Left ventricular mass (LVM) can be accurately and directly determined using cardiac magnetic resonance (CMR). It is an important predictor of cardiovascular morbidity and mortality1. LVM is known to increase in proportion to overall body size and differs by gender. Thus, to assess an individual’s risk for a cardiovascular event based on LVM, an adjustment for the patient’s body size should be done.
An LV mass "index" is derived by dividing LV mass by factors that include body height and/or weight. The normal range of LV mass index can then be derived from a reference sample of individuals believed to be free of significant risk factors that could otherwise cause LVM enlargement. CMR measured LVM divided by body surface area (BSA) is frequently used clinically to account for body size. However, this adjustment was not derived based on its ability to remove the effect of body size from LVM and the most commonly used formula for computing BSA is based on a study of 9 individuals published in 1916 and its validity and accuracy is unclear.
In MESA2, an adjustment is derived based on an optimal function to predict LVM from height, weight and gender in a healthy subsample. This subsample consisted of MESA participants who were free of diabetes or impaired fasting glucose, hypertension and were of normal weight2. The formula derived from this sample is
(1) predicted LVM = 6.82 x (height in meters)0.561 x (weight in kilograms)0.608 for women, and 8.17 x (height in meters)0.561 x (weight in kilograms)0.608 for men.
(2) The indexed LVM is written as 100xLVM/predicted LVM, so that the scale is in %. This index is called the LVM%predicted.
This index has the nice property that a value of 100 indicates that the indexed LVM is exactly equal to what is predicted by height, weight and gender and values above/below 100 indicates that the LVM is greater/smaller than the predicted LVM. This simple interpretation is an important advantage over other indexed LVM, such as LVM/BSA.
In MESA, nearly all excess risk of cardiac events occurred at LV mass 36% greater than LV mass predicted by height, weight and gender (LV mass % predicted, >136%). LV hypertrophy >136% of normal population corresponds approximately to LV mass >95% tile of a normal population, defined as LV hypertrophy in the statements below:
Individuals with LV hypertrophy had 2.7 times greater risk for hard events (MI or CHD death)
than individuals without LV hypertrophy over 15 years in fully adjusted models.
Individuals with LV hypertrophy had 5.4 times greater risk to develop heart failure than individuals without LV hypertrophy over 15 years in fully adjusted models.
Individuals with LV hypertrophy had 7.5 times greater risk for other CV death than individuals without LV hypertrophy over 15 years in fully adjusted models.
The interpretation of elevated LV mass values needs to take into account other characteristics such as relevant symptoms and other signs of disease, rather than in isolation. The excess risk for LV hypertrophy does not account for elevated risk associated with atypical/ genetic causes of LV hypertrophy (e.g., hypertrophic cardiomyopathy, amyloidosis, Fabry disease, etc).
REFERENCES:
https://www.ncbi.nlm.nih.gov/pubmed/20107905 Int J Cardiovasc Imaging. 2010 Apr;26(4):459-68
http://www.onlinejacc.org/content/52/25/2148 J Am Coll Cardiol. 2008 Dec 16;52(25):2148-55
https://www.ncbi.nlm.nih.gov/pubmed/31453766 Radiology. 2019 Aug 27:182871. doi: 10.1148/radiol.2019182871.