Combined Lung and Cranial Ultrasound for Early Identification and Severity Stratification of Neonatal Respiratory Distress Syndrome

Summary

This protocol describes a standardized bedside approach combining lung and cranial ultrasound for early identification and severity stratification of neonatal respiratory distress syndrome. The method integrates lung aeration scoring with cerebral hemodynamic assessment to support reproducible risk evaluation, treatment-escalation decisions, and dynamic monitoring in neonatal intensive care.

Abstract

Early identification and severity stratification of neonatal respiratory distress syndrome (NRDS) are essential for timely respiratory support and prevention of complications. This prospective observational study evaluated the clinical utility of a combined lung ultrasound (LUS) and cranial ultrasound (CrUS) framework for multidimensional bedside risk assessment in 137 neonates with respiratory distress. Baseline LUS and CrUS examinations were performed within 6 h of admission, followed by dynamic LUS monitoring at 24 h and 72 h. The integrated model (Clinical + LUS + CrUS) demonstrated superior diagnostic performance (AUC = 0.941). Optimal neonatal LUS (nLUS) thresholds were 8.5 for NRDS diagnosis, 9.5 for surfactant therapy, and 12.0 for prediction of invasive mechanical ventilation. In addition, improvement in nLUS at 24 h was significantly correlated with oxygenation response (r = 0.52, p < 0.001). These findings indicate that combined LUS-CrUS assessment provides a non-invasive and dynamic strategy for early NRDS identification and management optimization while enabling concurrent neurological risk monitoring.

Introduction

Neonatal respiratory distress syndrome (NRDS) is a major cause of respiratory failure and early neonatal mortality, particularly in preterm infants¹. The condition is primarily driven by pulmonary surfactant deficiency and structural lung immaturity², and its incidence remains high despite advances in perinatal care and respiratory support strategies³. Although pulmonary surfactant replacement therapy and non-invasive ventilation have improved survival in very low birth weight infants⁴, NRDS continues to impose substantial short-term and long-term burdens. In the acute phase, severe hypoxemia and acidosis are common, while persistent alveolar collapse and inflammatory injury may contribute to later complications, including bronchopulmonary dysplasia and neurodevelopmental impairment⁵. Therefore, accurate early identification and severity stratification within the first postnatal day are critical for improving outcomes and guiding individualized management⁶.

Traditional assessment of NRDS relies on clinical signs, arterial blood gas analysis, and chest radiography. However, these methods have important limitations in neonatal intensive care practice. Arterial blood gas analysis is invasive, and repeated sampling may increase the risk of anemia and infection in vulnerable neonates⁷. Chest radiography remains widely used, but radiographic findings may not fully synchronize with early clinical deterioration, and repeated exposure raises concerns about cumulative radiation, especially in preterm infants. In addition, radiography is a static imaging method and cannot provide real-time bedside monitoring to support rapid adjustment of respiratory support or the timing of surfactant administration⁸. These limitations create a clear need for a non-invasive, repeatable, and dynamic bedside assessment pathway.

Bedside lung ultrasound (LUS) has emerged as an effective tool for neonatal respiratory evaluation because it can detect pulmonary aeration loss, interstitial syndrome, consolidation, and pleural-line abnormalities with high sensitivity and specificity⁹. Standardized lung ultrasound scoring further enables semi-quantitative assessment of pulmonary injury severity and dynamic follow-up of treatment response¹⁰. At the same time, cranial ultrasound (CrUS) plays an essential role in monitoring cerebral status in preterm and critically ill neonates. Respiratory dysfunction can impair cerebral autoregulation through hypoxemia, hypercapnia, and hemodynamic instability, thereby increasing the risk of intracranial pressure fluctuation and intraventricular hemorrhage¹¹. Doppler-based CrUS can provide real-time information on cerebral blood flow parameters, including resistance index (RI) and pulsatility index (PI), while also allowing bedside evaluation of ventricular morphology¹².

However, pulmonary and cerebral ultrasound assessments are often performed separately in routine practice, which limits early integrated risk interpretation in neonates with respiratory distress¹³. A combined framework that links pulmonary structural severity with cerebral hemodynamic response may provide a more clinically useful and physiologically complete assessment of early NRDS progression¹⁴. To improve reproducibility and clinical transferability, the present study applies a predefined dual-axis bedside assessment workflow with fixed examination time points (within 6 h, 24 h, and 72 h), standardized ultrasound scoring output, and unified outcome-oriented risk evaluation. This study therefore proposes a dual-axis stratification model integrating lung ultrasound severity scoring with cranial Doppler parameters and examines whether this combined approach improves early identification and severity assessment of NRDS. Specifically, the study evaluates correlations between neonatal lung ultrasound score (nLUS) and cerebral Doppler indices and assesses the predictive value of the combined model for surfactant administration and escalation to invasive mechanical ventilation. Through dynamic bedside monitoring, this work aims to provide a practical and clinically applicable pathway for early risk stratification and precision management in neonates with respiratory distress.

Protocol

Ethics Statement

All procedures were performed in accordance with the institutional neonatal intensive care unit (NICU) guidelines and approved clinical protocols for neonatal imaging and monitoring. Ethical approval for this study was obtained from the Institutional Ethics Committee of Ganzhou Maternal and Child Health Hospital before study initiation (Approval No.: (2023) 101). Written informed consent was obtained from the parents or legal guardians of all enrolled neonates before ultrasound examination and data collection. Consent status was recorded in the case report form (CRF) before any study-specific imaging or data extraction was performed.

1. Study Design and Case Enrollment

1. Screen and enroll eligible neonates
   1. Conduct the study in the NICU using a single-center observational design.
   2. Screen neonates admitted with respiratory distress consecutively during the study period.
   3. Review the admission record, respiratory symptoms, oxygen requirement, and initial clinical diagnosis before enrollment.
   4. Confirm eligibility using the predefined inclusion and exclusion criteria before the first ultrasound examination.
   5. Assign a unique study identification number to each enrolled neonate.
   6. Record screening status, enrollment time, gestational age, birth weight, respiratory support mode, treatment interventions, and planned ultrasound time points in a unified CRF.
2. Perform baseline and follow-up ultrasound examinations
   1. Perform baseline lung ultrasound (LUS) and cranial ultrasound (CrUS) within 6 h of NICU admission.
   2. Perform baseline LUS before or as close as possible to major treatment escalation, including surfactant administration or invasive mechanical ventilation, when clinically feasible.
   3. Perform follow-up LUS examinations at 24 ± 4 h and 72 ± 8 h after baseline imaging.
   4. Confirm that the neonate is clinically stable enough for bedside ultrasound before each scan.
   5. Keep the neonate in a supine position for anterior lung and cranial scanning and use gentle lateral repositioning for lateral or posterior lung zones when clinically tolerated.
   6. Delay non-urgent imaging if the neonate has severe instability, emergency intervention, or unsafe handling conditions.
   7. Record the actual scan time, respiratory support mode, oxygen requirement, and major treatment events at each ultrasound time point.
   8. Document the reason for any delayed or incomplete examination if scheduled imaging cannot be completed within the target time window.
   9. Follow the standardized workflow shown in Figure 1 for study screening, diagnostic grouping, ultrasound timing, and follow-up assessment.

2. Diagnostic Reference and Clinical Definitions

1. Classify NRDS and non-NRDS cases
   1. Diagnose NRDS using a composite clinical reference standard based on respiratory distress manifestations, oxygen or respiratory support requirement, oxygenation impairment, compatible imaging findings, and final clinical diagnosis.
   2. Differentiate non-NRDS respiratory disorders including transient tachypnea of the newborn, infection-related pneumonia, meconium aspiration syndrome, pneumothorax, and congenital cardiopulmonary abnormalities.
   3. Re-evaluate uncertain cases using follow-up clinical course, repeat imaging, oxygenation response, and treatment response.
   4. Adjudicate discrepant or uncertain cases through consensus review by two senior neonatologists who are not involved in ultrasound scoring.
2. Define severity categories
   1. Classify cases as severe NRDS if invasive mechanical ventilation, escalation of respiratory support, or repeat surfactant administration occurs during early hospitalization.
   2. Categorize NRDS severity into mild, moderate, and severe strata using respiratory support intensity, oxygen requirement, oxygenation impairment, and treatment burden.
   3. Generate binary severity comparisons for ROC-based cutoff analyses, including mild versus moderate, moderate versus severe, and non-severe versus severe NRDS.
3. Categorize cranial ultrasound findings
   1. Classify CrUS findings as normal or abnormal for neurologic risk stratification.
   2. Record intraventricular hemorrhage-related abnormalities, ventricular enlargement, and abnormal periventricular white matter echogenicity using the predefined CrUS reporting form.
   3. Measure Doppler-derived cerebral hemodynamic parameters in the anterior cerebral artery when a stable waveform is obtained.
   4. Calculate the resistance index (RI) using the formula: RI = (peak systolic velocity − end-diastolic velocity) / peak systolic velocity.
   5. Calculate the pulsatility index (PI) using the formula: PI = (peak systolic velocity − end-diastolic velocity) / mean flow velocity.
   6. Repeat each Doppler measurement three times during the same examination and record the mean RI and PI values in the dataset.
   7. Treat RI and PI as supplemental Doppler-derived variables for neurologic risk stratification rather than as standalone diagnostic criteria for NRDS.

3. Ultrasound Acquisition, Timing, and Image Storage

1. Acquire ultrasound images
   1. Perform all ultrasound examinations at the bedside using a standardized acquisition workflow.
   2. Record study ID, imaging time point, actual scan time, respiratory support mode, and major clinical interventions during each scan.
2. Configure ultrasound settings
   1. Perform LUS using a high-frequency linear probe with a frequency range of 10–14 MHz.
   2. Perform CrUS using a micro-convex or sector probe with a frequency range of 5–8 MHz.
   3. Set LUS imaging depth to 3–5 cm and position the pleural line in the upper third of the image.
   4. Set CrUS imaging depth to 6–8 cm to include ventricular and periventricular structures.
   5. Adjust gain and focus to optimize image visualization while avoiding over-gain.
   6. Adjust pulsed-wave Doppler settings to obtain stable waveforms without aliasing.
   7. Maintain the insonation angle as low as feasible during Doppler acquisition.
3. Store ultrasound data
   1. Store all images and clips in a de-identified digital archive.
   2. Use standardized file naming including study ID, modality, time point, and operator code.

4. Lung Ultrasound Acquisition and Scoring

1. Perform lung ultrasound scanning
   1. Perform LUS using a fixed 12-region scanning scheme including anterior, lateral, and posterior lung zones.
   2. Scan upper and lower regions bilaterally using a consistent sequence across cases, starting from the right anterior zones, followed by the right lateral and posterior zones, and then repeating the same sequence on the left side.
   3. Perform anterior scanning in the supine position and lateral/posterior scanning using gentle lateral repositioning when clinically feasible. Keep the neonate in the incubator or radiant warmer during scanning whenever possible and avoid unnecessary handling or prolonged repositioning.
   4. For unstable neonates, complete anterior and lateral scanning first and perform posterior scanning only when gentle repositioning is clinically tolerated. Document any unscanned region and the reason for incomplete acquisition.
   5. Record pleural-line abnormalities, B-line pattern, confluent B-lines, consolidation, and atelectatic change.
2. Calculate neonatal lung ultrasound score
   1. Assign regional scores from 0 to 3 using predefined scoring criteria.
   2. Assign score 0 for normal aeration with A-lines.
   3. Assign score 1 for focal or sparse B-lines.
   4. Assign score 2 for confluent B-lines or white lung.
   5. Assign score 3 for consolidation with or without air bronchograms.
   6. Calculate total neonatal lung ultrasound score (nLUS) as the sum of all regional scores.
   7. Record regional scores and total nLUS at baseline, 24 h, and 72 h.
3. Handle incomplete posterior scanning
   1. Document missing posterior regions and reasons for incomplete scanning if repositioning is contraindicated or clinical instability is present.
   2. Retain incomplete cases for sensitivity analyses rather than excluding them.
   3. Use the standardized scoring rubric provided in Supplementary Table S1 during operator training and reviewer calibration.

5. Cranial Ultrasound Acquisition and Neurologic Risk Assessment

1. Acquire cranial ultrasound images
   1. Perform CrUS primarily through the anterior fontanelle with the neonate in the supine position whenever clinically feasible.
   2. Maintain minimal handling and keep the head in a stable neutral position during CrUS acquisition to reduce motion artifacts and physiologic fluctuation.
   3. Acquire standard coronal and sagittal imaging planes through the anterior fontanelle using a consistent scanning sequence.
   4. Assess ventricular morphology, parenchymal echogenicity, and hemorrhage-related findings.
   5. Record structural findings using a predefined reporting format.
2. Assess neurologic abnormalities
   1. Assess IVH-related abnormalities, ventricular enlargement, and abnormal periventricular white matter echogenicity.
   2. Classify IVH findings using the predefined neonatal cranial ultrasound grading framework.
   3. Categorize ventricular enlargement and periventricular echogenicity using predefined reporting criteria.
3. Perform Doppler measurements
   1. Perform pulsed-wave Doppler assessment in the anterior cerebral artery when clinically feasible.
   2. Obtain Doppler measurements during clinically stable periods with minimal handling and stable cardiorespiratory status.
   3. Avoid Doppler acquisition during crying, marked movement, oxygen desaturation, bradycardia, or immediately after major respiratory support adjustment.
   4. Measure RI and PI three times during the same examination.
   5. Enter the mean value into the dataset.

6. Quality Control, Blinding, and Interobserver Consistency

1. Standardize operator training
   1. Complete standardized operator training before study initiation.
   2. Train operators in LUS scanning sequence, nLUS scoring, CrUS structural assessment, and Doppler measurements.
   3. Use a small set of representative training images to calibrate regional LUS scoring, CrUS structural interpretation, and Doppler measurement procedures before formal image review.
2. Perform blinded image interpretation
   1. Interpret images in a blinded manner.
   2. Restrict assessor access to clinical outcomes, severity grouping, surfactant administration status, respiratory support escalation, invasive ventilation status, and integrated model results during scoring.
   3. Separate research image scoring from bedside treatment decision-making. Ultrasound images used for study scoring are reviewed independently by trained assessors who are not involved in clinical treatment decisions whenever feasible.
   4. Base clinical treatment decisions on routine NICU assessment, including respiratory status, oxygen requirement, blood gas results, and attending neonatologist judgment, rather than on the blinded research scoring results.
   5. Release research scoring results for analysis only after clinical grouping, treatment data, and follow-up outcomes have been recorded in the study database.
3. Evaluate image quality
   1. Classify LUS images as evaluable only when pleural-line and vertical artifacts are interpretable.
   2. Classify CrUS images as evaluable only when ventricular and periventricular structures are sufficiently visualized.
   3. Code non-evaluable parameters as missing and document the reason.
   4. Document the reason for non-evaluable images, including motion artifact, incomplete lung-zone acquisition, poor acoustic window, unstable clinical status, or interrupted examination.
4. Assess reproducibility
   1. Perform repeat blinded review using a randomly selected subset of scans.
   2. Perform repeat scoring after a washout interval for intraobserver assessment.
   3. Evaluate agreement for total nLUS using intraclass correlation coefficient (ICC).
   4. Evaluate agreement for categorical CrUS findings using Cohen’s kappa.

7. Variables and Outcomes

1. Record baseline and treatment variables
   1. Record gestational age, birth weight, sex, mode of delivery, antenatal corticosteroid exposure, Apgar scores, and age at NICU admission.
   2. Record oxygen requirement, respiratory support mode, oxygenation-related parameters, and escalation of respiratory support.
   3. Record ultrasound variables including total nLUS, regional findings, dynamic nLUS changes, CrUS findings, RI, and PI.
   4. Record surfactant administration, repeat surfactant dosing, invasive mechanical ventilation, respiratory support duration, and hospital stay duration.
2. Define study outcomes
   1. Define primary outcomes as early NRDS identification and severity progression.
   2. Assess primary severity progression outcomes within the first 72 h after baseline imaging.
   3. Define secondary outcomes as repeat surfactant administration, ventilation-related outcomes, respiratory support duration, hospital stay duration, and progression of abnormal CrUS findings.
   4. Analyze dynamic response using changes in nLUS at 24 h and 72 h.

8. Statistical Analysis

1. Summarize study variables
   1. Summarize normally distributed continuous variables as mean ± standard deviation.
   2. Summarize non-normally distributed variables as median and interquartile range.
   3. Summarize categorical variables as counts and percentages.
2. Perform diagnostic and predictive analyses
   1. Perform receiver operating characteristic (ROC) analysis for diagnostic and predictive evaluation.
   2. Calculate AUC, sensitivity, specificity, positive predictive value, and negative predictive value.
   3. Determine nLUS cutoff values using the Youden index.
   4. Evaluate cutoff performance for NRDS identification, surfactant administration, severity stratification, escalation of respiratory support, and invasive mechanical ventilation prediction.
3. Construct multivariable models
   1. Construct logistic regression models using stepwise analysis.
   2. Include gestational age, birth weight, and baseline respiratory support mode as prespecified confounders.
   3. Compare model discrimination and calibration across models.
   4. Perform decision curve analysis to estimate net clinical benefit.
4. Perform sensitivity and subgroup analyses
   1. Perform complete-case analysis for all primary endpoints.
   2. Perform sensitivity analyses for alternative severity definitions, incomplete regional LUS data, and imaging time-window deviations.
   3. Perform subgroup analyses according to gestational age strata and ventilation strategy when sample size permits.
   4. Use two-tailed statistical testing and define statistical significance as p < 0.05.
   5. Perform all analyses using validated statistical analysis software, with software names and version information provided in the Table of Materials.

Results

Baseline characteristics:

A total of 137 neonates with respiratory distress met the screening criteria and were included in the final analysis. Based on the composite diagnostic reference standard, 92 neonates were classified as NRDS and 45 as non-NRDS respiratory distress. The study population was predominantly preterm, consistent with the NICU admission profile during the study period.

Compared with the non-NRDS group, neonates in the NRDS group ...

Discussion

This study developed and evaluated a structured bedside ultrasound pathway for neonatal respiratory distress syndrome (NRDS), using lung ultrasound (LUS) as the primary respiratory severity axis and cranial ultrasound (CrUS) as a neurologic risk axis¹⁵. Across the early admission window and short-term follow-up, the framework supported three practical tasks in NICU care: early NRDS identification, severity stratification with escalation prediction, and dynamic response monitoring after surfactant ...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
CrUS-Structural-Doppler-Worksheet-v1.0	Study-generated	version date: 21 Apr 2026; XLSX format	Worksheet used to record anterior fontanelle views, structural CrUS findings, IVH-related abnormality, ventricular enlargement, periventricular echogenicity, RI, PI, and Doppler evaluability.
De-identified image archive naming convention	Study-generated	ImageArchive-NamingRule-v1.0; version date: 21 Apr 2026	Naming rule for saved images and clips: StudyID_Modality_TimePoint _RegionOrPlane_FileType_OperatorCode. Example: NRDS001_LUS_T0_RAU_Cine_OP01.
High-frequency linear ultrasound transducer	GE HealthCare	12L-RS linear array probe; catalog / part no. 5499501	Linear probe used for neonatal lung ultrasound. Use for pleural-line visualization, A-line/B-line assessment, subpleural consolidation detection, and regional nLUS scoring.
Lung ultrasound scoring worksheet	Study-generated	nLUS-12Region-Worksheet-v1.0; version date: 21 Apr 2026; XLSX format	Worksheet used to enter 12 regional LUS scores, total nLUS, missing regions, reason for incomplete scanning, and ΔnLUS at 24 h and 72 h.
Micro-convex ultrasound transducer	GE HealthCare	8C-RS micro-convex array probe; catalog / part no. 5499508	Micro-convex probe used for cranial ultrasound through the anterior fontanelle and for pulsed-wave Doppler acquisition when feasible.
Probe cleaning and disinfection SOP	Study-generated	SOP-LUS-CrUS-Disinfection-v1.0; version date: 21 Apr 2026	Defines cleaning sequence before and after bedside ultrasound, including gel removal, probe-handle wiping, machine surface wiping, and documentation of cleaning completion.
ROC and diagnostic statistics software	MedCalc Software Ltd.	MedCalc Statistical Software 20.2	Used for ROC analysis, AUC calculation, AUC comparison, Youden-index cutoff selection, sensitivity, specificity, PPV, NPV, LR+, and LR−.
R package for decision curve analysis	CRAN / open-source R package	rmda 1.6 or dcurves 0.5.0	Use one package consistently for decision curve analysis. Record the exact package and version in the final analysis log.
R package for ROC analysis or visualization	CRAN / open-source R package	pROC 1.18.5	Optional R package for secondary ROC visualization or sensitivity checks. Use the same ROC method as the primary analysis plan if results are reported.
R package for plotting	CRAN / open-source R package	ggplot2 3.4.4	Used for reproducible plots, including predictor ranking, calibration curves, and trajectory plots when figures are generated in R.
Routine ultrasound coupling gel	Parker Laboratories	Aquasonic 100 Ultrasound Transmission Gel; product no. 01-08; 0.25 L dispenser, 12 per box	Use for routine intact-skin bedside lung ultrasound when local infection-control policy permits non-sterile gel. Use a consistent gel type across examinations to reduce image-quality variability.
Statistical analysis software	IBM	SPSS Statistics 26.0	Used for descriptive statistics, group comparisons, logistic regression, and basic model summaries.
Statistical computing environment	R Foundation for Statistical Computing	R 4.3.2	Used for calibration plots, decision curve analysis, sensitivity analyses, and reproducible figure generation when applicable. Freeze package versions before final analysis.
Sterile ultrasound coupling gel	Parker Laboratories	Sterile Aquasonic 100 Ultrasound Transmission Gel; product no. 01-01; 20 g sterile overwrapped foil pouch, 48 per box	Use for neonatal cranial ultrasound or any examination requiring sterile single-use gel according to NICU infection-control policy. Use one pouch per patient whenever sterility is indicated.
Surface disinfectant wipes for ultrasound equipment	PDI Healthcare	Super Sani-Cloth Germicidal Disposable Wipes; product no. Q55172	Use for external surface disinfection of ultrasound probe handles, machine contact surfaces, and bedside equipment according to local NICU infection-control policy. Avoid using disinfectants on probe surfaces unless compatible with the manufacturer’s probe-care instructions.