FRCS Surgical Physiology & Pharmacology 2 — Questions and Answers
Question 1: What is the acid-base disturbance in a patient who has been vomiting profusely with gastric losses?
- Metabolic acidosis with respiratory compensation
- Metabolic alkalosis with respiratory compensation (hypoventilation) (Correct answer)
- Respiratory alkalosis with renal compensation
- Respiratory acidosis with metabolic compensation
Correct answer: Metabolic alkalosis with respiratory compensation (hypoventilation)
Loss of gastric HCl through vomiting results in metabolic alkalosis (loss of H+ and Cl−). The kidneys compensate by retaining HCO3− and excreting less acid. Hypochloraemia and hypokalaemia accompany the alkalosis. Respiratory compensation (hypoventilation to retain CO2) is limited.
Question 2: A patient develops bronchospasm, hypotension, and urticaria 10 minutes after induction of anaesthesia. What is the first-line treatment?
- IV hydrocortisone alone
- IV adrenaline 0.5mg IM (1:1000) (Correct answer)
- IV chlorphenamine
- Nebulised salbutamol
Correct answer: IV adrenaline 0.5mg IM (1:1000)
Anaphylaxis management requires adrenaline 0.5mg IM (1:1000) as the first-line treatment. Adrenaline reverses bronchoconstriction (beta-2 effect), causes vasoconstriction (alpha-1 effect), and reduces histamine release. IV fluids, antihistamines, and corticosteroids are secondary measures.
Question 3: What does the term 'oxygen delivery' (DO2) represent and how is it calculated?
- The volume of oxygen inhaled per breath
- Cardiac output × arterial oxygen content — represents the rate of oxygen delivery to tissues (Correct answer)
- The fraction of inspired oxygen reaching the alveoli
- SpO2 × heart rate
Correct answer: Cardiac output × arterial oxygen content — represents the rate of oxygen delivery to tissues
Oxygen delivery (DO2) = cardiac output (CO) × arterial oxygen content (CaO2), where CaO2 = (Hb × 1.34 × SaO2) + (0.003 × PaO2). Normal DO2 is approximately 1000 mL/min. Reduced DO2 from any combination of low CO, low haemoglobin, or low SaO2 leads to tissue hypoxia.
Question 4: What is the mechanism of action of morphine?
- Blocks sodium channels in peripheral nerves
- Binds to mu opioid receptors, reducing pain transmission and altering pain perception (Correct answer)
- Inhibits cyclooxygenase (COX) enzymes
- Blocks NMDA receptors in the spinal cord
Correct answer: Binds to mu opioid receptors, reducing pain transmission and altering pain perception
Morphine exerts its analgesic effects primarily through mu (μ) opioid receptor agonism in the CNS (brain and spinal cord), reducing the transmission and perception of pain. It also acts on peripheral mu receptors. Mu receptor activation causes analgesia, sedation, respiratory depression, and constipation.
Question 5: A patient with a tension pneumothorax develops cardiovascular collapse. What is the immediate management before chest drain insertion?
- Chest X-ray to confirm before any intervention
- Needle decompression via 2nd intercostal space, midclavicular line (Correct answer)
- Emergency thoracotomy
- Pericardiocentesis
Correct answer: Needle decompression via 2nd intercostal space, midclavicular line
Tension pneumothorax is a clinical emergency causing cardiovascular collapse from mediastinal shift and compression of the contralateral lung and great vessels. Immediate needle decompression (14G cannula, 2nd ICS, MCL or 4th-5th ICS, anterior axillary line) relieves pressure. Chest X-ray must not delay treatment.
Question 6: What is the effect of hypothermia on the oxyhaemoglobin dissociation curve?
- Shifts the curve right, promoting oxygen offloading
- Shifts the curve left, increasing haemoglobin's affinity for oxygen (Correct answer)
- Flattens the curve at all saturations
- No significant effect below 35°C
Correct answer: Shifts the curve left, increasing haemoglobin's affinity for oxygen
Hypothermia shifts the oxyhaemoglobin dissociation curve to the left, increasing haemoglobin's affinity for oxygen. While this enhances oxygen loading in the lungs, it impairs oxygen offloading to tissues. Other left-shifters include alkalosis, reduced CO2, and reduced 2,3-DPG.
FRCS Surgical Physiology & Pharmacology 2
What is the acid-base disturbance in a patient who has been vomiting profusely with gastric losses?