I read with interest the correspondence [1] regarding the concentration of chlorhexidine solutions used for skin preparation before central axial blocks. Reference was made to a court case where it was suggested that only 0.1 ml of chlorhexidine could cause arachnoiditis. The authors expressed concern that using higher concentrations (0.5% versus 2%) of chlorhexidine may lead to more of it being carried into cerebrospinal fluid by the tip of spinal and epidural needles.
I would like to highlight that while 0.1 ml appears to be a small volume, when comparing it to the dimensions of spinal and epidural needles it is actually a large volume.
One can theoretically calculate the volume of chlorhexidine that could be carried on the tip of spinal and epidural needles. When we insert a needle it will “cut” a cylindrical volume of this solution and take it to the neural tissues. We can calculate the volume of this “cylinder of chlorhexidine” that will sit on the tip of the needle by using the formula: volume of a cylinder = height x Pi x radius x radius (Where the constant Pi =3.143).Using a digital calliper ( Guogen, China) I found the diameter of a spinal needle ( 26 G, Vygon, UK ) to be 0.045 cm and that of an epidural needle (16G , Portex, UK) to be 0.163 cm (the corresponding radii would be half these measurements). Let us assume that we have a 0.1 cm layer of chlorhexidine on the skin. This would be the height of the cylinder of chlorhexidine.
Using these values in the formula, the volume of chlorhexidine on the tip the spinal needle would be 0.00016 ml. Similarly, the epidural needle could carry a volume of 0.0021 cm. These are magnitudes smaller than the 0.1 ml referred to in the case. In reality, it is likely that the actual volumes are even smaller as the chlorhexidine should be dried on the skin (i.e. much less than 0.1 cm thickness) , the needle tips are not flat but are pointed, and subcutaneous tissue and ligaments would rub away some or all of this volume.
The spinal cord is a very vital part of the body and one should make every effort to minimise introducing anything that can harm it. However, when assessing the risks of skin preparations, we should take into account the actual volumes that may reach neuronal tissue.
References: 1.Scott M, Stones J, Payne N. Antiseptic solutions for central neuraxial blockade: which concentration of chlorhexidine in alcohol should we use? Br J Anaesth 2009; 103:456-57
Should We Replace Stylets in Dural Puncture Epidurals ?
BJA 10 January 2009
Reducing post dural puncture headaches ( PDPH) is a goal common for anesthesiologists and neurologists. To reduce PDPH, neurologists replace stylets before pulling out spinal needles. This practice is based on a prospective randomized study by Strupp et al. [1] which found that replacing stylets after lumbar punctures reduced the incidence of PDPH from 16.3% to 5%. They hypothesize that cerebrospinal fluid (CSF) flow may drag an arachnoid fiber into the spinal needle, which when withdrawn, “threads” the fiber through the dural hole to form a “wick” from which CSF continues to leak and cause PDPH. When a stylet is replaced, it presumably pushes out these arachnoid fibers and prevents them from interfering with dural hole closure.
Replacing stylets after spinal anesthesia lumbar punctures has not been recommended because anesthesiologists inject fluid (anaesthetic agent) which may push arachnoid fibers away from needles.
However, there is a resurgence of a novel anaesthetic technique called “dural puncture epidural” (DPE) [2, 3] where replacement of spinal needle stylets may be appropriate. The DPE technique is same as a combined spinal epidural except that the spinal needle is used only to make a dural puncture without injecting any anaesthetic drug. Epidural drug may pass through this hole and provide better sacral block [3].
It is apparent that in the DPE technique, the spinal needle is used in a similar manner to a lumbar puncture needle used by neurologists where CSF is obtained, but nothing is injected. Therefore, it is possible that replacing the stylet when performing a DPE may reduce the incidence of PDPH associated with the technique.
Regrettably, the study by Strupp et al. has not been repeated (personal communication , Strupp) and the needle size used in their study was 21 G, and therefore its applicability to DPE may be questioned. In this respect, centers that choose to use the DPE technique may be in an ideal position to conduct similar studies to see if replacing stylets will reduce the incidence of PDPH.
References:
1. Strupp M, Brandt T, Muller A. Incidence of post-lumbar puncture syndrome reduced by reinserting the stylet: a randomized prospective study of 600 patients. J Neurol 1998;245:528–9.
2. Suzuki N, Koganemaru M, Onizuka S, Takasaki M. Dural puncture with a 26-gauge spinal needle affects spread of epidural anesthesia. Anesth Analg. 1996;82(5):1040-2.
3. Cappiello E, O’Rourke N, Segal S, Tsen LC. A Randomized Trial of Dural Puncture Epidural Technique Compared with the Standard Epidural Technique for Labor Analgesia. Anesth Analg 2008;107:1646 –51.
Introducer Needle Dural Puncture
Anaesthesia 01 Feb 2009
Erskine [1] explains how Luer connection hubs in current spinal needle introducer needles shorten the maximum achievable depth of spinal needles by up to 22 mm. In some patients the introducer needle may therefore prevent the spinal needle reaching the dura. As a solution, he promotes using 'Sise type' introducer needles which, instead of a Luer hub, have a 2 mm thick disc which only minimally limits spinal needle depth. However, a clinical situation I encountered highlights a potential problem that may arise with Sise type introducer needles. I was assisting a colleague giving a spinal anaesthetic to a moderately built patient, in the sitting position, for an elective Caesarean Section. He first inserted the introducer needle (depth 30 mm, Pajunk, Germany) and then was about to insert the spinal needle when I noticed cerebrospinal fluid appear at the hub of the introducer needle. Because the introducer needle had a Luer hub, we were able to use it as a 'short spinal needle' and local anaesthetic was injected via it, resulting in a successful spinal block. Had we used a Sise type introducer needle, we would not have been able to do this as it does not have a Luer hub and the patient would have had to be subjected to a repeat procedure with a new needle. Therefore, though the Sise type introducer may help when the dura is deep, it will be unsuitable when the dura is unexpectedly superficial. There are similar reports in the literature of the use of introducer needles as spinal needles [2, 3]. However, it is difficult to estimate if this occurs frequently enough in clinical practice to become an impediment to the widespread use of Sise type introducer needles.
References:
Erskine RJ. Assessment of a new disposable introducer for spinal needles. Anesthesia 2008; 63: 1238–40.
Loader J. Dural puncture with an introducer needle. Anaesthesia 2008; 63: 326–7.
Malaiya AK, Kulkarni A, Reynolds F. Dural puncture with the introducer of a 26 gauge atraumatic spinal needle.International Journal of Obstetric Anesthesia 1995; 4: 257.
" Epidural Needle Air "
BJA 26 February 2007
Loss of resistance (LOR) to saline is a common technique used to locate the epidural space[1]. I would like to draw attention to the fact that even when saline is used, a small amount of air is also injected from the Tuohy needle.
When performing an epidural, the Tuohy needle with its stylet, is inserted into the ligaments. When the stylet is removed, it leaves an empty space in the Tuohy needle which gets filled with room air. When the LOR syringe with saline is attached, this air is trapped within the Tuohy needle. As the needle enters the epidural space, this trapped air will be the first to be discharged followed by the saline.
Using a saline filled 1 ml syringe (Becton Dickinson, Madrid, Spain) to prime the hollow space in a 16 G, 80 mm Tuohy needle ( B. Braun, Melsungen, Germany), I have found that such a needle would trap about 0.2 ml of "needle air".
The clinical consequences of such a small volume of air are not known. For an example, some believe, though unproven, that saline helps to push the dura away from the tip of the Tuohy needle[1]. However, as explained, it is "needle air" that is discharged from the tip of the Tuohy needle before saline. Air, being much less dense, will not be as effective as saline at pushing the dura away and may thus reduce the effectiveness of this mechanism.
Though unlikely, if clinical significance is ever found, "needle air" can be got rid of when performing an epidural. After the stylet is removed, one can insert a long spinal needle into the Tuohy needle and inject saline to flush out the "needle air". A saline filled LOR syringe can then be attached to the primed Tuohy needle that will now be free of "needle air".
References: 1. Wantman A, Hancox N, Howell P. Techniques for identifying the epidural space: a survey of practice amongst anaesthetists in the UK. Anaesthesia 2006; 61: 370 – 75
Light Shock
BJA 4 January 2007
I wish to report an incident where I experienced an electric shock when adjusting an overhead operating light. As I touched the metal handle of the light (Hanaulux 2500, Siemens, Germany), I felt a strong "current" pass through my arm. An electrical fault with the light was assumed and no one touched it during the rest of the otherwise uneventful surgical procedure.
Afterwards, the operating light was tested and to our astonishment, no current leak was found. I then remembered that when adjusting the light, my arm had brushed against a metal drip stand onto which a fluid warmer (Ranger 245, Arizant Healthcare, U.S.A.) was attached. This fluid warmer was plugged into an extension box (extension cable with multiple sockets) which we examined. We found that the cable going into the extension box was markedly twisted. On opening the extension box, we found that the "Earth" wire of the cable supplying the box had got detached and alarmingly, that the "Live" wire of the cable was touching the place where the "Earth" wire should have been connected. This fault electrified the metal casing of the fluid warmer and the metal drip stand to which it was attached. When my arm touched the metal drip stand, the current would have passed from it, through my arm, onto the metal handle of the operating lamp.
Unfortunately, the theater electrical system did not have an isolated power supply or a residual current device, which could have afforded me protection [1].
In this incident it was assumed that the operating light was faulty and that by not touching it, that everything was safe. However, with hindsight, this assumption was a very dangerous one to have made. If the electrified drip stand or warmer was touched again, more shocks may have occurred with possibly fatal consequences.
The main safety lesson from this incident is that when an electrical shock occurs, one should not assume the cause of the fault. Instead, it would be safer to avoid touching any metal apparatus till they are declared to be safe by an professional electrician.
1. Courtney NM, McCoy EP, Scolaro RJ, Watt PA. A serious and repeatable electrical hazard - compressed electrical cord and an operating table. Anaesthesia and Intensive Care 2006; 34: 392-396
