Body Surface Area Calculator

Physiologic Basis

Body surface area is a useful metric across many clinical domains to optimize therapeutic outcomes.

To highlight this, consider two different patients; the first is 200cm tall and weights 100kg, the second is 120cm tall and 50kg. For weight based medications, these patients will have very different absolute medication requirements, just as they will require different tidal volumes to adequately ventilate. The same principle applies to the metabolic demands of the body, where body surface area has been a validated tool to standardize someones metabolic demands based off of their weight and height.

Body Surface Area is different to Body Mass Index (BMI), which provides a linear relationship between weight and height. For physiologic parameters like cardiac output, this is important. Metabolic demands and cardiac output more reliably follow allometric growth as opposed to a linear function like BMI, BSA better accounts for this power function to provide a more adequate idea of the metabolic demands of a patient.

Application to Practice

BSA provides an accurate estimate of a patient's metabolic requirements and physiologic normal values. In practice, many hemodynamic measurements that are measured via bedside ultrasound or pulmonary artery catheterization do not intrinsically factor in a patients height, weight and metabolic demands. When these values are corrected for by a patient's body surface area, the subsequent value is an indexed value that allows for a more nuanced characterization of patients in shock.

When assessing patients in shock or with pulmonary artery catheters, ensure you are using indexed values when it comes to cardiac output, stroke work, systemic vascular resistance, and others to accurately interpret values and characterize shock personalized to your patient.

List of calculators with indexed values:

• Cardiac Index

• Systemic Vascular Resistance Index

• Right ventricular stroke work index

• Left ventricular stroke work index.