Crit Care: Physiology of Shock (Distributive vs Cardiogenic) — MRCP Part 1

Crack Medicine
Jun 23
5 min read

TL;DR:

Shock physiology is a high-yield topic in MRCP Part 1, especially the distinction between distributive and cardiogenic shock. Candidates should focus on haemodynamic patterns, systemic vascular resistance (SVR), cardiac output changes, bedside findings, and lactate physiology. Understanding why distributive shock produces warm vasodilated states while cardiogenic shock causes cold vasoconstricted states is essential for exam success and clinical reasoning.

Why Shock Physiology Matters in MRCP Part 1

Shock is defined as a state of inadequate tissue perfusion leading to cellular dysfunction and organ failure. In the MRCP examination, shock is rarely tested as an isolated definition. Instead, questions integrate:

Haemodynamics
Critical care physiology
Cardiology
Acid–base balance
Renal dysfunction
Sepsis
Pharmacology

The exam frequently contrasts distributive shock with cardiogenic shock because they demonstrate opposite physiological mechanisms.

For broader preparation, review the official MRCP Part 1 revision hub:

👉 https://www.crackmedicine.com/mrcp-part-1/

You can also practise physiology-heavy questions in the free QBank:

👉 https://www.crackmedicine.com/qbank/

Understanding the Physiology of Shock

Tissue oxygen delivery (DO₂) depends on:

Cardiac output
Haemoglobin concentration
Oxygen saturation

When oxygen delivery becomes insufficient, the body activates compensatory mechanisms:

Sympathetic nervous system activation
Tachycardia
Peripheral vasoconstriction
Increased oxygen extraction
Renin–angiotensin–aldosterone system (RAAS) activation

Failure of compensation leads to:

Anaerobic metabolism
Lactate production
Metabolic acidosis
Cellular injury
Multi-organ dysfunction

The Five Most Tested Subtopics

1. Haemodynamic Profiles

This is the single highest-yield area for MRCP Part 1.

Parameter	Distributive Shock	Cardiogenic Shock
Cardiac output	High or normal early	Low
Systemic vascular resistance	Low	High
Pulmonary capillary wedge pressure	Low/normal	High
JVP	Usually normal/low	Raised
Extremities	Warm initially	Cold/clammy
Pulse pressure	Wide	Narrow
Common causes	Sepsis, anaphylaxis	MI, severe LV failure

Key Concept

Distributive shock = vascular tone problem
Cardiogenic shock = pump failure problem

This distinction underpins nearly all physiology questions in shock.

2. Distributive Shock Physiology

Distributive shock occurs because of pathological vasodilation and reduced SVR.

Major Causes

Septic shock
Anaphylactic shock
Neurogenic shock

Septic Shock

Inflammatory mediators including nitric oxide, TNF-alpha, and cytokines produce:

Widespread vasodilation
Capillary leak
Relative hypovolaemia
Maldistribution of blood flow

Typical Findings

Warm peripheries
Bounding pulse
Wide pulse pressure
Hypotension
Elevated lactate

Early septic shock may paradoxically show high cardiac output, which is a favourite MRCP exam point.

Why are patients warm?

Because vasodilation increases peripheral blood flow despite impaired tissue oxygen utilisation.

Neurogenic Shock

Loss of sympathetic tone following spinal cord injury causes:

Vasodilation
Hypotension
Bradycardia

The associated bradycardia helps distinguish neurogenic shock from most other forms of shock.

3. Cardiogenic Shock Physiology

Cardiogenic shock occurs when the heart fails to maintain adequate cardiac output.

Common Causes

Acute myocardial infarction
Severe left ventricular failure
Myocarditis
Mechanical complications of MI
Arrhythmias

Physiological Consequences

Reduced cardiac output triggers compensatory mechanisms:

Sympathetic activation
Peripheral vasoconstriction
Increased SVR
RAAS activation
Sodium and water retention

These mechanisms initially preserve blood pressure but increase afterload, often worsening myocardial performance.

Key Clinical Features

Raised JVP
Pulmonary oedema
Cool extremities
Narrow pulse pressure
Oliguria

Why does pulmonary oedema occur?

Elevated left ventricular filling pressures increase pulmonary capillary hydrostatic pressure, forcing fluid into alveoli.

4. Lactate and Tissue Hypoperfusion

Lactate physiology is heavily tested in MRCP critical care questions.

Elevated lactate may reflect:

Anaerobic metabolism
Reduced oxygen delivery
Impaired oxygen utilisation
Hepatic dysfunction
Beta-agonist therapy

Important MRCP Point

A patient may remain hypotensive with a normal lactate, or develop severe hyperlactataemia before blood pressure falls significantly.

Always interpret lactate within the clinical context.

For integrated acid–base revision, see:

👉 https://www.crackmedicine.com/blog/acid-base-disorders-mrcp-part-1/

5. Mixed Venous Oxygen Saturation (SvO₂)

SvO₂ reflects the balance between oxygen delivery and oxygen consumption.

Cardiogenic Shock

Low cardiac output
Increased tissue oxygen extraction
Low SvO₂

Early Septic Shock

High cardiac output
Impaired tissue oxygen extraction
Relatively high SvO₂

This concept commonly appears in advanced physiology questions.

10 Rapid Revision Facts for MRCP Part 1

Septic shock typically causes low SVR.
Cardiogenic shock usually causes high SVR.
Warm extremities suggest distributive shock.
Raised JVP suggests cardiogenic shock.
Neurogenic shock may produce bradycardia.
Early septic shock can have high cardiac output.
Pulmonary oedema suggests elevated left-sided pressures.
Lactate does not always equal anaerobic metabolism alone.
SvO₂ is often low in cardiogenic shock.
Cardiogenic shock activates RAAS and sympathetic pathways.

Mini-Case MCQ

A 72-year-old man presents following a large anterior STEMI. Examination shows:

BP 80/50 mmHg
Cool clammy extremities
Raised JVP
Bibasal crackles

Which haemodynamic profile is most likely?

A. High cardiac output, low SVRB. Low cardiac output, low SVRC. Low cardiac output, high SVRD. High cardiac output, high SvO₂

Answer: C. Low cardiac output, high SVR

Explanation

This patient has classic cardiogenic shock.

The failing myocardium causes reduced cardiac output. In response, the sympathetic nervous system increases SVR to maintain blood pressure. Raised filling pressures produce pulmonary oedema and elevated JVP.

This pattern contrasts with early distributive shock, where vasodilation lowers SVR.

Practise more integrated MRCP-style questions here:

👉 https://www.crackmedicine.com/mock-tests/

Common Exam Pitfalls

Confusing septic shock with hypovolaemic shock because both cause hypotension
Forgetting that septic shock may initially present with warm peripheries
Missing neurogenic shock because of associated bradycardia
Assuming lactate elevation always reflects tissue hypoxia alone
Forgetting that compensatory vasoconstriction raises SVR in cardiogenic shock

Practical Study Checklist

Before the MRCP exam, ensure you can:

Differentiate all major shock types haemodynamically
Interpret cardiac output and SVR changes rapidly
Recognise warm versus cold shock patterns
Understand preload and afterload physiology
Interpret lactate clinically
Identify causes of elevated JVP
Apply physiology to bedside findings
Solve integrated critical care MCQs efficiently

Recommended Revision Strategy

Memorise the haemodynamic comparison table.
Revise shock alongside acid–base disorders.
Practise daily timed MCQs.
Use spaced repetition for physiology facts.
Focus on mechanisms rather than memorising isolated findings.

For structured video teaching, visit:

👉 https://www.crackmedicine.com/lectures/

Medical trainee preparing for MRCP Part 1 critical care physiology revision

FAQs

What is the key difference between distributive and cardiogenic shock?

Distributive shock results primarily from pathological vasodilation and reduced SVR, whereas cardiogenic shock results from impaired cardiac pumping and reduced cardiac output.

Why are patients warm in septic shock?

Inflammatory mediators produce widespread vasodilation, increasing peripheral blood flow despite impaired tissue oxygen utilisation.

Why is SVR high in cardiogenic shock?

Reduced cardiac output activates sympathetic and RAAS pathways, causing peripheral vasoconstriction to maintain blood pressure.

Does elevated lactate always mean anaerobic metabolism?

No. Lactate may also rise because of beta-agonist therapy, hepatic dysfunction, mitochondrial dysfunction, or impaired clearance in sepsis.

Why is neurogenic shock associated with bradycardia?

Loss of sympathetic tone after spinal cord injury reduces heart rate and vascular tone simultaneously.

Ready to start?

Shock physiology remains one of the most clinically relevant and repeatedly tested topics in MRCP Part 1. Candidates who understand haemodynamic relationships rather than memorising isolated facts perform significantly better in integrated physiology questions.

Ready to strengthen your critical care physiology for MRCP Part 1?

Boost your revision with:

High-yield MRCP notes
Topic-wise QBank practice
Full mock examinations
Consultant-led lecture series

Start your preparation today with Crack Medicine: