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Fluid administration is one of the basic components in the management of neurosurgical patients. Despite advances and extensive research in the field of neurosciences, there is still a debate on the ideal fluid. Issues related to adequate volume replacement and effects on the intracranial pressure persist. Studies have demonstrated the harmful effects of colloids over crystalloids. Normal saline has remained a fluid of choice, but there is now emerging evidence that it too is not free from its harmful effects. Hypertonic saline has also been accepted by many practitioners, but its use and administration requires close monitoring and vigilance. There is now growing evidence on the use of balanced solutions for neurosurgical patients. However, this evidence comes from a small number of studies. This chapter tries to briefly cover various clinical situations in neurosciences with respect to fluid administration.
Introduction
Neurosurgical patients form a special population that poses challenges to anesthetists and intensivists when it comes to fluid administration. Issues pertaining to elevated intracranial pressure (ICP) and intraoperative blood losses have to be dealt with more compositely. Earlier, it was believed that fluid intake restriction up to 1 liter daily in patients undergoing craniotomy maintains good homeostasis. Larger volumes may result in expansion of the extracellular space and may result in brain edema,[1] although it was cautioned that fluid restriction might be dangerous in patients receiving hyperoncotic fluids, diuretics, and dexamethasone. However, the practice has changed over the years, and it is now believed that a generally restrictive fluid strategy could be dangerous as these neurosurgical patients so often receive mannitol and diuretics to prevent rise in ICP and reduce brain edema. The effect of anesthetics may be additive in causing systemic hypotension, which risks compromising the cerebral perfusion pressure (CPP), reducing the cerebral oxygenation and producing intracranial complications. These neurosurgical patients are also not excluded from the controversy over colloids versus crystalloids.[2]
This chapter deals with various neurosurgical situations in which fluid administration is vital to the overall management of the patient. Although the main focus of the chapter is on perioperative fluid management in different clinical scenarios, it also discusses the therapeutic roles of fluid aimed at reduction of ICP and improving CPP.
General principles
Crystalloids or colloids along with blood and blood products are routinely used during neurosurgical procedures. It has, in general, become common practice in neuroanesthesia to avoid fluids that are hypo-osmolar or contain glucose, because of the view that the free water produced by hypo-osmolar and glucose-containing solutions results in brain edema. Thus the fluids of choice include 0.9% normal saline and lactated Ringer’s solution, both being nearly equiosmolar to normal plasma. For the purpose of achieving brain relaxation in the intraoperative period, hypertonic fluids such as mannitol and hypertonic saline are frequently used. It is by virtue of these hypertonic fluids that water is drawn from intracellular and interstitial compartments into the intravascular compartment. This results in relaxation of the brain and increased compliance. However, it is essential that the blood–brain barrier be intact for hypertonic fluids to produce their effect on the brain.[3]
In general, sufficient fluids should be administered to neurosurgical patients to maintain good cardiac output and hemodynamic stability. It is accepted that respiratory variations in arterial pressure during mechanical ventilation reflect volume status and fluid responsiveness of the patients.[4] These “dynamic” hemodynamic parameters, such as the stroke volume, arterial pulse pressure, and the variations during positive pressure mechanical ventilation, are considered accurate by many authors in predicting volume status.[5]
In the following, we describe some of the situations in neurosurgery in which fluid administration requires special considerations.
Supratentorial tumor surgery
Administration of fluids in patients undergoing craniotomy for supratentorial surgery is not only for the purpose of replacement but also to provide brain relaxation. Whereas mannitol and hypertonic saline are popular fluids used for providing brain relaxation, normal saline remains the choice of fluid for maintaining the volume status of the patients. There remains a controversy over which fluid to use for intraoperative brain relaxation: mannitol or hypertonic saline. In a recent meta-analysis, the authors found that hypertonic saline significantly reduced the risk of tense brain, but the quality of evidence was low and the findings were from a limited number of studies.[6] Normal saline remains the fluid of choice to maintain intraoperative volume status. However, amidst the controversy of crystalloid versus colloid for neurosurgical patients, a recent study by Xia and colleagues compared goal-directed crystalloid and goal-directed colloid therapy in patients undergoing craniotomy.[7] Based on a study conducted on 40 patients, the authors concluded that goal-directed hydroxyethyl starch therapy was not superior to goal-directed lactate Ringer’s solution therapy for brain relaxation and cerebral metabolism. The authors found that less fluid volume was needed to maintain the target stroke volume variation in the colloid group when compared with the crystalloid group.
Subarachnoid hemorrhage
Fluid management in patients with subarachnoid hemorrhage (SAH) aims to maintain a good fluid balance and correction of hyponatremia, which accompanies the cerebral salt wasting syndrome often associated with SAH. This also formed the basis of “triple-H” therapy. The standard triple-H therapy consisted of hypervolemia, hemodilution, and hypertension. However, in a recent exploratory analysis on 413 patients enrolled in the CONSCIOUS-1 trial, the authors found that administration of colloid and maintenance of a positive fluid balance during the period of vasospasm after SAH was associated with poor outcome.[8] In a systematic review by Dankbaar and colleagues, the authors concluded that controlled studies did not offer evidence that supported the use of triple-H therapy or any of its components in improving cerebral blood flow. They also found from uncontrolled studies that “hypertension” was possibly the most effective component of triple-H therapy in increasing cerebral blood flow.[9]
When considering patients with severe brain injury, a pilot study demonstrated that balanced solutions reduced the incidence of hyperchloremic acidosis when compared with administration of chloride-rich solutions.[10] Similar findings have also been reported by Lehmann and colleagues, who found that in the management of patients with SAH, saline-based fluids resulted in a greater proportion of patients with hyperchloremic acidosis, hyperosmolality, and positive fluid balance when compared with balanced solutions.[11] Although evidence is accumulating in favor of balanced solutions rather than normal saline, it is still too early for any conclusive statement to be made in support of any type of fluid to be recommended for patients with SAH.
Pituitary surgery
Surgical procedures related to the pituitary pose challenges in terms of fluid and electrolyte imbalance, which may frequently be related. Diabetes insipidus (DI) may be observed both during the intraoperative and postoperative period. Fluid balance needs close monitoring to avoid fluid overload, electrolyte imbalance, and complications. Syndrome of inappropriate antidiuretic hormone secretion (SIADH) is another postoperative complication associated with pituitary surgery.[12] The management of these syndromes is beyond the scope of this chapter; however, it cannot be overemphasized that fluid and electrolyte disturbances occur during the treatment of patients with pituitary tumors. Half normal saline and 5% dextrose are fluids of choice in cases of DI, whereas, in the case of SIADH, fluid restriction is the most appropriate treatment. A comparison of these two conditions is given in Table 26.1. It is important to check serum osmolality frequently, to guide the administration of amount and type of fluid.
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