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Canadian Journal of Anesthesia 47:503-505 (2000)
© Canadian Anesthesiologists' Society, 2000
Reports of Investigation

Air or nitrous oxide for loss-of-resistance epidural technique?

Vandana Gaur, MD, Rakesh Kumar Gupta, MD, Anil Agarwal, MD, Mukesh Tripathi, MD and Atul Gaur, MD

From the Department of Anaesthesiology & Critical Care Medicine and Radiodiagnosis, Sanjay Gandhi Post Graduate Institute of Medical Sciences, Lucknow, India.

Address correspondence to: Dr. Atul Gaur, Type IV/12, SGPGIMS Campus, Sanjay Gandhi Post Graduate Institute of Medical Sciences, Lucknow, UP, India 226 014. E-mail: gaur_atul{at}hotmail.com


   Abstract
TOP
 Abstract
Introduction
 Methods
 Results
 Discussion
 References
 
Purpose: To compare the spinal-epidural spread of gas following loss of resistance (LOR) technique for detection of the epidural space using air or nitrous oxide (N2O).

Methods: Comparison of the spread of air and N2O in the epidural space following LOR technique was performed by using Magnetic Resonance Imaging (MRI). Ten adult patients ASA grade I served as their own control in this prospective study. A control MRI (MRI Contr-1) of the dorsolumbar spine was performed. Then, an 18 gauge epidural needle was introduced at the L3-4 intervertebral space using 0.14 ml•kg-1 N2O for LOR and the MRI (MRI-N2O) was repeated. Forty eight hours later, an MRI scan (Contr-2 MRI) was performed and, subsequently, an 18 gauge epidural needle was introduced, using 0.14 ml•kg–1 air for LOR followed by an MRI (MRI-Air) scan. The volumetric measurements of gas pockets were done using a formula.

Results: Gas bubbles after N2O were few and small compared with larger gas pockets occupying up to three vertebral segments after the use of air for LOR. The volume of air in the epidural space was 2.96 ± 0.93 ml compared with 0.35 ± 0.32 ml N2O.

Conclusion: The use of N2O for LOR technique of detecting the epidural space produced very small bubbles detected by MRI compared with the use of air under similar conditions.


   Introduction
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 Abstract
 Introduction
Methods
 Results
 Discussion
 References
 
LOSS of resistance (LOR) to injection of air is a common, simple and established technique for identifying the epidural space, but it may be associated with complications such as missed segments, neurological damage and venous air embolism.1–5 Complications may increase with the use of large volumes of air in difficult cases or when confirmation of the correct placement of the epidural needle is required. In a survey of practicing anesthesiologists in India, it was found that the use of 6 to 10 ml air for LOR was common practice. Attempts have been made to circumvent these complications using alternative methods for LOR, but they, too, have disadvantages such as the failure to detect subarachnoid puncture, failed epidural, high block and dilution of local anaesthetic.6–9

Air pockets and increased pressure inside the epidural space may contribute to complications following LOR technique. Nitrous oxide being a more soluble gas may provide an alternative. This prospective study was performed, in ten adult patients listed for pain therapy, after the use of air or N2O in the epidural space for LOR so as to compare the epidural spread of gas by MRI.


   Methods
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 Abstract
 Introduction
 Methods
Results
 Discussion
 References
 
After institutional ethical committee approval and written informed consent, 10 adult patients with aged 35.9 ± 7.34 yr and M:F ratio 2:3 served as their own control were included in this prospective study (TableGo).


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  		TABLE Demographic data
 
An MRI of the dorsolumbar spine was performed on a 2 T (Tesla) super conducting system operating at 1.5 T (Magnetom SP, Siemens, Germany) using an oval spine coil. T1 weighted (TR/TE = 500/15) proton density (PD) and T2 weighted images (TR/TE = 2500/20-80) were obtained in coronal and axial planes, with a slice thickness of 3 mm, interslice gap of 0.3 mm and 195x256 matrix. T2* weighted images (TR/TE/FA = 500/8/120) in axial plane were also performed to confirm the presence of gas (gas is used to denote air or N2O in this study) in the epidural space. The initial MRI scans served as the first control (MRI-Contr1) for patients. Nitrous oxide was collected from a Boyel Cadet anesthesia machine through a sterile bacteria-viral filter and two three-way connector assembly. Subsequently an 18 gauge epidural needle was introduced at the L3-4 intervertebral space using 0.14 ml•kg-1 N2O for LOR and ten minutes later another MRI was performed (MRI- N2O). Forty-eight hours later, the MRI was repeated (MRI Contr2). Following this, an 18 gauge epidural needle was introduced at the L3-4 space with 0.14 ml•kg–1 air for LOR. Ten minutes later another MRI (MRI-Air) was performed. In each patient, the epidural administration of either gas (air or N2O) was followed by the administration of 18 ml normal saline through the epidural needle to simulate the clinical identification of the epidural space by LOR and was followed by the administration of local anesthetic.

Each patient served as his/her own control and the sequence of MRI scans were MRI-Contr1, MRI-N2O followed 48 hr later by MRI-Contr2 and MRI-Air.

The control MRI images were compared with MRI- N2O and MRI-Air at the same spinal levels. The images were evaluated for the presence, size and location of gas pockets within the epidural space, around nerve root sheaths and also in the intervertebral foramina. Volumetric measurements of the air and N2O pockets were done by a combination of longitudinal and transverse planes using the formula V = 1/2 (L x AP X W) where V is the volume in ml, L is the length, AP is the anterio posterior dimension and W is the width. MRI interpretation was done by a radiologist blinded to antecedent treatment. Volumes of gas pockets of air and nitrous oxide were compared using the unpaired student's t test.

On day three after completion of the study patients received epidural steroid diluted in normal saline through L2-3 intervertebral space. No untoward side effects due to the epidural procedures were encountered in any patient.


   Results
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 Abstract
 Introduction
 Methods
 Results
Discussion
 References
 
The demographic data are detailed in the TableGo. Epidural space of the dorsolumbar spine in the MRI-Contr1 and Contr2 scans, did not show any pockets of N2O or air. A few small gas pockets in the epidural space were seen in MRI- N2O (Figure 1Go) and large air pockets were seen in MRI-Air (Figure 2Go). In five patients residual air pockets extended up to three vertebral segments and displaced the thecal sac anteriorly. In these patients, on coronal section, bilateral vertical air pockets were seen, and on axial section, air was compressing the thecal sac anteriorly while occupying the posterio-lateral aspect. In another four patients, on coronal section, an air pocket was seen spreading up to two vertebral segments on one or both sides of the theca. In one patient, on coronal section, an air pocket was seen occupying only one vertebral segment. Air was also seen in the intervertebral foramina in four patients. The volume of air in the epidural space was 2.96 ± 0.93 ml compared with 0.35 ± 0.32 ml N2O (P < 0.001).



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  		FIGURE 1 PD weighted coronal MRI scan (MRI-N2O) after use of N2O for LOR, shows N2O bubbles in left epidural space from L2 to L4 (arrows).

 


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  		FIGURE 2 PD weighted coronal MRI scan (MRI-Air) after use of air for LOR, shows thick column of air on both sides of thecal sac causing compression (arrows).

 

   Discussion
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 Abstract
 Introduction
 Methods
 Results
 Discussion
References
 
Epidural gas pockets were larger with the use of air than with N2O for LOR technique. Air pockets were large displacing the thecal sac and were seen to enter the intervertebral foramina.

The phenomenon of confined epidural air pockets becoming further enlarged when N2O is simultaneously used as a part of general anesthesia and its associated complications is well reported.3–5,10 Theoretically, the use of N2O for LOR should alleviate this problem. On the contrary, the size of N2O pockets should become further reduced due to their greater solubility compared with air. Hypothetically, the small gas bubbles should pose less interference with the diffusion of local anaesthetic to the surrounding neuronal tissue, hence resulting in a better quality of block.

In conclusion, our study suggests that LOR with N2O might be associated with fewer complications if the size and extent of distribution of bubbles determine the quality of epidural block.

Accepted for publication February 4, 2000.


   References
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 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
1 Bromage PR. Unblocked segments in epidural analgesia for relief of pain in labour. Br J Anaesth 1972; 44: 676–9.[Abstract/Free Full Text]

2 Crawford JS. The second thousand epidural blocks in an obstetric hospital practice. Br J Anaesth 1972; 44: 1277–87.[Abstract/Free Full Text]

3 Deam RK, Scott DA. Neurological damage resulting from extracorporeal shock wave lithotripsy when air is used to locate the epidural space. Anaesth Intensive Care 1993; 21: 455–7.[Medline]

4 Nay PG, Milaszkiewicz R, Jothilingam S. Extradural air as a cause of paraplegia following lumbar analgesia. Anaesthesia 1993; 48: 402–4.[Medline]

5 Cone A, Stott S. Neurological complications following epidural analgesia with air used for location (Letter). Anaesth Intensive Care 1993; 21: 890–1.[Medline]

6 Valentine SJ, Jarvis AP, Shutt LE. Comparative study of the effects of air or saline to identify the extradural space. Br J Anaesth 1991; 66: 224–7.[Abstract/Free Full Text]

7 Candido KD, Winnie AP. A dual-chambered syringe that allows identification of the epidural space using the loss of resistance technique with air and with saline. Reg Anesth 1992; 17: 163–5.[Medline]

8 Eldor J, Guedj P. Combined hanging drop loss-of resistance technique for identification of the epidural space (Letter). Reg Anesth 1991; 16: 299–300.

9 Taylor DR. Water-saline controversy of deficient regional anesthesia training? (Letter) Anesth Analg 1994; 78: 1203–4.[Free Full Text]

10 Petty R, Stevens R, Erickson S, Lucio J, Kao T-C. Inhalation of nitrous oxide expands epidural air bubbles. Reg Anesth 1996; 21: 144–8.




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