Pre-op - General Measures Flashcards
What relationship does the rate of stomach emptying have to stomach volume?
Proportional. Empties at 1-3%.
What is the t1/2 clearance of water and clear fluids from the stomach?
10-20 mins. By convention a patient must be fasted of clear fluids for at least 2 hours prior to induction of general anaesthesia to ensure complete clearance.
What is the order (fastest to slowest) in which the 3 main macronutrients are cleared by the stomach?
Carbohydrate (4 hours) > Protein > Fat (6 hours)
Why is milk considered a solid for fasting purposes?
It congeals in stomach acid.
What is the fasting time for breast milk? Why?
4 hours. Human breast milk contains less protein and fat than cow’s milk.
What factors predispose to regurgitation?
LOS incompetence - GORD, hiatus hernia, pregnancy, opioids, alcohol, anti-muscarinics
Delayed gastric emptying/full stomach - Shock/trauma, fear, anxiety, pain, opioids, recent meal, pyloric stenosis
Obstructed/abnormal peristalsis - Small and large bowel obstruction, gastric tumour, pyloric stenosis, peritonitis, ileus (pseudo-obstruction/gut paralysis)
How does ranitidine reduce the risk of aspiration?
Ranitidine is an H2 antagonist, and competitively blocks histaminergic H2 receptors on BL membrane of parietal cells, which otherwise increases levels of intracellular cAMP, in turn stimulating H+/K+ ATPase
Name the factors in the Virchow’s Triad. What do these predispose to?
Venous stasis, vessel wall injury and hypercoagulability. Venous thrombi.
Name some precipitating factors for VTE peri-operatively.
Patient factors (general) - Age >60, pregnancy, puerperium, OCP/HRT, immobility, travel >3 h within 4 weeks of surgery.
Patient factors (specific) - prev VTE, obesity, cancer, heart/respiratory failure, acute concurrent illness, recent CVE/MI, varicose veins/phlebitis, trauma, haematological disease, nephrotic syndrome, IBD.
Surgical factors - surgery duration >30 mins, major joint/trauma surgery, abdo/pelvic surgery.
How does enoxaparin help prevent VTE? (i.e. mechanism of action)
It activates anti-thrombin III, which forms a complex with Factor Xa thereby inhibiting it.
What is fondaparinux?
A synthetic pentasaccharide which acts similarly to LMWH, but is a specific indirect inhibitor of Xa.
When should a patient stop HRT before their surgery?
4 weeks, if possible.
