Strain measures of the left ventricle and left atrium are composite measures of left heart geometry and function

This study demonstrates that left ventricular and left atrial strain measures are composite indicators of cardiac geometry and function, particularly driven by mitral annular plane systolic excursion (MAPSE) and chamber dimensions, which explains their strong prognostic value.

The Gist

Imagine your heart as a sophisticated, two-chambered pump system: the Left Ventricle (LV) is the main engine that pushes blood out to your body, and the Left Atrium (LA) is the receiving tank that collects blood before it gets pushed into the engine.

For a long time, doctors have used complex "strain" measurements to check how well these parts are stretching and squeezing. Think of strain like a "stretchiness score." If a rubber band is healthy, it stretches easily and snaps back. If it's tired or damaged, it doesn't stretch as well.

This paper asks a simple but profound question: Are these "stretchiness scores" actually measuring the muscle's strength, or are they just measuring how big the pump is and how far it moves?

Here is the breakdown of their findings using a few creative analogies:

1. The "Elevator" Analogy (MAPSE)

The researchers focused on a specific movement called MAPSE. Imagine the heart's floor (the mitral valve) is an elevator. When the heart squeezes, this floor moves up toward the ceiling.

• The Finding: The study found that the "stretchiness score" (Strain) is almost entirely determined by how far the elevator moves (MAPSE) and how tall the room is (the size of the heart).
• The Metaphor: If you have a tall room and the elevator moves up 10 inches, that's a big movement. If you have a short room and the elevator moves up 10 inches, that's a huge percentage of the room's height. The "strain" score is just a math formula that combines Distance Moved + Room Size. It doesn't necessarily tell you if the elevator motor is strong; it just tells you how much the elevator moved relative to the room size.

2. The "Rubber Band" Connection (Left Atrium vs. Left Ventricle)

The Left Atrium and Left Ventricle are connected by a shared floor (the valve). When the engine (LV) pulls up, it pulls the receiving tank (LA) with it.

• The Finding: The "stretchiness" of the receiving tank (Left Atrial Strain) is highly correlated with the movement of the engine floor.
• The Metaphor: Think of the two chambers as two balloons connected by a straw. If the engine balloon shrinks and pulls the floor up, the receiving balloon stretches. The study shows that if the receiving balloon looks "stretched out" (high strain), it's often because the engine isn't pushing hard enough, causing the floor to move less, which forces the receiving tank to stretch more to compensate. It's a chain reaction, not an isolated event.

3. The "Composite Score" Revelation

The biggest takeaway is that these fancy "strain" numbers are composite measures.

• The Metaphor: Imagine you are judging a gymnast. You have a score for "Flexibility." But the study found that your "Flexibility Score" is actually just a mix of how tall the gymnast is and how far they can reach.
• Why this matters: Doctors often use these strain scores to diagnose heart failure. This paper says, "Be careful! A low score might not mean the muscle is weak; it might just mean the heart is very large (dilated)." The score is a mix of Function (how hard it works) and Geometry (how big it is).

4. The "Simple Shortcut"

The researchers discovered a clever shortcut.

• The Metaphor: Instead of using a complex, expensive 3D scanner to measure exactly how much the heart muscle stretches (which can be tricky and vary between machines), you can just measure how far the elevator moves and how tall the room is.
• The Result: You can calculate the "stretchiness score" with almost perfect accuracy using just those two simple measurements. This is like realizing you don't need a supercomputer to know how fast a car is going; you just need to know the distance it traveled and the time it took.

The Bottom Line for Everyday Life

This study tells us that when doctors look at these complex "strain" numbers on a heart report, they are actually looking at a package deal:

1. How much the heart moves (The elevator).
2. How big the heart is (The room size).

If a patient has a "bad" strain score, it might be because their heart muscle is weak, OR because their heart has become enlarged and dilated. The two are so tightly linked that you can't easily separate them.

Why is this good news?
It means we can simplify things. We don't always need the most complex, expensive software to get the answer. Simple, reliable measurements of heart movement and size can tell us just as much about a patient's heart health as the fancy strain numbers. It helps doctors interpret results more clearly, ensuring they don't mistake a "big heart" for a "weak heart" without looking at the whole picture.

Technical Summary

1. Problem Statement

Cardiac imaging increasingly utilizes deformation analysis (strain) to assess left ventricular (LV) and left atrial (LA) function. Specifically, Global Longitudinal Strain (GLS), Global Circumferential Strain (GCS), and Left Atrial Strain (LAS) are used as sensitive markers for systolic and diastolic dysfunction, often providing prognostic value beyond traditional metrics like Ejection Fraction (LVEF).

However, the physiologic basis for the strong correlations observed between these strain measures and between strain and chamber geometry remains poorly understood.

• It is unclear to what extent LAS (a marker of diastolic dysfunction) is driven by intrinsic atrial pathology versus its coupling with LV systolic function and filling pressures.
• The relationship between strain, atrioventricular plane displacement (MAPSE), and chamber dimensions needs clarification to ensure these measures are interpreted correctly in research and clinical practice.

2. Methodology

Study Design & Population:

• Type: Retrospective observational study.
• Population: 66 patients who underwent clinically indicated Cardiovascular Magnetic Resonance (CMR) imaging between 2017 and 2021.
• Selection Criteria: Patients were stratified to represent a wide spectrum of LVEF (ranging from <30% to >60%).
• Exclusions: Distinct myocardial pathologies (hypertrophic cardiomyopathy, amyloidosis, etc.), atrial fibrillation during scan, and insufficient image quality for strain tracking.

Imaging Protocol:

• Modality: 1.5T or 3T CMR with balanced steady-state free precession (bSSFP) cine imaging.
• Views: Short-axis and long-axis (2-, 3-, and 4-chamber) views with 30 phases per cardiac cycle.

Image Analysis:

• Software: Segment (Medviso AB).
• Strain Analysis:
  • LV: GLS and GCS calculated via semi-automated feature tracking.
  • LA: LAS (reservoir strain) calculated from end-diastole to end-systole.
• Geometric Measures:
  • MAPSE: Mitral Annular Plane Systolic Excursion (distance traveled by mitral annulus).
  • Dimensions: LV and LA volumes (EDV, ESV), mass, length, and diameters.
  • Longitudinal Contribution: Calculated as maximal LV short-axis area $\times$ MAPSE.

Statistical Analysis:

• Univariable and multivariable linear regression to assess correlations.
• Multivariable models were adjusted for multicollinearity.
• Bland-Altman plots used for agreement analysis.

3. Key Contributions & Hypothesis

The study hypothesizes that LAS is primarily explained by variations in MAPSE and LA size, and that the correlation between LAS and GLS is driven by their shared dependence on atrioventricular plane displacement and chamber geometry.

The paper contributes a unified geometric framework suggesting that:

1. Strain measures are not isolated functional metrics but composite measures of atrioventricular plane displacement (MAPSE) and chamber size.
2. LAS is a composite marker reflecting both reduced myocardial function (via MAPSE) and LA enlargement (via filling pressures).
3. GLS can be accurately estimated using simple geometric measurements (MAPSE indexed to LV length), offering a vendor-independent alternative to complex strain tracking.

4. Key Results

Left Ventricular Strain (GLS & GCS):

• Strong Correlation: GLS and GCS are highly correlated ($R^2=0.86$).
• Geometric Drivers: GLS is highly correlated with MAPSE, LV mass, and LV volumes.
• Predictive Model: In multivariable analysis, MAPSE and LV End-Systolic Volume (LVESV) provided the strongest model for GLS ($R^2=0.89$).
• Indexing: The correlation between GLS and MAPSE significantly strengthens when MAPSE is indexed to LV Length (LVL).
• Formula: A simple linear formula was derived to estimate GLS:
  $$GLS = -\left( \frac{\text{MAPSE}}{\text{LVL}} \right) \times 100$$
  This estimation showed high accuracy (bias 3.7 ± 1.9 %-points).

Left Atrial Strain (LAS):

• Correlations: LAS correlated with GLS ($R^2=0.51$) and MAPSE ($R^2=0.46$).
• Geometric Drivers: The strongest association for LAS was found with LA End-Diastolic Volume (LAEDV) and MAPSE ($R^2=0.67$).
• Physiologic Mechanism: The study confirms that LAS is not an isolated marker of diastolic function but is coupled to LV systolic function (via MAPSE) and LA volume (via filling pressures).

Longitudinal vs. Circumferential Contribution:

• Longitudinal motion contributed ~62% to LV stroke volume.
• Surprisingly, the percentage of longitudinal contribution to stroke volume did not correlate with GLS or MAPSE, suggesting these strain measures reflect global deformation rather than the specific mechanical contribution to volume ejection.

5. Significance and Clinical Implications

• Reinterpretation of Strain: Clinicians and researchers should view strain measures (GLS, GCS, LAS) as composite metrics reflecting both myocardial function and chamber geometry (size). A change in strain may result from a change in geometry (e.g., dilation) rather than intrinsic myocyte dysfunction.
• Simplified Assessment: In cases of poor image quality where feature-tracking strain is unreliable, MAPSE indexed to LV length can serve as a robust, vendor-independent surrogate for GLS.
• Understanding Diastolic Dysfunction: The findings clarify that LAS is a "composite" marker. A reduction in LAS may indicate increased filling pressures (due to LA dilation) or reduced LV systolic function (reduced MAPSE), rather than isolated atrial pathology.
• Prognostic Value: The high correlation between strain and geometric measures of known prognostic significance (e.g., LVESV, LAEDV) likely explains why strain measures are such powerful prognostic tools.

Limitations Noted:

• The study excluded hypertrophic cardiomyopathy and valvular diseases, potentially limiting generalizability to those specific populations.
• LA geometry is irregular, which may have introduced variability in LAS measurements compared to the more uniform LV.
• The cohort had a low proportion of female patients, though the authors argue geometric scaling remains consistent across sexes for hearts of similar size.