Getting an accurate weight for patients before procedures that require anesthesia is crucial as it affects medication dosage, ventilator parameters, airway control, intravenous complications, and fluid management.1 Anesthesiologists use various formulas and calculations to determine the appropriate dosage of anesthetic drugs for each patient. These calculations consider the patient’s age, gender, weight, and comorbidities that can affect the metabolism of the drugs. Drug dosage calculations generally use one of the following three methods: set dosage schema, weight-based dosing, or body surface area (BSA)-based dosing.4 Because many anesthetic medicines are delivered intravenously, an incorrect body mass assessment might result in the wrong dose, resulting in substantial side effects and comorbidities. 4 For instance, under-dosing on anesthetic drugs may result in inadequate anesthesia, pain, and suffering during and after surgery. 4 Alternatively, if the patient’s weight is overestimated and the anesthesiologist administers a higher dose of anesthetic medicines than is necessary, the patient may experience respiratory or cardiovascular complications.4 As a result, an accurate weight is crucial for safe and effective anesthesia.
The majority of individuals may be weighed and measured prior to undergoing anesthesia. Alternately, patients may be questioned, in which case they often provide relatively accurate weight estimates.1 Nevertheless, patients cannot always be measured or provide estimations due to mental capacity, immobility, emergency trauma, or unconsciousness. Physicians can estimate a patient’s weight if needed.1 However, the accuracy of the estimation varies from observer to observer, which might exacerbate dose mistakes, particularly in emergencies.1
Anesthesiologists also face substantial challenges when treating morbidly obese individuals who require pharmacologic adjustments owing to obesity-specific pharmacokinetic variations.3 Numerous studies have shown the impact of obesity on metabolic, cardiovascular, and pulmonary function and the increased risk of anesthesia.3 Fat mass per kilogram of total body weight can substantially impact the apparent volume of distribution of certain medications – in other words, how much reaches the tissues. In addition, changes in cardiac output, total blood volume, and regional blood flow may impact the peak plasma concentration, clearance, and elimination half-life of several anesthetics.3 For instance, cardiac and respiratory function abnormalities associated with morbid obesity exacerbate the adverse effects of anesthetics and reduce the therapeutic window. 3 To compensate for some of the anthropometric and physiological changes caused by obesity, dosing guides other than total body weight, such as ideal body weight, body surface area, body mass index, and lean body weight, have been utilized. 3
Dosing anesthetic drugs during anesthesia can be challenging without an accurate body weight. Using estimations may result in overdosing or underdosing, jeopardizing the patient’s safety and recovery. It is important to note that drugs generally administered by a set dosage schedule may need dose adjustments to account for body weight, composition, and concomitant pathology variations. This is particularly important for morbidly obese patients with changes in body composition, cardiac output, and regional blood flow. A thorough understanding of the most frequently used anesthetic drugs, including induction agents, opioids, inhalation agents, and neuromuscular blockers, in relation to body mass is critical for safe and effective management during procedures.
References
- Coe, T.R., Halkes, M., Houghton, K. and Jefferson, D. (1999), The accuracy of visual estimation of weight and height in pre-operative supine patients. Anaesthesia, 54: 582-586. https://doi.org/10.1046/j.1365-2044.1999.00838.x
- Hebbes CP, Thompson JP. Pharmacokinetics of anaesthetic drugs at extremes of body weight. BJA Educ. 2018 Dec;18(12):364-370. doi: 10.1016/j.bjae.2018.09.001. Epub 2018 Oct 26. PMID: 33456803; PMCID: PMC7808064.
- Ingrande, J., & Lemmens, H. J. (2010). Dose adjustment of anaesthetics in the morbidly obese. British journal of anaesthesia, 105 Suppl 1, i16–i23. https://doi.org/10.1093/bja/aeq312
- Nafiu, O. O., Mills, K., & Tremper, K. K. (2018). Some Cautionary Tales About Ideal Body Weight Dosing of Anesthetic Medications: It Is Not All That Ideal!. Anesthesia and Analgesia, 127(2), 586–588. https://doi.org/10.1213/ANE.0000000000002662