Obesity Does Not Affect Propofol Pharmacokinetics During Hypothermic Cardiopulmonary Bypass

Impact of Obesity on Hepatic Drug Clearance: What are the Influential Variables?

Drug clearance in obese subjects varies widely among different drugs and across subjects with different severity of obesity. This study investigates correlations between plasma clearance (CLp) and drug- and patient-related characteristics in obese subjects, and evaluates the systematic accuracy of common weight-based dosing methods. A physiologically-based pharmacokinetic (PBPK) modeling approach that uses recent information on obesity-related changes in physiology was used to simulate CLp for a normal-weight subject (body mass index [BMI] = 20) and subjects with various severities of obesity (BMI 25-60) for hypothetical hepatically cleared drugs with a wide range of properties. Influential variables for CLp change were investigated. For each drug and obese subject, the exponent that yields perfect allometric scaling of CLp from normal-weight subjects was assessed. Among all variables, BMI and relative changes in enzyme activity resulting from obesity proved highly correlated with obesity-related CLp changes. Drugs bound to α1-acid glycoprotein (AAG) had lower CLp changes compared to drugs bound to human serum albumin (HSA). Lower extraction ratios (ER) corresponded to higher CLp changes compared to higher ER. The allometric exponent for perfect scaling ranged from -3.84 to 3.34 illustrating that none of the scaling methods performed well in all situations. While all three dosing methods are generally systematically accurate for drugs with unchanged or up to 50% increased enzyme activity in subjects with a BMI below 30 kg/m2, in any of the other cases, information on the different drug properties and severity of obesity is required to select an appropriate dosing method for individuals with obesity.

Drug pharmacokinetics in the obese population: challenging common assumptions on predictors of obesity-related parameter changes

INTRODUCTION

Obesity is associated with many physiological changes. We review available evidence regarding five commonly accepted assumptions to a priori predict the impact of obesity on drug pharmacokinetics (PK).

AREAS COVERED

The investigated assumptions are: 1) lean body weight is the preferred descriptor of clearance and dose adjustments; 2) volume of distribution increases for lipophilic, but not for hydrophilic drugs; 3) CYP-3A4 activity is suppressed and UGT activity is increased, implying decreased and increased dose requirements for substrates of these enzyme systems, respectively; 4) glomerular filtration rate is enhanced, necessitating higher doses for drugs cleared through glomerular filtration; 5) drug dosing information from obese adults can be extrapolated to obese adolescents.

EXPERT OPINION

Available literature contradicts, or at least limits the generalizability, of all five assumptions. Clinical studies should focus on quantifying the impact of duration and severity of obesity on drug PK in adults and adolescents, and also include oral bioavailability and pharmacodynamics in these studies. Physiologically-based PK approaches can be used to predict PK changes for individual drugs, but can also be used to define in general terms based on patient characteristics and drug properties, when certain assumptions can or cannot be expected to be systematically accurate.

Propofol pharmacokinetics and pharmacodynamics-a perspective in minimally invasive extracorporeal circulation

There is limited evidence as to the pharmacokinetic changes expected in adults with extracorporeal technologies. Drugs may be taken up by various components of the cardiopulmonary bypass circuit itself. Issues include the increased volume of the circuit leading to haemodilution; the sequestration of lipophilic drugs within the circuit tubing; and the absorption of proteins, especially albumin, onto the circuit, which can result in increased free drug. However, in this context, the aspect of pharmacokinetics and pharmacodynamics during minimally invasive extracorporeal circulation has not been described and evidenced by scientific studies. In this single-centre control study of 60 patients undergoing isolated coronary artery bypass grafting, we present the results focused on postoperative albumin values and intraoperative propofol dosages in patients undergoing surgery with minimally invasive (n = 30) versus conventional extracorporeal circulation (n = 30). In the minimally invasive extracorporeal circulation group, a lower propofol dosage titrated to a bispectral index of 40-45 was used during coronary artery bypass grafting, and an improvement of postoperative concentration of serum albumin was observed compared to the conventional extracorporeal circulation group.

Volatile Anesthesia Versus Total Intravenous Anesthesia During Cardiopulmonary Bypass: A Narrative Review on the Technical Challenges and Considerations

The Mortality in Cardiac Surgery Randomized Controlled Trial of Volatile Anesthetics (MYRIAD) demonstrated that cardiac surgery with either volatile anesthesia or intravenous anesthesia techniques can be comparable with respect to morbidity and mortality. Maintaining anesthesia during cardiopulmonary bypass (CPB) with either approach requires appreciation of the nuances that are unique to each. This narrative review addresses these technical challenges and other considerations.

Perioperative Care of the Obese Cardiac Surgical Patient

Morbid obesity is associated with impairment of cardiovascular, pulmonary, gastrointestinal, and renal physiology with significant perioperative consequences and has been linked with higher morbidity and mortality after cardiac surgery. Cardiac surgery patients have a higher incidence of difficult airway and difficult laryngoscopy than general surgery patients do, and obesity is associated with difficult mask ventilation and direct laryngoscopy. Positioning injuries occur more frequently because obese patients are at greater risk of pressure injury, such as rhabdomyolysis and compartment syndrome. Despite the association between obesity and several chronic disease states, the effects of obesity on perioperative outcomes are conflicting. Studies examining outcomes of overweight and obese patients in cardiac surgery have reported varying results. An "obesity paradox" has been described, in which the mortality for overweight and obese patients is lower compared with patients of normal weight. This review describes the physiologic abnormalities and clinical implications of obesity in cardiac surgery and summarizes recommendations for anesthesiologists to optimize perioperative care of the obese cardiac surgical patient.