POPE is one of the most common postoperative complications after liver resection. Previous studies have reported various risk factors for POPE, including age, body mass index, chronic obstructive pulmonary disease, resection site of liver, resected liver weight, operative time, and transfusion [2, 3, 11, 12]. The present study revealed abdominal incision, blood loss, diaphragm incision, and intraoperative infusion volume as independent risk factors for CR-POPE. CTR on POD1 was identified as an independent predictor. Moreover, estimated POPE volume using CT volumetry correlated with CTR on POD1. CT volumetry for POPE is a novel approach that allows continuous quantification of volume.
Generally, PE is classified as exudative (due to imbalances in hydrostatic or oncotic pressures) or transudative (inflammatory) effusion based on the pathophysiology [13, 14]. POPE following coronary artery bypass surgery is classified as exudative, but POPE following hepatobiliary surgery is uncertain. PE after liver resection might have a dual etiology of both hypoalbuminemia with cirrhosis and direct inflammatory responses to the thoracic cavity. Clinically, most POPE tends to arise in the right-sided pleural cavity. Risk factors including abdominal incision, resection site, incision in the diaphragm to reach the liver, and tumor invasion to the diaphragm, indicated the influence of local inflammation. Another strongly associated factor, was postoperative complications excluding POPE, such as bile leakage or cut-end abscess, which demonstrate local or systemic inflammation, leading to secondary POPE. However, since biochemical data for PE were lacking, it remains unclear whether POPE is a result of other complications or a sign of complications, or both.
Postoperative hypoalbuminemia is also reportedly associated with POPE [15]. We therefore investigated the impact of hemodynamics on CR-POPE and whether CTR could predict CR-POPE. Excessive infusion volume has been reported to worsen postoperative pulmonary function [16, 17]. Surgical invasion has direct effects on fluid retention. Plasma volume decreases with general anesthesia alone [18]. Under such conditions of intraoperative hemodynamics, fluid distributes into the interstitium. A Danish multicenter randomized trial showed that a higher volume of intraoperative fluid infusion resulted in sodium retention and weight gain for 2–3 days after surgery, then declined [19]. This accumulation of fluid and delayed clearance may cause excessive intravascular volume loading, leading to pulmonary edema and PE. Postoperative enlargement of the CTR is a sign of overloading and cardiomegaly. In the present study, the CR-POPE group showed a significantly higher infusion volume and lower urine volume intraoperatively. Furthermore, infusion volume correlated with both estimated POPE volume and CTR. This demonstrated that an excessive fluid balance resulted in POPE. Perioperative fluid management should be administered appropriately according to blood loss [20]. However, if the operation takes a longer time, total infusion volume will be correspondingly higher. In hepatobiliary surgery, bleeding from the liver parenchyma sometimes requires massive infusion or transfusion. Attention must be paid to the infusion volume, particularly in patients with poor cardiac, respiratory, or renal function or the elderly.
Adjusting postoperative infusion volume and early administration of diuretics could inhibit excessive overloading. On the other hand, goal-directed fluid therapy that requires perioperative fluid restriction has become widespread, and is being trialed for liver resection [21–23]. Transition of surgery and anesthesia might affect the occurrence of postoperative complications. Actually, the present cohort showed a lower incidence of CR-POPE than previous reports [1–3].
In the present study, abdominal incision was an independent factor. Recent progress in minimally invasive surgery has led to increases in laparoscopic surgery [24]. Conversely, the occurrence of comorbidities has decreased in laparoscopic surgery [25]. The higher estimated blood loss, transfusion and biliary reconstruction in the CR-POPE group may support secondary POPE due to other postoperative complications. In fact, postoperative complications were independently associated with CR-POPE.
The CTR is a well-established value in the clinical setting for evaluating cardiac morphology radiologically [4]. Rayner et al. reported that CTR on cardiac radiographs correlated with left ventricular mass on echocardiography [26]. Previous studies have reported the prognostic utility of the CTR. A larger CTR could be a predictive marker of sudden death in patients with chronic heart failure and long-term survival rate in patients with hemodialysis [5, 6]. Those studies demonstrated that hemodynamics had an effect on CTR. Further, a larger CTR is a prognostic factor for short-term postoperative outcomes in cardiac surgery [27, 28]. However, no investigations with non-cardiac surgery have been conducted and this is the first to show an impact of CTR on postoperative outcomes.
This retrospective study showed several limitations. Imaging conditions for chest radiography might vary depending on the status of the patient. Postoperative abdominal pain alters respiratory functions. Patients with chronic obstructive pulmonary disease often have a smaller CTR, whereas patients with chronic heart failure have a larger CTR. The advantage of CTR measurement is that this simple method can be applied using chest radiographs. Several advanced imaging modalities have emerged recently, such as CT, magnetic resonance imaging, and radioisotope imaging [29]. However, radiographs cannot replace these options because of the convenience, speed, and low exposure. Radiographs can be taken anywhere using mobile equipment without patient transfer. Although the rate of increase in CTR from pre- to postoperatively is also useful, here we offer a use for postoperative CTR on POD1 with clinical utility. In the present study, the properties of PE using pleural fluid were not examined and further investigations should be conducted in the future.