MRCS Part B Revision · Applied surgical science and critical care
Shock 1 — MRCS Part B Applied surgical science and critical care
By Dr Richard Miller, MBChB FRCS · Reviewed
Shock 1 is an applied surgical science and critical care station. Three of the seventeen examined stations in the MRCS Part B OSCE fall in this area. These stations ask you to interpret data and manage a sick surgical patient: a chart, a blood gas, an imaging study or a deteriorating patient on the ward, and the physiology underneath the decision.
What you need to know for the Shock 1 station
Shock
Hypovolaemic (blood loss, fluid loss)
Hypovolemia is a direct loss of effective circulating blood volume leading to:
- A rapid, weak, thready pulse due to decreased blood flow combined with tachycardia
- Cool, clammy skin due to vasoconstriction and stimulation of vasoconstriction
- Rapid and shallow breathing due to sympathetic nervous system stimulation and acidosis
- Hypothermia due to decreased perfusion and evaporation of sweat
- Thirst and dry mouth, due to fluid depletion
- Cold and mottled skin (Livedo reticularis), especially extremities, due to insufficient perfusion of the skin
- The severity of hemorrhagic shock can be graded on a 1-4 scale on the physical signs. This approximates to the effective loss of blood volume. The shock index (heart rate divided by systolic blood pressure) is a stronger predictor of the impact of blood loss than heart rate and blood pressure alone. This relationship has not been well established in pregnancy-related bleeding.
Cardiogenic (true cardiogenic and obstructive)
Symptoms of cardiogenic shock include:
- Distended jugular veins due to increased jugular venous pressure
- Weak or absent pulse
- Arrhythmia, often tachycardia
- Pulsus paradoxus in case of tamponade
Distributive (Septic, endocrine, neurogenic)
Distributive shock includes infectious, anaphylactic, endocrine and neurogenic causes. The SIRS features typically occur in early septic shock.
Stages of Shock
There are four stages of shock. As it is a complex and continuous condition there is no sudden transition from one stage to the next. At a cellular level shock is the process of oxygen demand becoming greater than oxygen supply.
Initial
During this stage, the state of hypoperfusion causes hypoxia. Due to the lack of oxygen, the cells produce lactic acid (causing lactic acidosis).
Compensatory
This stage is characterised by the body employing physiological mechanisms, including neural, hormonal and bio-chemical mechanisms in an attempt to reverse the condition. As a result of the acidosis, the person will begin to hyperventilate in order to rid the body of carbon dioxide (CO2). CO2 indirectly acts to acidify the blood and by removing it the body is attempting to raise the pH of the blood. The baroreceptors in the arteries detect the resulting hypotension, and cause the release of adrenaline and noradrenaline. NA causes predominately vasoconstriction with a mild increase in heart rate, whereas adrenaline predominately causes an increase in heart rate with a small effect on the vascular tone; the combined effect results in an increase in blood pressure. The renin–angiotensin axis is activated, and anti-diuretic hormone (ADH/vasopressin) is released to conserve fluid via the kidneys. These hormones cause the vasoconstriction of the kidneys, gastrointestinal tract, and other organs to divert blood to the heart, lungs and brain. The lack of blood to the renal system causes the characteristic low urine production.
Progressive
Should the cause of shock not be successfully treated, the shock will proceed to the progressive stage and the compensatory mechanisms begin to fail. Due to the decreased perfusion of the cells, sodium ions build up within while potassium ions leak out. As anaerobic metabolism continues, increasing the body's metabolic acidosis, the arteriolar smooth muscle and precapillary sphincters relax such that blood remains in the capillaries. Due to this, the hydrostatic pressure will increase and, combined with histamine release, this will lead to leakage of fluid and protein into the surrounding tissues. As this fluid is lost, the blood concentration and viscosity increase, causing sludging of the micro-circulation. The prolonged vasoconstriction will also cause the vital organs to be compromised due to reduced perfusion.
Refractory
At this stage, the vital organs have failed and the shock can no longer be reversed. Brain damage and cell death are occurring, and death will occur imminently.
What are you asked at the Shock 1 station?
The station runs to 16 questions over nine minutes. These are the questions as they are put to you; the model answers are in the question bank.
- How would you assess this patient?
- The patient is stabilised and secondary surgery is performed including radiographs of her painful thigh. What do the radiographs show?
- What volume of blood can be lost from a femoral fracture?
- If we estimate that the patient has lost approximately 1.75L of blood and has not received any transfusion what signs and symptoms might they display?
- Define the term ‘shock’
- What types of shock do you know?
- Define hypovolaemic shock
- What are the causes of hypovolaemic shock?
- What clinical signs would help you to identify a patient with hypovolaemic shock?
- What would the central venous pressure (CVP) of a patient with hypovolaemic shock demonstrate?
- How is hypovolaemic shock classified?
- What volume of blood loss would a patient presenting with heart rate of 110, a reduced pulse pressure, normal BP, urine output of 25mls/hr and mild anxiety have?
And 4 more at this station.
How is the applied surgical science and critical care station marked in MRCS Part B?
Each of these three stations is marked out of 20, split 12 marks for clinical knowledge and its application, 4 for clinical and technical skill and 4 for professionalism. No communication marks are available. The three cover critical care management, interpretation of clinical data, and interpretation of visual information.
FAQ
What does the Shock 1 station ask?
It opens with "How would you assess this patient?" and runs to 16 questions over nine minutes. Each of these three stations is marked out of 20, split 12 marks for clinical knowledge and its application, 4 for clinical and technical skill and 4 for professionalism. No communication marks are available. The three cover critical care management, interpretation of clinical data, and interpretation of visual information.
What counts as applied surgical science in MRCS Part B?
Three of the seventeen examined stations: critical care management, interpretation of clinical data such as blood results and charts, and interpretation of visual information such as imaging and traces.
How is an applied science station marked?
Out of 20, with 12 marks for clinical knowledge and its application, 4 for clinical and technical skill and 4 for professionalism. Unlike the communication stations, none of the marks are for how you say it.
How much physiology do I need?
Enough to explain the decision you are making. The station rewards applying physiology to the patient in front of you rather than reciting it, so practise talking through why a number changes your management.
How many stations are in the MRCS Part B OSCE?
Seventeen examined stations of nine minutes each, with a minute to read the task before each one. Two preparation stations and at least one rest station bring the circuit to about twenty, and the exam takes about three and a half hours.
What is the pass mark for MRCS Part B?
There is no published pass mark. The cut score is set separately for Applied Knowledge and Applied Skills, for each circuit, by borderline regression. Published pass rates across the 2024/25 diets ranged from 51% to 66%.
Can I fail a station and still pass?
Yes. There is no rule about how many stations you may fail: the cut score applies to your total mark in each component, so a weak station costs the marks you lost on it and strong stations elsewhere can make them back. Applied Knowledge and Applied Skills are passed separately and must both be passed at the same sitting, so a strong anatomy performance cannot rescue a weak communication one.
Dr Richard Miller, MBChB FRCS
Station summaries are reviewed against the current intercollegiate MRCS syllabus and the published marking blueprint. Guidance changes between diets: check the royal colleges' own pages before relying on a date, a fee or a threshold.
Practise this station
The question bank carries the model answer to every question above, with the rest of the applied surgical science and critical care stations.
More applied surgical science and critical care stations