This post kicks off a deep-dive in to evidence-based preparation of acute wounds, from the nuanced details of needle selection to antiseptic use prior to suturing.  I hope you enjoy!

You are getting ready to inject some anesthetic in to a fresh laceration, to facilitate painless cleansing prior to primary closure.  You open the suture cart and take a look at the variety of needles available.

Which needle?
What size needle would you use to inject local anesthetic in your patient’s wound, and why?

Which one will you choose, and why?

At least a few recent studies have delved in to the nuances of how to minimize injection pain when considering local infilitration of anesthetics.

This well-designed Norwegian study used a group of healthy volunteers to compare injection pain when given subcutaneous 1% lidocaine on the abdomen using a 21 G vs a 23 gauge vs a 27 gauge needle. The pain experienced was reported by the subjects on a Visual Analog Scale (VAS).  The study controlled for: angle of injection (45 degrees), volume of injection (3ml), rate of injection (over 20 seconds) and depth of injection (19 mm). They also controlled for host factors (no chronic painkiller use, no overlying eczema, no serious underlying medical disease).  All study participants were well-incentivized (they got a pair of movie tickets…would you have signed up?).

What do you think this study showed? You may be surprised.  Oddly, though the study recruited sufficient subjects to actually be overpowered, no statistical difference emerged with respect to the VAS ratings.

This seems counter-intuitive.  Smaller needles hurt less, right?  To hash this out, the authors also asked the participants to rate each of the injections from most to least painful. Using this verbal rating scale, a statistical difference favoring the 27 G needle did emerge.

What’s the take-away here? 

A 27 G needle is very likely less painful,

but maybe not so much less painful that it actually matters if you use a 21G or 23G.

Also, keep in mind that the rate of flow through a needle is proportional to the fourth power of the radius of that needle. If that makes your brain hurt, just remember that this means: all other factors being equal, the flow rate through a 27 gauge needle is just a fraction of the flow rate through a 23 gauge needle.

So perhaps, its not really fair to just think about the needle size in isolation when thinking about pain.  Maybe it’s the injection speed that makes the difference. The same study group went to task to answer this question.

The study design was very similar. This time, they gave 4.5 ml of 1% lidocaine over a period of either 15 seconds, 30 seconds, or 45 seconds.   The results: no difference nor meaningful trend was determined between the 3 injection speeds. When the authors further interviewed their subjects, it appears that the subjects could definitely sense more severe pain over a shorter period (15 seconds) compared with less severe pain delivered over a longer period (45 seconds).

Some subjects felt that less severe pain dragging out over a longer period of time was actually worse then a shorter duration of more severe pain. As it turns out, there appears to be marked variation in the way people take their pain. I suppose then the right question to ask a patient before you begin an injection of local anesthetic should then be, “Are you a rip-the-bandaid-off-fast kind-of-person or a remove-the-bandaid-slow kind of person?”

More recently, a dermatologist published a method he uses to minimize needle insertion pain accounting for needle size and injection speed.  He described what he coined the “parallel, minimal needle insertion technique.”  He reported that in a group of 146 patients receiving anesthesia of the head and neck for Moh’s microsurgery, over half the patients reported no pain on a 1-10 VAS, resulting in a mean VAS pain score of less than one.

How does he do it?  His technique involves use of a 30 G needle, and buffered 0.5% lidocaine with 1:200,000 epinephrine (we’ll talk about buffering in a future post). He inserts the needle tip parallel to the skin with the bevel down, applying constant, light pressure on the syringe to slowly deliver the anesthetic, as if performing a wheal. He then advances the needle within the blanched area of skin where the anesthetic has already penetrated.

Here’s a video of the technique, as adapted for an emergency medicine application: abscess incision and drainage.

Clearly, many problems exist with a “study” in which a single operator performs the intervention (namely, generalizability and reproducibility).  Still, there may be something to be said for a kinder, gentler delivery of anesthetic.


But I digress.  Back to the academic angle:  to summarize, these studies don’t prove a significant benefit in pain reduction based on either the needle size or injection speed.

Where does this leave us?  If it’s not the needle size nor the speed of injection that’s causing your patient pain, maybe it’s more a matter of what’s inside of it.  In Part II of this series, we’ll discuss in detail what you are using for your anesthetic and how this affects your patient’s pain experience.