Introduction
Gastrointestinal surgery is a high-risk operation [
1,
2]. Although the surgical operation and perioperative treatment have been greatly improved, the incidence of postoperative complications and surgical mortality are still high, especially in older patients. Older patients tend to have severe internal disorders and low system function before surgery, which could lead to a higher risk of major complications, such as pulmonary complications, seriously affecting the postoperative rapid recovery of patients and prolonging the length of hospital stay [
3].
Enhanced recovery after surgery (ERAS) protocols are increasingly widely used in the perioperative treatment and can greatly improve patient outcomes, shorten postoperative hospital stays, reduce perioperative complications, and decrease readmission rates to the hospital by 30–50% [
4]. Perioperative fluid management plays a pivotal role in the implementation of ERAS protocols.
Numerous observational studies have reported a strong association between both hypovolemia and overloaded intraoperative fluid infusion and an increased risk of postoperative complications [
5‐
7]. Previous studies have shown a strong correlation between appropriate perioperative fluid management and a reduction in the incidence of postoperative pulmonary complications in patients who undergo major gastrointestinal surgery [
8,
9].
Adequate fluid intake to maintain cardiac output and blood pressure can ensure tissue perfusion, but it does not indicate a good state of microcirculation and tissue oxygenation. Therefore, static indicators cannot accurately guide fluid perfusion in the perioperative period. With the development of science and technology, dynamic indicators have been widely used in perioperative fluid management, such as stroke volume variation (SVV), and many good effects have been achieved [
10]. Previous studies have shown that SVV can predict liquid reactions to some extent.
It is important to monitor not only systemic responses to fluid therapy but also microcirculation and homeostasis. However, research focusing on patients’ terminal tissue perfusion and oxygenation when receiving perioperative fluid management based on more advanced dynamic indicators is lacking. Plasma colloid osmotic pressure (COP) is an important factor in maintaining the balance of fluid flow between the extravascular and intravascular lumens. COP can inhibit the movement of water from intravascular to extravascular and allows interstitial fluid to infiltrate back into the blood vessels from the postcapillary venule, which plays an important role in stabilizing blood volume and preventing tissue edema. A report on pulmonary edema seems to indicate that the maintenance of a normal COP may be of greater importance in critically ill patients [
11]. In our study, we hypothesized that the intraoperative use of COP measurements might be valuable for fluid intake balance and be significant for reducing postoperative pulmonary complications.
Discussion
This study shows that COP- and SVV-based GDT protocols can more accurately improve intraoperative pulmonary edema, lowering the incidence of postoperative pulmonary complications of major abdominal surgery. Importantly, intraoperative COP- and SVV-based GDT reduced postoperative pulmonary complications of grade 2 and higher severity, which contributed to improving the outcome of major abdominal surgery, and conducive to shortening postoperative hospital stay (Table
4).
Some studies have shown that perioperative GDT seems to be more beneficial for patients with higher surgical risk [
13,
14]. Many studies have also confirmed the important role of GDTs in major abdominal surgery [
15,
16]. It is suggested that GDT guided by SVV can improve the intravascular volume status by controlling the fluid volume as a form of fluid therapy [
17,
18], as we found in Group S1 of our study. Compared with the preoperative lactic acid level (T0), the lactic level of the patients in the S1 group decreased to varying degrees after we started GDT guided by SVV (Fig.
3), which indicates that the rapid and targeted intake of sufficient fluid can satisfy tissue perfusion as quickly as possible and reduce blood lactate levels. However, we also found that in the SVV-guided infusion Group S1, lactate levels decreased significantly at T1 and T2 but increased again at T3 (all values were within normal ranges) (Fig.
3). The lactate of patients in the S2 group remained at a lower level. This means that although SVV is a good monitor for fluid infusion in terms of effective circulating blood volume and the response of the cardiovascular system to fluid, it has no good guiding value for providing sensitive monitoring for long-term homeostasis and does not provide good guidance or advice on the choice and use of liquid types.
Therefore, for patients during major abdominal surgery, the perioperative GDT involves not only the maintenance of effective circulating blood volume but also the water balance inside and outside blood vessels. It is important to choose the right type of liquid and reduce the amount of intraoperative fluid infusion and tissue edema by using a reasonable amount of fluid according to the dynamic parameters. According to theoretical analysis, COP is important for maintaining patient fluid balance by influencing fluid flow in and out of blood vessels, according to Starling’s equation. It can more accurately monitor tissue perfusion status.
Studies have shown that increased microvascular permeability in older patients with gastrointestinal diseases leads to extravasation of fluid and protein into the alveoli [
2,
19]. COP is mainly provided by serum total protein. This means that the preoperative COP of older patients undergoing gastrointestinal surgery is worse, suggesting that we should pay more attention to the stability of the internal environment. It is suggested that patients with low COP at admission have no significant difference in vital signs, but their hospital stay is significantly longer than those of individuals with normal COP. A previous study showed that COP below 20 mmHg increases interstitial fluid volume and exposes tissues to edema, which in diverse ways may interfere with normal functions [
20]. Animal models have shown that existing low COP doubles the fluid leakage from capillaries to the interstitium compared to a similar magnitude increase in hydrostatic pressure. Fluid accumulates in the extracellular space of the lung tissue, forcing the alveoli to collapse and exudate, resulting in alveolar dead space and intrapulmonary shunt, affecting oxygen exchange and increasing the risk of postoperative respiratory failure, pulmonary infection, and acute respiratory distress syndrome [
21]. Depressed COP could contribute to pulmonary interstitial fluid overload, which can be assessed by oxygenation indices (PaO
2/FiO
2 ratio) and chest ultrasound techniques. As we found in our research, patients in the infusion group without COP monitoring presented urorrhagia, an increase in extravascular lung water (EVLW), and a decrease in oxygenation index. As shown in Table
6, the patients in Groups C and S1 had higher urine production rates than those in Group S2. The COP of patients in the S2 group was maintained above 20 mmHg, while the COP of patients in Group C and Group S1 was not monitored. By comparison, the PaO
2/FiO
2 ratio of patients in Group S2 was significantly higher than that in Group C at T3, and there was no significant decrease compared with that before surgery (Fig.
2). Moreover, among the three groups, only the S2 group had no significant increase in LIS 3 h after surgery (Table
5).
In addition, recent studies have shown that the release of several inflammatory mediators caused by gastrointestinal disease and its correlative excessive COP lead to the degradation of endothelial glycocalyx [
22], which increases endothelial permeability, resulting in pulmonary edema and worsening of gas exchange [
23]. Therefore, our research indicated that the maintenance of a normal COP is of greater importance in gastrointestinal surgery.
Furthermore, research shows that GDT based on a combination of dynamic indicators of liquid reactivity and other optimized parameters was more accurate than that based on dynamic indicators alone [
24]. Accordingly, as we used in Group S2 of our study, we chose both the COP and the SVV to achieve appropriate fluid loading, which not only controls the total volume of fluid but also maintains the balance of the internal and external volumes of blood vessels. As shown in Fig.
2 of our research results, the PaO
2/FiO
2 ratio of the S2 group was significantly higher than that of the C group at T2 and T3, and compared with patients in the other two groups, after 3 h of surgery, there was no significant decrease in the PaO
2/FiO
2 ratio, indicating that GDT guided by this combination had more obvious lung protection at the later stage.
Nevertheless, the type and timing of fluid infusion may be important, as we see in Table
6, since patients in the S2 group received more colloid boluses early during surgery, suggesting an earlier optimization of tissue perfusion, and less fluid accumulated in the extracellular space. This may be associated with the “no absorption rule.” Numerous studies have shown that contrary to what we previously thought, except for the renal cortex and medulla, there is no continuous fluid absorption of downstream microvessels under stable circulation conditions; the effect of interstitial protein on fluid flow is very small; and transcapillary flow is also lower than previously thought [
25,
26]. It has been clinically observed that the use of crystalloids and colloids does not improve existing tissue edema, most likely because persistent low COP can lead to the inverse relationship between the interstitial protein concentration gradient near the vessel wall and the driving force, the venous ends of continuous capillaries do not undergo fluid reabsorption, and only a small fraction of the solution filtered into the interstitium is returned to circulation through the lymphatic system [
27]. Therefore, dynamic monitoring of COP during perioperative fluid therapy is necessary for patients with chronic low COP before surgery. The normal level of COP should be restored as soon as possible, and the stability of COP should be maintained, which is of great significance for the prevention of postoperative pulmonary edema. In addition, given the significantly lower-than-normal COP at the beginning of surgery and the small differences observed at the end of the surgery, it is difficult to bring the value back to normal or even higher. This is also consistent with our research results, as shown in Table
4. The incidence of postoperative pulmonary edema in the S2 group was significantly lower than that in the C group.
Our study has some limitations. First, COP and SVV were monitored only intraoperatively. GDT should be administered throughout the perioperative period, which may lead to more dramatic improvements in patient recovery. Second, this is a single-center study, and multicenter studies may reduce research errors and make the results more accurate. Third, some factors, such as dietary habits, may affect the accuracy of the results, although we have tried to account for many potential confounders in the experiment.
Future research may focus on the influence of the timing of infusion of different fluid types on the COP to further uncover the qualitative and quantitative influence of fluid types and use timing on microcirculatory perfusion and tissue edema.
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