Ultrasound for Neuraxial Anaesthesia

Most of the time that we palpate the spine (70%) we are incorrect at assessing the level of the spine that we are palpating blindly. In the future ultrasound guidance will be standard of care for spinal anesthesia, according to Jose Carvlho, from Toronto at the Western Anaesthesia Symposium.

Does the line drawn across the iliac crests really cross the spine at L4? Actually it is usually at the level of L2,3 (or even higher). In addition, using the palpation technique usually results in a fishing expedition for the subarachnoid space, and the distance from skin to dura is much shorter than we think. Experienced operators state that they rarely have to reach for “the long needle” when using US for obese patients.

US is  very useful in patients that have had back surgery and have kyphoscoliosis – you can identify the the rods and spaces that have been spared. In addition, some patients have abnormal anatomy of the ligmentum flavum, and this might result you in using a different interspace or avoid, for example, placing an epidural. NICE, in the UK, first advocated the use of US in 2008; perhaps prematurely.

Spinal ultrasound is very easy – there are two patterns – the transverse approach that looks like the batman (or flying bat) sign. The saggital plane is used for identifying the sacrum – it looks like a saw (the teeth of the saw – peaks and troughs): you can then find the interspaces. A curvilinear probe is used – it is the same transducer that the obstetricians use – low frequency but high penetration.

The saggital view is used first – it is paramedic – this gives you the saw sign. The first trough seen is the L5,S1 interspace. This tells you where you are! You can place a mark adjacent to the level that you have chosen – and this is your horizontal level.

When you do the transverse scan – you see the sharp (paper cut) dural space – and you can measure, clearly, from the skin, the depth of the dura. Then you mark at the level that you had a great view of the space (vertically). You have now 2 marks on the skin – draw intersecting lines – and voila the point that you find is the needle insertion site!

Depth estimation is very accurate with ultrasound, but not as much with obese patients as you tend to compress fat – and underestimate the depth. Dr Carvalho is now using a paramedian approach to offset this problem.

A huge advantage of US is that you often find that some interspaces are very difficult to access (“bone, bone, bone”), some, in the same patient, are easy. Read here for more information.

Finally, Dr Carvalho is currently using ultrasound to look at stomach contents – whether the stomach of a laboring patient is full or empty. Two groups, one in France and one in Toronto are actively researching this field. Abstracts here and here. Food an air (full stomach) demonstrates a “frosted glass” sign.

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