Factors Affecting Oral Drug Absorption and Distribution
The position of the patient when the drug is taken (sitting vs. standing) does not significantly affect an orally-administered drug's absorption and/or distribution in the body. While patient position may have minor effects on gastric emptying rates, it is not considered a major determinant of drug absorption compared to the other factors listed 1.
Major Factors Affecting Oral Drug Absorption and Distribution
1. Patient's Age
- Age significantly affects drug pharmacokinetics through multiple mechanisms 1:
- Reduced gastric acid production and emptying in older adults
- Decreased splanchnic blood flow and GI motility
- Changes in body composition (increased fat mass, decreased total body water)
- Reduced hepatic blood flow (20-30% decrease)
- Decreased plasma albumin levels affecting drug binding
- Reduced renal function affecting drug elimination
2. Integrity of the Gastrointestinal Tract
- Critical for drug absorption as it determines 1:
- Surface area available for absorption
- Activity of gut wall transporters (P-glycoprotein, etc.)
- First-pass metabolism capability
- Presence of metabolizing enzymes
- Mucosal barrier function
3. Regional Blood Flow
- Significantly impacts drug absorption and distribution 1:
- Hepatic blood flow directly affects clearance of drugs with high extraction ratios
- Splanchnic blood flow determines rate of drug delivery to the liver
- Cardiac output affects tissue perfusion and drug distribution
- Regional blood flow changes can alter bioavailability of drugs
Why Patient Position Has Minimal Impact
While some research suggests patient position can influence gastric emptying rates 2, 3, these effects are:
- Relatively minor compared to the physiological factors listed above
- Primarily affect the rate rather than the extent of absorption
- Often clinically insignificant for most medications
- Easily overwhelmed by other factors like food intake, drug formulation, and patient physiology
Clinical Implications
When considering factors affecting drug absorption and distribution, clinicians should focus on:
- Age-related physiological changes that may require dose adjustments
- Gastrointestinal disease states that might impair absorption
- Hepatic and renal function that affect drug metabolism and elimination
- Drug interactions affecting metabolizing enzymes (CYP450) and transporters (P-gp)
Understanding these factors is essential for optimizing drug therapy and minimizing adverse effects, particularly in special populations like elderly patients or those with gastrointestinal disorders.