Body Mass Index (BMI) is one of the most commonly used measurements in healthcare, including in anesthesiology. BMI allows healthcare providers to obtain a quick estimate of a person’s weight relative to their height. Although it does not present a complete explanation of a patient’s health, modern use of BMI helps anesthesiologists assess risks, plan dosing, and make important decisions about how to manage a patient’s anesthesia care before, during, and after surgery.

BMI is calculated by dividing a person’s weight in kilograms by the square of their height in meters; the normal range is 18.5-24.9kg/m².¹ It is used to categorize individuals as underweight, normal weight, overweight, or obese. Although BMI does not directly measure body fat, it is widely used as a screening tool for estimating a patient’s risk of certain conditions.

High BMI is associated with many medical conditions that make anesthetic management more complex. Obesity is linked to an increased risk of airway management challenges, respiratory complications such as obstructive sleep apnea, difficulty with intravenous access and regional blocks, aspiration, and delayed gastric emptying². Patients with extremely high levels of body fat may also require specialized equipment, such as longer needles for spinal or epidural anesthesia, bariatric operating tables, and additional monitoring for oxygen levels and airway pressure.

Alternatively, patients with low BMI have on average lower nutritional reserves, higher sensitivity to certain medications, lower blood volume, and heightened risk of overall complications following surgery.³ A primary use of BMI in the perioperative setting is providing an estimate of the risk for a patient to undergo anesthesia.

BMI also influences how anesthetic drugs are dosed. Some medications are based on ideal body weight, others on total body weight, and some require simple adjustment calculations. Anesthesiologists use these formulas to safely deliver the right amount of medication while minimizing side effects.

Before surgery, anesthesiologists evaluate BMI along with other patient factors such as age, medical history, and activity level. Patients with very high or low BMI may be referred for preoperative optimization, such as weight stabilization, smoking cessation, or sleep studies for suspected sleep apnea.⁴ While BMI alone does not determine safety, it helps evaluate potential issues early so they can be managed proactively.

BMI is a simple but powerful tool that helps anesthesiologists assess and plan safe care for surgical patients. Whether a patient has a high, low, or normal BMI, individualized anesthetic strategies help ensure the safest possible outcome.

References

1. CDC. About Adult BMI. Centers for Disease Control and Prevention. Updated June 2022. https://www.cdc.gov/healthyweight/assessing/bmi/adult_bmi/index.html
2. Centers for Disease Control and Prevention (CDC). Health Effects of Overweight and Obesity. Centers for Disease Control and Prevention. Updated June 3, 2022. Accessed June 5, 2025. https://www.cdc.gov/obesity/basics/consequences.html
3. Martínez-Ortega AJ, Piñar-Gutiérrez A, Serrano-Aguayo P, et al. Perioperative Nutritional Support: A Review of Current Literature. Nutrients. 2022;14(8):1601. Published 2022 Apr 12. doi:10.3390/nu14081601
4. Chung F, Abdullah HR, Liao P. STOP-Bang questionnaire: a practical approach to screen for obstructive sleep apnea. Chest. 2016;149(3):631-638. doi:10.1378/chest.15-0903