Gen Surg: Metabolic response to surgery and Principles of fluid and electrolyte balance Flashcards
WHy can hypovolaemia occur in response to injury?
- Blood loss
- Sequestration of protein-rich fluid into interstitial space (third-space loss)
Why does oliguria with sodium and water retention occur commonly after major surgery?
Due to ADH and aldosterone release
What factors play a role in ADH secretion in surgery?
- Nerve impulses from site of injury
- Atrial stretch receptors responding to reduced circulating volume
- Aortic/carotid baroreceptor responding to reduced pressure
- Increased plasma osmolality
- Higher centre input - pain, emotion, anxiety
Why is aldosterone released as a response to surgery?
- Activation of RAAS
- ACTH release in response to hypovolaemia and hypotension
- Direct adrenal cortex stimulation
How long after surgery does ADH/aldosterone stay raised?
48-72 hours - causes oliguria and increased plasma osmolality
What are urinary changes which occur in metabolic response to injury?
- Oliguria - due to ADH and aldosterone secretion
- Decreased urinary sodium/increased potassium
- Increased urinary osmolality
How much does total energy expenditure increase by following surgery?
10-30%
Why are patients frequently pyrexial following surgery for the first 24-48 hours?
Pro-inlammatory cytokine release, which reset temperature-regulating centres in the hypothalamus.
By roughly what percentage does BMR increase by per 1oC increase in temperature?
10%
Why does starvation occur following surgery?
- Reduced nutritional intake due to illness
- Fasting prior to surgery
- Fasting after surgery
- Loss of appetite
What are the metabolic effects of acute starvation?
- Glycogenolysis and gluconeogenesis in the liver - glucose more readily available
- Lipolysis
These processes can supply normal energy requirments of the body for up to 10 hours
What are metabolic effects of chronic starvation?
- Muscle catabolism -> release of amino acids -> converted to glucose in liver
- FFAs -> ketones -> used as energy
Ketone use compensates for chronic starvation until fat stores deplete, meaning that muscle catabolism begins again
Why is anaemia common after surgery?
- Bleeding
- Haemodilution
- Impaired RBC production
Why does RBC production become inhibited following surgery?
- Reduced EPO production by the kidneys
- Reduced iron availibility due to increased ferritin and reduced transferrin binding capacity
What can increase insensible fluid loss in a surgical patient?
- Hyperventilation
- Intubation/non-humidified high-flow oxygen
- Pyrexia - from the skin
What are third space losses?
Occurs when too much fluid moves from the intravascular space (blood vessels) into the interstitial or “third” space-the nonfunctional area between cells. This can cause potentially serious problems such as edema, reduced cardiac output, and hypotension.
Why do obstructions which occur high up in the upper GI tract result in greater fluid losses?
Fluids secreted by the upper GI tract fail to reach the absorptive areas of the distal jejunum and ileum
Why do you get fluid losses from paralytic ileus?
Fluid can’t be reabsorbed in distal jejunum and ileum
What types of fluids can be used for fluid optimisation in a pre-operative surgical context?
- 0.9% saline
- Dextrose 4% + Saline 0.18%
- Hartmann’s solution
- Ringer’s lactate