Dose adjustment of anaesthetics in the morbidly obese

Abstract

Anaesthesiologists must be prepared to deal with pharmacokinetic and pharmacodynamic (PD) differences in morbidly obese individuals. As drug administration based on total body weight can result in overdose, weight-based dosing scalars must be considered. Conversely, administration of drugs based on ideal body weight can result in a sub-therapeutic dose. Changes in cardiac output and alterations in body composition affect the distribution of numerous anaesthetic drugs. With the exception of neuromuscular antagonists, lean body weight is the optimal dosing scalar for most drugs used in anaesthesia including opioids and anaesthetic induction agents. The increased incidence of obstructive sleep apnoea and fat deposition in the pharynx and chest wall places the morbidly obese at increased risk for adverse respiratory events secondary to anaesthetic agents, thus altering the PD properties of these drugs. Awareness of the pharmacology of the commonly used anaesthetic agents including induction agents, opioids, inhalation agents and neuromuscular blockers is necessary for safe and effective care of morbidly obese patients.

Perioperative management of the severely obese patient: a selective pathophysiological review

“…The potential for incorrect dosing is high due to the impact of increases in cardiac output, extracellular fluid volume, fat mass, and lean body weight (LBW) on pharmacokinetics. 64,65 The increase in LBW is counterintuitive. Although the ratio of LBW to total body weight (TBW) is lower in the obese, the absolute value of LBW is higher than in non-obese subjects of the same sex and height.…”

“…Data are available to provide some guidance on appropriate dosing for common perioperative medications (Table 3). 64,65,[67][68][69][70][71][72][73][74] In Table 4, there is an outline of the calculation methods for the various dosing weights.…”

The 5th National Audit Project (NAP5) on accidental awareness during general anaesthesia: summary of main findings and risk factors

“…Certain types of surgery (e.g. cardiac, obstetric), female sex [21,22], higher ASA physical status [21,23,24] and obesity [25][26][27] are all implicated. The notion of an intrinsic, possibly genetic, resistance to anaesthesia has also been raised, with up to 11% of patients with AAGA having a previous history [8,27] and an important minority of AAGA cases having no apparent cause [9,28].…”

Rocuronio en obesos: ¿Debe dosificarse por peso real o peso ideal?

“…Se ha recomendado que las dosis de los relajantes musculares en obesos sean calculada en base al peso ideal (IBW) debido a que al usar el peso corporal total (TBW) se teme una excesiva prolongación de la duración de acción 14,17 . Los estudios clínicos de duración de los bloqueadores neuromusculares en la literatura no han utilizado el aná-lisis de sobrevida (análisis tiempo-evento), excepto Sluga 23 , quien utilizó test de Log-rank.…”

“…Se ha sugerido que las dosis de rocuronio en obesos sean calculadas en base al peso ideal 7,8,11,14,16 pero esta recomendación está basada más bien en la prudencia que en la evidencia científica 17 . Por esta razón, se decidió diseñar un estudio con el objetivo de comparar el comportamiento del rocuronio al administrarlo en base a peso real (TBW) y a peso ideal (IBW) en pacientes obesos.…”