Description

Propofol is the most used intravenous agent for induction and maintenance of anaesthesia as part of total intravenous anaesthesia (TIVA) regimen.

In the morbidly obese, various factors, such as, increased body fat content, lean body weight, cardiac output, total blood volume, and alterations in regional blood flow; which adversely/unpredictably affect the volume of distribution, clearance and elimination of intravenous anesthetic drugs, thereby making administration of TIVA difficult to control.

A major concern with propofol dosing based on total body weight (TBW) in the obese patients is disproportionate drug administration leading to undue drug accumulation in body with potential overdosing, delayed recovery from anaesthesia, and adverse hemodynamic outcome. Studies on propofol dose regimen for TIVA recommended that LBW should be used for calculating bolus dose during induction of anaesthesia and TBW or ABW for arriving at a infusion dose required for maintenance of anaesthesia.

Propofol requirement for induction of anaesthesia is based on LBW is especially relevant for the morbidly obese patients as because their surplus fat mass increases volume of distribution of propofol, which, in the face of decreased blood flow to adipose tissue; imposes the burden of potential drug accumulation. This may result in increased drug delivery to non-adipose tissue during induction of anaesthesia and possibly leading to undesirable rarefaction of depth-of-anaesthesia and attendant adverse haemodynamic effects.

Conversely, during the maintenance phase of propofol TIVA, the volume of distribution and clearance of propofol increases and correlates linearly with TBW. In this respect, controlling propofol delivery in the morbidly obese with Eleveld allometric PK model, which utilizes TBW as weight parameter; has been found to be superior to other models that employs other weight dosing scalars.

The use of ABW in Schnider and Marsh model takes into consideration drug distribution to lean tissues as well as a proportion to the body fat weight, thus accounting for lipid solubility dynamics of propofol. ABW is calculated by adding 40% excess fat weight (FW) to IBW.

In obese patients propofol delivery using the Eleveld allometric PK model by incorporating TBW has been found to be superior to other models using other dosing scalars.

Target-controlled infusion (TCI) forms the core of standard-of-care method used for administration of propofol TIVA. TCI system typically includes a microprocessor-controlled syringe pump that is designed to achieve a defined plasma concentration of the drug based on patient response and multi-compartment pharmacokinetic (PK) model.

While TCI systems are designed to deliver propofol at a rate based on a predetermined plasma concentration, they do not take into consideration patient's pharmacodynamic profile. Hence, it is difficult to determine whether the target plasma concentration achieved has produced adequate anaesthesia depth. In the absence of reliable depth of anaesthesia monitors, during the maintenance phase of propofol TIVA, the desired plasma concentration achieved may either result in intraoperative awareness due to under-dosing or delayed recovery from anaesthesia because of over-dosing.

Currently, an array of research on automated propofol delivery using computer-controlled closed loop anaesthesia delivery systems which deliver propofol based on patient's frontal cortex electrical activity as determined by bispectral index (BIS); have amply exhibited that these systems deliver propofol and maintain depth of anaesthesia with far more precision as compared to manual administration.

Liu et al used a BIS-guided dual loop anaesthesia delivery system to determine requirement of propofol and remifenatnil in the obese versus lean patients. The propofol delivery was controlled by a closed loop set through TCI pump. Propofol was delivered with dose calculation by TBW and based on the Schnider model. The propofol dosage delivered as per TBW in real-time was analyzed post hoc on a IBW scale. The propofol requirements for induction and maintenance based on TBW was equivalent both in obese and lean patients.

CLADS is a BIS-guided automated closed-loop anaesthesia delivery system developed by Puri, which delivers propofol using a non-TCI infusion pump. This system uses a control algorithm that is based on the relationship between diverse rates of propofol infusion and BIS variable. CLADS regulates the propofol infusion rate to maintain a predetermined BIS target (BIS=50) and is independent of plasma propofol concentration status. CLADS is uniquely versatile in that it can calculate propofol dosage delivered both on basis of TBW or IBW.

Whereas, in a study comparing CLADS administered propofol versus desflurane-GA in morbid obese patients undergoing bariatric surgery (unpublished data) the propofol maintenance dosage based on ABW was 5.5 + 1.3 mg kg-1 h-1; Liu et al reported a median propofol consumption of 5.2 [4.1, 6] mg kg-1 h-1 with their dual-loop closed loop anaesthesia delivery system that also utilized TBW based administration of propofol and remifentanil. In both the study, BIS was used as an input control actuator to close the feedback loop joining the patient, the delivery system, and the infusion flow system.

Although maintenance of propofol TIVA based on TBW is well established, the dosing schedule based on ABW is not well explored. Since the ABW takes into consideration a certain percentage of FW in addition to IBW and not the complete FW as in TBW, we hypothesize that propofol dosing using ABW will result in lower propofol requirement as compared to TBW for maintaining equivalent anesthetic depth. Since CLADS gives an objective assessment of propofol dose delivered and anaesthesia depth consistency, this randomized study aim to compare the maintenance requirements of propofol in obese patients given propofol dosing based on TBW versus ABW