Misleading indexed hemodynamic parameters: the clinical importance of discordant BMI and BSA at extremes of weight

e widespread availability of minimally invasive hemodynamic monitors encourages evaluation of cardiac output (CO) and stroke volume (SV) in real time, and they are integral to goal-directed therapy, algorithmbased treatment plans. Patients with extremes of body surface area (BSA) are at risk of having their indices erroneously evaluated despite adequate non-indexed values. We evaluated the use of body mass index (BMI) in this population and identifi ed a discordance of BSA and BMI values at extreme weights. We recommend unindexed values in these patients when deciding treatment options. Physiologic parameters are indexed or normalized by dividing the absolute values by the BSA. Th e cardiac and stroke indices are CO/BSA and SV/BSA, respectively. Large BSAs due to morbid obesity can distort indexed values. A normal CO or SV when divided by an extreme BSA will yield a low cardiac index or stroke index despite even supraphysiologic actual hemodynamic performance [1]. Consequently, this may lead to therapeutic interventions opposite to patients’ needs. BMI itself, even when extreme, does not cause clinically signifi cant increases in CO or SV [2]. However, BSA and BMI are not interchangeable. As evidenced by Table 1, BSA and BMI do not always correlate. Th e surface area of a short person with extreme weight can be signifi cantly less than a taller, thinner person. Similarly, the BSA of a tall person compared with a shorter person with proportional weights would be much greater but their BMIs would be similar. Th e BSA and BMI of a simulated patient with height and weight varying (DuBois and DuBois formula) are presented in Table 1. Th e bold values illustrate the BSA at which patients are in danger of being misclassifi ed as having a falsely low cardiac index despite adequate CO, given standard algorithms [1]. Th e table reveals BMI– BSA discordance, most prominently observed in very tall or short persons. For example, patients weighing 181 kg may be mis classifi ed using BSA if they are taller than 178 cm despite a decrease in their BMI with increasing height. However, a person weighing 181 kg who is shorter