FRCS Surgical Physiology 2

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?

Accepted Answer

Cardiogenic shock

Answer

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?

Accepted Answer

Approximately 200 ml/L

Answer

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?

Accepted Answer

Myogenic response of afferent arterioles

Answer

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.)

Accepted Answer

420 mmol

Answer

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?

Accepted Answer

Factor V

Answer

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.

FRCS Part A Practice Test

FRCS Part A Practice Test

FRCS Surgical Physiology 2