11.2 Fluid Resuscitation, Acid-Base Disturbances & Surgical Shock

Intravenous Fluid Resuscitation and Maintenance

Appropriate fluid management is critical in the perioperative period to maintain intravascular volume, tissue perfusion, and electrolyte balance. Fluids are categorized as crystalloids (aqueous solutions of mineral salts) and colloids (larger insoluble molecules like albumin). Crystalloids are the mainstay of initial resuscitation.

Normal Saline (0.9% NaCl) contains 154 mEq/L of both sodium and chloride. While widely used, massive administration can lead to hyperchloremic non-anion gap metabolic acidosis due to the high chloride content exceeding physiological levels (normal plasma chloride is ~98-106 mEq/L). Lactated Ringer's (LR) is a balanced crystalloid containing sodium, chloride, potassium, calcium, and lactate. The lactate is metabolized by the liver into bicarbonate, helping to buffer acidosis. LR is often preferred for large volume resuscitation in trauma or major surgery to avoid hyperchloremic acidosis, though it is generally avoided in severe liver failure or hyperkalemia due to its potassium content.

Calculating Maintenance Fluids The 4-2-1 rule is the standard formula used to calculate hourly maintenance fluid rates based on a patient's weight:

• 4 mL/kg/hr for the first 10 kg of body weight.
• 2 mL/kg/hr for the second 10 kg of body weight.
• 1 mL/kg/hr for every kilogram over 20 kg.

Example Calculation: For a 75 kg patient: First 10 kg: 10 × 4 = 40 mL/hr Next 10 kg: 10 × 2 = 20 mL/hr Remaining 55 kg: 55 × 1 = 55 mL/hr Total maintenance rate = 40 + 20 + 55 = 115 mL/hr. (Alternatively, for adults over 20 kg, you can simply add 40 to the patient's total weight in kg: 75 + 40 = 115 mL/hr).

Acid-Base Disturbances in Surgery

Surgical patients are highly susceptible to acid-base derangements secondary to fluid losses, physiological stress, or underlying pathology.

• Metabolic Alkalosis: Most commonly seen in patients with severe vomiting, prolonged nasogastric (NG) tube suctioning, or volume contraction. Loss of gastric hydrochloric acid (HCl) leads to hypochloremic, hypokalemic metabolic alkalosis. Management involves correcting the volume deficit and electrolyte imbalances, typically with Normal Saline supplemented with potassium.
• Metabolic Acidosis: Often a marker of severe pathology. An elevated anion gap metabolic acidosis in a surgical patient strongly suggests tissue hypoperfusion leading to lactic acidosis (e.g., hemorrhagic shock, severe sepsis, or ischemic bowel). Management focuses on reversing the underlying cause, improving tissue oxygenation, and restoring adequate perfusion.

Surgical Shock and Hemorrhage

Shock is defined as a state of inadequate tissue perfusion leading to cellular hypoxia. In surgical patients, it is classified into four main categories:

1. Hypovolemic Shock: Most common in surgery, primarily due to acute blood loss (hemorrhagic shock) or massive fluid shifts/losses (e.g., severe burns, aggressive diuresis).
2. Distributive Shock: Characterized by profound vasodilation. Septic shock (from intra-abdominal infections, severe SSIs) is the most frequent surgical cause. Neurogenic shock occurs with spinal cord injuries above T6, presenting uniquely with hypotension and bradycardia (loss of sympathetic tone).
3. Cardiogenic Shock: Pump failure, often secondary to a massive myocardial infarction or severe arrhythmias in the perioperative period.
4. Obstructive Shock: Extracardiac obstruction to blood flow, such as cardiac tamponade, tension pneumothorax, or massive pulmonary embolism.

Hemorrhagic Shock Classification and Management The Advanced Trauma Life Support (ATLS) guidelines classify hemorrhagic shock into four classes to guide resuscitation efforts based on the estimated percentage of blood volume lost.

• Class I (Up to 15% loss): Minimal tachycardia, normal blood pressure, pulse pressure, and urine output. Crystalloid resuscitation is sufficient.
• Class II (15-30% loss): Tachycardia (>100 bpm), tachypnea, decreased pulse pressure, but normal systolic blood pressure. Urine output may slightly decrease. Fluid resuscitation typically begins with crystalloids, and blood may be needed.
• Class III (31-40% loss): Marked tachycardia (>120 bpm), tachypnea, decreased systolic blood pressure, and significant oliguria. Confusion or anxiety is common. Resuscitation requires immediate blood transfusion alongside crystalloids, often triggering massive transfusion protocols.
• Class IV (>40% loss): Immediately life-threatening. Severe tachycardia (>140 bpm), profound hypotension, negligible urine output, and lethargy. Requires immediate, massive blood transfusion and definitive surgical intervention to control hemorrhage.

Massive Transfusion Protocol (MTP): For patients in severe hemorrhagic shock (Class III or IV), the paradigm has shifted away from aggressive crystalloid resuscitation, which can exacerbate coagulopathy, hypothermia, and acidosis (the "lethal triad"). Instead, early administration of blood products in a balanced ratio—typically 1 unit of Packed Red Blood Cells (PRBCs) : 1 unit of Fresh Frozen Plasma (FFP) : 1 unit of Platelets—is recommended to closely mimic whole blood and address trauma-induced coagulopathy.