If you have given more than a thousand intramuscular injections, you have probably hit bone at least once or twice. This is not the “Eureka!” kind of discovery that one gets from striking oil or seeing that crimson flashback from a successful and intentional arterial puncture. Pricking the periosteum is more of a fingernails-on-the-blackboard moment. Even if somehow you managed to ignore the bone-chilling vibrations that traveled up the syringe, the patient's response told you that going too deep is a bad thing.
In 2007, the Centers for Disease Control and Prevention released a set of guidelines that include a recommendation for longer needles when giving intramuscular injections (Epidemiology and Prevention of Immunization Practices: The Pink Book, 10th ed., Feb. 2007), up to 1 inch for the deltoid and 11/4 inch for the thigh. Their concern was that a failure to penetrate well into the muscle would result in suboptimal vaccine absorption.
At a lunchtime staff meeting that I somehow neglected to attend (it must have been a warm sunny day—or maybe just not raining) we decided that we should comply with the CDC's guidelines and we ordered a couple of boxes of longer needles. Trying to be a good soldier and a team player, I agreed to give these big boys a try. It was an ugly week.
One of my secrets for giving minimally painful injections is to prevent the patient and/or parent from seeing the business end of the syringe. However, even with my best sleight-of-hand techniques, I was having trouble hiding the longer needles. Whenever the patient caught a glimpse of these ice picks, the anxiety level in the room escalated and it was downhill from there.
With longer needles, I was uncomfortably out of control. Wielding an extra few millimeters of steel, I felt like a jousting knight of the Round Table galloping down the runway, my 10-foot lance wobbling in the wind, never quite sure if I was going to hit my intended target. When I did strike pay dirt, watching a 1-inch needle flexing back and forth in the thigh of a squirming toddler was more than I could take. I wasn't going to wait until I had performed an unintended bone marrow biopsy, and we halted the experiment.
It wasn't the first time I had ignored the CDC's recommendation, but I admit that I have harbored a few second thoughts. However, recently I have been rescued from my insecurity by William C. Lippert and Dr. Eric J. Wall, whose article, “Optimal Intramuscular Needle-Penetration Depth,” reassured me that I can stick with my shorter needles (Pediatrics 2008;122:e556–63). Using CT and MRI scans, they have determined that if one followed the CDC guidelines one would overpenetrate the thigh of 11% of the patients using a 1-inch needle and 39% of the patients if a 11/4-incher were used. For the deltoid, the numbers were more troubling. Overpenetration would occur 11% of the time with a 5/8-inch needle, 55% for a 7/8-inch, and a walloping 63% of the time with a 1-inch needle. Ouch!
The authors, neither of whom is a pediatrician, recommend shorter needles for thigh injections and offer weight- and gender-based guidelines to aid in selection of the correct length. In their opinion, the current CDC guidelines for deltoid injections were still appropriate. However, the CDC's Advisory Committee on Immunization Practices has decided to ignore this new creatively obtained evidence. (See story, page 8—Ed.) And so, I will continue to ignore the committees and inject to the beat of my own drummer. I will temper the new evidence with my own experience. In the discussion portion of the paper, Mr. Lippert and Dr. Wall buried a pearl that supports my rogue practice. They said that in light of the great variability in extremity dimensions, “clinical judgment be used and an assortment of different lengths be available.”
Three cheers for a combination of common sense and carefully done science.