Anesthesiologists must be able to evaluate and optimize volume status and electrolyte balance in the perioperative period. The primary goals of intravenous fluid therapy are the preservation of intravascular volume and the maintenance of left ventricular filling pressure and cardiac output to ensure adequate oxygen delivery to tissues.
The average adult man is approximately 60% water by weight, whereas the average woman is approximately 50%. This is referred to as total body water, and it is divided into two major fluid compartments: intracellular fluid (ICF = 40% total body weight) and extracellular fluid (ECF = 20% total body weight). ECF is further subdivided into the interstitial (15% total body weight) and intravascular components (5% total body weight).
Blood plasma is the major component of intravascular fluid volume contained in the vascular endothelium. Electrolytes are freely exchanged between the intravascular space and the interstitium, maintaining near-equilibrium state between the two compartments. Plasma proteins such as albumin do not cross endothelium freely and therefore provide osmotic forces.
PREOPERATIVE EVALUATION OF INTRAVASCULAR FLUID VOLUME
Determining the fluid volume status of a patient can be challenging. Detailed patient history, physical examination, and laboratory data aid in accurately gauging volume status.
Nil per os (NPO) status, nausea and vomiting, diarrhea, bowel preparation, hemorrhage, burns, history of weight change, and high urine output are all common causes of preoperative hypovolemia. Hyperventilation, fever, and diaphoresis are often overlooked causes of hypovolemia. Tachycardia, orthostatic hypotension, and low urine output with concentrated urine are nonspecific signs of dehydration. Physical examination findings, suggestive of hypovolemia, include dry mucous membranes, flat neck veins, orthostatic hypotension, concentrated urine, and poor skin turgor. In babies, sunken fontanelles indicate hypovolemia.
Hematocrit is often elevated with dehydration. Hypovolemic shock can cause tissue hypoperfusion leading to metabolic acidosis and elevated lactate production. If renal function is normal during dehydration, sodium is retained, leading to low urine sodium and high urinary specific gravity (>1.025 in adults), and an elevated blood urea nitrogen: creatinine ratio (BUN/creatinine ratio >20).
PERIOPERATIVE FLUID THERAPY
Perioperative fluid therapy entails the replacement of preexisting fluid deficits, administration of maintenance fluids, and replacement of surgical losses.
Compensatory intravascular volume expansion (CVE) counteracts venodilation and cardiac depression from anesthesia as well as the hemodynamic effects of positive-pressure ventilation. CVE with 5–7 mL/kg of a balanced salt solution should occur prior to, or simultaneously with induction of general anesthesia provided there are no patient comorbidities prohibiting fluid administration.
Hourly maintenance of fluid requirements can be estimated using the “4-2-1 rule” (Table 96-1). This hourly rate can also be calculated for any person weighing more than 20 kg as [weight (in kg) + 40]. Maintenance fluid requirements take into account ongoing losses secondary to continued ...