Discussion
In this study, we compared the findings from CXR, which is typically conducted to evaluate lung expansion in preterm and term infants treated with HFOV, with that of POCUS, which we performed in our NICU The level of the right hemidiaphragm, which we assessed for lung expansion using both CXR and POCUS, was comparable in both methods.
Unlike conventional ventilation, HFOV has a lung protective effect by keeping hyperinflation at a minimum with low tidal volumes. However, optimal lung volume must be ensured for effective ventilation and oxygenation. Many experimental and clinical studies have been conducted to evaluate optimal lung volume during HFOV. However, even though these sophisticated devices used in studies provided more exact information than SaO2 and diaphragm level measurements, they could not be used in clinical practice due to practical difficulties[5-7,9-13].
Although measuring lung capacity during HFOV is challenging, CXR can be used for approximation. Chest radiography is one of the most commonly used imaging modalities in the NICU. CXR, which is mostly performed in the anteroposterior supine position, is sufficient for the evaluation of the chest wall, heart, lungs, diaphragm, catheter, and endotracheal tubes. Because optimizing lung expansion is an important aspect of the HFOV strategy, CXR should be performed frequently in the early phases of HFOV, and subsequently at least daily as the patients become more stable. Furthermore, if a small adjustment in mean airway pressure leads to a significant increase in the patient’s oxygen requirement, CXR should be performed to assess lung volume [14]. If conditions do not improve or deteriorate despite performing lung opening maneuvers to restore sufficient end-expiratory volume, a CXR should be taken [15].
The position of the diaphragm and the relative flattening of the diaphragm are two criteria to consider lung expansion. The right hemidiaphragm should be located between the lower border of the 8th to 10th ribs for optimal lung expansion. Although more advanced technologies are needed for more precise lung volume estimation, CXR, which is routinely employed in all neonates who require mechanical ventilation, appears to be a more practical overall strategy [7,13,14]. However, substantial concerns have recently emerged regarding the harmful consequences of radiation exposure in diagnostic procedures (radiography, computed tomography, angiography, etc.) that entail a high risk of malignancy development. The risk of malignancy may increase later in life due to both the effect of radiation exposure on cells in the neonatal period and the fact that children who are growing are more radiosensitive. Newborns in critical condition are still regularly subjected to radiographic imaging during the management of pulmonary diseases. The desire to reduce exposure and various disadvantages of CXR have highlighted the POCUS in NICU. POCUS has become a more favorable technology than radiography due to its point-of-care applicability, diagnostic reliability, faster detection compared to CXR, lack of radiation damage, repeatability, ability to evaluate disease course and response to treatment, and low cost [16].
POCUS should become part of the clinical examination and should be performed by clinicians who can properly interpret the images to avoid time loss for newborns and should be regarded as an ethical choice rather than an alternative method to radiography. For the clinicians to perform POCUS, they should be trained for 6-8 weeks and have evaluated about 20-30 patients [17,18]. In our study, POCUS was performed by a neonatologist who had previously been trained and used ultrasonography in her daily practice.
In this study, we evaluated the extent of lung expansion using ultrasonography to reduce radiation damage in babies on HFOV in our unit. With CXR, the right hemidiaphragm was recorded at the median 9th rib (8-11) while with POCUS it was recorded at the 7th (6-8.5) rib. We associated the difference of about two ribs with the anatomical structure of the chest wall and the rib level of the right hemidiaphragm being counted posteriorly with CXR and anteriorly with USG. Such a good correlation of these measurements with each other shows that lung aeration can be evaluated following adjustments in HFOV using USG instead of CXR. Furthermore, the measurements taken at different postnatal ages of patients in our study at different weeks of gestation and with varied lung pathologies, as well as their correlation with CXR findings, enhance the method’s clinical applicability regardless of gestational age and pathology.
Our study is the first in this field as no other studies have been conducted on this subject so far. However, randomized controlled studies in larger patient groups are required to routinely employ ultrasonography in the evaluation of appropriate lung volume, to respond to patients more promptly, to reduce radiation exposure and time loss, and  to improve this method further.