trainees to record how much air is in the syringe before pushing it into the epidural space. They also record how much air refills the syringe once they release the thumb from the plunger.
We aimed to determine the prevalence of the air-refill phenomenon and its utility in confirming the LOR to air in epidural labour analgesia. Therefore, we reviewed the labour analgesia special charts and conducted this retrospective observational study.
METHODS
After obtaining approval from the ethics committee in a tertiary medical centre on March 29, 2021 (IRB Number: CE21083A). We collected charts of all patients who received labour epidural analgesia from 01 July 2020 to 31 December 2020 and recorded the volume of air pushed in by the syringe and the air volume that refilled the syringe. We also recorded the patients’ age, height, body weight (BW), body mass index (BMI), the epidural set brand (Portex/B Braun), the category of operators (residents, fellows, attending anaesthesiologists), and whether reinsertion of epidural catheter was conducted. We defined any visible amount (>0.5 ml) of air-refill at LOR as air-refill positive, and no air-refill of the syringe as air-refill negative. Successful labour epidural analgesia was defined by satisfactory pain relief. Patients who received epidural catheter reinsertion, later on, was defined as failure.
RESULTS
There were 373 patients who received labour epidural analgesia in our institution from 01 July 2020 to 31 December 2020 [Table 1]. Of the 373 patients, there were 159 patients for whom the air-refill data were properly recorded [Figure 1]. In our institution, epidural analgesia is the only option for labour analgesia. We included all patients who received labour epidural analgesia and did not exclude any patients. For patients who underwent reinsertion of epidural catheter, we only included the first attempt. Among these 159 patients, 147 (92.5%) were cases of success and 12 (7.5%) were cases of failure, whereas 151 (95%) were air-refill positive and 8 (5%) were air-refill negative. The sensitivity, specificity, positive predictive value, and negative predictive value were 94.6%, 0%, 92.1%, and 0%, respectively. Dural puncture occurred in four cases (2.5%) among the 159 patients.
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Table 1: Patient characteristics
Figure 1: Flowchart of study sample
DISCUSSION
We noticed that the air-refill phenomenon only happens when using a B Braun air syringe and does not occur when using a Portex epidural set. We designed a pressure measuring device to determine how much air pressure is needed to push the plunger back for B Braun (green) and Portex (blue) syringes [Figure 2]. This device connects the cuff pressure gauge with three syringes, including a B Braun syringe, a Portex syringe, and a regular syringe in a series. When 3 ml of air was injected into the system via the regular syringe we found that the plunger could be pushed back into the B Braun syringe when the pressure was as low as 6–8 cmH2O, but the Portex syringe needed up to 100 cmH2O [Supplemental Video Files 2 and 3]. This explains why air-refill was only seen in B Braun syringes, but not in Portex syringes.
Figure 2: Air pressure measuring device
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According to a previous study,[8] the epidural pressure is 7.69–7.91 mmHg (10.46–10.76 cmH2O) in the lumbar area and 3.84–6.40 mmHg (5.22–8.70 cmH2O) in the thoracic area. We speculate the air-refill phenomenon will likely occur more often in the lumbar area than in the thoracic area. Furthermore, epidural pressure has been reported to be higher in pre-labour women.[9] As such, the epidural pressure in the lumbar area of a pre-labour woman will exceed 10.46–10.76 cmH2O, which is sufficient to produce the air-refill phenomenon in a B Braun syringe. This explains why the incidence of positive air-refills was as high as 95% in 159 of our pre-labour women. Although pre-labour women have higher average epidural pressure, the individual epidural pressure was still different from patient to patient. In our study, eight negative air-refill cases turned out to be success cases, probably because these eight patients had lower epidural pressure that was not greater than 6–8 cmH2O and could not push the plunger back in LOR. With a high sensitivity (94.6%) and positive predictive value (92.1%), but poor specificity and negative predictive value, this air-refill test can only be used as a supplementary test alongside conventional LOR. When an air-refill test is positive, we can be strongly confident about the correct needle location. However, a negative air-refill test does not mean that the needle is incorrectly positioned. Therefore, this test is only useful in air-refill positive cases. Nevertheless, 95% of pre-labour women in our study were air-refill positive cases.
In 2021, we asked our 12 residents to complete an anonymous questionnaire. Ten out of the twelve residents said this air-refill test was either helpful or very helpful in their learning processes. We think this is a valuable teaching tool and would therefore like to introduce it to anaesthesiologists worldwide. The air-refill test has allowed the subjective feeling of LOR that is felt only by trainees to become observable evidence for both supervisors and trainees.
The limitations of our study were the retrospective nature of the investigation, the definition of failure analgesia as receiving reinsertion may have been inadequate, and the amount of missing data was not small. However, the failure rate within the missing data and collected data were similar (17/214 = 7.9%, 12/159 = 7.5%), [Figure 1]. We postulate that our recorded cases are still representative of the whole population.
CONCLUSION
As the prevalence of positive air-refills in pre-labour women was high, we recommended the air-refill test as a good confirmatory adjunct and teaching tool in labour epidural analgesia. The air-refill test could serve as a teaching tool for residents in labour epidural analgesia.
Ethics approval
The study was approved by the Institutional Review Board of Taichung Veterans General Hospital (IRB: CE21083A). Written informed consent has been obtained from the patient for publication of video material in a scientific journal.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
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