Threshold levels of extreme body surface area that may cause a misrepresentation of cardiac performance

Introduction

Stroke volume (SV) and cardiac output (CO) are standardized into the stroke index (SI) and cardiac index (CI) by dividing by the patient's body surface area (BSA). Commonly used algorithms in hemodynamic management suggest diverse therapeutic interventions based on low, high, or normal SI or CI. Morbid obesity is increasingly common. When morbidly obese patients have their SV or CO indexed, high absolute values become low SI and CI, and may be misleadingly low. This would then cause therapeutic interventions opposite to their needs.

Methods

BSA was derived using the Mosteller formula on the metric equivalent of simulated patients ranging from 5 ft to 7 ft and 100 lbs to 700 lbs. A priori, we defined normal CO = 4 to 8, normal CI = 2.5 to 5.0, normal SV = 60 to 100 ml/beat, and normal SI = 33 to 47. Algebraic analysis was used to determine BSA levels that would classify an SV or CO as abnormal.

Results

Critical BSA thresholds (T) are presented in Table 1. For example, at SV = 100, a BSA higher than 3.03 (to the second decimal place) would classify the patient as having a low SI.

Table 1

Conclusions

Patients with extreme BSAs are increasingly encountered in the ICU, especially larger BSAs related to obesity. We provide threshold values where extreme BSAs will classify high SV or CO values as low indexed values. The ranges considered normal for SI and CI may be inappropriate for patients with extreme BSAs, particularly in the obese. We caution against relying solely on the SI and CI to assess hemodynamic performance. Instead, the SV and CO along with other physiological parameters should also be considered before making therapeutic decisions.