Management of Fluids, Electrolytes, and Blood Products in Neurosurgical Patients

Abstract

The major fluid compartments of the body are the intracellular compartment and the extracellular compartment, which is subdivided into intravascular and interstitial spaces. The volume of the individual compartments may change in a disease state or as the body adapts to environmental stress. In peripheral tissues, the primary determinant of fluid movement across capillaries (i.e., between the intravascular and interstitial spaces) is the oncotic gradient produced by large plasma proteins such as albumin. Unlike the peripheral tissues, the brain and spinal cord are isolated from the intravascular compartment by the blood–brain barrier.

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Overview

• The major fluid compartments of the body are the intracellular compartment and the extracellular compartment, which is subdivided into intravascular and interstitial spaces.

• The volume of the individual compartments may change in a disease state or as the body adapts to environmental stress.

• In peripheral tissues, the primary determinant of fluid movement across capillaries (i.e., between the intravascular and interstitial spaces) is the oncotic gradient produced by large plasma proteins such as albumin.

• Unlike the peripheral tissues, the brain and spinal cord are isolated from the intravascular compartment by the blood–brain barrier.

• The primary determinant of water movement across the intact blood–brain barrier is the osmotic pressure gradient produced by osmotically active particles including plasma sodium and other electrolytes.

• Intravenous infusion of solutions hyperosmolar to plasma (e.g., 3% sodium ­chloride, mannitol) will lead to a decrease in brain water content and intracranial pressure (ICP). Administration of excess free water (e.g., hypoosmolar or dextrose-containing electrolyte-free solutions) will lead to increased brain water content and ICP.

• Osmotically active particles as well as plasma proteins may “leak” into the cerebral tissue where the blood–brain barrier has been disrupted and thus contribute to worsening cerebral edema in such regions.

• Intravenous administration of hyperosmolar solutions results in a decrease in water content in brain where blood brain barrier is intact to make room for the injured brain.

Implications for the Neurosurgical Patient

Perioperative fluid management in neurosurgical patients poses special challenges.

• The presence and treatment of elevated ICP, surgical bleeding, and a variety of pathophysiological derangements associated with neurologic injury may lead to significant hypovolemia, electrolyte abnormalities, anemia, and coagulopathy.

Care must be taken to:

• Maintain hemodynamic stability, optimal cerebral perfusion pressure, and oxygen delivery to the CNS tissue and

• Minimize the impact of fluid resuscitation on the development or exacerbation of cerebral edema.

The goals of fluid resuscitation are (see Tables 4.1–4.5):

Table 4.1 Commonly used IV solutions

Table 4.2 Common causes of hyponatremia

Table 4.3 Common causes of hypernatremia

Table 4.4 Common causes of hypokalemia

Table 4.5 Considerations for assessing intravascular volume

• Restore intravascular volume and cerebral perfusion pressure and

• Achieve a slightly hyperosmolar state.

The clinician can choose from a variety of intravenous fluids including crystalloid, hypertonic saline, colloid, and blood products as dictated by the clinical scenario. The typical initial fluid choice for an elective craniotomy is a combination of Lactated Ringer’s and 0.9% saline (Table 4.6).

Table 4.6 Indications for commonly used intravenous fluids and blood products

Concerns and Risks (Table 4.7)

Table 4.7 Concerns and risks of fluid management in neurosurgical patients

Anemia

• Has been associated with worse neurologic outcome in cardiopulmonary bypass surgery and with perioperative visual loss in prone spine surgery.

• The ideal hematocrit for optimizing cerebral blood flow and oxygen delivery in focal ischemia model is currently believed to be 30–34%. Higher hematocrit results in increased blood viscosity; hematocrit  ≤  25% results in decreased oxygen-carrying capacity.

• Normovolemic hemoglobin levels of 7–9 g/dL appear to be safe for the general ICU patient population.

• There is insufficient evidence to allow recommendations regarding:

  • The “safe” level of anemia for patients with neurologic injury; or

  • Whether correction of anemia by transfusing red cells has beneficial or detrimental effects on neurologic outcome.

Transfusion of Blood Products

Current concerns regarding blood product transfusion in the developed world focus more on the immunomodulating effects of transfusion rather than transmission of infectious agents (Table 4.8). Transfusion-related acute lung injury (TRALI) is thought to be the leading cause of transfusion-related mortality.

Table 4.8 Examples of risks associated with transfusion of blood products

Special Circumstances (Table 4.9)

Table 4.9 Special circumstances

Key Points

• Fluid management goal is a euvolemic, slightly hyperosmolar state.

• Avoid hypoosmolar fluids and dextrose-containing solutions unless needed to treat hypoglycemia.

• Consider using hypertonic saline, unless contraindicated, to treat elevated ICP in a hypovolemic, hemodynamically unstable patient.

• In anemic patients with neurologic injury, there is insufficient evidence regarding transfusion thresholds. Do not use an arbitrary hemoglobin number; weigh risks of transfusion (e.g.,TRALI, immunosuppression) with benefits of oxygen delivery to injured CNS tissue.