⚡ Metabolic Response to Surgery
Ebb & flow phases, hormonal changes, substrate mobilisation, cytokines, and strategies to attenuate the response — Cuthbertson’s description — MRCS high-yield.
The Ebb & Flow Phases
The metabolic response to surgery was first described by Sir David Cuthbertson in the 1930s. It is a complex, systemic physiological reaction triggered by tissue injury and surgical stress. Its primary evolutionary purpose is to conserve circulating blood volume and mobilise energy substrates to ensure survival and facilitate tissue repair.
Cuthbertson’s Description — Why It Matters Surgically
Understanding this response explains a wide range of post-operative physiological changes that might otherwise seem puzzling: why patients have a fever on day 2; why they retain fluid; why wound healing is impaired in poorly controlled diabetes; why blood glucose rises even without diabetes; why nitrogen excretion spikes post-operatively; and why malnourished patients have worse outcomes.
The response is proportional to the severity of surgical insult — a minor day-case procedure causes a minimal response; major trauma or emergency surgery causes a massive, prolonged response that can result in multi-organ failure if unchecked.
The ebb phase is the immediate protective mechanism designed to minimise energy demands and preserve blood volume. The body enters a state of controlled hibernation.
- ↓ Basal metabolic rate — energy demands reduced to conserve resources
- ↓ Oxygen consumption — reduced cellular metabolic activity
- ↓ Body temperature — hypothermia; reduced thermoregulation
- ↑ Sympathetic nervous system activity — catecholamine surge; vasoconstriction; tachycardia; priority blood flow to vital organs
- ↑ ACTH and cortisol — HPA axis activated
- ↑ ADH and aldosterone — sodium and water retention; preservation of intravascular volume
- Hyperglycaemia begins — glycogenolysis from catecholamines; glucose released for brain and wound
The ebb phase mirrors the physiological response to haemorrhagic shock — the body does not yet know whether the injury will be lethal, so it conserves everything.
Driven by stress hormones (cortisol, catecholamines, glucagon), the body switches to a hypermetabolic state to provide energy and building blocks for tissue repair.
- ↑ Cardiac output — increased tissue oxygen delivery
- ↑ Resting energy expenditure — metabolic rate elevated 10–50% above normal
- ↑ Body temperature — fever (cytokine-driven; IL-6 peaks at 12–24h)
- Skeletal muscle breakdown (proteolysis) → amino acids released for hepatic acute-phase protein synthesis and gluconeogenesis
- Fat mobilisation (lipolysis) → free fatty acids released for direct energy oxidation
- Hyperglycaemia — hepatic glucose overproduction + peripheral insulin resistance
- Negative nitrogen balance — up to 20g nitrogen/day excreted in urine
- Sodium and water retention continues → weight gain from fluid retention (counterintuitive to the catabolic state)
Once the acute stress resolves, growth hormone and androgens take over to rebuild the depleted stores of fat and protein.
- ↑ GH and androgens — drive protein synthesis; rebuild skeletal muscle and fat stores
- Positive nitrogen balance — net protein synthesis; nitrogen retention
- Diuresis — retained fluid mobilised; return to pre-operative weight
- Fatigue and weakness — reflects extent of muscle mass lost in catabolic phase
- Appetite returns — oral nutrition drives anabolic recovery
- Duration depends on severity of surgery, nutritional status, and absence of complications
| Feature | Ebb Phase (0–24h) | Catabolic Phase (Days 1–10) | Anabolic Phase (Days 10–60) |
|---|---|---|---|
| Metabolic rate | ↓ (reduced) | ↑↑ (hypermetabolic) | Returning to normal |
| Body temperature | ↓ (hypothermia) | ↑ (fever) | Normal |
| Cardiac output | ↓ (shock-like) | ↑ (hyperdynamic) | Normal |
| Dominant hormones | Catecholamines, ADH, aldosterone | Cortisol, glucagon, catecholamines | Growth hormone, androgens, insulin |
| Nitrogen balance | Slightly negative | Markedly negative (up to −20g/day) | Positive |
| Blood glucose | ↑ (glycogenolysis) | ↑↑ (insulin resistance + gluconeogenesis) | Normalising |
| Fluid balance | Na⁺ + H₂O retention | Continued retention; K⁺ loss | Diuresis — fluid mobilised |
| Clinical picture | Pale, cold, tachycardic, oliguric | Feverish, catabolic, hyperglycaemic, losing muscle | Improving, regaining weight, fatigue improving |