While several studies have demonstrated the negative effects of positive-pressure pneumoperitoneum on cardiovascular and organ perfusion, many institutions still continue to use standard pressure pneumoperitoneum at 12–14 mmHg due to its surgical space convenience. Unfavorable consequences are not expected during most elective laparoscopic operations in healthy or low-risk individuals. However, increased intraabdominal pressure has a significant clinical impact on high-risk patients including the elderly population, cardiac dysfunction patients or critically ill patients.6,13
Our study results consistent with previous study results indicating that CI, SVI, MAP, and end-tidal CO2 levels were not significantly different between low and standard pressure groups.14–16 Despite transient reductions in CI and SVI at the beginning of insufflation, the levels recovered after gas insufflation.14 We did not expect any between-group differences in these parameters to confound hemodynamic effects, volume status, and CO2 outcomes. In our study, the heart rate in the low pressure group trended significantly lower than the standard pressure group. This difference has not been reported in previous studies.14,16 One effect of low pressure pneumoperitoneum was reduced postoperative pain due to lower visceral pain secondary to peritoneal stretch receptors.14 However, we excluded pain or hypercarbia effects from the outcome since all subjects received fentanyl i.v. maintenance and QL block during surgery for intra and postoperative pain management. Additionally, normal end-tidal CO2 value, level of BIS and TOF were maintained at comparable levels during pneumoperitoneum and surgery in both groups. We assumed the higher heart rate in the standard pressure group was a response to the higher inflammatory response due to higher pneumoperitoneum pressure.
Kidney function is very sensitive to increases in intraabdominal pressure. Even a slight pressure increase of 10 mmHg has shown to affect the kidney, and pressures as high as 20 mmHg have disrupted the kidney function.7,17 An animal study showed that CO2 pneumoperitoneum of 12–18 mmHg induced renal cell apoptosis in the outer medulla and cortex.17 In humans, increased intra-abdominal pressure caused hypoperfusion in the abdominal or splanchnic regions with or without hypotension. Research on animals showed that 12 mmHg pneumoperitoneum resulted in hypoperfusion that induced the release of inflammatory cytokines and neutrophil migration.7,18 Advanced venous congestion and decreased renal blood flow leads to tissue hypoperfusion or ischemia that triggers an inflammatory response. After desufflation, reperfusion occurs when renal blood flow is normalized. This leads to oxidative stress that stimulates the synthesis of inflammatory cytokines, which have been postulated to mediate the association between blood flow changes and endothelial-epithelial injury.6,19
Increased intraabdominal pressure causes mechanical compression of the inferior vena cava, renal vasculature, and parenchyma.20,21 It increases sympathetic activity, which is regulated through CO2-mediated baroreceptors, and can lead to renal cortical vasoconstriction and its sequelae.18,19 Kidney autoregulation is influenced by vascular (myogenic) and tubuloglomerular feedback (TGF). Vascular factors affect autoregulation of renal perfusion through blood flow and pressure on blood vessels, which depend on cardiac output and blood pressure (as long as it is on the threshold of autoregulation).22 Under normal conditions, blood flow is laminar, which gives constant pressure to the blood vessel walls. Changes in blood flow cause shear stress due to turbulent or oscillatory flow. Shear stress stimulates the pro-inflammatory transcription factor NF-𝜿B signalling pathway and the synthesis of nitric oxide through VEGFR-2 activation, which is present on the surface of endothelial cells.23–26 Furthermore, stimulation of inflammatory responses plays a role in endothelial and epithelial cell activation, injury, repair, and apoptosis. Shear stress in the form of a constant or uniform laminar flow has a protective effect on the endothelium. Low or turbulent shear stress caused by impaired blood flow will stimulate the inflammatory response, increasing the expression of endothelial adhesion molecules and their interactions with neutrophils and monocytes in the endothelium.23
During pneumoperitoneum insufflation, an increase in RI indicates that increased intra-abdominal pressure causes a decrease in interlobar arterial blood flow, stimulating a systemic inflammatory response that triggers the release of IL-6.23 Our study showed a higher release of IL-6 during the increasing intra-abdominal pressure in the standard pressure group than in the low pressure group. Pneumoperitoneum insufflation using CO2 gas is not ideal, as the high solubility of CO2 gas makes it readily absorbable by tissue, resulting in sympathetic stimulation such as tachycardia. Although CO2 and surgical techniques can contribute to the release of pro-inflammatory cytokines,27 our study showed that an acute, slight increase in intraabdominal pressure results in significantly increased IL-6 levels. Furthermore, using a low pressure pneumoperitoneum can attenuate this response.
Studies on the impact of low versus standard pressure pneumoperitoneum have shown various results. A laparoscopic cholecystectomy study performed with low and standard pressures showed no differences in the increase of IL-6, IL-8, and IL-10. Our study results mirrored another laparoscopy study that found significantly higher IL-1, IL-6, and CRP levels in the standard-pressure pneumoperitoneum group than in the low pressure pneumoperitoneum group.19 Yap et al. found that laparoscopic donor nephrectomy resulted in nearly 50% of their subjects showing an increase in tumor necrosis factor-alpha (TNF-𝛼) excretion at both 5 and 24 hours and increased urine neutrophil gelatinase-associated lipocalin (NGAL) after donor nephrectomy without any significant differences were observed in cardiopulmonary parameters. These results also suggested that elevated cytokine content may be due in part to increased endogenous production. The results of this study validate the previously published study demonstrating that animal models of AKI nephrectomy resulted in increased TNF-𝛼, IL-6, and monocyte chemoattractant protein-1 expression. IL-6 has been shown to induce neutrophil infiltration with increased macrophage infiltration.28 An animal study showed extrarenal IL-6 production from the liver after unilateral nephrectomy.29 As urine output and serum creatinine were within the normal limit before and after the procedure, our results suggest that the increased plasma IL-6 was due to increased endogenous production and not decreased renal excretion.
As hypothesized, we found that increasing plasma syndecan-1 corresponded to elevated plasma IL-6. IL-6 is a proinflammatory cytokine that causes syndecan-1 activation and shedding from the endothelial surface of blood vessels into the bloodstream. In accordance with the degree of inflammation that occurs, the shedding of syndecan-1 increased in both levels of pneumoperitoneum pressure compared to the baseline conditions. However, the increasing plasma syndecan-1 level was lower and proximal tubular cell syndecan-1 expression was higher in the low pressure group than in the standard pressure group. There is a significant increase in syndecan-1 glycocalyx product degradation after major surgery in humans and animals.30 The duration of laparoscopic nephrectomy is longer than open nephrectomy, and the addition of high pressure pneumoperitoneum use leads to longer and more profound warm ischemia that contributes to syndecan-1 shedding.31
The renal tubular epithelium can not only be passively injured but can also produce an active response to inflammation. The release of proinflammatory and chemotactic cytokines activates T-cells and their co-stimulating molecules. Proximal tubular cells respond to T-cell ligands through cell surface receptor activation.19 The increasing syndecan-1 expression and its shedding into the blood are considered an adaptive response to repair and early cell injury.32 Syndecan-1 plays a role in the process of re-epithelialization during inflammation and is involved in promoting renal tubular epithelial cell survival in animal models of ischemia/reperfusion and human kidney transplantation. In early renal injury, tubular epithelial cells increase syndecan-1 regulation to repair injured cells. In response to a mild inflammatory condition, increasing tubular syndecan-1 expression results in better re-epithelialization in allografts, and correlates with less proteinuria and tubular atrophy, lower serum creatinine, and lower risk of delayed graft function. Syndecan-1 becomes a tubular marker that correlates with kidney graft function and survival.33 In further injury, epithelial cells will increasingly lose syndecan-1 due to their decreasing ability to proliferate and regenerate this factor. The sustained elevating plasma syndecan-1 and low syndecan-1 expression correlate with the degree of kidney tubular function loss.34 Syndecan-1 expression in the proximal renal tubules is related to the degree of proteinuria in various kidney diseases, therefore the plasma syndecan-1 could become an early sign of renal tubular injury.35
Vascular endothelial growth factor-A (VEGF-A) is a strong angiogenic cytokine that has a role in maintaining the microvascular system and increasing vascular permeability. One regulator of VEGF-A is VEGFR-2, which is expressed during ischemic or inflammatory conditions.36,37 When inflammation occurs, IL-6 and activated syndecan-1 in the endothelial cells stimulate the synthesis of VEGF-A molecules and its binding to VEGFR-2 on the endothelial surface. This increases VEGFR-2 phosphorylation in order to repair the endothelial injury.34,38 Plasma syndecan-1 levels are hypothesized to correlate with plasma soluble VEGF-A as a marker of endothelial damage and plasma creatinine and urea as a marker of kidney function.34 In a normal human kidney, VEGFR-2 is expressed on glomerular endothelial cells and peritubular capillaries, as well as tubular epithelial cells at a low degree. Regulation of protein expression through the VEGFR-2 receptor is important for the survival of kidney endothelial cell tissue after ischemic injury.36,39
The synthesis and activation of VEGFR-2 in baseline conditions occurs but is very mild. Our study found an increase in the synthesis and higher activation of VEGFR-2 in the standard pressure than the low pressure group. The level of plasma sVEGFR-2 was significantly higher when standard pressure was used. In comparison, the low pressure pneumoperitoneum attenuated the inflammatory response and produced lower sVEGFR-2 levels. Activation of VEGFR-2 as the marker of vascular endothelial permeability depends on the extent of inflammation and results in an increase in endothelial permeability and increased levels of plasma sVEGFR-2. Plasma soluble VEGFR-2 is the result of an increase in alternative splicing of mRNA or as a proteolytic product of membrane-bound VEGFR-2 released into the bloodstream. During ischemia-reperfusion injury, VEGFR-2 mRNA expression and sVEGFR-2 increase as a response of VEGFR-2 receptors. Increased VEGFR-2 expression is a direct effect of VEGF released by ischemic tubular epithelial cells to the adjacent endothelial cells to maintain capillary blood supply and promote tubular cell survival and recovery. As a comparison, previous studies have shown that at laparoscopic sites, there is an increase in protective VGEF-mRNA expression as a response to injured tissue repair.36,40,41
Our observed the increase in tubular epithelial cell VEGFR-2 expression may suggest that inflammatory responses occurring in circulation reach the extracellular matrix and renal tubules. Tubular epithelial cell VEGFR-2 expression was higher in the standard than in the low pressure group. The low pressure group produced less injury to the kidney due to less inflammation and less stimulation of VEGFR-2 in the renal endothelial and tubular epithelial cells. A previous study showed that overstimulation of VEGFR-2 occurring before unilateral nephrectomy induced endothelial proliferation, abnormal angiogenesis, extracellular matrix deposition, and acute tubulointerstitial injury in experimental animals.39 There is a hypothesis that syndecan-1 acts as a VEGFR-2 co-receptor and has a role in modulating VEGF-VEGFR-2 signals for endothelial cell proliferation and survival. It has been proposed that syndecan-1 and VEGFR-2 act as new markers for AKI and its treatment.34,42
From electron microscopy examination, the low pressure group showed intact tubular cell membranes with clear cell boundaries and intact brush borders. These morphologies were healthier when compared to the standard pressure group, which showed greater injury, tenuous tubular cell membranes, brush borders detached from the cell body, and more vacuolization. The extracellular matrix peritubular endothelial cell was also more edematous in the standard pressure pneumoperitoneum group. These results support the preference of low pressure pneumoperitoneum, which also results in a lower degree of ischemia and tissue inflammation, and reduced endothelial and tubular epithelial cell injury. Perioperative ischemia and reperfusion cause injury to donor kidney epithelial cells that can continue to induce a response from the vascular endothelium.31,33 Animals treated with various CO2 pneumoperitoneum pressure gradients indicate that increased intraabdominal pressure causes reperfusion ischemic injury leading to cell apoptosis.17 Damage or loss of tubular epithelial cells is the main histological finding of tissue damage that occurs in renal ischemia-reperfusion injury. In humans, acute tubular necrosis is observed in 44% of open nephrectomy and 45% of laparoscopic patients. In patients undergoing laparoscopy nephrectomy, 54% of renal biopsy specimens taken showed subcapsular cortical injury. These injuries indicate that pneumoperitoneum and mechanical injury during laparoscopic surgical manipulation causes acute tubular necrosis accompanied by peritubular capillary congestion.41
In our study, urinary KIM-1 level was lower during low pressure pneumoperitoneum than standard pressure pneumoperitoneum. The increasing urinary KIM-1 during pneumoperitoneum expressed the proximal tubule cell stress injury accompanied by the formation of debris and apoptotic cells. This process causes an increase in KIM-1 molecule synthesis that will be released into the lumen of the tubule and detection in the urine.43 The reversible tubular injury that was represented by KIM-1 returning to baseline levels 2 hours after desufflation that may be due to the short length of the pneumoperitoneum duration during laparoscopy procedure.
We found syndecan-1 to be expressed in proximal and distal tubular epithelial cells, with negative syndecan-1 expression within the glomerular or peritubular vasculature. This result was similar to that of Adepu et al., who found syndecan-1 in the basolateral layer in proximal tubular epithelial cells in human kidney biopsy samples and hypothesized that the increase in plasma syndecan-1 levels was partly derived from an extravascular source such as the renal tubular epithelial cells.33 Our study on living donor patients showed contradictory results to a previous animal study that showed the presence of syndecan-1 protein in the glomerulus and peritubular capillaries.34 Syndecan-1 may not have been detected in the glomerular endothelium because the dominant proteoglycan expression in glomerular endothelial cells are syndecan-4, perlecan, and glypican according to research on human glomerular endothelial culture cells in vitro. Other studies also show the dominant proteoglycan layer in IgA nephropathy to be of perlecan and biglycan.35,37 Urinary syndecan-1 can be used as another alternative to detect extravascular shedding of the glycocalyx layer.
Our study showed that laparoscopic donor nephrectomy resulted in increased plasma IL-6, syndecan-1, and sVEGFR-2 during pneumoperitoneum and 2 hours after gas desufflation. IL-6 as a mediator of the extrarenal effects of AKI is a clinically important finding, since it may lead to the use of cytokine-binding proteins and other anti-inflammatory agents to improve outcome beyond what current supportive renal measures can offer. Despite the absence of syndecan-1 from the glomerular and peritubular endothelial glycocalyx, it was found in the membrane of the proximal and distal tubules and was important for renal tubular cell survival during inflammation. VEGFR-2 can be a sensitive marker to detect endothelial injury due to perfusion disturbance and inflammation. Both increasing plasma syndecan-1 and sVEGFR-2 levels, rather than plasma creatinine, BUN or urine output, can be interpreted as an early warning of the underlying injury.
Endothelial and renal tubular responses are the earliest signs of hypoperfusion and inflammation due to increased intra-abdominal pressure.3,7,18 From our study results, although the duration of pneumoperitoneum was relatively short, the inflammatory reaction and presence of endothelial and renal tubular markers to inflammation were higher, especially when standard pressure, and not low pressure pneumoperitoneum was used. The usage of low pressure pneumoperitoneum can attenuate this systemic inflammatory and vascular response. Inhibiting syndecan-1 shedding and the release of VEGFR-2 are believed to have renal-protective roles.31,34,39,42 Syndecan-1 and VEGFR-2 are the early markers of renal tubular and vascular endothelial response due to a systemic inflammation, however, the inhibition of syndecan-1 shedding and sVEGFR-2 response to endothelial injury in preventing or reducing kidney injury demands further experimental and clinical studies. It is also important to evaluate the risks and benefits between low and standard pressure in operator’s point of view related to operative comfort such as space for dissection, and vision while using suction.