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
What are sources of fluid loss in surgery patients?
- Insesible loss
- Urine
- Gut
- Third space
What increases the risk of third space losses in surgery?
Greater extent of surgery and tissue trauma
What are the main causes of GI fluid loss in a surgical patient?
- Obstruction
- Ileus
- Fistulae
- Diarrhoea
How many grams of sodium per 100ml does 0.9% NaCl contain?
9g
If you were to give a rapid infusion of 1000ml 5% dextrose solution, how would it distribute in the body within 30-60 minutes of the infusion?
- 670 ml into ICF
- 260 ml into ECF
- 70 ml into IVF
If you were to give a rapid infusion of 1000 ml saline, how would it distribute in the body within 30-60 minutes of the infusion?
- 786 into ECF
- 214 into IVF
How does Ringer’s lactate work?
Closely matches ECF compostition - physiological concentrations of sodium and lactate. After infusion, lactate is broken down leading to bicarb generation. This solution reduces the risk of hyperchloraemia
What are examples of colloid solutions?
- 4.5% albumin
- Staches
- Gelfusine
- Haemaccel
How do colloid solutions work?
They remain largely in the intravascular space until the colloid particles are removed by the reticuloendothelial system. Intravascular half-life is 6-24 hours
What are risks with using colloid solutions?
- Coagulopathy
- Anaphylaxis
- Pruritis
- Reticuloendothelial dysfuntion
How would you manage someone who is hypovolaemic following surgery?
250ml bolus saline, reassess -> if still hypovolaemic, give more
What are the main causes of hypoovolaemic hypernatraemia in surgery?
- Decreased oral intake
- Nausea and vomiting
- Diarrhoea
- Increased insensible loss
- Severe burns
- Diuresis
What are causes of hypervolaemic hypernatraemia in surgery?
- Excessive sodium loading
- Mineralocorticoid activity
How would you manage someone with hypovolaemic hypernatraemia?
Replace intravascular volume, then slowly replace water deficit
If, when trying to correct hypovolaemic hypernatraemia, you tried to replace water deficits to quickly, what could happen?
Cerebral oedema - cells in the brain adapt to hypernatraemic state, so when this is corrected rapidly, it results in a rise in intracellular volume, leading to cerebral oedema, seizures and coma
What patients might need fluid optimisation in a pre-operative setting?
- Illness that has affected absorption - D&V, intestinal obstruction, biliary colic, gastroenteritis
- Those with poor renal function
- Low body weight
- Children
- Illness that has caused reduced fluid intake - pancreatitis, chest infection etc.
How would you monitor fluid optimisation in a pre-operative setting?
- Skin turgor and mucosal hydration
- 1 hrly urine output - 0.5ml/kg/hr
- Monitor serum urea
What are causes of hyperkalaemia in surgery?
- Metabolic acidosis
- Massive blood transfusion
- Rhabdomyolysis
- Massive tissue damage
- Drugs
- AKI/CKD
What are causes of hypokalaemia in a surgical context?
Increased potassium excretion and losses
- Vomiting
- Fistulae
- Diarrhoea
- Ileus
- Intestinal obstruction
- Metabolic alakalosis
- Diuretics
How would you treat hyperkalaemia?
- Treat cause
- Calcium gluconate - protect the heart
- Dextrose + insulin
- Salbutamol nebs
- Consider haemodialysis
How would you manage someone with hypokalaemia?
Oral/IV replacement therapy
What is the maximum rate of administration advised for potassium replacement?
20 mmol/h
What factors contribute to a sugical patient developing hypomagnesemia?
Decreased oral intake + IV fluids for several days
What are common causes of lactic acidosis in surgery?
- Shock
- Severe hypoxaemia
- Severe haemorrhage/anaemia
- Liver failure
What are causes of metabolic acidosis in surgical cases?
- Lactic acidosis
- DKA
- Starvation ketoacidosis
- AKI/CKD
- Poisonining
- Diarrhoea
- Intestinal fistulae
- Hyperchloraemic acidosis
What is the commonest cause of metabolic acidosis in surgical patients?
Lactic acidosis due to hypovolaemia and impaired tissue oxygen delivery
What are causes of fluid loss following surgery?
- Haemorrhage
- Vomiting
- NG drainage
- Diarrhoea
- Sweating
- Evaporation
- Third space loss
What are common causes of metabolic alkalosis in surgical patients?
- Vomiting
- Loss of gastric secretions
- Diuretics
What are common causes of respiratory alkalosis in a surgical patient?
- Pain
- Apprehension/fear -> hyperventilation
- Pneumonia
- CNS disorders - meningitis, encephalitis
- PE
- Septicaemia
- Liver failure
What are causes of respiratory acidosis in surgical patients?
- Opiod drugs
- Head injury
- Severe asthma
- COPD
- Severe chest infection
