35.1 Basic Surgical Principles I: The Surgery TOS, Fluids and Electrolytes, Acid-Base Balance, the Metabolic Response to Injury, Surgical Nutrition and Blood Transfusion

Why this area matters

The Surgery TOS has nine areas:

• basic surgical principles (20%);
• three abdominal areas (esophagus, stomach and small bowel; biliary tract, liver, pancreas and spleen; colon, rectum, anus and appendix);
• head and neck, thyroid and ENT;
• breast, hernia, and skin and soft tissue;
• three specialty areas (neurosurgery and ophthalmology; thoracic, cardiovascular, urologic and gynecologic surgery; plastic, orthopedic and pediatric surgery).

Each item follows a clinical scenario: risk factors, history and examination, diagnostic tests, treatment options and operations, preoperative preparation, postoperative care, prognosis, complications, and prevention.

Fluids and electrolytes

• Body water is about 60% of body weight in men (less in women and older adults):
  • two-thirds is intracellular;
  • one-third is extracellular, of which about a quarter is plasma and three-quarters is interstitial.
• Surgical fluid losses:
  • third-space losses (bowel obstruction, peritonitis, pancreatitis, burns);
  • GI losses: vomiting and NG suction lose H+, Cl- and K+, while diarrhea, fistulas and ileostomies lose HCO3- and K+;
  • insensible losses (fever adds about 10% per degree Celsius).

Fluid and Electrolyte Therapy: Maintenance Requirements & Resuscitation Crystalloids

Maintenance Fluid Calculations: The Holliday-Segar 4-2-1 Rule

Daily water and solute turnover must balance obligatory urinary, fecal, and insensible cutaneous/pulmonary losses. The classic Holliday-Segar (4-2-1 rule) dictates hourly maintenance infusion rates:

• First $10\text{ kg}$ body weight: $4\text{ mL/kg/hour}$ ($100\text{ mL/kg/day}$)
• Second $10\text{ kg}$ body weight ($11-20\text{ kg}$): $2\text{ mL/kg/hour}$ ($50\text{ mL/kg/day}$)
• Each additional kilogram above $20\text{ kg}$: $1\text{ mL/kg/hour}$ ($20\text{ mL/kg/day}$)

Calculation Example for a $70\text{ kg}$ Adult: $\text{Rate} = (10\text{ kg} \times 4) + (10\text{ kg} \times 2) + (50\text{ kg} \times 1) = 40 + 20 + 50 = 110\text{ mL/hour}$ (or approximately $2,500\text{ to }2,600\text{ mL/day}$).

Standard maintenance fluid in fasting non-resuscitation post-surgical patients is $D_5 0.45\%\text{ NaCl} + 20\text{ mEq/L KCl}$ (supplying dextrose to prevent starvation ketosis and spare muscle protein catabolism, hypotonic sodium to match free water clearance, and potassium to cover renal excretion once adequate urine output is verified).

Resuscitation Crystalloids: 0.9% Normal Saline versus Balanced Salt Solutions

• 0.9% Normal Saline: Contains $154\text{ mEq/L Na}^+$ and $154\text{ mEq/L Cl}^-$ with an osmolarity of $308\text{ mOsm/L}$. Massive infusion triggers hyperchloremic non-anion gap metabolic acidosis because excess serum chloride forces intracellular bicarbonate egress and renal bicarbonate elimination, reducing renal cortical perfusion.
• Lactated Ringer's (LR): A physiologically balanced crystalloid containing $130\text{ mEq/L Na}^+$, $109\text{ mEq/L Cl}^-$, $28\text{ mEq/L lactate}$, $4\text{ mEq/L K}^+$, and $2.7\text{ mEq/L Ca}^{2+}$. Hepatic conversion of lactate to bicarbonate buffers acidosis. LR is the preferred resuscitation fluid in surgical trauma, severe pancreatitis, and burn shock (unless severe hyperkalemia or traumatic brain injury is present).

Electrolyte disorders in surgical patients

Acid-base balance

1. Look at the pH: below 7.35 is acidemia, above 7.45 is alkalemia.
2. Identify the primary process: low HCO3- means metabolic acidosis, high pCO2 respiratory acidosis, high HCO3- metabolic alkalosis, and low pCO2 respiratory alkalosis.
3. Check compensation:
   • in metabolic acidosis, the expected pCO2 is 1.5 x HCO3- + 8 (plus or minus 2) (Winter's formula);
   • a measured value outside this range means a second respiratory disorder.
4. Calculate the anion gap: Na - (Cl + HCO3-), normal about 8 to 12.
   • High-gap acidosis: lactic acidosis from shock or ischemic bowel, ketoacidosis, renal failure, toxins (methanol, ethylene glycol, salicylates).
   • Normal-gap (hyperchloremic) acidosis: diarrhea, pancreatic or small bowel fistulas, ureterosigmoidostomy, renal tubular acidosis and large volumes of normal saline.
5. Typical surgical patterns:
   • pyloric obstruction or prolonged NG suction: hypochloremic, hypokalemic metabolic alkalosis with paradoxical aciduria, treated with saline and potassium;
   • mesenteric ischemia or septic shock: lactic acidosis;
   • pain and anxiety: respiratory alkalosis;
   • oversedation or splinting: respiratory acidosis.

The endocrine and metabolic response to injury

• Ebb phase (first 24 to 48 hours): decreased metabolic rate, cardiac output and temperature, as the body conserves energy.
• Flow phase: a hypermetabolic, catabolic state driven by catecholamines, cortisol, glucagon, ADH, aldosterone and cytokines (IL-1, IL-6, TNF-alpha). It causes:
  • hyperglycemia (insulin resistance, gluconeogenesis);
  • muscle protein breakdown and negative nitrogen balance;
  • lipolysis;
  • sodium and water retention (low urine output after surgery);
  • an acute-phase response (raised CRP, low albumin).
• Clinical meaning: the response is proportional to the injury. Excess or prolonged responses lead to SIRS, organ dysfunction and wasting. Enhanced recovery (ERAS) programs limit it with minimal fasting, carbohydrate loading, regional anesthesia, opioid-sparing analgesia, early feeding and early mobilization.

Surgical nutrition

• Assessment:
  • recent weight loss (more than 10% in 6 months is significant);
  • intake;
  • BMI;
  • muscle wasting;
  • tools such as Subjective Global Assessment or NRS-2002;
  • albumin is an inflammatory marker, not a reliable nutrition marker.
• Requirements: about 25 to 30 kcal/kg/day and 1.2 to 2.0 g/kg/day of protein (more in burns and sepsis).
• Enteral nutrition ("if the gut works, use it"):
  • preserves the gut barrier and lowers infection compared with parenteral feeding;
  • routes are oral supplements, nasogastric or nasojejunal tubes, gastrostomy and jejunostomy;
  • start early (within 24 to 48 hours) after most operations and in critical illness.
• Parenteral nutrition: for a non-functioning gut (prolonged ileus, obstruction, short bowel, high-output fistula). Complications are:
  • catheter infection;
  • hyperglycemia;
  • electrolyte disorders;
  • liver dysfunction;
  • gut atrophy.
• Refeeding syndrome:
  • when feeding starts after starvation, insulin drives phosphate, potassium and magnesium into cells;
  • causes hypophosphatemia, heart failure, arrhythmias, respiratory failure and Wernicke encephalopathy;
  • prevented by starting at low calories, giving thiamine and monitoring electrolytes.

Blood transfusion

• Before transfusion: check blood type and crossmatch, identify the patient and unit at the bedside, obtain consent, and monitor vital signs.
• Reactions:
  • acute hemolytic reaction: ABO incompatibility, usually a clerical error. Fever, chills, flank pain, hypotension, hemoglobinuria and DIC. Stop the transfusion at once, give fluids to protect the kidneys, and return the unit for testing;
  • febrile non-hemolytic reaction: cytokines from white cells, prevented by leukoreduction;
  • allergic urticaria: antihistamines. Anaphylaxis occurs in IgA deficiency;
  • TRALI (transfusion-related acute lung injury): non-cardiogenic edema within 6 hours;
  • TACO (transfusion-associated circulatory overload): heart failure, treated with diuretics;
  • delayed hemolytic reaction: days later;
  • transfusion-transmitted infections: HBV, HCV, HIV, syphilis, malaria and bacteria in platelets.
• Massive transfusion complications: hypothermia, dilutional coagulopathy, hypocalcemia (citrate binds calcium), hyperkalemia and acidosis. Use warmed products, balanced ratios and calcium replacement.
• Patients who refuse blood, such as Jehovah's Witnesses: a competent adult's refusal is respected. Use blood-conservation strategies (iron, erythropoietin, cell salvage if acceptable, meticulous hemostasis).