The Left Parasternal Window
©Walter Rasmussen, R.D.C.S
An acoustic window is a point on the body surface where ultrasound will penetrate to the internal anatomy of interest.
The left parasternal window typically ranges from the left 4th to 5th intercostal spaces, adjacent to the left side of the sternum and provides an anterior to posterior view of the heart from its base to apex. Dependent on variations of individual internal anatomy, it may be necessary to move the transducer to lower or higher interspaces in order to find this window.
In a very small percentage of patients, the parasternal window will yield no images at all and the search for the parasternal views will have to be expanded to the other side of the sternum or almost anywhere on the anterior chest wall. The parasternal long or short axis view is sometimes only visible from the subcostal window. The first place to look depends the patient’s medical history.
For hearts displaced by chronic lung disease or post-operative lung lobectomy, the subcostal window in the upper abdomen might yield some clues for where to look for the parasternal window as it can for or abnormalities where the heart occupies the right side of the chest.
Finding a good parasternal view can be very challenging, even with patients who have normal anatomy and are of average height and weight. The sonographer can overcome some of these challenges by positioning the patient initially lying on their left side and steepening the position if the image is still faint. If the heart appears to lie directly under the sternum, the patient can be positioned face-up on their back or even turned to their right side, causing it to shift in position sufficiently for better imaging.
When the heart is difficult to image, the primary reason is rib-shadowing or overlying lung tissue. Breathing techniques which push the heart from behind the rib or shrink lung tissue are useful in difficult imaging situations. This consists of noticing at what point in the patient’s breathing cycle the image begins to clear and then instructing the patient to exhale or inhale more deeply and pausing respiration for a few seconds while the image is acquired.
Prior heart or lung surgery, pleural effusions, pericardial effusion, chronic lung disease, (pulmonary hypertension, COPD, ILD), and congenital abnormalities can make imaging the heart more difficult.