Presley Regional Trauma Center Department of Surgery University of Tennessee Health Science Center Memphis, Tennessee
Body Fluid Compartments
Total Body Water • Approximates 60% of total body weight • Composed of the intracellular and extracellular compartments
• The intracellular compartment or intracellular volume (ICV) constitutes 40% of total body weight • Extracellular volume (ECV) makes up the remaining 20%
ECV • Composed of interstitial fluid (IF) and the intravascular or plasma volume (PV) • The PV constitutes 25% of ECV (5% of total body weight) – remainder is IF • Red cell volume, approximately 2 to 3% of TBW, is part of the ICV • Total blood volume is approximately 7 to 8% of total body weight
Requirements • Sufficient water is required to replace obligatory GU losses of approximately 1L/day and GI losses of 100 - 200ml/day • Insensible water losses must also be considered in estimating maintenance fluid - Amount to 8 to 12ml/kg/day - Equally divided into respiratory and cutaneous water loss - Cutaneous losses increase by 10% for each degree of temperature greater than 37°C
Electrolytes • Daily sodium intake approaches 100 to 250mEq/day • Balanced by sodium losses in sweat, stool, and urine • Renal conservation of sodium is extraordinary • In cases of profound volume depletion, urinary losses of sodium may be less than 1mEq/day
Electrolytes • In the perioperative period, adequate maintenance of sodium may be achieved with an intake of 1 to 2mEq/kg/day • Normal potassium intake is approximately 40 to 120mEq/day • 10-15% is excreted as normal urinary losses • With normal renal function, body potassium stores can be maintained with an intake of approximately 0.5 to 1.0mEq/kg/day
Perioperative Fluid Requirements
Perioperative • Appropriate management of fluids and electrolytes in the perioperative period requires a flexible yet systematic approach
• Ensures that fluid administration is appropriately tailored to the patient's changing requirements
Perioperative • The amount of fluids administered in the immediate post-op period must take into account the existing deficit, maintenance requirements, and any ongoing losses • Estimation of the existing deficit must incorporate an approximation of intra-op blood loss as well as fluid losses from evaporative and third-space fluid sequestration
Losses • Extravascular fluid sequestration represents an important source of intra-op fluid loss • Extensive dissection at the operative site induces a localized capillary leak, resulting in extravasation of intravascular fluid into the interstitium with edema formation • The loss of intravascular volume via this route depends on the extent of exposure and degree of dissection
Losses • For example, estimated intravascular fluid losses associated with IHR are 4ml/kg/h, while losses during AAA repair may be as high as 8ml/kg/h • This capillary leak may persist as long as 24h into the post-op period and should be considered as part of ongoing losses in the immediate post-op period
Losses • GI losses (stomas, tubes/drains, or fistulae) comprise ongoing fluid losses • These losses may be accurately estimated by closely following recorded hourly outputs • The electrolyte composition of the output depends on the source of effluent • Replacement fluids should be chosen to best approximate the composition of the ongoing losses
Post-op • Post-op fluid orders should take into account the overall fluid balance in the OR as an estimate of the existing deficit along with maintenance fluid requirements and any ongoing losses • The preferred approach is to reassess the patient frequently to determine volume status
Post-op • Evaluation of heart rate, blood pressure, and most importantly, hourly urine output provides an excellent measure of intravascular volume status • Orders for IVF should be rewritten frequently to maintain a normal heart rate, a urine output of approximately 1ml/kg/h, and adequate blood pressure
Disorders of Sodium Homeostasis
Sodium • Maintenance of a normal serum [sodium] is intimately associated with control of plasma osmolarity
• Posm= 2×Plasma [Na+] + [Gluc]/20 + [BUN]/3 • Plasma [Na+] alone provides no information about the total content of sodium in the body but simply provides an estimate of the relative amounts of free water and sodium
Osmolarity • Maintenance of the plasma osmolarity within normal limits depends on the ability of the kidneys to excrete water, thus preventing hypoosmolarity, and on a normal thirst mechanism with access to water to prevent hypernatremia
