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Local anaesthetics best served warm (Australian Doctor)

23-Aug-2011

Sophie Cousins

LOCAL anaesthetics are less painful when warmed to body temperature before injection, research finds.

Patients reported significantly less pain when local anaesthetics had been warmed to 37°C, compared with when injected at room temperature, researchers concluded after assessing 18 studies with a total of more than 800 patients.

The finding held true both for subcutaneous and intradermal injections, and for buffered and unbuffered anaesthetics, researchers wrote last month in the Annals of Emergency Medicine.

"There are important clinical implications to these findings," they said. "Warming of local anaesthetic solutions can be performed in almost any clinical setting in which local anaesthetics are used by utilising equipment already available for other purposes, such as thermostatically controlled water baths or incubators."

The studies included in the meta-analysis used a variety of warming approaches, such as fluid warmers, baby food warmers, a warming tray and a syringe warmer. It is unknown, however, whether simply warming the anaesthetic between a doctor's hands would suffice.

Patients in the studies were asked to rate the amount of pain they felt on injection by using a pain scale, which was measured from 0 to 100. There was an average pain reduction of 11 when anaesthetics were warmed, compared with when were left at room temperature.

An explanation for the findings could be that as temperature increases, more anaesthetic passes through cell membranes, resulting in a faster onset of effect, the researchers said. Alternatively, colder solutions might stimulate nociceptors to a greater degree than warmer solutions. Meanwhile, they dismissed concerns that local anaesthetics might degrade on heating.

Annals of Emergency Medicine 2011; 58:86-98.

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Another one of the things we have been teaching/doing for years finally proved right with research!

Pain can also be helped by inclusion of sodium bicarbonate. Pain may also be precipitated by keeping it the fridge - the LA not the bicarb!

For my undergrad research proposal I looked at the issues surrounding the warming of LA prior to injection - would have liked the opportunity to do the trial - but hey ho, not to be be. One of the things that also came up was that there was noticeable reduction in the amount of time to complete anaethesia with warmed LA. Because I had to reduce the number of variables in the proposal I didn't look at buffered (Sodium bicarb) LA, but then we weren't using that in our training anyway. From memory I think that opthamology and a lot of dentists warm LA prior to injection.

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A baby bottle warmer etc. sounds sophisticated. I know of podiatric surgeons who have for many years warmed cartridges of LA (in the packing) by simply keeping them in the trouser pocket of a scrub suit.

This method is not recommended for use with a syringe and a needle intact!

Bill Liggins

I have always 'felt' that it was the relative temperature of the LA solution and blood that was the significant factor, ie if the LA was colder or warmer than the blood the injection would be more 'painful'.

What about the temperature of the needle? Should it also be at the same temperature as the blood or surrounding tissues?

Might the injection result in even less pain if the temperature of the tissues and blood was reduced to that of the LA solution?

Might their even be a temperature, either below or above normal body temperature at which the pain of injection was minimal.

Is this just relevant to local anaesthetics or does it include all injections?

Are injections of LA more painful than other injections?

How painful are LA injections?


In real terms how great is the difference in the level of 'pain' with and without the warmer LA?

Is it worth the effort?

Oops! I seem to have slipped into cynicism. Maybe I just need to get out more.


Bill

Ive been carrying out weekly nail surgery sessions for years and have been a true beleiver in this. Especially in the winter months i walk round all day with the LA cartridges in my scrubs pocket, it works a treat and takes effect much quicker too.....I also warm the toe a little too if cold in the winter

Well, there's 10 areas of research for you!

Bill Liggins

I would imagine the sharpness of the needle and its gauge has an impact. The guage of the needle on the syringe would also have an impact on injection speed. Best to keep the guage middle (25G) of the road with a smaller diameter syringe (3ml) - rather than a 25G with a 30ml syringe.

UWS published a paper from an honours student some time ago reporting that the best method of reducing needlestick pain during a digital block was ice. EMLA was not the best way according to that piece of research. Sodium Bicarb was not evaluated from memory - but we use this and it definitely helps buffer the tissue PH especially in tissue which which is inflammed or infected.

I don't think temperature is the best way to evaluate this - the needlestick is most likely the painful part as when the LA is injected anaesthesia is pretty much immediate. Lignocaine plain has an extremely fast onset. I would imagine the pain on injection of the agent is most likely due to the speed at which it is injected and to some degree its initial temperature. A good, slow injection is relatively painless and with the right needle gauge the needlestick pain is minimal.

I would imagine that it is somewhat relevant to all injections however tissue PH, agent PH, speed of injection, gauge of needle, diameter of ampoule would also be variables that are extremely relevant.

Yes - try injecting yourself with linocaine plain and bupivacaine plan - due to the pKa value of the agent and its action of onset is the most likely reason.

Give yourself one - fast then slow. Post your results. :)

Of course...if you care about your patients pain levels that is. Otherwise just inject away....

i always run the cartidges under the hot tap or put them in your trouser pocket for 1/2 hour it seems to sting less if the la is warm u dont need an 80 quid cartridge warmer just drop in a cup of hot water then dry. simples if you care lots and lots u could always spray with ethyl chloride bp which comes in 50 ml fine jet glass containers this cryo analgesic helps reduce the pain of needle stick.

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An assistant I worked with a number of years ago used to warm the cartridges by tucking them into her bra.

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I have been doing this for years - nothing new there.

However, putting the cartridges in one's bra is not a good idea as it can give a perculiar lop-sided appearance.

regards

Catfoot

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yea but did she let u get them out.

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I wouldn't dare - I'd be in need of medical attention myself if I tried that!

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As was mentioned above speed does have a lot to do with it. Dentists have figured this out and created a computer controlled injection system to control the speed.

http://milesci.com/dental_compudent.html

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I was going to ask if I could just warm them under my arm-pit but that is much funnier

Since being advised on my LA course in 1972 that warming the LA might be helpful, I bought and used a dental LA warmer (I would add a picture if I could remeber how).
About 4 years ago there was a shortage of Citanset and so I stopped using cartidge syringes and it was impractical to ensure the similar safe warming of Marcain 10 mL plastic polyamp steripacks (they did not fit the warmer) and so just kept them in a warm room.
I have not noted patients reporting more discomfort.

I think therefore that it is just important (as has already been said) not to have the LA too cold - as that surely would sting.

I think EMLA and other creams are marginal in their help compared to other factors.

The worst contirbuter to a patient having problems from the LA experience is having had a previous clinician traumatise them (some must use a blunt knitting needle and inject water - by the sound of it).

I also think that the whole range of vairiables need to be managed for a good LA technique;
fine needle, good calm clincian giving reassurance, non-hysterical patient (with some faith in the clinician) and good skill in administration.

Tom