In this episode, I’ll discuss how obesity affects SC and IM medication administration.
The thickness of the subcutaneous layer increases with obesity and this has potential implications for both subcutaneous and intramuscular medication administration.
The AJHP recently published a review article on the implications of obesity for drug administration and absorption from subcutaneous and intramuscular injections.
The most significant issue highlighted by this review article is:
Since for a successful IM injection, the needle must enter the muscle by at least 5 mm, a BMI above 30 is likely to decrease the chance of a successful IM injection if the needle length is unchanged from the standard length. This complication is especially significant in male patients.
Since IM absorption is usually more rapid than with the SC route, an injection that is intended to be given IM but instead is given SC can alter the rate and extent of medication absorption.
This could become of clinical significance in the administration of vaccines, benzodiazepines, and antipsychotics.
Unfortunately, the authors of the review do not specify the ideal needle length to use in obese patients to achieve a successful IM administration of these medications.
A previous study has been published on this topic in JAMA:
Among healthy adults of the age range studied, the following needle lengths appear to be appropriate for true deltoid intramuscular immunization:
For men across the weight ranges studied (59-118 kg), use of a 25-mm (1-in) needle would result in at least 5 mm of muscle penetration in all subjects.
For women who weighed less than 60 kg, a 16-mm (5/8-in) needle would be sufficient to achieve muscle penetration of 5 mm.
For women between 60 and 90 kg, a 25-mm (1-in) needle would be sufficient
Women greater than 90 kg would require a 38-mm (1.5-in) needle to ensure intramuscular administration.
Yet another article has published a formula that can be applied to estimate the deltoid subcutaneous fat pad thickness.
According to this formula, A 1-inch needle is expected to reach the deltoid in 85.3% of women less than 200 pounds, and 98.6% of men less than 260 pounds and a 1.5-inch needle is expected to reach the deltoid in 76.7% of women greater than 200 pounds, and 75.0% of men greater than 260 pounds.
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