Explanation for drug not absorbed from abdomen in fatty person
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Why a Drug May Not Be Absorbed Properly from Subcutaneous Tissue in an Obese Person
This is a classic pharmacokinetics question. The short answer: subcutaneous absorption depends on blood flow to the injection site, and adipose (fatty) tissue is poorly perfused.
The Core Principle: Absorption = Blood Flow
As stated clearly in multiple pharmacology textbooks:
"Absorption of drugs from depots in subcutaneous or muscle tissue is directly dependent on the drug formulation and the blood flow to the depot."
- Barash's Clinical Anesthesia, 9e
"Subcutaneous and intramuscular absorption depend on drug diffusion from the site of injection to the bloodstream. The rate at which a drug enters the bloodstream depends on both blood flow to the injected tissue and the injectate formulation."
- Morgan & Mikhail's Clinical Anesthesiology, 7e
Why Fatty/Adipose Tissue Has Poor Blood Flow
Adipose tissue belongs to the "vessel-poor group" (also called the slow tissue group) in pharmacokinetic models. This classification is based on:
| Tissue Group | Perfusion | Examples |
|---|---|---|
| Vessel-rich group | Very high blood flow per gram | Brain, lungs, kidneys, heart |
| Lean/fast tissue group | Moderate blood flow | Skeletal muscle, most splanchnic organs |
| Vessel-poor group | Low blood flow per gram | Adipose tissue, cartilage, tendons |
"The vessel-poor group (slow tissue group) is comprised of tissues that have a large capacity for drug uptake but a limited tissue perfusion... the slowly equilibrating tissues contain the majority of adipose tissue."
- Barash's Clinical Anesthesia, 9e
What Happens in an Obese Person
When you inject a drug subcutaneously into a fatty abdominal wall of an obese person:
- The needle may inadvertently deposit the drug deep into adipose tissue rather than into the intended subcutaneous layer (which sits just below the dermis).
- Adipose tissue has very sparse capillary density relative to other tissues.
- With low blood flow, the drug sits in the fatty depot and diffuses slowly - or not at all - into the systemic circulation.
- This results in delayed, reduced, or unpredictable absorption and a lower peak plasma concentration than expected.
Classic Clinical Example: Subcutaneous Insulin & Heparin
This is especially relevant in clinical practice with subcutaneous insulin or heparin:
"Drug absorption following subcutaneous administration is more unpredictable because of the variability of subcutaneous blood flow, especially in the context of certain disease pathologies; this is the primary reason that subcutaneous heparin and regular insulin administered perioperatively have variable times of onset and maximum effect."
- Barash's Clinical Anesthesia, 9e
In obese patients, the standard needle length (typically 4-8 mm) may not clear the thick fat layer to reach the more vascularized subdermal tissue, so the drug ends up deeper in the avascular fat depot.
Summary Table
| Factor | Consequence in Obese Patient |
|---|---|
| Thick adipose layer under skin | Needle may deposit drug into fat rather than subcutaneous tissue |
| Adipose tissue = vessel-poor group | Very low blood flow, slow drug diffusion into capillaries |
| Poor capillary density in fat | Drug lingers as a depot with minimal systemic uptake |
| Result | Delayed/reduced absorption, unpredictable onset |
Key Takeaway
The fundamental reason is that adipose tissue is relatively avascular - it belongs to the vessel-poor group with low blood flow per gram of tissue. A drug injected into a fatty depot cannot readily cross into the circulation because there are too few capillaries to pick it up. This is why longer needles, rotation of injection sites, or alternative routes (intramuscular or IV) are used in obese patients to ensure reliable drug delivery.
- Barash's Clinical Anesthesia, 9e - Intramuscular and Subcutaneous Administration
- Morgan & Mikhail's Clinical Anesthesiology, 7e - Absorption (Parenteral routes)
- Katzung's Basic & Clinical Pharmacology, 16e - Blood Flow at Site of Administration
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