Metabolic Response to Surgery - High-Yield 10-Mark Notes

DEFINITION

TRIGGERS (Afferent Limb)

• Pain (nociceptive signals via spinal cord to hypothalamus)
• Tissue damage / hypoxia / acidosis
• Hypovolemia (baroreceptor stimulation)
• Inflammatory cytokines (IL-1, IL-6, TNF-α)
• Immobility
• Preoperative fasting

PHASES (Classic - Cuthbertson)

NEUROENDOCRINE RESPONSE (Efferent Limb)

CENTRAL MECHANISM: INSULIN RESISTANCE

• Definition: A normal insulin concentration produces a subnormal biological response.
• Mechanism:
  • Reduced activation of GLUT-4 transporter in muscle cells
  • Decreased peripheral glucose uptake
  • Increased gluconeogenesis from amino acids (muscle protein)
• Consequences:
  • Hyperglycemia
  • Loss of lean muscle mass
  • Negative nitrogen balance
  • Free radical formation in non-insulin-sensitive tissues
  • Independent predictor of LOS and postoperative complications

• Laparoscopic cholecystectomy: 2.5-fold decrease in insulin sensitivity
• Open colorectal resection: 3.5-fold greater insulin resistance vs laparoscopic

METABOLIC CHANGES: ORGAN-WISE

Carbohydrate Metabolism

• Glycogenolysis + gluconeogenesis → Hyperglycemia
• Peripheral glucose uptake reduced despite high plasma glucose
• In fed state, insulin normally rises 6-8x basal to stop glucose production; this is blunted postoperatively

Protein Metabolism

• Net protein catabolism - muscle protein broken down → amino acids → gluconeogenesis substrate
• Negative nitrogen balance (can be increased several-fold over baseline)
• Loss of lean body mass → reduced muscle function, impaired mobilization
• Elevated stress hormones cause net catabolism even if adequate glucose provided

Fat Metabolism

• Preferential lipid oxidation (ketone body formation)
• Lipolysis → free fatty acids as energy substrate
• Loss of adaptive ketogenesis (impaired in severe stress - distinguishes stress response from simple starvation)

Fluid & Electrolytes

• ADH + Aldosterone → Na+ and water retention → oliguria, edema
• Hypoalbuminemia (dilutional + decreased synthesis)
• K+ losses

INFLAMMATORY RESPONSE

• Cytokines: IL-1, IL-6, TNF-α released from macrophages at wound site
• Cytokines reach hypothalamus → fever, acute phase proteins (CRP, fibrinogen) ↑
• Complement activation
• Neutrophil activation → ROS production
• In extreme cases: SIRS (Systemic Inflammatory Response Syndrome)

FACTORS THAT WORSEN INSULIN RESISTANCE

• Pre-existing comorbidities: Metabolic syndrome, diabetes, obesity, cancer
• Pain - directly reduces insulin sensitivity (↑ cortisol, epinephrine, free fatty acids)
• Prolonged fasting (traditional NPO)
• Major / open surgery vs laparoscopic
• Sepsis / infection

CLINICAL CONSEQUENCES

• Hyperglycemia → impaired wound healing, increased infection risk
• Muscle wasting → prolonged bed rest, DVT risk, delayed recovery
• Fluid retention → pulmonary edema risk
• Negative nitrogen balance → poor anastomotic healing, impaired immune function
• Increased LOS (insulin resistance = independent predictor)

MODIFYING / ATTENUATING THE RESPONSE

NUTRITIONAL REQUIREMENTS POST-SURGERY

• Energy: Up to 40 kcal/kg/day in severe stress
• Nitrogen: Increased (to offset negative nitrogen balance)
• Route: Enteral > Parenteral (when gut is functioning)

EXAM ANSWER TEMPLATES

• Define the response → triggers → neuroendocrine changes (list hormones + effects) → central role of insulin resistance → metabolic consequences (carbohydrate/protein/fat/fluid)

• Definition → mechanism (GLUT-4, gluconeogenesis) → hyperglycemia → muscle catabolism → negative nitrogen balance → clinical impact → how to reduce it

• Preop carb loading → epidural analgesia → MIS/laparoscopic approach → ERAS → early feeding → mobilization → glycemic control → table format scores well

• Ebb phase (features) → Flow catabolic (features) → Flow anabolic (features) → table format

• Quote: energy needs up to 40 kcal/kg/day in stress; increased nitrogen requirements; enteral preferred; early feeding reduces catabolism

HIGH-YIELD ONE-LINERS

1. Insulin resistance is the central metabolic abnormality after surgery.
2. GLUT-4 downregulation prevents muscle from taking up glucose even with high plasma insulin.
3. Muscle protein is catabolized to provide gluconeogenic substrates - amino acids.
4. Pain directly increases insulin resistance via cortisol and catecholamine release.
5. More invasive surgery = more insulin resistance: open colorectal > laparoscopic cholecystectomy by 3.5x.
6. Carbohydrate loading 2 hours before surgery is the single most evidence-based intervention to reduce the catabolic response.
7. ADH + Aldosterone cause postoperative oliguria and Na+ retention - do not confuse with renal failure.
8. Preop fasting + surgery together = most severe form of stress catabolism.
9. Adaptive ketogenesis is LOST in surgical stress (unlike simple starvation where it is preserved).
10. Insulin resistance is an independent predictor of length of hospital stay.