Patients
The study was performed between June and August in 2010 in an 8 bed adult general ICU of the university-affiliated Vittorio Emanuele Hospital in Catania, Italy. All consecutive adult ICU patients requiring mechanical ventilation and haemodynamic monitoring with trans-pulmonary thermodilution in the opinion of the attending physician were enrolled. Our critically ill patients are admitted to the ICU from the emergency department, from post-surgery and post-procedure settings, and from the trauma centre. These patients are referred from all of the ordinary wards in the hospital when their haemodynamic status worsens to a level with the critical need for invasive ventilation. A lung ultrasound study was performed in all patients immediately after they obtained a trans-pulmonary thermodilution haemodynamic profile (PiCCO System). The two methodologies were performed separately by two independent operators. Particularly, the operator performing the lung ultrasound study was totally unaware of the results of the haemodynamic invasive analysis. Lung ultrasonography was performed by an anaesthesiology resident trained in lung ultrasonography under the direct supervision of a senior physician who had more than 5 years of experience in critical care ultrasonography.
Both lung ultrasonography and haemodynamic monitoring are part of routine management protocols in our ICU, and their application was totally independent from the study protocol. All data were treated anonymously, as represented by the standardized protocol applied in our institution for any patient admitted for treatment. The informed consent of patients was waived because the patients were all unconscious and the study did not involve additional procedures to the standardized clinical protocols, apart from the anonymous treatment of data.
Lung ultrasound
The sonographic examinations were performed with the patients in the supine position. Four quadrants were examined in each hemithorax: anterior–superior, anterior-basal (between the parasternal line and the anterior axillary line), lateral-superior and lateral-basal (between the anterior axillary line and the posterior axillary line). A longitudinally oriented probe was used. In total, 8 quadrants were studied in each patient. As previously defined, the B-lines are an “echoic, coherent bundle with a narrow basis, spreading from the pleural line to the edge of the screen without fading”. When 3 or more B-lines were visualized in a single chest area, the quadrant was defined as “positive,” as defined in recently published guidelines.
The ultrasound examinations were performed using a single ultrasound system (Acuson Cypress, Siemens Medical Solutions USA Inc., Mountain View, CA, USA) equipped with a 5–7 MHz phased-array probe (Acuson 3V2c).
PiCCO system
The trans-pulmonary thermodilution technique (Pulsion PiCCO Plus, Pulsion Medical Systems, München, Germany) was used for haemodynamic monitoring.
In all patients, a 5 Fr arterial catheter was placed in the femoral artery (PV2015LZQ-A Pulsion Medical Systems, München, Germany), and an 8.5 Fr multi-lumen central venous catheter was placed in the internal jugular vein or subclavian vein (Arrow International, Reading, CA, USA). The position of the central venous catheter was verified using a standard chest radiogram.
For the calibration of the system, 3 injections of 15 ml cold saline were performed. Cardiac output was measured, whereas EVLW was computed by the machine. For the calculation of the EVLW index, the actual and predicted body weights were used. For the predicted body weight (PBW) calculation, the following formulas were used: PBW (kg) = 0.91 (height cm—152.4) +50 (for males) and PBW (kg) = 0.91 (height cm—152.4) +45.5 (for females) [14].
Statistical analysis
Data are expressed as the median and the range.
The Spearman (rho) rank correlation test was used to analyse the correlation between the number of chest quadrants positive for B-lines at lung ultrasound and the EVLW index values.
Sensitivity and specificity of the number of positive quadrants predictive of the EVLW index >10 ml/kg were adjudicated by using the receiver operative characteristic (ROC) curve. A p value of <0.05 was considered statistically significant. The statistical analysis was performed using the software MedCalc version 11.6.1.0 (MedCalc Software, Mariakerke, Belgium).