Acute respiratory distress syndrome (ARDS) is an acute form of respiratory insufficiency and a common cause of respiratory failure. In Europe, the incidence is approximately 7–25 cases per 100,000 inhabitants per year [1]. ARDS occurs in 23% of all ventilated patients and in about 10% of all intensive care patients, with around 5.5 cases per intensive care bed. The syndrome is characterized by diffuse alveolar damage, increased permeability of the pulmonary capillaries, and the presence of pulmonary edema with accumulation of extravascular lung water (EVLW) in the interstitium of the lung tissue and in the alveoli. If ARDS is not treated immediately, the course is rapid and mortality is high.
Early diagnosis remains a challenge
Only about 60% of ARDS cases are recognized in time [2]. Especially in pulmonary edema, nonspecific signs – which often become reliable only at a later stage – significantly complicate the diagnosis. It is crucial not only to correctly assess the extent of pulmonary edema but also to establish a differential diagnosis to determine whether it is permeability-related edema typical of ARDS or pulmonary edema of cardiac origin. The therapeutic approaches would be entirely different.
Diagnostic methods: limitations despite progress
Over the years, various definitions and diagnostic procedures have been developed to describe the difficult-to-classify ARDS syndrome, enable early diagnosis, and differentiate levels of severity.
However, classification and grading of ARDS according to the current Berlin Definition [3] show significant limitations: sensitivity and specificity are limited, changes in the patient’s condition cannot be assessed promptly at the bedside, and early detection is often not possible. The lack of information for predicting the development of ARDS and pulmonary vascular permeability results in missed or delayed diagnoses in most patients and delayed treatment, which may negatively affect outcomes [4].
Radiological imaging is an important component of diagnostics but also comes with limitations and risks. Interpretation of the chest X-ray concerning pulmonary edema is complex. It has been reported that marked inter-observer errors are frequent [5] [6] [7]. CT scans offer detailed images but are always associated with high radiation exposure [8] and a risky transport of critically ill patients. Blood gas analyses are easy to perform and reliable, but they are influenced by many ventilator settings [9] [10]. Pulmonary artery catheters can differentiate the causes of pulmonary edema, but they are invasive and associated with numerous risks [11]. Lung ultrasound is a relatively new method for assessing pulmonary edema, yet effective only when performed by trained users [12] and tracking of lung water changes is difficult [13]. Each of these procedures therefore has its specific limitations, which continue to make ARDS diagnosis and severity classification complex.
Advanced hemodynamic monitoring as the key?
Two parameters obtained by transpulmonary thermodilution within advanced hemodynamic monitoring (e.g., PiCCO) play a particularly important role. The extravascular lung water index (ELWI) enables highly accurate determination of extravascular lung water and a precise assessment of the extent of pulmonary edema [14]. On average, ELWI can even predict the progression of acute lung injury in at-risk patients approximately 2.6 days before ARDS criteria are met [15]. Another parameter, the pulmonary vascular permeability index (PVPI), supports the necessary differential diagnosis between cardiogenic and permeability-related pulmonary edema [16]. At present, transpulmonary thermodilution is the only technology that allows simultaneous determination of ELWI and PVPI.
The clinical significance of ELWI and PVPI is increasingly recognized. Studies confirm that these indices can not only predict the severity of lung injury and mortality rates [17][18], but also improve the effectiveness of fluid management strategies in ARDS patients [19]. Including them in ARDS diagnostic criteria could lead to earlier and more accurate diagnoses and thereby to improved treatment outcomes.
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