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The neonatal lung: Lung imaging using ultrasound and electric impedance tomography


Key points

  • Lung ultrasound and electrical impedance tomography are point-of-care imaging modalities that provide continuous, radiation-free bedside lung imaging.

  • Lung ultrasound relies on the interpretation of reproducible ultrasound artifact patterns that correspond to the degree of lung aeration. It has an established role in the diagnosis of numerous neonatal respiratory disorders.

  • Electrical impedance tomography analyzes variations in electrical bioimpedance to provide detailed information on total and regional ventilation and aeration characteristics.

  • Both tools have an established role in neonatal research and may improve the care of newborn infants by optimizing respiratory support.

Abbreviations

B-mode Brightness mode
BPD Bronchopulmonary dysplasia
CT Computed tomography
CXR Chest radiography
EEV End-expiratory volume
EIT Electrical impedance tomography
LU Lung ultrasound
LUS Lung ultrasound scoring system
M-mode Motion mode
NICU Neonatal intensive care unit
RDS Respiratory distress syndrome
TTN Transient tachypnea of the newborn

Introduction

Lung imaging is essential to guide neonatal respiratory support. Chest radiographs (CXRs) provide a static picture of the lung, expose infants to ionizing radiation, and correlate poorly with lung volume. Lung ultrasound (LU) and electrical impedance tomography (EIT) are emerging point-of-care imaging modalities. Continuous, radiation-free images make these attractive tools to assess the newborn lung. This chapter will outline the principles, current research, and applications of both modalities.

Lung ultrasound

History of neonatal lung ultrasound

The high acoustic impedance associated with the aerated lung initially relegated LU to a role as an adjunct to CXRs. Despite early recognition that the presence of air and liquid influences ultrasound propagation, the diagnostic use of LU is a recent development. Ultrasound interaction with the reflective pleura generates reproducible artifacts which vary proportionally with lung aeration. LU is easy to learn and concurs with CXR when distinguishing between the various causes of respiratory distress. This has led to LU being adopted by adult, pediatric and most recently neonatal intensive care clinicians.

Lung ultrasound technique

High-frequency microlinear transducers are favored for LU due to their higher resolution and ability to scan the small newborn chest. Detailed examination requires both longitudinal and transverse transducer orientation. Longitudinal orientation allows visualization across multiple intercostal spaces. In contrast, transverse orientation facilitates detailed assessment of pleural line integrity.

Homogeneous lung disorders including respiratory distress syndrome (RDS) secondary to primary surfactant deficiency and lung immaturity, where gravity-dependent aeration differences have not yet evolved, are accurately assessed by imaging only the anterior and lateral lung. Posterior imaging improves the diagnostic accuracy for nonhomogeneous lung disorders such as bronchopulmonary dysplasia (BPD).

Key features of neonatal lung ultrasound

Lung sliding

During real-time scanning, the pleural surface moves with respiration. This phenomenon known as “lung sliding” is an important indicator of pleural apposition and tidal ventilation.

M-mode

B-mode (brightness mode) imaging is usually accompanied by M-mode (motion mode) confirmation of pleural sliding. The image acquired along a single line of sight is displayed over time, allowing for assessment of moving structures using a still image. Normal lung sliding produces the “seashore” sign ( Fig. 18.1 ). This indicates apposition of the parietal and visceral pleura and when present, excludes pneumothorax.

Fig. 18.1, The “seashore” sign. The M-mode cursor is positioned between the rib space and the corresponding M-mode trace is displayed below. The horizontal lines seen in the upper portion of the trace represent the static chest wall and are likened to waves in the sea. The grainy, sand-like appearance of the lower portion of the trace represents the lung ultrasound artifact moving within the M-mode plane.

A-lines

A-lines result from a strong reverberation artifact between the pleura and ultrasound transducer. They are echogenic, horizontal lines which are equidistant from the pleura to the transducer ( Fig. 18.2 ). A-lines occur in normal lung but are also dominant in pneumothorax.

Fig. 18.2, A-lines. The pleural line is seen as a thin, bright, echogenic line (large arrow) . A-lines (small arrows) formed due to reverberation artifact are seen inferior to the pleural line.

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