7) extremes of weight Flashcards
(24 cards)
What are the key weight classifications used in prescribing at extremes of weight?
- Actual Body Weight (ABW): Current weight including fat, muscle, bone, and water.
- Total Body Weight (TBW): Same as ABW, used interchangeably.
- Lean Body Weight (LBW): Weight excluding fat; includes muscle, organs, bones, and water.
- Ideal Body Weight (IBW): Estimated weight based on height, used for assessing drug dosing.
- Adjusted Body Weight (ABW): Modified for obese individuals to account for excess fat while considering lean mass.
How is obesity defined clinically, and what are its limitations?
Obesity is often defined by BMI ≥30. Limitations: BMI does not account for fat distribution, muscle mass, or the health status of fat (visceral vs. subcutaneous).
What are the key physiological changes in obesity that affect pharmacokinetics?
1) Total Body Water: Increased, leading to increased volume of distribution (Vd) for water-soluble drugs.
2) Fat Mass: Increased, which alters Vd for lipophilic drugs.
3) Lean Body Mass: Increased, potentially leading to greater clearance of drugs metabolized by the liver.
4) Plasma Proteins: Increased, leading to more plasma protein binding and less free drug.
5) Cardiac Output: Increased, potentially enhancing drug clearance.
How do physiological changes in obesity affect drug dosing and clearance?
- Increased fat mass: Affects lipophilic drug dosing by increasing drug sequestration in fat tissue.
- Increased lean body mass: Leads to increased clearance of drugs that are metabolized by the liver.
- Total body water increase: Affects the distribution of hydrophilic drugs.
How does malnutrition impact drug pharmacokinetics?
- Decreased plasma proteins: Leads to decreased protein binding, increasing the free drug concentration and potentially increasing the risk of toxicity.
- Reduced lean body mass: Affects drug metabolism and clearance due to decreased hepatic blood flow and enzyme activity.
- Increased drug sensitivity: Malnourished individuals may have altered response to drugs, requiring dose adjustments.
What specific pharmacokinetic changes occur in malnutrition that require dosing adjustments?
- Decreased albumin levels: Less plasma protein for drug binding, increasing the unbound drug concentration.
- Decreased liver function: Slower metabolism of drugs, requiring dose reductions in liver-metabolized drugs (e.g., warfarin).
- Changes in renal function: Reduced renal clearance, requiring dose adjustments for renally excreted drugs.
Why is it important to adjust drug dosing in patients with extreme obesity?
Obesity alters pharmacokinetic parameters (e.g., Vd, clearance) and increases the volume of distribution for lipophilic drugs. Dose adjustments are essential to avoid subtherapeutic effects or toxicity. Guideline recommendations: Adjust doses based on adjusted body weight (ABW).
How does body weight affect the dosing of paracetamol in both low and high body weight patients?
Low body weight (<50 kg): Risk of toxicity due to other factors like liver function and malnutrition.
High body weight: Standard doses may be less effective for pain relief, though toxicity is less of a concern.
What specific considerations should be made when prescribing paracetamol in underweight patients?
Clinical review: Check for conditions like liver disease or alcohol use that may increase toxicity risk. Lower doses may be required if the patient has liver dysfunction or long-term alcohol use.
How does body weight affect the pharmacokinetics of DOACs like apixaban?
- Obesity (BMI > 40kg/mg or weighs more than 120 kg): Can reduce apixaban exposure by approximately 30%.
- Low body weight (<50 kg): Increases exposure to apixaban, raising the risk of bleeding.
- Renal function: In underweight patients, renal function can be overestimated, leading to improper dosing.
What pharmacokinetic considerations are important when prescribing DOACs for obese patients?
Obesity alters the volume of distribution (Vd) for lipophilic drugs, which may reduce the effectiveness of DOACs. At BMI >50 kg/m², plasma levels should be monitored, and switching to VKA therapy may be considered if necessary.
How does extreme obesity affect the pharmacodynamics of emergency hormonal contraception (EHC)?
Levonorgestrel (LNG): Less effective in women with BMI >26 kg/m² or weight >70 kg. Ulipristal acetate (UPA-EC): Less effective in women with BMI >30 kg/m² or weight >85 kg. Adjustments, like a double dose of LNG or using UPA-EC, may be necessary.
What role does adjusted body weight (ABW) play in prescribing drugs for obese patients?
ABW helps to more accurately estimate the appropriate dose by considering both fat and lean mass. Adjustments using ABW ensure drugs are neither underdosed nor overdosed in obese patients.
How does bariatric surgery affect drug absorption and dosing?
Post-bariatric surgery: Altered gastric pH, surface area, and transit time can reduce the absorption of lipophilic drugs like rivaroxaban. Monitoring of drug levels is essential, and switching to VKAs may be appropriate for some patients.
Why should drug dosing be adjusted in pediatric and elderly patients with extreme weight?
- Pediatric patients: Higher drug metabolism and renal clearance in children may necessitate higher doses in some cases.
- Elderly patients: Decreased hepatic and renal function may require dose reductions, even in normal-weight individuals.
How does extreme weight influence the clearance of lipophilic and hydrophilic drugs?
- Lipophilic drugs: Increased fat mass leads to reduced clearance and potentially prolonged drug effects.
- Hydrophilic drugs: Increased total body water can increase the Vd and alter clearance, requiring adjustments for dosing.
What is the significance of body weight in prescribing NOACs for patients with extreme obesity or low body weight?
Obesity: Affects volume of distribution for NOACs, leading to potential underdosing. Plasma monitoring may be needed for patients with BMI >40 kg/m². Low body weight: Increases exposure to NOACs, increasing bleeding risk, requiring dose reduction in some cases.
How does low body weight affect the use of NOACs like apixaban and edoxaban?
Low body weight (<60 kg): Requires dose reduction for apixaban and edoxaban, especially in older patients or those with reduced renal function. Special care is needed in underweight patients as they may have altered drug metabolism and clearance.
Why might switching to warfarin (VKA) be considered for patients with a BMI >50 kg/m² on DOACs?
Increased weight affects the volume of distribution and metabolism of DOACs. Warfarin has well-established dosing guidelines and monitoring protocols, which may be more effective for patients with extreme obesity.
What are the major risks associated with prescribing anticoagulation therapy (DOACs or VKAs) for patients with extreme weight?
Increased bleeding risk in underweight patients and underdosing risk in obese patients due to altered pharmacokinetics. Close monitoring of anticoagulant effects (INR, plasma levels) is essential for both groups to ensure safety and efficacy.
What adjustments should be made for drugs with narrow therapeutic windows (e.g., warfarin) in obese or underweight patients?
Obese patients: May need higher doses due to increased clearance, but monitoring is essential. Underweight patients: May require lower doses as their plasma concentrations are higher due to decreased lean mass and lower metabolic clearance.
How does extreme weight affect the pharmacokinetics of drugs that are metabolized by the liver?
Obesity can increase hepatic blood flow, which may result in faster metabolism and require higher drug doses for liver-metabolized drugs. Underweight patients with decreased liver function may metabolize drugs more slowly, requiring dose reductions.
What factors must be considered when prescribing drugs for pediatric patients with extreme weight?
Increased clearance: Children have faster metabolism, meaning drug doses may need to be higher, even in overweight patients. Drug distribution: Body composition in pediatric patients (higher water content) may affect drug distribution and necessitate dosing adjustments.
What are the dosing considerations for elderly patients with extreme weight (either low or high)?
Obese elderly: May require lower doses due to reduced hepatic and renal function despite increased body weight. Underweight elderly: May require dose reductions due to reduced protein binding and higher drug concentrations.
