General Surgery
Metabolic Response to Injury and Fluid Balance
MBBS surgery notes on the metabolic response to injury and fluid balance: ebb and flow phases, stress hormones, cytokines and fluid therapy, mapped to NMC codes SU1.1 to SU1.3.
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Metabolic Response to Injury and Fluid Balance
MBBS surgery notes on the metabolic response to injury and fluid balance: ebb and flow phases, stress hormones, cytokines and fluid therapy, mapped to NMC codes SU1.1 to SU1.3.
This opening chapter explains how the body reacts to surgical trauma and how the surgeon keeps fluid and electrolytes in balance around an operation. It covers the ebb and flow phases, the hormonal and cytokine mediators, and the practical rules for maintenance, replacement and resuscitation fluids.
High-yield: Metabolic Response to Injury and Fluid Balance
- The metabolic response to injury has two classic phases: an early ebb phase of hypoperfusion and low metabolism, followed by a flow phase of catabolism and hypermetabolism.
- The flow phase is driven by catecholamines, cortisol, glucagon and antidiuretic hormone, producing insulin resistance and a raised blood glucose despite adequate feeding.
- Cytokines, chiefly interleukin-1, interleukin-6 and tumour necrosis factor alpha, mediate the acute phase response and the fever seen after major surgery.
- The acute phase reactant C-reactive protein rises within hours of injury and is a useful marker of the inflammatory response and its resolution.
- Catabolism causes muscle protein breakdown, a negative nitrogen balance and loss of lean body mass, worst in the first week after major injury.
- Antidiuretic hormone and aldosterone drive postoperative sodium and water retention, so routine day-one fluids should be modest to avoid overload.
- Total body water is about 60% of body weight in men and 50% in women, split two-thirds intracellular and one-third extracellular.
- The extracellular fluid is divided into interstitial fluid (about three-quarters) and plasma (about one-quarter).
- Daily maintenance needs in an adult are roughly 25 to 30 mL/kg of water, 1 mmol/kg each of sodium and potassium, and 50 to 100 g of glucose.
- Third-space losses into the gut lumen, peritoneum and injured tissue can be large after bowel surgery and must be replaced with balanced crystalloid.
- Balanced crystalloids such as Hartmann solution are preferred to 0.9% saline for resuscitation, as large saline volumes cause a hyperchloraemic metabolic acidosis.
- Urine output of at least 0.5 mL/kg/hour is a simple bedside guide to adequate renal perfusion and volume status.
- The best measures of adequate resuscitation combine trends in heart rate, blood pressure, urine output, capillary refill and lactate clearance.
- Enhanced recovery programmes limit prolonged fasting and avoid fluid overload, both of which slow return of gut function.
- Hyperkalaemia is the most dangerous acute electrolyte disturbance and is treated with calcium gluconate for cardiac protection plus insulin and dextrose to shift potassium.
Ebb versus flow phase of the injury response
- **Ebb phase:** First few hours: reduced cardiac output, hypoperfusion, low core temperature and depressed metabolism as the body conserves resources.
- **Flow phase:** Days after: hypermetabolism, catabolism, insulin resistance, raised glucose and a negative nitrogen balance driven by stress hormones.
- **Key mediators:** Catecholamines, cortisol, glucagon, antidiuretic hormone and the cytokines interleukin-6, interleukin-1 and tumour necrosis factor alpha.
- **Fluid rule:** About 25 to 30 mL/kg water and 1 mmol/kg each of sodium and potassium daily, with balanced crystalloid for losses.
NMC competencies in this chapter
- **SU1.1:** Metabolic response to injury: phases and mediators
- **SU1.2:** Fluid and electrolyte balance in the surgical patient
- **SU1.3:** Assessment and correction of fluid and electrolyte disturbances
Frequently Asked Questions
What are the two phases of the metabolic response to injury?
The ebb phase in the first hours brings hypoperfusion and reduced metabolism, and the flow phase over the following days brings hypermetabolism, catabolism and insulin resistance driven by stress hormones.
Why is balanced crystalloid preferred over normal saline?
Large volumes of 0.9% saline deliver a high chloride load and cause a hyperchloraemic metabolic acidosis, whereas balanced solutions such as Hartmann more closely match plasma electrolytes.
How much maintenance fluid does a stable adult need each day?
Roughly 25 to 30 mL/kg of water with about 1 mmol/kg each of sodium and potassium and 50 to 100 g of glucose, adjusted for ongoing losses and clinical status.
What urine output signals adequate resuscitation?
A urine output of at least 0.5 mL/kg/hour, read together with heart rate, blood pressure, capillary refill and falling lactate, suggests adequate organ perfusion.
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