15.2 Left Ventricular Systolic Function: Simpson Biplane, 5/6 Area x Length & FS

15.2 Left Ventricular Systolic Function: Simpson Biplane, 5/6 Area x Length & FS

Clinical Core: Quantitative assessment of left ventricular systolic performance is a central requirement of every pediatric echocardiographic examination. Systolic function guides clinical decision-making across dilated and hypertrophic cardiomyopathies, congenital valve lesions, oncology cardiotoxicity surveillance, and postoperative critical care. While Fractional Shortening (FS) remains widely utilized due to its rapid bedside acquisition, its reliance on a single minor-axis linear dimension makes it profoundly deceptive in children with altered ventricular geometry, conduction abnormalities, or regional wall motion discrepancies. The Biplane Simpson's Method of Discs serves as the clinical standard for Ejection Fraction (EF) determination, complemented by the 5/6 Area-Length method in truncated or univentricular morphologies.

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Fractional Shortening (FS): Mathematical Foundations & Normal Values

Fractional Shortening measures the percentage change in the left ventricular internal minor-axis diameter between end-diastole and end-systole:

$FS (\%) = \frac{\text{LVEDD} - \text{LVESD}}{\text{LVEDD}} \times 100$

• Normal Pediatric Reference Range: 28% to 44% (mean $\approx 36%$; values $<28%$ define systolic dysfunction, while values $>44%$ represent hyperdynamic contractility).
• Measurement Plane: Direct 2D parasternal long-axis (PLAX) view, or short-axis view at the tips of the mitral leaflets, oriented strictly perpendicular to the LV long axis.

Fractional Shortening Geometric Assumption:

       End-Diastole (LVEDD)               End-Systole (LVESD)
     ┌──────────────────────┐           ┌──────────────────┐
     │         IVS          │           │       IVS        │
     ├──────────────────────┤           ├──────────────────┤
     │  ◄────── D ──────►   │    ───►   │  ◄──── d ────►   │
     │      (Cavity)        │           │     (Cavity)     │
     ├──────────────────────┤           ├──────────────────┤
     │        LVPW          │           │       LVPW       │
     └──────────────────────┘           └──────────────────┘
   Fundamental Assumption: Uniform, symmetric radial contraction along one line!

Critical Pitfalls & Failure Modes of Fractional Shortening

Because FS samples a single line of interrogation, it assumes that the minor axis accurately represents the global contractile function of the entire three-dimensional ventricle. This assumption fails completely in four common clinical scenarios:

1. Regional Wall Motion Abnormalities (RWMAs):

   • In ALCAPA (Anomalous Left Coronary Artery from the Pulmonary Artery), the anterolateral and apical segments may be completely akinetic or scarred, while the basal inferior wall supplied by the normal RCA is hyperdynamic. Placing minor-axis calipers across the basal ventricle generates a falsely "normal" FS ($>32%$) in an infant experiencing catastrophic global heart failure.
   • In Kawasaki disease with LAD coronary thrombosis, apical infarction is completely missed by minor-axis basal interrogation.

2. Ventricular Conduction Delays & Dyssynchrony:

   • In Left Bundle Branch Block (LBBB) or post-surgical Right Ventricular Pacing, electromechanical dyssynchrony produces early, paradoxical backward motion of the septum ("septal bounce" or beaking). The uncoordinated septal kinematics decouple septal excursion from posterior wall thickening, invalidating linear FS.

3. Right Ventricular Pressure & Volume Overload:

   • In large atrial septal defects (ASDs) or severe pulmonary regurgitation after Tetralogy of Fallot repair, massive RV volume overload causes diastolic flattening of the interventricular septum (reverse curvature), producing a D-shaped left ventricle in diastole. In pulmonary arterial hypertension, systolic pressure overload causes systolic septal flattening. This abnormal septal motion falsely depresses FS ($<24%$) despite robust, hyperdynamic LV intrinsic free-wall contractility.

4. Altered Geometry & Single Ventricles:

   • Dilated cardiomyopathy transforms the LV from an elongated prolate ellipsoid into a spherical globe. In single-ventricle hearts (hypoplastic left heart syndrome or tricuspid atresia), geometric asymmetry precludes minor-axis linear modeling.

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Left Ventricular Volumetric Ejection Fraction (LVEF) Methodologies

Ejection Fraction defines the volumetric fraction of blood pumped from the left ventricle with each contraction:

$EF (\%) = \frac{\text{EDV} - \text{ESV}}{\text{EDV}} \times 100$

• Normal Pediatric Reference Range: 55% to 70% (mean $\approx 62%$; values $<55%$ define systolic dysfunction; values $<40%$ signify severe systolic depression).

Comparison of Ventricular Volumetric Models:

1. Biplane Simpson's Discs:        2. 5/6 Area-Length ("Bullet"):      3. Teichholz (Contraindicated):
       ┌──────────┐                         ┌──────────┐                    ┌──────────┐
       │  Disc 20 │ (Apex)                  │Hemisphere│ (Apex)             │ Single   │
       ├──────────┤                         ├──────────┤                    │ Diameter │
       │  Disc 19 │                         │          │                    │ Cubed    │
       ├──────────┤                         │ Cylinder │                    │          │
       │   ...    │                         │  (Area)  │ (Mid-Papillary)    │          │
       ├──────────┤                         │          │                    │          │
       │  Disc 1  │ (Base)                  └──────────┘                    └──────────┘
   20 Independent Slices              V = (5/6) × Area × Length             V = [7/(2.4+D)] × D³
 (Accounts for Asymmetry)            (Ideal for Conical/Single V)       (Extreme Error in CHD)

1. Biplane Simpson's Method of Discs (ASE Gold Standard)

• Mathematical Model: Divides the left ventricular cavity along its long axis into 20 contiguous cylindrical discs of equal height. The volume of each elliptical cylinder disc is calculated from orthogonal diameters measured in the Apical 4-Chamber (A4C) and Apical 2-Chamber (A2C) views:

$V = \sum_{i=1}^{20} \pi \times \frac{a_i}{2} \times \frac{b_i}{2} \times h$

where $a_i$ and $b_i$ are the orthogonal diameters in the two views, and $h$ is disc height.

• Pediatric Advantages: Biplane Simpson does not assume a fixed geometric shape. By integrating orthogonal planes, it directly compensates for asymmetric chamber remodeling, regional infarcts, and septal flattening.
• Technical Rules & Artifacts:
  • Apical Foreshortening: The ultrasound probe must be placed at the true apex (maximizing long-axis length) rather than a rib interspace higher. Foreshortening cuts the LV short, underestimating true end-diastolic volume (EDV) and falsely elevating calculated EF.
  • Compact Endocardial Tracing: Tracing must track the compact myocardial border, excluding the papillary muscles and trabeculations from the wall (including them within the cavity volume).

2. The 5/6 Area-Length ("Bullet") Method

• Mathematical Model: Approximates the left ventricle as a combination of a cylinder (basal and mid two-thirds) and a hemisphere (apical one-third):

$V = \frac{5}{6} \times A \times L$

where $A$ is the cross-sectional endocardial area in the parasternal short-axis view at the mid-papillary muscle level, and $L$ is the long-axis length from the apical endocardial tip to the midpoint of the mitral valve annular plane measured in the apical 4-chamber view.

• Pediatric Clinical Indications:
  • Indicated when the apical 2-chamber view is suboptimally visualized, preventing an accurate biplane Simpson trace.
  • Highly validated and standard for functionally univentricular hearts (single morphologic right or left ventricles) and truncated, conical ventricles.

3. The Teichholz Formula: Prohibited in Pediatric Practice

• Mathematical Formula: Derives 3D volume from a single 1D minor-axis linear diameter ($D$):

$V = \frac{7.0}{2.4 + D} \times D^3$

• Why Teichholz is Contraindicated: The formula relies on the rigid mathematical assumption that the ventricle conforms to a normal prolate ellipsoid with a uniform 2:1 length-to-diameter ratio. In congenital heart disease, volume overload, and single-ventricle physiology, chamber geometry is severely distorted. In abnormally shaped or dilated ventricles, Teichholz generates volumetric errors exceeding 30% to 50%. The ASE pediatric guidelines strictly prohibit its use.

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Left Ventricular Mass & Relative Wall Thickness (RWT)

Left ventricular mass reflects total myocardial muscle weight and hypertrophic remodeling in response to chronic pressure overload (e.g., aortic coarctation, aortic stenosis, hypertension) or volume overload:

Devereux Equation for Pediatric LV Mass

$\text{LV Mass } (\text{g}) = 0.8 \times \left( 1.04 \times \left[ (\text{LVEDD} + \text{IVSd} + \text{LVPWd})^3 - \text{LVEDD}^3 \right] \right) + 0.6$

• Indexing: In children, raw LV mass is indexed to height raised to the allometric power of 2.7 ($\text{LVMI } [\text{g/m}^{2.7}]$) or converted into a Z-score based on BSA.

Relative Wall Thickness (RWT)

Relative wall thickness categorizes left ventricular remodeling independent of absolute body mass:

$RWT = \frac{2 \times \text{LVPWd}}{\text{LVEDD}}$

• Normal Pediatric RWT: $< 0.42$
• Concentric Remodeling: $RWT \ge 0.42$ with normal indexed LV mass.
• Concentric Hypertrophy: $RWT \ge 0.42$ with elevated indexed LV mass (e.g., severe aortic stenosis, hypertrophic cardiomyopathy).
• Eccentric Hypertrophy: $RWT < 0.42$ with elevated indexed LV mass (e.g., severe aortic or mitral regurgitation, large ventricular septal defect).

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Regional Wall Motion Analysis: The 17-Segment Model & WMSI

While global myocardial depression is typical of viral myocarditis or dilated cardiomyopathy, segmental wall motion discrepancies signify localized ischemic injury or anomalous coronary architecture.

Pediatric 17-Segment Model (AHA Polar Bullseye Distribution):

                       Basal Anterior (1)
                 Basal                       Basal
            Anteroseptal (2)             Anterolateral (6)
                       Mid Anterior (7)
                 Mid                       Mid
            Anteroseptal (8)          Anterolateral (12)
                      Apical Anterior (13)
              Apical                       Apical
             Septal (14)    Apex (17)    Lateral (16)
                      Apical Inferior (15)
                 Mid                       Mid
            Inferoseptal (9)           Inferolateral (11)
                       Mid Inferior (10)
                 Basal                       Basal
            Inferoseptal (3)             Inferolateral (5)
                       Basal Inferior (4)

Segmental Scoring Scale

• 1 = Normal or Hyperkinetic: Radial wall thickening $\ge 40%$ and brisk inward systolic endocardial motion.
• 2 = Hypokinetic: Reduced systolic wall thickening ($<30%$) and sluggish inward motion.
• 3 = Akinetic: Complete absence of systolic wall thickening ($<10%$) and absence of inward motion.
• 4 = Dyskinetic: Paradoxical systolic thinning and outward systolic expansion during ventricular contraction.
• 5 = Aneurysmal: Persistent diastolic and systolic outward pouching or scarred deformity.

Wall Motion Score Index (WMSI)

$WMSI = \frac{\sum \text{Segmental Scores}}{\text{Total Number of Visualized Segments}}$

• Normal Value: 1.0 (all visualized segments score 1).
• Pathological Significance: A WMSI $>1.0$ confirms regional wall motion abnormalities; values $\ge 1.5$ to $1.7$ identify extensive myocardial infarction territory.

Pediatric Etiologies of Segmental Wall Motion Abnormalities

1. ALCAPA (Bland-White-Garland Syndrome):
   • Anomalous left coronary artery arising from the low-pressure pulmonary artery causes retrograde coronary steal into the PA.
   • Echocardiographic Signature: Severe hypokinesis, akinesis, or dyskinesis of the anterolateral and apical segments (segments 1, 6, 7, 12, 13, 16) with prominent endocardial fibroelastosis, severe ischemic mitral regurgitation, and compensatory hyperkinesis of the basal inferior and inferoseptal walls supplied by the dilated, normal RCA.
2. Kawasaki Disease Coronary Thrombosis:
   • In-situ thrombosis of giant aneurysms leads to transmural myocardial infarction in infants and toddlers.
   • LAD Occlusion: Akinesis of the anterior, anteroseptal, and apical segments.
   • RCA Occlusion: Akinesis of the inferior and inferoseptal segments.
   • LCx Occlusion: Akinesis of the inferolateral segments.
3. Postoperative Arterial Switch Operation (ASO):
   • Coronary buttons relocated to the neoaorta can kink, stretch, or become compressed by the anterior Lecompte maneuver, causing acute postoperative regional akinesis.
4. Anthracycline Cardiotoxicity:
   • Progressive doxorubicin-induced myocyte loss produces early regional strain deficits and delayed segmental wall motion impairment before global EF falls below $55%$.

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Left Ventricular Systolic Assessment Methodologies Table

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Clinical Alerts & Diagnostic Pearls

[!WARNING] The Paradoxical Septal Motion Trap in RV Volume Overload: A 6-year-old child with a large secundum ASD has a calculated M-mode Fractional Shortening of 22% (reported as "moderate LV systolic dysfunction"). However, 2D inspection reveals vigorous radial contraction of the posterior and lateral walls with diastolic septal flattening due to massive RV volume overload. The low FS is an artifact of abnormal septal kinematics! Biplane Simpson calculation reveals a completely normal EF of 64%. Never rely on FS in the presence of altered septal motion.

[!TIP] Detecting Apical Foreshortening: To ensure the true apex is visualized during biplane Simpson acquisition, translate the transducer one intercostal space lower and more lateral until the apex appears thin, tapering, and stationary during systole, rather than thick and moving inward. Foreshortening cuts through the muscular mid-wall, truncating cavity length and falsely inflating calculated EF.

[!NOTE] Infant ALCAPA Clinical Presentation: Any infant presenting with dilated cardiomyopathy, unexplained crying or diaphoresis during feeding (angina), or severe mitral regurgitation must have coronary origins interrogated and regional wall motion scrutinized. If the anterolateral wall is akinetic while the basal inferior wall is hyperdynamic, ALCAPA is the diagnosis until proven otherwise.