CBME Competency Spotlight: SU1.1 — Metabolic Response to Injury

Competency Code: SU1.1 –  “Describe basic concepts of homeostasis, enumerate the metabolic changes in injury and their mediators.”

Domain: Knowledge (K)    Level: KH (Knows How)  Core Competency: Yes Vertical Integration: Physiology, Biochemistry

Assessment Method: Written / Viva Voce

If you’ve just started reading Surgery in your undergraduate or postgraduate course, SU1.1 is usually the very first competency you’ll encounter — and for good reason. Before you can understand shock, sepsis, wound healing, or post-op complications, you need to understand what happens inside the body the moment it is injured. This competency is the physiological foundation for almost everything else in surgery.

Why This Competency Matters

Every surgical patient — whether they’ve had a road traffic accident, perforation of hollow viscus,  burn, or an elective surgery — undergoes a predictable sequence of metabolic changes. Examiners love this topic because it connects three subjects at once: Physiology (homeostasis), Biochemistry (metabolic pathways), and Surgery (clinical relevance). That’s exactly why it’s vertically integrated, and exactly why it shows up repeatedly in theory papers and viva.

The Core Concept: Homeostasis

Homeostasis is the body’s ability to maintain a stable internal environment — temperature, pH, fluid balance, and electrolyte concentration — despite external stress. In a healthy, uninjured person, this balance is maintained automatically through feedback loops involving the nervous and endocrine systems.

Injury disrupts this balance. The body’s response to injury is essentially an attempt to restore homeostasis, and that response happens in a well-defined, time-based pattern.

The Metabolic Response to Injury — Two Phases

This is the part most commonly asked in exams, so learn it phase-wise.

1. Ebb Phase (early phase — first 12–24 hours)

• Occurs immediately after injury
• Characterized by reduced metabolic rate, low cardiac output, hypothermia, and reduced oxygen consumption
• The body is essentially in a state of shock-like conservation
• Mediated primarily by catecholamines (adrenaline, noradrenaline)

2. Flow Phase (catabolic, followed by anabolic recovery)

The flow phase itself has two sub-parts:

1. a) Catabolic flow phase (days 2–7 approximately)

• Metabolic rate rises sharply — this is the hypermetabolic state
• Increased oxygen consumption, increased cardiac output
• Breakdown of protein, fat, and glycogen stores to supply energy and substrates for healing
• Negative nitrogen balance (muscle wasting)
• Hyperglycemia due to insulin resistance

1. b) Anabolic flow phase (recovery phase, can last weeks)

• Body shifts to rebuilding — protein synthesis resumes
• Positive nitrogen balance
• Restoration of fat stores and body weight

Key Mediators of the Metabolic Response

This is the most frequently asked viva sub-topic. Group your answer into three categories:

Neuroendocrine mediators

• Catecholamines (adrenaline, noradrenaline) — increase heart rate, mobilize glucose
• Cortisol — promotes gluconeogenesis, protein catabolism
• Glucagon — promotes glycogenolysis and gluconeogenesis
• Growth hormone and ADH — fluid retention, altered glucose metabolism
• Insulin — paradoxically resistance develops, contributing to hyperglycemia of injury

Cytokines (the inflammatory mediators)

• IL-1, IL-6, TNF-alpha — released by macrophages at the injury site
• Responsible for fever, acute phase protein synthesis (like CRP), and driving the catabolic state

Other mediators

• Prostaglandins and free radicals — contribute to local and systemic inflammatory effects

Clinical Relevance — Why Surgeons Care

Understanding this competency isn’t just theoretical. It directly explains:

• Why post-op patients often have transient hyperglycemia (even non-diabetics)
• Why early enteral nutrition is emphasized to counter catabolism
• Why prolonged catabolism delays wound healing
• Why trauma/burns patients need aggressive nutritional and metabolic support

Quick-Fire Viva Questions

1. What is the difference between the ebb and flow phase? Ebb = early, hypometabolic, low cardiac output. Flow = later, hypermetabolic, catabolic then anabolic.
2. Name the three most important cytokines in the metabolic response to injury. IL-1, IL-6, and TNF-alpha.
3. Why does hyperglycemia occur after major surgery even in non-diabetic patients? Due to insulin resistance combined with increased cortisol, glucagon, and catecholamine-driven gluconeogenesis.
4. What is negative nitrogen balance and when does it occur? It occurs during the catabolic flow phase, when protein breakdown exceeds protein synthesis, indicating net muscle/tissue loss.

Exam Tip

When this competency is asked as a long question, structure your answer as: Definition of homeostasis → Ebb phase → Flow phase (catabolic then anabolic) → Mediators (neuroendocrine, cytokines) → Clinical significance. Examiners specifically reward this structure because it mirrors how the competency itself is phrased in the curriculum document.

Test yourself on this topic — take our quiz on Metabolic Response to Surgery, or revisit the related OSCE station on post-operative complications.