Abstract
Perioperative fluid management in neurosurgery is optimal with a strategic goal of therapy and a comprehension of the underlying pathophysiology.
The aim of therapy is maintenance of normovolemia, and as a result cerebral and spinal cord perfusion pressures.
Isotonic crystalloids are a rational choice for optimal outcomes.
Early detection and correction of fluid abnormalities helps in the preservation and restoration of neurologic function.
Keywords
Cerebral perfusion, Fluid management, Isotonic crystalloids, Maintenance normovolemia, Neurosurgery, Perioperative
Outline
Introduction 815
Anatomy and Physiology 815
Pathophysiology 817
Definitions 817
Choice of Fluids 818
Hypertonic Fluids 818
Isotonic Fluids 820
Colloids 820
Hypotonic Fluids 821
Fluid Management 822
Commonly Encountered Fluid Abnormalities 823
Conclusion 824
Clinical Pearls 825
References 825
Introduction
Fluid management in the perioperative period of a neurosurgical patient presents to the neuroanesthesiologist unique challenges. The aim of therapy is maintenance of normovolemia and as a result cerebral and spinal cord perfusion pressures; however, these patients may be volume depleted due to diuretic therapy; intraoperatively they are exposed to the vasodilatory effects of potent anesthetic agents, third space losses, perioperative volume losses, and occasionally complicated by the onset of diabetes insipidus.
The goal of initial fluid therapy in a patient with neurologic injury is to restore intravascular volume, optimize hemodynamics, and thus maintain tissue perfusion, integrity, and function. Most commonly asanguineous fluids are given to replace sensible and insensible losses. The choice of fluids will influence cerebral metabolism and volume. The selection of fluids for resuscitation in patients who have decreased intracranial compliance requires an understanding of the relationship among intravenous fluids, brain water, and intracranial pressure. Fluid management strategies need to be individualized and guided by an understanding of the underlying pathophysiological mechanisms due to the lack of clinical outcome evidence ( Box 49.1 ).
Maintenance of normovolemia and as a result cerebral and spinal cord perfusion pressures
Restore intravascular volume
Optimize hemodynamics
Maintain tissue perfusion, integrity, and function
Anatomy and Physiology
Water comprises 60% of the total body weight of an adult. The total body water is divided functionally into the extracellular fluid [(ECF), which makes up 20% of body weight] and the intracellular fluid [(ICF), which makes up 40% of body weight] spaces separated by the cell membrane with its active sodium pump, which ensures that sodium remains mainly in the ECF. The cell, however, contains large anions such as proteins and glycogen, which cannot exit and therefore draw in K + ions to maintain electrical neutrality (Gibbs–Donnan equilibrium). These mechanisms ensure that Na + and its anions Cl − and <SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='HCO3−’>HCO3−HCO3−
HCO 3 −
, are the mainstay of the ECF osmolality, and K + has the corresponding function in the ICF. The ECF is further divided into the intravascular and the interstitial fluid spaces. The intravascular space has its own intracellular component in the form of red blood cells (hematocrit = ∼40%) and white cells and an extracellular element in the form of plasma (55–60% of the total blood volume ) ( Fig. 49.1 ).
Fluid movements between vasculature and tissues : Earnest Starling described the forces that determine the movement of water between vasculature and tissues, which was formalized subsequently into Starling equation, as follows:
Q f = K f S [ ( P c − P t ) −δ ( π c − π t ) ]
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