The parasternal long axis, often abbreviated PLAX, is the first view most clinicians obtain during a cardiac ultrasound exam. Acquired by placing the ultrasound probe along the left side of the sternum with its marker pointed toward the right shoulder, PLAX provides a lengthwise cross-section of the heart that reveals the left ventricle, the aortic and mitral valves, the left atrium, part of the right ventricular outflow tract, and the root of the aorta in a single frame. Because it packs so much anatomy into one image, PLAX functions as both a quick-look screening tool and a gateway to more detailed measurements, making it relevant to anyone who has had, is about to have, or is learning to perform an echocardiogram.
What You Actually See in the PLAX View
Picture a slice through the heart from upper right to lower left. At the top of the screen sits the right ventricular outflow tract, which appears as a crescent of muscle wrapping around the front of the heart. Beneath it lies the interventricular septum, and below that the left ventricular cavity, bounded on the far side by the posterior wall. To the right side of the image, the aortic root fans open into the ascending aorta, and the two aortic valve leaflets visible in this plane can be seen opening and closing with each heartbeat. Just behind the aortic root, the left atrium fills the lower-right corner. The mitral valve sits between the left atrium and the left ventricle, its two leaflets sweeping open during filling and snapping shut during contraction.
This single frame gives an immediate sense of whether the heart’s main chambers are the right size, whether the walls are thickening or thinning normally, and whether the valves are opening and closing the way they should. A trained eye can spot a large pericardial effusion behind the heart, a thickened septum suggesting hypertrophic cardiomyopathy, or a flail mitral valve leaflet in seconds.
How the View Is Obtained
The patient typically lies on their left side with the left arm raised, which shifts the heart closer to the chest wall and opens a wider acoustic window between the ribs. The ultrasound probe goes in the third or fourth intercostal space, just to the left of the sternum, with its orientation marker aimed toward the patient’s right shoulder. Slight tilts, rotations, or sliding a rib space up or down help bring all the key structures into alignment. When done correctly, the septum and posterior wall run roughly parallel across the screen and the aortic root lines up neatly with the left ventricular outflow tract.
Getting this alignment right matters more than it might seem. If the probe is angled even a few degrees off-axis, the left ventricle can look deceptively small or the septum can appear thicker than it really is. An expert proposal from German echocardiography specialists highlighted that improper technique in standard views is a common source of diagnostic error, emphasizing the need for methodological rigor even in this seemingly straightforward first step of the exam.
Key Measurements Clinicians Take from PLAX
PLAX is the standard view for a suite of left ventricular measurements. With a single cursor line drawn across the ventricle just beyond the tips of the mitral valve leaflets, the sonographer captures the thickness of the interventricular septum, the internal diameter of the left ventricle, and the thickness of the posterior wall, all in both diastole (when the heart is filling) and systole (when it is contracting). These six numbers feed into calculations of how well the left ventricle squeezes and how much blood it moves per beat.
The aortic root and left atrial dimensions also come from this view. Measuring the aortic sinus diameter and comparing it to the left atrial dimension gives a ratio that helps flag left atrial enlargement, which can be an early sign of valve disease, high blood pressure, or heart failure. In veterinary echocardiography, these same PLAX-derived ratios carry diagnostic weight. A study of dogs with myxomatous mitral valve disease found that the left atrial-to-aortic dimension ratio in healthy dogs ranged from about 1.8 to 2.4, and that dogs with mitral valve disease had ratios significantly higher than that range.
1PubMed. Two-dimensional, long-axis echocardiographic ratios for assessment of left atrial and ventricular size in dogsSheep echocardiography follows a similar playbook. Researchers studying Ghezel sheep used the right parasternal long-axis four-chamber view to measure left atrial diameter in diastole and systole, mitral valve annulus dimensions, and left ventricular volumes calculated from the same view using a volume estimation method built into the ultrasound machine. A separate long-axis view of the left ventricular outflow tract provided the aortic sinus and aortic valve dimensions.
2PubMed Central. Two-dimensional and M-mode echocardiographic parameters in Ghezel sheepM-Mode Through PLAX
One of the oldest and still most commonly used techniques is to activate M-mode while in the PLAX view. M-mode fires a single ultrasound beam along a fixed line and plots the returning echoes over time, creating a scrolling graph of how structures move with each heartbeat. By placing that line through the left ventricle at the level of the mitral valve chordae, you get a precise tracing of wall motion and chamber size that updates with every cardiac cycle. This approach remains a workhorse for measuring fractional shortening, a quick index of how well the left ventricle contracts.
M-mode through the aortic valve and left atrium is equally important. In a normal heart, the two visible aortic leaflets open wide during systole and form a box-shaped pattern on the M-mode tracing. In hypertrophic cardiomyopathy, the M-mode tracing through the mitral valve can reveal systolic anterior motion, a telltale sign in which the front leaflet of the mitral valve drifts toward the septum during contraction and partially blocks blood flow out of the heart. The M-mode tracing through the aortic valve may also show the leaflets partially closing in mid-systole, a secondary clue to the same obstruction.
Spotting Hypertrophic Cardiomyopathy
PLAX is often the first view where hypertrophic cardiomyopathy, a condition in which the heart muscle is abnormally thick, becomes apparent. A septum measuring 15 millimeters or more in diastole raises a red flag, especially when the posterior wall is normal thickness. The combination of a thick septum and a small or normal-sized left ventricular cavity is hard to miss on a well-aligned PLAX image.
Systolic anterior motion of the mitral valve, mentioned above, is an important wrinkle. In a study of 25 patients with hypertrophic cardiomyopathy, the distal portions of the mitral leaflets moved toward the septum during systole while the proximal portions of those same leaflets simultaneously bulged backward into the left atrium, overshooting the valve’s ring by 5 to 15 millimeters. That finding demonstrated that mitral valve prolapse and systolic anterior motion can coexist in the same patient, which matters because each abnormality generates a different type of valve leak.
3The American Journal of Cardiology. Simultaneous occurrence of mitral valve prolapse and systolic anterior motion in hypertrophic cardiomyopathyFor cats, hypertrophic cardiomyopathy is the single most common heart disease, and PLAX is central to the diagnosis. However, a recent study comparing different imaging methods for measuring left ventricular wall thickness in cats found that two-dimensional measurements and M-mode measurements from the same view could not be used interchangeably, particularly in cats with the disease, where the disagreement between methods was wide enough to affect clinical decisions. The researchers noted that wall thickening in affected cats was often unevenly distributed, with the septum frequently measuring thicker than the free wall, and recommended that two-dimensional measurement might be the better choice for identifying focal areas of thickening.
4Journal of Veterinary Internal Medicine. Effect of echocardiographic imaging view and methods on left ventricular wall-thickness measurements in normal cats and cats with hypertrophic cardiomyopathyColor Doppler and Valve Leaks
Adding color Doppler to the PLAX image turns blood flow into a visual map: red indicates flow toward the probe, blue indicates flow away, and turbulent or high-velocity jets light up as a mosaic of colors. This is where aortic regurgitation, mitral regurgitation, and abnormal flow across the septum become immediately visible.
For ventricular septal defects, the PLAX view can reveal a jet of color crossing through the septum from the left ventricle into the right ventricle. A study of patients with perimembranous ventricular septal defects found that seeing color flow across the septum in a standard parasternal long-axis view was associated with aortic regurgitation, with moderate sensitivity and specificity. The researchers also looked for aortic cusp override and abnormal cusp movement in the same view, both of which pointed toward valve involvement.
5PubMed. Echocardiographic characteristics of perimembranous ventricular septal defects associated with aortic regurgitationFor aortic regurgitation specifically, PLAX is arguably the best view to start with. A regurgitant jet shows up as a flame of color in the left ventricular outflow tract during diastole, and its width relative to the outflow tract gives a rough sense of severity. Mitral regurgitation jets aim in the other direction, shooting back into the left atrium during systole. PLAX provides a profile view of these jets, though the apical views usually offer better alignment with the Doppler beam for quantifying severity.
The Rule of Thirds
A widely taught shortcut called the “Rule of Thirds” proposes that, on PLAX, the right ventricular outflow tract, the aortic outflow tract, and the left atrium should each take up roughly one-third of the base of the heart, forming an approximate 1:1:1 ratio. If any of the three looks disproportionately large, the idea is that something may be wrong: an oversized right ventricular outflow tract might suggest pulmonary hypertension, an enlarged left atrium could point to mitral valve disease, and so on.
The concept is intuitive, but a cross-sectional study that actually tested its diagnostic performance found it has significant limitations. For detecting an abnormal right ventricular outflow tract, the Rule of Thirds showed only about 13% sensitivity but 84% specificity, meaning it rarely flagged a problem that was actually there, but when it did flag something, it was usually real. Interobserver agreement, or how consistently different examiners reached the same conclusion, was only fair. The researchers concluded that the Rule of Thirds works best as a “rule-in” tool and should not be relied upon alone when evaluating critically ill patients.
6PubMed. The diagnostic test characteristics of the “rule of thirds” on the parasternal long axis view: A cross-sectional studyIn practice, the Rule of Thirds remains a useful mental model for beginners who are still training their eyes, but it is no substitute for taking actual measurements. Its low sensitivity means a normal-looking ratio does not reliably exclude pathology.
PLAX as a Starting Point in Point-of-Care Ultrasound
In emergency departments and hospital wards, point-of-care ultrasound (POCUS) has made the PLAX view something that non-cardiologists increasingly perform. When a patient arrives in shock or with unexplained shortness of breath, a PLAX image can quickly answer several binary questions: is there a large pericardial effusion? Is the left ventricle squeezing well or barely moving? Is the right ventricle dilated enough to suggest a massive pulmonary embolism? These are not nuanced measurements. They are yes-or-no visual assessments, and PLAX is the view most commonly used to start answering them.
The tradeoff is image quality. Emergency physicians often cannot position the patient optimally, rib shadows may obstruct the view, and the exam may be performed on a portable machine with a lower-frequency probe. A suboptimal PLAX image can still be useful for ruling in a large effusion or severely depressed function, but it is much less reliable for the fine measurements that a formal echocardiography lab would report. Knowing what PLAX can and cannot tell you in a rushed bedside setting is a practical skill in itself.
Common Mistakes That Lead to Bad Images or Wrong Numbers
The most frequent error is being off-axis. If the ultrasound beam cuts through the ventricle obliquely rather than along its true long axis, the chamber will look bigger or smaller than it really is. The posterior wall may appear artificially thickened, or the septum may seem to merge into the right ventricular free wall. A good alignment check is to confirm that the aortic valve and the cardiac apex are both visible and that the septum and posterior wall are roughly parallel.
Another common pitfall involves the M-mode cursor. Placing it too close to the mitral valve tips leads to overestimation of septal thickness, because the chordal structures bulk up the apparent wall. Placing it too far toward the apex can miss regional abnormalities near the base. The standard recommendation is to position the cursor at or just beyond the mitral leaflet tips.
Gain settings also matter. Too much gain fills the left ventricular cavity with artifact that makes it look smaller. Too little gain makes the endocardial border disappear, leaving the sonographer guessing where the wall ends and the cavity begins. Some of these pitfalls are obvious in retrospect, but they can shift a measurement by several millimeters, which is enough to push a borderline reading into the “abnormal” category or mask genuine pathology.
Speckle Tracking and Strain Imaging from PLAX
Beyond simple measurements, modern software can track tiny speckle patterns in the ultrasound image frame by frame, calculating how much each segment of the heart wall deforms during contraction. This technique, called speckle-tracking strain imaging, has been applied to PLAX clips to measure longitudinal and radial strain, two distinct ways the heart muscle shortens and thickens as it squeezes.
Research in mice found that the parasternal long-axis view provided the most reproducible images for longitudinal strain analysis, making it the preferred window for tracking how well the inner layers of the heart muscle shorten along the length of the ventricle.
7PubMed Central. Echocardiographic Speckle-Tracking Based Strain Imaging for Rapid Cardiovascular Phenotyping in Mice A related study used PLAX-derived strain to detect early cardiac hypertrophy in a mouse model, finding that global radial strain and longitudinal strain from the parasternal long axis both decreased within three days of inducing hypertrophy, before conventional measurements like wall thickness or ejection fraction had budged.8PLoS ONE. Speckle Tracking Based Strain Analysis Is Sensitive for Early Detection of Pathological Cardiac Hypertrophy
In human clinical practice, strain imaging from PLAX is less commonly used than from the apical views, partly because the apical windows give a longer-axis view of the left ventricle that is better suited to tracking all of its segments. Still, PLAX strain has a niche in patients whose apical windows are poor, in follow-up of focal wall-motion abnormalities near the base, and in research protocols designed to capture radial deformation.
Why Some Patients Are Harder to Image
Body habitus is the single biggest determinant of PLAX image quality. Patients who carry more weight around the chest wall or who have hyperinflated lungs from conditions like emphysema often have narrow or absent parasternal windows because air and adipose tissue scatter the ultrasound beam before it reaches the heart. In these individuals, the sonographer may need to try different intercostal spaces, apply more pressure with the probe, or ask the patient to exhale and hold their breath to push lung tissue out of the way.
Chest wall deformities such as pectus excavatum can paradoxically improve parasternal windows by pressing the heart closer to the probe, but they also distort cardiac geometry, making the ventricle appear compressed. Post-surgical patients with sternal wires or mediastinal hardware may have acoustic shadows that obscure portions of the view. In all these scenarios, the interpreter needs to note the limitation and supplement PLAX with other windows or alternative imaging modalities when the view is suboptimal.
How PLAX Fits into the Full Echocardiographic Exam
A standard transthoracic echocardiogram uses a sequence of views obtained from several acoustic windows: parasternal, apical, subcostal, and suprasternal. PLAX is typically the first acquired, partly by convention and partly because it confirms that the machine settings and patient positioning are adequate before moving on. The sonographer then rotates the probe 90 degrees to obtain the parasternal short-axis view, which slices the heart like a loaf of bread and adds information about regional wall motion in a different plane.
After the parasternal views, the probe moves to the apex of the heart for the apical four-chamber, two-chamber, and long-axis views, which are better aligned with blood flow for Doppler measurements and give a fuller picture of all four chambers. Subcostal views come next, providing an alternative window that is especially useful in patients on ventilators or with difficult parasternal access. Each view adds something the others cannot provide, and PLAX’s role as the starting point means it sets the first impression of the exam. If the PLAX looks dramatically abnormal, the rest of the study is often tailored to characterize that specific abnormality in more detail.
In transesophageal echocardiography, where the probe sits inside the esophagus behind the heart, there is no direct equivalent of the transthoracic PLAX. The closest match is the mid-esophageal long-axis view, which shows a similar set of structures from a posterior perspective. Comparing findings between the two helps confirm diagnoses, particularly for mitral and aortic valve pathology where surgical planning depends on precise anatomy.