13.1 Standard Pediatric Acoustic Windows & Subcostal / Suprasternal Views

13.1 Standard Pediatric Acoustic Windows & Subcostal / Suprasternal Views

Clinical Core: Pediatric echocardiography is fundamentally defined by segmental anatomical analysis. Unlike adult transthoracic echocardiography—which focuses heavily on left ventricular systolic function and acquired degenerative valve disease—the pediatric examination requires an exhaustive, multi-planar interrogation to establish spatial segment-to-segment connections, great artery branching patterns, ventricular septal integrity, and pulmonary venous drainage. Because neonates and young children lack ossified costal cartilage, have thin chest walls, and possess compliant abdominal musculature, pediatric sonographers utilize acoustic windows that are virtually inaccessible in adults. Mastering systematic sweeps across these windows is essential for unmasking complex congenital heart disease (CHD).

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Systematic Pediatric Acoustic Windows & Scanning Conventions

The Five Primary Acoustic Windows

A complete pediatric transthoracic echocardiographic examination interrogates the heart and great vessels through five primary acoustic windows:

1. Subcostal (Subxiphoid) Window: The primary diagnostic foundation in neonates and young infants. The compliant abdominal wall and acoustic window of the left liver lobe allow comprehensive evaluation of visceral and atrial situs, systemic/pulmonary venous connections, interatrial and interventricular septa, and outflow tracts without lung interference.
2. Parasternal Window: Located in the 2nd to 4th left intercostal spaces adjacent to the sternum. Interrogates long-axis (PLAX) and short-axis (PSAX) planes to define great vessel alignment, ventricular septal anatomy, atrioventricular valve morphology, and coronary artery origins.
3. Apical Window: Positioned at the cardiac point of maximal impulse (PMI). Interrogates the crux of the heart, differential atrioventricular valve offset, morphologic right versus left ventricular identification, and provides parallel Doppler alignment for ventricular inflows and outflows.
4. Suprasternal Notch (SSN) Window: Positioned in the jugular notch above the manubrium with the neck hyperextended. Essential for defining aortic arch sidedness, transverse arch branching, aortic coarctation, patent ductus arteriosus, and the pulmonary venous confluence ('crab view').
5. Right Parasternal Window: Positioned in the 2nd to 4th right intercostal spaces with the patient in the right lateral decubitus position. Highly valuable for parallel Continuous-Wave (CW) Doppler alignment across severe aortic stenosis, ascending aortic aneurysms, right-sided aortic arches, and anomalous systemic venous connections.

Pediatric Display Conventions

In accordance with American Society of Echocardiography (ASE) pediatric guidelines, modern pediatric echocardiographic displays orient the apex of the sector transducer at the top of the monitor (near field):

• Parasternal Long-Axis (PLAX): The transducer index mark is directed toward the patient's right shoulder (10 to 11 o'clock). Screen-right represents the base of the heart (aortic root and ascending aorta); screen-left represents the cardiac apex; near field represents the anterior right ventricle; and far field represents the left atrium, posterior mitral leaflet, coronary sinus, and descending thoracic aorta.
• Parasternal Short-Axis (PSAX): The transducer is rotated 90 degrees clockwise with the index mark directed toward the patient's left shoulder (1 to 2 o'clock). Screen-left represents the patient's right/anterior anatomy; screen-right represents the patient's left/posterior anatomy.

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The Systematic Parasternal Long-Axis (PLAX) Sweep

A static, single-frame PLAX view is inadequate for congenital heart assessment. A complete pediatric interrogation mandates a systematic three-tier tilt sweep across three distinct orthogonal planes:

                    Systematic PLAX Three-Tier Sweep:

       [Medial & Inferior Tilt]        ──► Right Ventricular Inflow Tract (RVIT)
                   │
       [Neutral Central Plane]         ──► Standard Mid-Ventricular PLAX
                   │
       [Lateral & Superior Tilt]       ──► Right Ventricular Outflow Tract (RVOT)

1. Right Ventricular Inflow View (RVIT / RV Inflow)

From the standard PLAX position, the transducer beam is tilted inferiorly and medially (aiming toward the patient's right hip and groin):

• Anatomical Structures Profiled: Right atrium (RA), tricuspid valve, right ventricular cavity and apex, Eustachian valve, Chiari network, and entry of the inferior vena cava (IVC) and coronary sinus (CS).
• Tricuspid Valve Leaflets: Primarily visualizes the anterior leaflet (superior/anterior) and posterior leaflet (inferior/posterior). Slight rotational manipulation brings the septal leaflet into view.
• Primary Diagnostic Utilities:
  • Tricuspid Regurgitation (TR) Jet Velocity: Provides optimal parallel Doppler beam alignment ($\theta \approx 0^\circ$) to record peak systolic TR velocity. Applying the modified Bernoulli equation ($\Delta P = 4v^2$) derives the peak systolic RV-to-RA pressure gradient, which when added to estimated right atrial pressure (RAP) yields Pulmonary Artery Systolic Pressure (PASP).
  • Ebstein Anomaly: Evaluates apical displacement of the tricuspid valve leaflets relative to the anterior mitral valve annulus. Apical displacement exceeding $8\text{ mm/m}^2$ body surface area confirms Ebstein anomaly, accompanied by functional 'atrialization' of the inlet RV and anterior leaflet tethering/sail-like excursion.
  • Inlet VSD & Tricuspid Dysplasia: Detects inlet ventricular septal defects beneath the tricuspid leaflets, tricuspid valve prolapse, and bacterial endocarditis vegetations.
  • Invasive Line Tracking: Profiles peripherally inserted central catheters (PICC lines), umbilical venous catheters (UVC), and transvenous pacing leads traversing the RA into the RV apex.

2. Standard Mid-Ventricular PLAX View (Neutral Plane)

The transducer beam is held perpendicular to the long axis of the left ventricle:

• Anatomical Structures Profiled: Anterior right ventricular free wall, RV cavity, interventricular septum (IVS: perimembranous and muscular segments), left ventricular cavity, inferolateral (posterior) LV wall, left ventricular outflow tract (LVOT), aortic valve (Right Coronary Cusp anteriorly, Non-Coronary Cusp posteriorly), aortic root, sinotubular junction, proximal tubular ascending aorta, mitral valve (anterior mitral leaflet in fibrous continuity with the posterior aortic root; posterior mitral leaflet and chordae), left atrium, coronary sinus, and descending thoracic aorta.
• Diagnostic Landmarks & Pathologies:
  • Malalignment VSDs & Aortic Override: Detects anterior malalignment VSD with overriding aorta in Tetralogy of Fallot (ToF), posterior malalignment VSD with subaortic stenosis in interrupted aortic arch or critical coarctation, and large conoventricular VSDs.
  • Fibrous Aorto-Mitral Continuity: Essential for confirming normal ventriculoarterial concordance. Loss of fibrous continuity between the anterior mitral leaflet and semilunar valve indicates Double Outlet Right Ventricle (DORV) or transposition complexes with a bilateral subarterial muscular conus.
  • Subaortic Obstruction: Profiles discrete fibrous subaortic membranes, fibromuscular tunnels, and systolic anterior motion (SAM) of the mitral valve in hypertrophic obstructive cardiomyopathy (HOCM).
  • Aortic Root Sizing: Standard acoustic window for 2D and M-mode Z-score measurements of the aortic annulus (mid-systole, inner-to-inner hinge points), sinuses of Valsalva, and sinotubular junction (end-diastole, leading-edge to leading-edge) in Marfan and Loeys-Dietz syndromes.
  • Coronary Sinus vs. Descending Aorta Differentiation: The Coronary Sinus (CS) appears in cross-section within the posterior atrioventricular groove, located anterior to the posterior pericardium. The Descending Thoracic Aorta (DA) appears as a circular, pulsatile structure located posterior to the left atrium and behind the posterior pericardial reflection. Massive dilation of the CS ($>5\text{ to }8\text{ mm}$) strongly indicates a Persistent Left Superior Vena Cava (PLSVC) draining via the coronary sinus.
  • Pericardial vs. Pleural Effusion: Free fluid tracking anterior to the descending thoracic aorta is contained within the pericardial space. Fluid extending posterior to the descending aorta represents a left pleural effusion.

3. Right Ventricular Outflow View (RVOT / PLAX RVOT)

From neutral PLAX, the transducer is tilted superiorly and laterally (aiming toward the patient's left shoulder):

• Anatomical Structures Profiled: RV infundibulum (conus arteriosus), pulmonary valve (PV), main pulmonary artery (MPA) trunk, and proximal right and left pulmonary artery branches.
• Diagnostic Utilities: Evaluates dynamic or fixed subpulmonary infundibular stenosis in ToF, pulmonary valve dysplasia/stenosis, pulmonary regurgitation, and visualizes the entering jet of a neonatal patent ductus arteriosus (PDA) into the distal MPA.

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Parasternal Short-Axis (PSAX) Sweep: Four Standard Levels

Rotating the transducer 90 degrees clockwise from PLAX directs the index mark toward the patient's left shoulder (1 to 2 o'clock). A continuous tilt sweep from base to apex evaluates four standardized acoustic levels:

                  PSAX Sweep Hierarchy (Base to Apex):

    [Level 1: Base / Aortic Valve]    ──► Great vessels, Coronary ostia, RVOT/MPA
                   │ (tilt inferiorly)
    [Level 2: Mitral Valve]           ──► 'Fish mouth' orifice, Clefts, Commissures
                   │ (tilt inferiorly)
    [Level 3: Papillary Muscles]      ──► Circular LV, Crescent RV, Septal flattening
                   │ (tilt inferiorly)
    [Level 4: Low Apical]             ──► Apical trabeculations, Muscular VSDs

Level 1: Aortic Valve Level (Base / Great Vessels)

The aortic root sits centrally, displaying the tri-radiate 'Mercedes-Benz' cusp coaptation sign in diastole:

• Cusp Anatomy: Right Coronary Cusp (RCC, anteriorly/superiorly), Left Coronary Cusp (LCC, posteriorly and screen-right), Non-Coronary Cusp (NCC, posteriorly and screen-left adjacent to the atrial septum).
• Coronary Artery Ostial Origins:
  • Left Main Coronary Artery (LMCA): Arises from the left sinus of Valsalva at the 2 to 4 o'clock position and bifurcates into the LAD and LCx.
  • Right Coronary Artery (RCA): Arises from the right sinus of Valsalva at the 10 to 12 o'clock position.
  • Pathology: Essential for detecting coronary aneurysms in Kawasaki Disease ($Z\text{-score} \ge 2.5$ for dilation, $\ge 5.0$ for giant aneurysms), Anomalous Left Coronary Artery from the Pulmonary Artery (ALCAPA), and anomalous aortic origin with an interarterial/intramural course.
• Circling Right Heart Anatomy: Clockwise around the aortic root from screen-left to screen-right sit the Right Atrium, Tricuspid Valve, RVOT, Pulmonary Valve, and Main Pulmonary Artery bifurcating into the RPA and LPA ('whale tail' appearance).
• VSD Clock-Face Localization:
  • Outlet / Supracristal / Subarterial VSD (12 to 2 o'clock): Located in the RVOT directly beneath the pulmonary and aortic valves. High risk for right coronary cusp prolapse and progressive aortic regurgitation.
  • Perimembranous VSD (9 to 11 o'clock): Located adjacent to the septal leaflet of the tricuspid valve, frequently accompanied by tricuspid tissue aneurysm formation.
  • Inlet VSD (7 to 9 o'clock): Located beneath the tricuspid septal and posterior leaflets.
• Bicuspid Aortic Valve (BAV): Assessed in systole for a 'football-shaped' orifice and in diastole for cusp fusion and raphe. Most common is RCC-LCC fusion (70% to 80%), followed by RCC-NCC fusion (20% to 25%).

Level 2: Mitral Valve Level

Tilting the beam slightly inferiorly profiles the left ventricle at the mitral valve level:

• Anatomical Structures: The LV appears as a thick muscular ring. In diastole, the leaflets open into a 'fish mouth' orifice with the broad Anterior Mitral Leaflet (AML) and crescentic Posterior Mitral Leaflet (PML: P1, P2, P3 scallops).
• Diagnostic Utilities: Identifies Cleft Mitral Valve in atrioventricular septal defects (AVSD), where the cleft (zone of apposition) points directly toward the ventricular septum at 12 o'clock (contrasting with an isolated cleft pointing toward the LVOT). Also diagnoses Parachute Mitral Valve (all chordae inserting into a single papillary muscle producing mitral stenosis) and Double-Orifice Mitral Valve (DOMV).

Level 3: Mid-Papillary Muscle Level

Tilting further toward the apex profiles the mid-ventricular cavity:

• Anatomical Structures: Shows the circular LV containing the Posteromedial Papillary Muscle (4 to 5 o'clock) and Anterolateral Papillary Muscle (7 to 8 o'clock), with the thin, crescentic RV wrapped anteriorly.
• Interventricular Septal Kinetics & Hemodynamic Overload:
  • Normal: Circular LV contour maintained in both systole and diastole.
  • RV Volume Overload (Diastolic-Only Flattening): In large ASDs or severe PR, excessive RV diastolic filling flattens the IVS toward the LV during diastole ('D-shaped' LV), but the septum springs back to a normal circular shape during systole.
  • RV Pressure Overload (Systolic & Diastolic Flattening): In severe pulmonary stenosis, Eisenmenger syndrome, or primary pulmonary hypertension, RV pressure equals or exceeds LV pressure throughout the entire cardiac cycle. The IVS remains flattened or bowed into the LV during both systole and diastole.
• Quantitative Ventricular Function: Standard site for 2D-guided M-mode or direct 2D measurements of LV internal dimensions in diastole (LVIDd) and systole (LVIDs) to calculate Fractional Shortening ($FS = [LVIDd - LVIDs]/LVIDd \times 100%$, normal 28% to 44%).

Level 4: Low Apical Level

Tilting to the most inferior and lateral angulation profiles the ventricular apex:

• Diagnostic Utilities: High-frequency, low-scale color sweeps detect apical muscular VSDs ('Swiss-cheese' septum) that escape detection on basal views. Also diagnoses Left Ventricular Non-Compaction (LVNC), demonstrating deep intertrabecular recesses filled with blood from the LV cavity and a non-compacted to compacted myocardial ratio $>2.0$.

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The Apical Acoustic Window: Crux & Alignment

Positioned over the apex (PMI) with the transducer index mark pointing toward the patient's left axilla (3 o'clock):

                      The Apical View Family:

                 [Apical 4-Chamber (A4C)]
           (Crux, AV offset, RV vs LV morphology)
                            │
         ┌──────────────────┴──────────────────┐
         ▼                                     ▼
    [Apical 5-Chamber]                   [Apical 2- & 3-Chamber]
    • Anterior tilt                      • 60° to 90° CCW rotation
    • LVOT & Aortic valve                • Inferior, anterior, anteroseptal walls
    • Parallel CW Doppler alignment      • LAA & Mitral-aortic continuity

1. Apical 4-Chamber (A4C) View

• Crux of the Heart & AV Valve Offset: Normal cardiac anatomy exhibits differential apical offset of the atrioventricular valves: the septal tricuspid leaflet inserts into the septum more apically than the anterior mitral leaflet by $5\text{ to }8\text{ mm/m}^2$ BSA ($2\text{ to }5\text{ mm}$ in infants), creating the intervening atrioventricular septum.
  • AVSD: Complete loss of offset; AV valves insert at the exact same horizontal planar level across the crest of the septum (co-planar insertion).
  • Ebstein Anomaly: Severe apical displacement of the tricuspid valve $>8\text{ mm/m}^2$, atrializing the inlet RV.
• Morphological Chamber Identification:
  • Morphologic Left Ventricle (LV): Bullet-shaped cavity, smooth septal endocardium without direct chordal attachments, two distinct papillary muscles, fine apical trabeculations, and fibrous continuity between mitral and aortic valves.
  • Morphologic Right Ventricle (RV): Triangular/crescentic cavity, coarse apical trabeculations, prominent moderator band traversing the apex to anterior papillary muscle, and a tri-leaflet valve with direct chordal attachments to the ventricular septum.
• The Fossa Ovalis Drop-Out Trap: The thin membrane of the fossa ovalis lies strictly parallel to the ultrasound beam in A4C, producing frequent false-positive acoustic drop-out mimicking an atrial septal defect in up to 30% of normal children. Never diagnose an ASD based solely on A4C drop-out!

2. Apical 5-Chamber (A5C), 2-Chamber (A2C) & 3-Chamber (A3C) Views

• Apical 5-Chamber: Angling anteriorly unfolds the LVOT, aortic valve, and proximal ascending aorta, providing optimal coaxial alignment ($\theta \le 15^\circ$) for Continuous-Wave Doppler quantification of aortic stenosis, subaortic membranes, and HOCM.
• Apical 2-Chamber: Rotating 60 degrees counterclockwise profiles the isolated LV anterior and inferior walls, left atrium, and left atrial appendage.
• Apical 3-Chamber (Apical Long-Axis): Rotating 90 degrees counterclockwise profiles the anteroseptal and inferolateral walls, LVOT, aortic valve, and mitral valve.

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The Subcostal (Subxiphoid) Window: Pediatric Cornerstone

In neonates and infants, the subcostal window is the premier acoustic window because the liver provides an expansive acoustic window directly into the inferior cardiac surface without intervening aerated lung.

                      Subcostal Sweep Spectrum:

    [Coronal Sweep (Index: Left, ~3 o'clock)]
    • Posterior: IVC, Hepatic veins, Eustachian valve, Pulmonary veins
    • Mid-Coronal: Gold standard ASD view (beam perpendicular to IAS)
    • Anterior: LVOT, RVOT, MPA bifurcation, Great artery crossover

    [Sagittal Sweep (Index: Cranial, ~12 o'clock)]
    • Right Sagittal: Bicaval view (IVC/SVC into RA)
    • Mid-Sagittal: Abdominal situs (Aorta vs IVC relationship to spine)
    • Left Sagittal: Ductal arch, Systemic venous connections

1. Subcostal Coronal Sweeps

With the transducer in the subxiphoid notch and index mark pointing toward the patient's left (3 o'clock), sweeping posterior to anterior reveals:

• Posterior Coronal: Profiles systemic veins (IVC, hepatic veins, Eustachian valve) and confirms pulmonary veins entering the posterior left atrium.
• Mid-Coronal (Subcostal 4-Chamber) — The Gold Standard for ASD: Because the ultrasound beam strikes the interatrial septum at a strictly perpendicular ($90^\circ$) angle, specular reflection is maximal, completely eliminating false drop-out. Delineates defect rims for transcatheter device closure suitability (superior-anterior, posterior, inferior-caval, and AV valve rims) and classifies ASD subtypes:
  • Secundum ASD: Centered in the fossa ovalis with surrounding tissue rims.
  • Primum ASD: Located at the base of the atrial septum adjacent to the AV valve rings, lacking an inferior tissue rim.
  • Sinus Venosus ASD (SVC-type): Located at the junction of the SVC and RA, strongly associated with Partial Anomalous Pulmonary Venous Connection (PAPVC) of the right upper pulmonary vein.
  • Coronary Sinus ASD (Unroofed CS): Defect in the roof separating the coronary sinus from the left atrium.
• Anterior Coronal: Evaluates LVOT, RVOT, and pulmonary bifurcation, confirming the normal perpendicular spiral crossover of the great arteries (RVOT courses anteriorly and superiorly, crossing the posterior LVOT). In d-Transposition of the Great Arteries (d-TGA), this crossover is absent; the great arteries exit in parallel.

2. Subcostal Sagittal Sweeps

Rotating the transducer 90 degrees with the index mark directed cranially toward the chin (12 o'clock), sweeping right to left:

• Right Sagittal (Bicaval View): Delineates the long axis of the RA, showing the SVC entering superiorly and the IVC entering inferiorly. Essential for sinus venosus ASD detection and central venous line positioning.
• Mid-Sagittal (Abdominal Situs Determination): The mandatory first step in pediatric segmental analysis. Assesses the spatial relationship of the descending aorta and IVC relative to the spine:
  • Situs Solitus: Pulsatile aorta lies to the left of the spine; collapsible IVC lies to the right and anterior to the aorta.
  • Situs Inversus: Mirror-image reversal (aorta right, IVC left).
  • Heterotaxy Syndromes (Isomerism):
    • Right Isomerism (Asplenia): Aorta and IVC are positioned on the same side of the spine (juxtaposition of great vessels).
    • Left Isomerism (Polysplenia): Characterized by Interrupted IVC with Azygos or Hemiazygos Continuation. The intrahepatic IVC is absent; hepatic veins drain directly into the RA, while lower-body systemic venous blood courses via an enlarged azygos vein running posterior to the aorta into the SVC.

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The Suprasternal Notch (SSN) Window: Arch & Pulmonary Confluence

With a shoulder roll hyperextending the neck, the transducer is seated in the jugular notch:

                  Suprasternal Notch Interrogations:

       [SSN Long-Axis: 'Candy Cane']       [SSN Short-Axis: 'Crab View']
       • Transverse aortic arch            • Cross-section of aortic arch
       • Isthmus & Coarctation shelf       • RPA long-axis retro-aortic
       • 3 Head & Neck branches            • All 4 Pulmonary Veins into LA
       • Circular RPA under arch           • SVC & Left Innominate Vein

1. Suprasternal Long-Axis View ('Candy Cane' View)

The index mark is directed toward the patient's left scapula (12 to 1 o'clock):

• Anatomical Structures: Profiles the entire thoracic aortic arch: ascending aorta, transverse arch, aortic isthmus, and descending aorta, with the circular RPA nestled beneath the arch.
• Branching Order & Arch Sidedness (The Contralateral Innominate Rule): In a normal Left Aortic Arch, three vessels arise sequentially: (1) Innominate Artery (courses to the right and bifurcates), (2) Left Common Carotid Artery, and (3) Left Subclavian Artery. The first branch always courses toward the side opposite the arch.
• Aortic Coarctation: Delineates the aortic isthmus (between left subclavian artery and ductal insertion). A discrete posterior shelf produces marked systolic color aliasing and Continuous-Wave Doppler demonstrates systolic acceleration with a persistent diastolic forward runoff ('sawtooth' waveform).
• Interrupted Aortic Arch (IAA): Luminal discontinuity classified by Celoria-Patton into Type A (distal to left subclavian), Type B (between left carotid and left subclavian, highly associated with 22q11 deletion), and Type C (between innominate and left carotid).
• Candy Cane vs. Ductal Hockey Stick: The aortic arch ('candy cane') has a tight curvature and gives off three head/neck branches. The ductal arch ('hockey stick') has an open, flat curvature, arises from the RVOT/MPA, and gives off zero head/neck branches.

2. Suprasternal Short-Axis View ('Crab View')

Rotating 90 degrees clockwise with index mark toward the patient's right shoulder (3 o'clock):

• Anatomical Structures: Displays the transverse aorta in cross-section, with the RPA coursing horizontally beneath it. Directly inferior, all four pulmonary veins drain into the four corners of the left atrium, resembling the outstretched legs of a crab.
• Systemic Veins & TAPVC: Visualizes the SVC to the right and the horizontal Left Innominate Vein above the arch. In Supracardiac Total Anomalous Pulmonary Venous Connection (TAPVC), pulmonary veins unite into a common retrocardiac confluence draining via a vertical vein, causing massive dilation of the innominate vein and SVC.

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Summary Matrix: Pediatric Acoustic Windows & Pathologies

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Clinical Pearls & Sonographic Traps

[!WARNING] The Fossa Ovalis False Drop-Out Trap: In the Apical 4-Chamber view, the ultrasound beam runs parallel to the thin tissue flap of the fossa ovalis, producing specular reflection loss and false-positive acoustic drop-out in up to 30% of normal children. Never diagnose an ASD based on A4C alone. Always confirm defect rims in the Subcostal Mid-Coronal view, where the ultrasound beam impinges perpendicularly ($90^\circ$) upon the interatrial septum, and verify trans-septal flow with color Doppler optimized to a $25\text{ to }35\text{ cm/s}$ Nyquist scale.

[!TIP] Differentiating Aortic vs. Ductal Arch in Cyanotic Neonates: Mistaking a large patent ductus arteriosus (PDA) arch for a normal aortic arch can mask critical coarctation or interrupted aortic arch. The Aortic Arch ('candy cane') has a tight curvature and gives off the three brachiocephalic head/neck branches. The Ductal Arch ('hockey stick') has a broad, flat contour, arises directly from the RVOT/MPA trunk, and gives off zero head/neck branches.

[!NOTE] D-Shaped Septal Hemodynamics on PSAX: Always record M-mode or high-frame-rate 2D clips of the IVS in PSAX at the papillary muscle level with simultaneous ECG. Diastolic-only septal flattening coinciding with the T-P segment indicates RV volume overload (ASD, severe PR). Persistent septal flattening across both systole (QRS-T) and diastole confirms that right ventricular systolic pressure equals or exceeds left ventricular pressure (severe PS, pulmonary arterial hypertension).