Correlation Between Lung UltraSound Score and Hypoxemia for Interstitial Syndrome in Emergency Department

Justification for the research

Interstial lung disease (ILD) is characterized by a diffuse affection of the lung interstitial tissue. It can be secondary to acute conditions such as pneumonia or pulmonary oedema either of haemodynamic aetiology either permeability induced. ILD can also result from chronic diffuse parenchymal lung disease (DPLD). It is admitted that acute ILD leads to decreased pulmonary aeration and therefore to hypoxemia.

In front of a patient presenting in Emergency Department (ED) with altered oxygen saturation, arterial blood is classically sampled. Arterial Blood Gases (ABG) procedure, in addition to be painful for the patient, is invasive. ABG however gives access, among other results, to the Arterial Pressure in Oxygen (PaO2), which evaluates the level of hypoxemia. Although the correlation between ABG results and prognosis hasn't been widely evaluated, a rather recent study supported the absence of correlation between those for Acute Heart Failure (AHF). Authors however did not report the correlation between severity of symptoms at patient's hospital admission and hypoxemia. Yet, according to the Berlin definition, hypoxemia represented by the ratio PaO2/FiO2 is a severity indicator in Acute Respiratory Distress Syndrome (ARDS).

Lung ultrasound is a simple tool, non-invasive, non-irradiating, low cost and easily available at the patient's bedside. In addition, ultrasound equipment has become wildly available in the ED and more and more portable. Simultaneously, the interest of lung ultrasound for critically ill patient has been demonstrated. The ILD ultrasonographic diagnosis is based on the presence of multiple ultrasound artefacts called B-lines. B-lines appear when the ratio between air and water is abnormal. Different anatomic substrates have been recognized as being at the origin of this ultrasound pattern. In acute pulmonary oedema the presence of B-lines is related to the presence of extra-vascular water. B-lines presence is also attributed to the thickening of sub-pleural interlobular septa. B-lines are therefore a representation of ILD and accessible using ultrasound. Additionally, more and more ultrasound machines are provided with options to quantify the number of B-lines.

Regarding the importance of B-lines and their interconnection to ILD, a score has been created to evaluate ILD. The Lung Ultrasound Score (LUSS) is based on B-lines quantitative assessment. The B-lines detection is performed in six thoracic zones bilaterally. In each zone, ultrasound lung aeration status is determined scoring from zero to four. A normal aeration of the lung with presence of A lines, persistent pleural sliding and less than three B-lines scores zero point. An ILD, resulting in moderate loss of aeration with more than three spaced B-lines, 7mm apart, scores one point. An alveolar-interstitial syndrome meaning a severe loss of aeration with coalescent B-lines, less then 3mm apart, scores two. An alveolar consolidation leading to a complete loss of aeration with an ultrasonographic tissue like pattern, scores three. A total score over thirty-six can be calculated using this method. Initially developed on an experimental model resembling various pathological conditions encountered in the critically ill, LLUS proved to be correlated to the different states of aeration of the lung. Literature findings are consistent with LUSS being an efficient tool to measure lung aeration in different intensive care situations: re-aeration antibiotics-induced in Ventilator Associated Pneumonia, assessment of PEEP-induced lung recruitment. LUSS realized during weaning trial has been showed to be able to predict of post-extubation distress. In the Intensive Care Unit (ICU), LUSS is efficient to quantify lesions and predict mortality associated with ARDS.