Left ventricular (LV) fractional shortening (FS) represents a relatively underutilized, yet valuable, measure of left ventricular systolic function. While ejection fraction (EF) reigns supreme as the primary indicator of systolic performance, FS offers a unique perspective, particularly in specific clinical scenarios. This article delves into the intricacies of LV fractional shortening, exploring its calculation, normal ranges, clinical significance, limitations, and comparisons with EF. We will also address its measurement using various echocardiographic techniques.
What is Fractional Shortening?
Fractional shortening is a quantitative assessment of the LV's ability to contract during systole. It describes the percentage change in LV end-diastolic dimension (LVEDD) relative to LV end-systolic dimension (LVESD). In essence, it quantifies the extent to which the left ventricle shortens during its contraction phase. A higher FS indicates a more vigorous contraction and, generally, better systolic function. Unlike EF, which focuses on the volume of blood ejected, FS directly reflects the linear shortening of the LV.
LV Fractional Shortening vs. Ejection Fraction: A Comparative Analysis
Both FS and EF are indices of LV systolic function, but they differ in their methodology and what they primarily reflect. EF represents the percentage of blood ejected from the LV during each contraction, calculated as (EDV – ESV) / EDV, where EDV is end-diastolic volume and ESV is end-systolic volume. It is a volumetric measure.
FS, on the other hand, is a linear measure, focusing on the dimensional changes of the LV. It is calculated as (LVEDD – LVESD) / LVEDD x 100%. While both reflect systolic performance, they don't always correlate perfectly. The discrepancy arises primarily due to variations in LV geometry and the presence of regional wall motion abnormalities (RWMAs).
Under ideal conditions – a normal, uniformly contracting LV without significant RWMAs – FS and EF exhibit a strong correlation. However, in the presence of RWMAs, such as those seen in myocardial infarction or cardiomyopathy, the correlation weakens. This is because EF can be relatively preserved despite regional dysfunction, while FS will be more significantly affected by the reduced shortening in the affected segments. Therefore, FS may be more sensitive to regional abnormalities in LV contraction than EF. This sensitivity, however, also presents a limitation, as it makes FS more susceptible to variations in measurement technique and observer variability.
Furthermore, FS is significantly influenced by preload and afterload. Increased preload (increased EDV) can lead to a seemingly lower FS, even if the contractility is normal, simply because the ventricle has more to shorten from. Similarly, increased afterload (increased resistance to ejection) reduces the extent of LV shortening, thus lowering FS. EF, while also affected by preload and afterload, is less directly influenced by these factors compared to FS.
How to Calculate Fractional Shortening
Calculating FS requires accurate measurement of LVEDD and LVESD. These measurements are typically obtained from M-mode echocardiography, specifically from the parasternal long-axis view. The calculation is straightforward:
FS (%) = [(LVEDD – LVESD) / LVEDD] x 100
Accurate measurements are crucial. Slight variations in the positioning of the M-mode cursor can significantly impact the results. Therefore, meticulous attention to detail during echocardiographic acquisition and measurement is paramount. The use of automated software can help mitigate some of this variability, but careful review by an experienced echocardiographer remains essential.
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