Pitfalls & Limitations
A common limitation when assessing cardiac function is foreshortening of the ventricle, particularly in the PLAX and A4C views (Figure 28,29). Foreshortening occurs when the imaging plane cuts through the heart at an angle that misses the true apex, making the LV appear shorter and more rounded than it actually is. This leads to underestimation of LV size, overestimation of LVfx and poor visualization of true wall motion. To minimize this, adjust your probe position, often one intercostal space lower, if you notice a truncated LV. That said, in some patients, a foreshortened view may be the only achievable window due to body habitus, lung interference, or cooperation. In such cases, it is essential to recognize the limitation and interpret findings with caution. This is why the heart should always be imaged in as many views as possible.
Figure 28: Comparison of PLAX foreshortened VS not foreshortened.
Figure 29: Comparison of A4C foreshortened VS not foreshortened
PoCUS relies primarily on qualitative assessment of LV systolic function. While visual evaluation and “eyeballing” EF is a valuable and necessary skill, it is inherently subjective and can vary significantly between users. Studies have shown that qualitative assessment alone can miss borderline or mild systolic dysfunction [7, 25]. Beginners may find it especially challenging. Comparing PoCUS impressions to formal echocardiography reports is a practical way to increase accuracy and build clinical confidence over time.
EPSS can be a helpful quantitative supplement, but it has important limitations. Obtaining an accurate EPSS measurement can be technically challenging, particularly in pediatric patients as it requires placing the M-mode cursor through the tip of the AMVL. Errors in positioning can lead to inaccurate measurements. It is also important to reiterate that EPSS should never be used as a single measure of LV systolic function. EPSS should be interpreted only in conjunction with other functional assessments
Additionally, EPSS is unreliable in the presence of mitral valve pathology, aortic regurgitation, LV dilation and IVS motion abnormality/arrythmia. Structural abnormalities or altered leaflet motion can distort EPSS measurements and reduce their reliability for assessing systolic function. In these cases, the EPSS should not be used. EPSS alone is insufficient to guide decisions regarding cardiology referral or formal echocardiography without corroborating functional findings.
More generally, if significant structural, valvular, or electric dysfunction is present, PoCUS 2D assessment of LVfx may also be unreliable. When blood is not flowing normally, the usual 2D indicators, such as wall thickening or change in chamber size, can appear preserved even though effective stroke volume is reduced and may provide false reassurance about systolic function.
FS and EF can also provide helpful quantitative data, but it is important remember that these linear dimensions rely on key assumptions. When these conditions are not met, FS and EF derived from M-mode may not accurately reflect true LVfx. Moreover, because EF calculations involve cubing linear dimensions, they are more sensitive to variations in heart size and shape than FS. Therefore, these measurements should always be interpreted in the context of the overall clinical and imaging picture.
Arrhythmias also limit the accuracy of PoCUS assessments [7]. Bradycardia can make interpretation of LV systolic function challenging, as slow contractions may mimic systolic dysfunction or give the appearance of hypokinesis of the LV walls. Conversely, tachycardia can make it difficult to accurately assess function and wall motion due to reduced filling time and motion blur and may also create the illusion of hyperdynamic function. In cases of tachycardia it is important to focus on the ventricular wall motion and not the speed of contraction, and consider slowing the clip playback to assess function more accurately. Other arrhythmias with variable heart rates, such as bundle branch blocks, fibrillation, and frequent ectopy can complicate both qualitative and quantitative evaluation by causing beat-to-beat variability in ventricular filling and contraction, making both visual interpretation and consistent measurements difficult to obtain.
Figure 30. PLAX(a) and PSAX(b) views in a child with tachycardiac mimicking hyperdynamic function.
Figure 31. PLAX view demonstrating beat-to-beat variation in LV filling due to frequent premature ventricular contractions, illustrating the challenge of visual interpretation of LVfx in presence of arrythmias.
As with all cardiac ultrasound, patient factors can significantly affect image quality. Lung interference (figure 32), body habitus, and poor cooperation (especially in children) may limit visualization. Try multiple child positions (e.g., supine, left lateral decubitus) and use caregiver involvement, such as having the child sit in a parent’s arms, to increase comfort and cooperation. Utilize the most accessible windows, such as subcostal views, if other views are limited.
Figure 32. PLAX view with lung interference
Finally, formal echocardiography should be pursued when image quality is suboptimal, interpretation is uncertain, or there are clinical concerns for cardiac dysfunction or injury, even if PoCUS findings appear normal [19].