We conducted this prospective, observational feasibility study in the Amsterdam UMC—VU University Medical Center Amsterdam (VUmc Amsterdam, The Netherlands), a tertiary hospital. The local Human Subjects Committee of the VUmc approved the study (METc 16/128), and written informed consent was obtained from all participants. Patient inclusion started June 2016 and ended March 2017. Patients were included on the general ward. The study included consecutive adult patients (age ≥ 18 years) scheduled for elective major abdominal (e.g., gastrointestinal, vascular, or renal) surgery with an intermediate or high risk for the development of postoperative pulmonary complications according to the Assess Respiratory risk In Surgical patients in CATalonia (ARISCAT) risk score ≥ 26 (https://www.mdcalc.com/ariscat-score-postoperative-pulmonary-complications) [15, 16]. Exclusion criteria were trauma or emergency surgery.
Patient characteristics
Patient characteristics were retrieved from the patient data management system and included age, gender, weight, length and body mass index (BMI), American Society of Anesthesiology (ASA) classification, the ARISCAT score, co morbidities, alcohol use, and smoker status. Perioperative parameters were also retrieved and included type of surgery, ICU referral, and total in-hospital length of stay. Patient and laboratory data included the following: arterial oxygen saturation, mode of ventilation, O2 supplementation, sputum cultures, temperature, and leukocyte count.
Chest X-ray
CXR was performed according to standard clinical practice (e.g., after placement of chest tube or central venous line placement) and when demanded by the treating physician for clinical reasons. Anteroposterior bedside CXRs were obtained using a DRX-Revolution mobile X-ray unit (Carestream Health, Inc. © Toronto, Canada). CXR findings, assessed by a radiologist blinded to the LUS findings, were retrospectively retrieved from the patient data management system (PDMS). The Nomenclature Committee of the Fleischner Society recommended terminology was used to describe pathological entities according to the diagnostic criteria for bedside CXR [17, 18].
Lung ultrasound
LUS was routinely performed by a trained member of the research team (n = 3), which consisted of two dedicated investigators and one ICU fellow. LUS was performed upon admission after surgery on postoperative day (POD) 0, and on all consecutive PODs 1–3. Daily LUS was intended, but ultimately could not be performed in a large proportion of the time because of limited researcher availability, e.g., admission after working hours and follow-up days in the weekends. All ultrasonographers were trained according to The Netherlands Society of Intensive Care Programme for Intensive Care Ultrasound (NVIC ICARUS) training course (https://www.nvic.nl). The ultrasonographer was blinded to clinical details and CXR findings, and did not contribute to the diagnostic and treatment strategy of the patient. We used a CX50 ultrasound machine (Koninklijke Philips NV®, Eindhoven, The Netherlands) with both the cardiac phased array (1-5 MHz) and the linear vascular probe (> 10 MHz). The ultrasonographer could choose a particular probe for a particular view in a particular patient, according to individual preference. Lung sliding was determined using the vascular probe. LUS views were obtained according to the BLUE protocol [13, 19].
LUS was performed according the BLUE protocol and adjusted for the postoperative setting [12]. BLUE profile for each BLUE point (Fig. 1) and BLUE profile per hemithorax (BLUE 1 and 2) was determined as follows: A; B; A′; B′; or C profile. A profile means predominantly A lines (Fig. 2). B profile means predominantly multiple (> 2) anterior diffuse B lines indicating interstitial syndrome (Fig. 3). A′ or B′ means the corresponding BLUE profile with absence of, or abolished lung sliding. C profile means anterior alveolar consolidation (Fig. 4). Furthermore, we determined the postero-lateral alveolar and/or pleural syndrome (PLAPS) (Figs. 1, 5) at the lateral sub-posterior sides of the chest and scored them as positive or negative. When positive, consolidation and/or pleural effusion were scored separately and the diagnosis of atelectasis was added to the flowchart. The final conclusion was made according to the bedside LUS protocol in cardiothoracic patients, as published in the previous research [12]. For differentiation between pneumonia and atelectasis, the ultrasonographer was also allowed to look for fever, leukocyte count, and C-reactive protein (CRP) in the PDMS (data also available to the radiologist).
Postoperative pulmonary complications
The incidence of PPCs detected with CXR was retrieved by a research team member using the PDMS. PPCs were scored according to reported radiology findings of on-demand CXR studies ordered by the treating physician at the ward. PPCs were scored as previously defined by Canet et al.: respiratory infection defined as treatment with antibiotics for a suspected respiratory infection and at least one of the following criteria: new or changed sputum, new or changed lung opacities, fever, leukocyte count > 12,000/mm3; pleural effusion defined by blunting of the costophrenic angle, loss of the sharp silhouette of the ipsilateral hemidiaphragm in upright position, evidence of displacement of adjacent anatomical structures, or (in supine position) a hazy opacity in one hemithorax with preserved vascular shadows; atelectasis defined by lung opacification with shift of the mediastinum, hilum, or hemidiaphragm towards the affected area and compensatory over-inflation in the adjacent non-atelectatic lung; and pneumothorax defined by the air in the pleural space with no vascular bed surrounding the visceral pleura, all demonstrated by the CXR [15, 16].
Clinically relevant postoperative pulmonary complications
Clinically relevant PPCs, defined as a PPC that required treatment, as judged by the treating physician, were also reported [12]. We considered the following initiated treatments for scoring clinically relevant PPCs: re-intubation; ICU admission; bronchoscopy; extra bronchodilator therapy; thoracic drain placement; and the use of diuretics and/or antibiotics. Scores of clinically relevant PPCs were retrospectively retrieved from the PDMS according to initiated treatment by the treating physicians for PPCs at the ward. For example: a found pneumothorax which did not require drainage was counted as a PPC but not as a crPPC. The physician who diagnosed clinically relevant PPCs took the following into account: physical examination, conventional monitoring, laboratory results, CXR, and/or CT scan results when available. This reflected daily clinical practice. The treating physicians were blinded to LUS findings.
Statistical analysis
We included as many consecutive patients as possible in the study period. We planned to include more than 90 patients to detect around 30 PPCs based on incidence rates of PPCs after surgery [16]. Results of this study will be used to plan a larger study on the effects of LUS on patient outcome after major abdominal surgery. We performed statistical data analyses using SPSS statistical software package version 22.0 (IBM, New York, NY, USA). Feasibility was reported as percentage of patients in which LUS was possible. Detection rates of PPCs during the study period were reported as frequencies. A PPC could only develop once. For example, if a pleural effusion was detected on POD1, it could not be scored again on POD2. However, a patient could develop more than one PPC during the study period and during 1 day. In addition, detection rates were compared according to both imaging modalities. Furthermore, an aggregated analysis was performed to compare the detection of PPCs in patients who had both; CXR and LUS were performed the same day during the study period. Relative risk was calculated comparing PPC detection with LUS and CXR. Discordant observations were compared for PPCs detected with LUS and CXR.