Effects of body surface area-indexed calculations in the morbidly obese: a mathematical analysis

Cardiac and stroke indices routinely are used to communicate the adequacy of circulation, especially by cardiac anesthesiologists. However, indexed values may be distorted when derived using conventional formulae on morbidly obese patients. In extreme cases, distortion of the raw value by the indexed value may suggest inappropriate therapeutic interventions. This study attempted to highlight threshold values of body surface area (BSA) that place morbidly obese patients at risk of being overtreated.

Mathematical analysis.

Simulated patients.

BSA was derived using the commonly used Mosteller and Dubois and Dubois formulae on a range of simulated patients. These simulated BSAs then were applied to normal cardiac output (CO) and stroke volume (SV) values to identify the threshold at which BSA-indexed values result in a change in classification to abnormal. Additionally, the effects of 7 different published BSA formulae were examined, using a range of height-weight combinations.

Critical thresholds at which BSA calculations would classify normal CO and SV as abnormal are presented in a tabular form. Among the 7 BSA formulae, there was substantial variation in predicted BSA at a given height-weight combination when values typically associated with morbid obesity are used.

In morbidly obese patients, cardiac and stroke indices can be misleading relative to the underlying raw values (CO and SV) as a result of distortion by widely used BSA formulae. The authors caution against relying on threshold cardiac and stroke indices as triggers for the initiation of hemodynamic therapies in the morbidly obese. Further research on what BSA formula should be used on patients with very extreme body types is warranted.