Morbidly obese patients undergoing general anesthesia represent a challenge for the anesthesiologist as multiple co-morbidity might compromise their physiological status. Therefore, special anesthetic approaches have to be considered when anesthetizing morbidly obese patients. In addition to the physiological challenges, pharmacological changes associated with obesity might lead to alterations in the distribution, binding and elimination of many drugs. The net pharmacokinetic effect in these patients is often uncertain, making drug titration even more difficult and unpredictable.
Beside kinetics, pharmacodynamic changes can be seen in morbidly obese patients. Additionally, many package inserts, including a wide variety of drugs like benzodiazepines, opioids, intravenous (i.v.) anesthetic agents, volatile anesthetic agents, muscle relaxants, local anesthetics and other drugs such as those influencing the cardiovascular system, explicitly provide per-kilogram adult-dosing guidelines. Doesn't this tell us that the drugs should be given per kilogram of body weight? That is the message, but it may be wrong. One of the problems in providing anesthesia for morbidly obese patients is how does obesity influence the pharmacokinetics and pharmacodynamics compared to non-obese patients. As a result, multiple questions have to be answered: Can pharmacokinetic-based drug administration be used as safely in obese patients as it is being used in the non-obese or do we need to correct them and in what way? What about pharmacodynamics and pharmacodynamic monitoring, if available?
Changes in pharmacokinetics
In clinical anesthesia practice, most drugs are given using standard-dosing guidelines without applying knowledge of their pharmacokinetics to control their administration.