FRCS Surgical Physiology 2 — Questions and Answers
Question 1: A patient in the intensive care unit has a central venous pressure of 14 mmHg, a mean arterial pressure of 55 mmHg, and a cardiac index of 1.8 L/min/m2. Which type of shock is most consistent with these findings?
- Cardiogenic shock (Correct answer)
- Hypovolaemic shock
- Septic shock (warm phase)
- Neurogenic shock
Correct answer: Cardiogenic shock
The combination of elevated CVP, low MAP, and low cardiac index is characteristic of cardiogenic shock. The heart is failing as a pump, leading to fluid backing up (high CVP) with inadequate forward flow (low CI). Hypovolaemic shock would show low CVP. Warm septic shock typically shows high CI with low SVR.
Question 2: What is the oxygen content of arterial blood (CaO2) in a patient with Hb 150 g/L, SaO2 98%, and PaO2 13 kPa?
- Approximately 200 ml/L (Correct answer)
- Approximately 150 ml/L
- Approximately 100 ml/L
- Approximately 250 ml/L
Correct answer: Approximately 200 ml/L
CaO2 = (Hb x 1.34 x SaO2) + (PaO2 x 0.023). With Hb 150 g/L: (150 x 1.34 x 0.98) + (13 x 0.023) = 196.9 + 0.3 = approximately 197 ml/L, which rounds to approximately 200 ml/L. The dissolved oxygen contribution is minimal compared to haemoglobin-bound oxygen.
Question 3: Which physiological mechanism is primarily responsible for autoregulation of renal blood flow between mean arterial pressures of 80-180 mmHg?
- Myogenic response of afferent arterioles (Correct answer)
- Sympathetic nervous system
- Renin-angiotensin system
- Tubuloglomerular feedback alone
Correct answer: Myogenic response of afferent arterioles
Renal autoregulation is primarily mediated by the myogenic response (Bayliss effect) of the afferent arterioles, which constrict in response to increased transmural pressure and dilate when pressure falls. Tubuloglomerular feedback (via macula densa sensing NaCl delivery) contributes but is secondary. These mechanisms maintain a constant GFR across a wide range of perfusion pressures.
Question 4: A 70 kg man has a serum sodium of 125 mmol/L. If the target sodium is 135 mmol/L, approximately how much sodium deficit does he have? (Assume total body water is 60% of body weight.)
- 420 mmol (Correct answer)
- 700 mmol
- 210 mmol
- 1000 mmol
Correct answer: 420 mmol
Sodium deficit = TBW x (target Na - actual Na) = (0.6 x 70) x (135 - 125) = 42 x 10 = 420 mmol. However, correction must be gradual (no more than 10-12 mmol/L in 24 hours) to avoid central pontine myelinolysis (osmotic demyelination syndrome).
Question 5: Which clotting factor is NOT vitamin K dependent?
- Factor V (Correct answer)
- Factor II
- Factor VII
- Factor X
Correct answer: Factor V
The vitamin K-dependent clotting factors are II, VII, IX, and X (mnemonic: 1972 — 10, 9, 7, 2). Proteins C and S are also vitamin K-dependent. Factor V is not vitamin K-dependent; it is a co-factor in the prothrombinase complex and is activated by thrombin.
Question 6: What is the daily basal fluid requirement for a 70 kg adult calculated using the Holliday-Segar formula?
- 2500 ml (Correct answer)
- 2000 ml
- 3000 ml
- 1500 ml
Correct answer: 2500 ml
Holliday-Segar formula: 100 ml/kg for first 10 kg (1000 ml) + 50 ml/kg for next 10 kg (500 ml) + 20 ml/kg for remaining 50 kg (1000 ml) = 2500 ml/day. This is a commonly used estimate for maintenance fluid prescribing, though actual requirements depend on losses and clinical context.
FRCS Surgical Physiology 2
A patient in the intensive care unit has a central venous pressure of 14 mmHg, a mean arterial pressure of 55 mmHg, and a cardiac index of 1.8 L/min/m2.
Which type of shock is most consistent with these findings?