The microdialysis principle
Microdialysis is a minimally invasive technique that allows for in vivo sampling of unbound compounds from the interstitial space. The microdialysis system includes a double-lumen microdialysis catheter, a syringe pump, and microvials for collection of dialysates. The catheter is placed in the tissue or cavity of interest, and the syringe pump ensures a constant flow of perfusion fluid in the catheter. At the tip of the catheter, molecules diffuse across a semipermeable membrane from the interstitial space to the perfusion fluid inside the catheter. The transfer of molecules is passive from high to low concentrations. In this way, the microdialysis catheter mimics a capillary. The perfusion fluid is collected in microvials and analysed bedside, thus providing a dynamic view of changes in concentrations of molecular substances within the interstitial spaces. It has proven to be a safe procedure with a low rate of minor complications and no major complications [11, 14].
Traditionally, glycerol, lactate, pyruvate and glucose are the substances that most frequently have been measured using the microdialysis principle [10, 13, 15,16,17,18,19,20,21,22,23] as commercially available equipment provides bedside measurements of these substances. Under anaerobic conditions (e.g. due to compromised perfusion of tissue) pyruvate is converted to lactate. In cases of low levels of oxygen or insufficient energy supply, high levels of lactate and low levels of glucose and pyruvate are observed. These changes may be an early indication of postoperative complications. When the body is in a catabolic state (e.g. following surgery), cleavage of triglyceride results in the release of glycerol [13, 24,25,26]. Glycerol may also derive from the breakdown of phospholipids (the major component of cell walls) when the cell is depleted of glucose or oxygen . Therefore, glycerol should increase in patients with postoperative complications; however, the opposite has also been demonstrated [11, 13], and the mechanism behind the increase and decrease in intraperitoneal glycerol concentration following surgery is not fully understood.
This prospective observational single-centre study was conducted at the surgical department of Odense University Hospital. The study was conducted according to the Declaration of Helsinki following approvals from the Danish Health and Medicines Authority (EudraCT No.: 2012-004398-22), the Regional Scientific Ethical Committees for Southern Denmark (ID: S-20130018), and the Danish Data Protection Agency (2008-58-0035).
Patients and procedures
Inclusion criteria included: more than 18 years of age; informed written consent obtained from the patient or relatives; contamination of two of the four abdominal quadrants with overt peritonitis. Exclusion criteria included known severe renal disease (estimated glomerular filtration rate (eGFR) < 30 mL/min/1.73 m2) and known intolerance to standard antibiotic regimens.
From July 15th 2013 to April 14th 2014, we registered 35 consecutive patients eligible for the study (Fig. 1). From this cohort we excluded 20 patients. 1 patient died prior to inclusion. 4 patients did not have peritonitis, and 1 patient had local peritonitis confined to one abdominal quadrant only. 3 were unable to consent and 1 patient refused to participate. In 2 cases, we lacked equipment. The attending surgeons did not include 4 patients because of various logistic reasons. Microdialysis data was lost from 1 patient, and 1 patient had accidental preterm removal of the microdialysis catheter. 2 patients were not included for reasons unknown. In total, we included 15 evaluable patients.
Comorbidities, including cardiovascular disease (CVD), chronic obstructive pulmonary disease (COPD), diabetes and active cancer, were registered. CVD was defined as hypertension, angina, arrhythmia, claudication, previous myocardial infarction, previous apoplexy, previous transitory cerebral ischaemia, previous cardiac bypass operation and/or previous coronary angiography.
It was left to the surgeon’s discretion to choose between laparoscopy and laparotomy. All patients received intravenous antibiotic prophylaxis with 3 g cefuroxim (Zinacef®, Actavis, Gentofte, Denmark) and 1.5 g metronidazole (Baxter A/S, Allerød, Denmark) either within 30 min prior to skin incision or when peritonitis was diagnosed during surgery. At the end of the surgical procedure, a microdialysis catheter was placed into the peritoneal cavity by use of a splittable introducer (Flocare Jejunokath®, Nutricia, Erlangen, Germany). The tip of the catheter was placed free floating in the most contaminated region of the abdomen. The microdialysis catheter was fixated to the skin to avoid dislocation. Every patient had a central venous line (CVL) inserted. Following surgery, patients were either transferred to an intensive care unit (ICU) or recovery ward depending on the clinical status of the individual patient.
A microdialysis catheter (CMA 65 custom-made microdialysis catheter, M Dialysis AB, Stockholm, Sweden) with a cut-off value of 100 kDa, a membrane length of 30 mm (material: polyethersulfone), and a shaft length of 310 mm was used in this study. The catheter was perfused continuously with Voluven® (Fresenius Kabi, Island Brygge, Denmark) via a pump (CMA 106 or 107 microdialysis syringe pump, CMA Microdialysis AB, Stockholm, Sweden) with a flow rate of 0.3 µL/min. Dialysates were analysed every 4th hour at the bedside for the concentrations of lactate, pyruvate, glucose and glycerol using an ISCUSflex microdialysis analyser (M Dialysis AB, Stockholm, Sweden). The lactate/pyruvate (L/P) ratio and lactate/glucose (L/G) ratio were calculated. The microdialysis continued for a maximum of 7 days.
Data collection and management
The American Society of Anesthesiologists (ASA) score , acute-phase reactants [C-reactive protein (CRP), white blood cell (WBC) count], acid–base status, and eGFR were obtained prior to surgery. Sequential Organ Failure Assessment (SOFA) score was registered on postoperative day (POD) 1 . We registered clinical events during a 30-day follow-up period (complications related to the surgical intervention, re-leakage from the GI tract or formation of intraabdominal fistula, intraabdominal abscess or empyema, intestinal ischemia, re-operation, mortality, and septic shock). Septic shock was defined as sepsis and continues hypotension (systolic blood pressure <90 mmHg or mean arterial pressure <70 mmHg or decrease in systolic blood pressure >40 mmHg or lactate concentration >4 mmol/L in phripheral blood) despite of fluid resuscitation or need for inotropic and/or pressor agents. Analyses of the dialysate from the microdialysis catheter were compared to the clinical course.
Continuous variables are presented as medians with interquartile range (IQR 25th, 75th) or range. Medians were compared using the Mann–Whitney U test. Data analysis was performed using IBM SPSS Statistics 21. A two-sided P value of less than 0.05 was considered statistically significant. We present data from POD 1–5 as all patients underwent at least 5 days of IPM.