Calculation Of Lung Inflation in Neonates

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Chest x-ray (CXR) are a simple and commonly used tool to assess the lungs of a neonate. Most neonates receiving assisted mechanical ventilation will have a CXR at some stage after starting treatment (many will have serial CXRs over time). Clinicians often use CXRs to help determine the impact of mechanical ventilation (use of a breathing machine to support diseased or sick lungs) on a patient’s lung. It is commonly taught that lung volume can be assessed through the number of posterior ribs seen above the diaphragm in a CXR (diaphragm position; the more ribs the larger the lung volume). This is commonly used in adults to assess for emphysematous changes, or hyperinflation, in chronic obstructive lung disease or asthma. Counting ribs on a CXR, as a mechanism of quickly assessing lung, volume is often recommended in neonatal ICU guidelines. Normal inflation of the lung is considered to be between a diaphragm position of the 8-9th posterior rib, with less than 7 ribs indicating underinflated lungs and over 10 ribs indicating overinflated lungs, although varying thresholds are used. Estimation of lung volume using diaphragm position assessment is also commonly used to titrate specific ventilation settings. Despite being used to guide clinically important decisions, there have been few studies that have assessed and analysed the relationship between the number of ribs above the diaphragm and lung volume. Those that have assessed this question, have either found no significant relationship,2 or have formulated a 3D structure of the lungs from multiple scans to accurately represent lung volume, a method which is not used in clinical practice. There is a need to determine whether this common practice (counting ribs on CXR) to guide important intensive care interventions is useful as a measure of lung volume in neonates. Computed Tomography (CT) is a gold standard measurement of lung aeration and volume. It provides detailed information regarding the aeration of the lungs (such as the absolute amount of lung tissue collapsed or overdistended) and can provide the value and measurements of lung volume in ml/kg for an individual. However, due to the associated increased radiation exposure, CT is not routinely performed in neonates. The aim of this retrospective observational study is therefore to evaluate the relationship between the number of the ribs above the diaphragm on a CXR and the lung volume and aeration as measured through a CT chest. This study will be completed through a retrospective analysis of neonatal chest CTs from a data bank at the Royal Children’s Hospital in Melbourne, Victoria. Lung volume and aeration measures will be derived from CT imaging and compared to diaphragm position on the scout CT image (which is the same as a CXR) performed at the start of every CT. The relationship between lung volume, lung aeration and diaphragm position will then be compared.

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