Parasternal Long- & Short-Axis Views

The Parasternal Window: Position and Transducer Placement

The parasternal window is typically the first window acquired in a comprehensive transthoracic echocardiogram (TTE). The patient is placed in the left lateral decubitus (LLD) position — often with the left arm raised behind the head to spread the ribs — which brings the heart closer to the chest wall and rotates lung tissue out of the acoustic path. The transducer is placed at the left sternal border, typically in the third or fourth intercostal space.

Parasternal Long-Axis (PLAX) View

With the index marker pointed toward the patient's right shoulder (roughly the 10-o'clock position), the transducer images the heart along its long axis. The PLAX view visualizes, from anterior (near field) to posterior (far field): the right ventricular outflow tract (RVOT), the interventricular septum, the left ventricle (LV) — its basal and mid segments only, never the true apex — the posterior wall, the mitral valve (anterior and posterior leaflets with their chordae), the left atrium (LA), and the aortic valve and root. Only the right coronary cusp (anterior) and non-coronary cusp (posterior) of the aortic valve are seen in this plane; the left coronary cusp lies out of the imaging plane. The descending thoracic aorta is often visible in short-axis cross-section posterior to the LA, and the pericardium is well seen as a bright line along the posterior LV wall.

PLAX is the primary window for several key linear measurements, obtained at end-diastole except where noted:

Tilting the transducer medially from PLAX (toward the patient's right hip) produces the RV inflow view, which profiles the tricuspid valve and right atrium — useful for aligning continuous-wave Doppler with a tricuspid regurgitant jet. Tilting laterally/superiorly produces the RV outflow view, showing the pulmonic valve and main pulmonary artery.

Parasternal Short-Axis (PSAX) Views

Rotating the transducer roughly 90 degrees clockwise from the PLAX position (index marker now pointed toward the patient's left shoulder) produces an axial, doughnut-shaped cross-section of the heart. Sweeping the beam from the cardiac base toward the apex passes through four standard levels:

• Aortic valve level (base): the tricuspid, three-leaflet aortic valve creates the classic 'Mercedes-Benz' sign; this level also shows the tricuspid valve, RVOT, pulmonic valve, main pulmonary artery and its bifurcation, and the interatrial septum. It is used to judge aortic valve morphology (tricuspid vs. bicuspid), coronary ostia, and aortic root size.
• Mitral valve level: the LV appears round, and the mitral leaflets open toward the full cross-sectional area of the LV in diastole in a 'fish-mouth' pattern. This level is used for direct planimetry of mitral valve area in mitral stenosis.
• Papillary muscle level: the round anterolateral and posteromedial papillary muscles bulge into a circular LV cavity. This is one of the three standard short-axis levels (basal, mid/papillary, apical) used for 16- or 17-segment wall-motion scoring and provides a quick global assessment of LV systolic function.
• Apex level: obtained just proximal to the true LV apex, distal to the papillary muscles.

Common Pitfalls and Color Doppler Screening

A frequent PLAX technical error is an off-axis or foreshortened image, where the transducer plane cuts obliquely through the LV rather than through its true long axis; this falsely narrows the LV cavity and can systematically underestimate LV internal dimensions and overestimate wall thickness, so sonographers confirm a true long-axis plane (mitral valve and aortic valve both centered, LV cavity at its widest) before freezing measurement images. Color Doppler is also applied in the parasternal window even though it is not used for quantitative velocity measurement: color flow overlaid on PLAX and PSAX helps localize and characterize regurgitant jets — for example, an eccentric aortic regurgitation jet directed toward the anterior mitral leaflet, or the origin and direction of a mitral regurgitation jet — screen for shunts, and flag turbulence for follow-up spectral interrogation from a better-aligned window. In the PSAX aortic valve level specifically, color Doppler is used to screen the interatrial septum for a shunt and to assess pulmonic and tricuspid regurgitation qualitatively.

Why the Parasternal Window Matters

Because the parasternal beam travels roughly perpendicular to blood flow through the mitral, aortic, and tricuspid valves, PSAX and PLAX are excellent for two-dimensional anatomy, chamber dimensions, and wall thickness — but they are poor windows for spectral Doppler velocity measurement, since a perpendicular intercept angle yields little to no Doppler shift. The one major exception is the LVOT diameter measurement in PLAX, which is a geometric (not velocity) measurement used together with an apically-derived LVOT velocity-time integral to calculate stroke volume via the continuity equation. Doppler velocity measurements — for stenosis gradients, regurgitant jets, and diastolic filling — are instead obtained from the apical window, covered in the next section.