Stroke Volume Variation-Guided Fluid Management Decreased Postoperative Pain in Patients Undergoing Spine Surgery

Introduction

The adequacy of intravascular fluid status is important for Anesthesiologists in order to maintain efficient cardiac output and tissue perfusion. Some textbooks suggest a protocol for perioperative fluid management in different kinds of patients to supply basic fluid requirements, replace preoperative deficits, and infuse intraoperative losses (blood loss, intravascular fluid shift, and evaporation) [1]. The strategy for infusing and monitoring fluid status preoperatively is based on the assumption of daily fluid needed, preoperative fasting duration, and the types of surgery, all of which might not been estimated clearly. Although central venous pressure (CVP) is traditionally regarded as a practical guide for fluid status, its sensitivity and specificity for the relationship with fluid responsiveness is poor when compared to other dynamic monitoring methods [2]. Recent publications suggested that goal-directed fluid management guided by perioperative dynamic monitoring reduces the related complication rate, the length of hospital stay, as well as the postoperative mortality rate, improve the healthcare quality and reduces medical cost [3, 4, 5]. These growing evidences on the benefits of perioperative fluid optimization gradually became and constructed one element of enhanced recovery after surgery (ERAS) program, which resulted in faster recoveries and better outcomes [6]. Stroke volume variation (SVV), one of hemodynamic measurement of pre-load responsiveness, derived from the minimally invasive arterial wave courter variations due to cyclically controlled mechanical ventilation-induced trans-pulmonary pressure changes. Its simplicity, accuracy and availability have been showed to provide a more accurate measure of fluid responsiveness than CVP does. Previous experiments have shown that optimization of fluid management, using goal-directed therapy in surgical patients is helpful in preventing or treating tissue hypoxia [7], ischemia- reperfusion injury, and furthermore decreasing postoperative infection and postoperative renal and gastrointestinal complications [4, 8]. Theoretically, once ischemia damages tissue, multiple chemical mediators release from the damaged tissue and inflammatory cells by nociceptive conditions. We hypothesized that optimizing the perioperative fluid management by goal- directed hemodynamic monitoring would improve the tissue perfusion and reduce the hypervolemia or hypovolemic induced hypoxia or ischemia, resulting in decreased postoperative pain intensity.

Results

Ninety-two patients were enrolled into the study. One patient did not meet the inclusion criteria due to moderate chronic obstructive pulmonary disease; 91 patients were included in the study analysis. No patient withdrew from the protocol. There were 46 and 45 patients in the SVV and CVP-guided groups, respectively. (Figure 2) Table 1 shows the patients' demographic data and history.

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There was no difference between the SVV-guided and the CVP-guided group in age, weight, height, gender, ASA classification, or the patient history of hypertension, cigarette smoking, or alcohol ingestion. Table 2 lists the perioperative measurements and comparisons between the two groups. There was no differences in the total volume of fluid infused, total blood loss, total blood volume infused, total urine output, and duration of surgery between the two groups. There was a statistically significant difference in the average hospital length of stay (days) between groups (SVV vs. CVP-guided group; 5.3 ± 1.3days vs. 7.2 ± 1.4 days, respectively, P = 0.003). There were no differences in the preoperative baseline measurements. However, the intraoperative and postoperative measurements from arterial blood showed that lactate and glucose levels were significantly lower in the SVV-guided group than in the CVP-guided group (P < 0.05). The incidence of minor complications did not differ between the two groups, and no patient encountered a major complication during the hospitalization. Table 3 shows postoperative pain intensity and cumulative morphine consumptions between the two groups. During movement or at rest, the postoperative pain intensities were significantly lower in the SVV-guided group than in the CVP-guided group from postoperative day one (POD1) until day three (POD3), nonetheless, the cumulative morphine consumption showed no difference between the groups. The incidence of postoperative nausea and vomiting (PONV) was similar between the groups.

We found that SVV guided perioperative fluid management for the major posterior spine surgery resulted in perioperative infusion of similar fluid volumes and lower plasma lactate and glucose levels during and after surgery. The postoperative pain intensities during movement and at rest were lower in the SVV-guided group than the CVP-guided group on postoperative day one to three. Many benefits are recognized for the goal- directed fluid therapy in the surgical patients. We employed perioperative goal-directed fluid therapy in major spine surgery and specifically utilized postoperative pain intensity as the main primary outcome. We found that SVV-guided fluid management improved postoperative pain condition (Table 3) and decreased intraoperative and postoperative lactate and glucose levels (Table 2). Although the underlying mechanism for reduced pain intensity with the SVV- guided fluid management patient is unclear and likely complex, recent studies suggest that an ischemic pain mechanism is a potential mechanism [11]. In animal studies, tissue acidosis and lactate increased in the incision wounds facilitated nocicept or activation and enhanced pain behaviors [12]. Previous studies have shown that optimization of hemodynamic and fluid management, using goal-directed therapy, in high risk patients is helpful in preventing and treating tissue hypoxia, thus reducing mortality and morbidity [7]. Additionally, tissue microcirculation and oxygen tension were improved by goal-directed colloid fluid infusion when compared to restrictive fluid infusion in both healthy and anastomosis colon [13]. We did not measure patients’ wound tissue pH or lactate due to the potential for increasing wound infection, nonetheless, we supposed optimizing haemodynamic with SVV-guided goal-directed fluid therapy in surgical patients would improves the tissue circulation, prevent ischemia-reperfusion injury, and thus, decrease postoperative pain intensity. Excessive postoperative pain intensities after major surgeries in adult’s leads to delayed ambulation, increased pulmonary complications, hospital lengths of stay, and lasting functional impairment [14, 15]. Our results show that pain intensity of the patients in the SVV-guided group was lower than in the CVP-guided group, regardless rest or movement. The results imply the approach to fluid optimization by goal-directed management decrease hypervolemia induced ischemia or hypervolemia induced edema, and thus, decrease pain intensity. It is unclear the reason why the postoperative morphine consumption by patients of the SVV-guided group demonstrated no significant difference when compared with those of CVP- guided group during postoperative three days even if the pain intensity showed significant difference between groups. The small sample size (the calculation based on the power of 0.8 and the measure for pain intensity on movement decreased by 20% during the postoperative day one) and the small effect size (= 0.24 for the morphine consumption at POD1) might make us hard to distinguish the effect from the random. In addition, as the pain intensity at rest (VASR) was consistently lower than three during the study period and the overall satisfaction score higher than 8.5, the better-than-average pain management would further limit the postoperative morphine consumption by patients. The position change from supine to prone might infer to an underlying physiologic and hemodynamic variations. Different prone positions also have different impact on the measurement of hemodynamic values [16]. However, Biais et al. demonstrated that only a slightly increase for pulse pressure variation or stroke volume variation has been observed during the change from the supine to the prone position, its accuracy and reliability did not be altered to predict the fluid responsiveness during the prone position [17]. The optimal threshold value or trigger value for the prone position is not yet been defined conclusively in the literatures, Biais et al. suggested 14% might be provide a better sensitive and specificity in their study with 30 participates. There is no significant modification with the trigger value in the supine position; we continued to use 13% as the trigger value as we do in the supine position. Our results showed no difference in the total volume of fluid infused between the two groups. The total perioperative volume of fluid infusion was primarily related to the duration of surgery, baseline fluid status of patient, the goals of the health provider set, and the volume and timing of blood loss/blood transfused. Previous literatures indicated that with SVV goal-directed fluid management the patients received more colloid during elective intra-abdominal surgery as compared to fluid management by traditional routine [18]. Additionally, Lopes et al. demonstrated that the average volume of fluids infused intra-operatively was greater in the group guided by pulse pressure variation [19]. On the contrary, Forget and colleagues found that intraoperative crystalloids and total volume infused were significantly lower in the goal-directed pulse oximetry plethysmography variability index-guided group [20]. The inconclusive results regarding fluid volume infused perioperative might be explained by the types and durations of different surgery, or by the variation of patient’s fluid status; hypervolemia in some patients and hypervolemia in other patients, or by the use of different protocols/algorithms, in which the fluid infusion rate varied at times. Lactate is the product of tissue hypo- perfusion or hypoxia-induced anaerobic glycol’s is. It provides an indirect but sensitive indicator of tissue perfusion and is obviously related with the sufficiency of intravascular fluid volume, organ hypoxia, and energy failure from blood flow reperfusion [21]. Lactate concentrations would bede creased by optimizing the fluid status and cardiac preload [4, 19]. High plasma lactate level (≥ 2 mmol/L) is associated with 2-5 fold increase in 28-days mortality rate as compared to those patients with lactate level ≤ 2 mmol/L [22]. Thus, serum lactate became a potentially useful indicator of tissue perfusion especially in sepsis or shock patients. Our results showed that the lactate concentration was lower in the SVV-guided group for intraoperative and postoperative measurements than in the CVP-guided group; though most lactate level were less than 2 mmol/L. Perioperative infusion of lactated Ringer’s solution has less of an effects on plasma lactate level, even in patients with impaired lactate clearance [23] Our protocol used the lactated Ringer’s solution as baseline and supplemental solution throughout the operative procedure in relatively health patients at a rate of 2-3 mL/kg/h during a period of 3-5 hours of surgery, which volume was expected had no inferential bias on serum lactate measurements. Glucose provides sufficient energy and prevents hypoglycemia during the perioperative period. Maintenance of glucose homeostasis is a major concern for health providers. Some authors suggest hyperglycemia and glucose variability are predictive of postoperative mortality and morbidity in pediatric patients [24]. Excessive oxidative stress overproduction, mitochondrial damage, or/ and coagulation impairment induced by hyperglycemia are potential mechanisms of action [25]. More strict glucose control is discussed, it remains controversial, with issues of safety and efficacy for the “intensive glucose control” (defined as target plasma blood glucose, 80-110 mg/dL) [26]. Our study was based on the traditional liberal blood glucose control and no patients received insulin therapy during anesthesia. Our results showed that the blood glucose concentration was modestly lower in the SVV-guided group than in the CVP-guided group (Table 2); and most patients’ blood glucose were be controlled less than 200 mg/dL in both groups, which means that optimizing fluid management guided by SVV during surgery provided better glucose control. Recent experiment reported that perioperative “high glucose control” with plasma glucose concentrations greater than 200 mg/Dl suppresses various aspects of immune function. These included chemo taxis, phagocytosis, and generation of reactive oxygen species, intracellular killing of bacteria, and increased circulating inflammatory cytokine concentration [27]. All of these conditions led to poor outcomes. The suggestion for perioperative glucose concentration has not yet been conclusive, further randomized controlled studies are necessary to explore the issues. The average length of hospital stay for major spine surgery was significantly different between the two groups (SVV-guided vs. CVP-guided group; 5.3 ± 1.3 vs. 7.2 ± 1.4 days, respectively, P = 0.003). A reduction of hospital stay would lead to health care cost savings. However, the criteria for patient administration and discharge are confounded by the individual surgeon decision-making, preoperative physical and economic conditions of the patient, and post-discharge factors [28]. A hospital related total budgets payment system in Taiwan would further impact the decision making for the timing of patient discharge from hospital. Our study has a few limitations. We did not include high-risk patients with major cardiac or pulmonary disease; thus, our results should not be extrapolated to those high-risk patients. Since the introduction of blood pressure-based or stroke volume-based variation into clinical use to monitor the fluid management in 1990, the possibility of “learning contamination” of individual anesthesiologist might be a potential source of bias [29]. In conclusion, the use of SVV-guided fluid management was associated with better control for lactate and glucose levels, and reduced postoperative pain at rest and on movement for the first postoperative three days after major spine surgery. The hospital length of stay was shorter in the SVV-guided group than CVP-guided group. The results suggest that SVV-guided fluid management leads to optimal fluid volume compared with CVP-guided and avoids the cascades of tissue hypo perfusion.