Cardiovascular Physiology Explained for the MRCEM Primary Exam: The Cardiac Cycle and ECG

Cardiovascular physiology is one of the most heavily tested areas in the MRCEM Primary physiology revision syllabus. Whether you are working through MRCEM Primary MCQs, tackling MRCEM SBA practice questions, or preparing for your MRCEM Primary mock exam, a solid understanding of the cardiac cycle and ECG is essential. This article breaks down the core concepts in a way that is practical, memorable, and directly aligned with what the MRCEM Primary exam actually tests.

What Is the Cardiac Cycle and Why Does It Matter for MRCEM Primary Physiology Revision?

The cardiac cycle describes the sequence of mechanical and electrical events that produce a single heartbeat. It encompasses everything from the onset of atrial depolarisation through to the end of ventricular relaxation, and it repeats approximately 60 to 100 times per minute at rest. For the MRCEM Primary exam, you need to understand the cycle not just as a list of phases, but as an integrated sequence where pressure, volume, valve movement, and electrical activity all interact.

The cycle is divided into two broad phases:

• Systole — the period of ventricular contraction and ejection
• Diastole — the period of ventricular relaxation and filling

At a resting heart rate of 75 beats per minute, the total cycle lasts approximately 0.8 seconds, with systole occupying around 0.3 seconds and diastole the remaining 0.5 seconds. As heart rate rises, diastole shortens disproportionately — a fact with significant clinical implications for coronary perfusion and cardiac output that frequently appears in MRCEM SBA exam questions.

How Does the Cardiac Cycle Progress Step by Step?

Understanding the sequential phases of the cardiac cycle is the foundation of MRCEM Primary basic sciences. The phases are as follows:

1. Late Diastole (Ventricular Filling)

At the start of the cycle, the heart is relaxed. Both the mitral and tricuspid valves are open, and blood flows passively from the atria into the ventricles. Approximately 70 to 80% of ventricular filling occurs passively during this phase. The remaining 20 to 30% is delivered by atrial contraction — sometimes called the atrial kick — which follows P-wave depolarisation on the ECG.

2. Isovolumetric Contraction

Following ventricular depolarisation (the QRS complex), the ventricles begin to contract. Intraventricular pressure rises sharply. All four valves are closed during this phase — the atrioventricular valves have shut because ventricular pressure now exceeds atrial pressure, but the semilunar valves remain closed because aortic and pulmonary pressures still exceed ventricular pressure. No blood enters or leaves the ventricle, so volume remains constant — hence the term isovolumetric.

3. Rapid Ejection

Once ventricular pressure exceeds aortic pressure (approximately 80 mmHg on the left side), the aortic valve opens and blood is ejected rapidly into the aorta. The left ventricle ejects roughly 70 ml per beat at rest — the stroke volume. Ejection fraction, the proportion of end-diastolic volume ejected, is normally 55 to 70% and is a key measure of left ventricular function that MRCEM SBA exam questions frequently probe.

4. Reduced Ejection and Isovolumetric Relaxation

As systole ends, ejection slows. The ventricle begins to relax and ventricular pressure falls below aortic pressure, causing the aortic valve to close — producing the dicrotic notch on the aortic pressure trace. With all valves again closed, the ventricle relaxes isovolumetrically until pressure falls below atrial pressure, at which point the mitral valve reopens and filling recommences.

How Do Pressure-Volume Loops Consolidate MRCEM Primary Physiology Revision?

The pressure-volume (PV) loop is a graphical representation of the cardiac cycle that brings together all of the above phases in a single diagram. It is a favourite tool in MRCEM Primary revision notes and online revision platforms because it elegantly demonstrates the effects of preload, afterload, and contractility on cardiac function.

• Preload — an increase in end-diastolic volume (e.g. from increased venous return) shifts the loop to the right, increasing stroke volume via the Frank-Starling mechanism.
• Afterload — an increase in aortic pressure narrows the loop, reducing stroke volume and increasing end-systolic volume.
• Contractility — a positive inotrope steepens the end-systolic pressure-volume relationship, increasing stroke volume without changing preload.

Being able to predict how the PV loop changes under different physiological and pathological conditions is a high-yield skill for the MRCEM SBA revision course and the MRCEM Primary exam itself.

What Do You Need to Know About the ECG for the MRCEM Primary Exam?

The ECG represents the sum of all cardiac electrical activity recorded from the body surface. For MRCEM Primary physiology revision, you need to understand both what each waveform represents and the ionic mechanisms that generate it.

The Standard Waveforms and Their Correlates

• P wave — atrial depolarisation. Generated by a wave of depolarisation spreading from the sinoatrial (SA) node through both atria. Duration normally less than 120 ms; amplitude less than 2.5 mm in lead II.
• PR interval — the time from onset of atrial depolarisation to onset of ventricular depolarisation. Normally 120 to 200 ms. Prolongation indicates delayed AV nodal conduction (first-degree AV block); shortening suggests accessory pathway conduction (e.g. Wolff-Parkinson-White syndrome).
• QRS complex — ventricular depolarisation. Normally less than 120 ms. Widening indicates aberrant conduction (e.g. bundle branch block or hyperkalaemia).
• ST segment — the isoelectric phase representing the plateau of the ventricular action potential (phase 2). Elevation or depression here is clinically critical.
• T wave — ventricular repolarisation. Notably, repolarisation proceeds in the opposite direction to depolarisation (from epicardium to endocardium), which is why the T wave is normally upright despite repolarisation being the reverse of depolarisation.
• QT interval — the total duration of ventricular electrical systole. Rate-corrected QTc is normally less than 440 ms in men and less than 460 ms in women. Prolongation predisposes to torsades de pointes.

The Ventricular Action Potential and Ion Channels

The shape of the cardiac action potential — particularly its prolonged plateau phase — distinguishes it from skeletal muscle and explains why the heart cannot undergo tetanic contraction. The key phases are:

1. Phase 0 — Rapid depolarisation: Fast voltage-gated sodium channels open, producing a steep upstroke.
2. Phase 1 — Early repolarisation: Sodium channels inactivate; transient outward potassium current begins.
3. Phase 2 — Plateau: L-type calcium channels open, balancing potassium efflux. This phase underpins the ST segment and is the target of calcium channel blockers.
4. Phase 3 — Rapid repolarisation: Calcium channels close; potassium efflux predominates, restoring resting membrane potential.
5. Phase 4 — Resting membrane potential: Approximately -90 mV in ventricular myocytes, maintained by the sodium-potassium ATPase.

SA node cells lack a true resting phase 4 — instead, they exhibit spontaneous depolarisation (pacemaker potential) driven by the funny current (I_f), which is blocked by ivabradine. This is a recurring point in MRCEM Primary pharmacology revision topics.

How Is MRCEM Primary Physiology Revision Tested in the SBA Format?

The MRCEM SBA exam uses single best answer questions to test your ability to apply physiological knowledge to clinical scenarios. You are unlikely to be asked simply to name the phases of the cardiac cycle — instead, questions will present a scenario and require you to integrate your knowledge. Common question themes include:

• Predicting the haemodynamic effect of a given intervention (e.g. fluid bolus, vasopressor, positive inotrope)
• Interpreting an ECG abnormality and linking it to an underlying ionic mechanism
• Explaining why coronary perfusion is at risk during tachycardia
• Calculating or interpreting cardiac output and its determinants (heart rate, stroke volume, preload, afterload, contractility)
• Linking drug mechanisms to specific phases of the action potential

Working through MRCEM SBA practice questions and MRCEM Primary exam questions that are mapped to the physiology syllabus is the most efficient way to identify gaps and consolidate your understanding before exam day.

Key Facts to Memorise for Your MRCEM Primary Exam Preparation

A well-organised MRCEM Primary study guide should include these high-yield facts as anchor points:

• Coronary perfusion to the left ventricle occurs predominantly in diastole
• Normal ejection fraction is 55 to 70%
• Normal cardiac output at rest is 4 to 8 L/min
• The Frank-Starling law states that stroke volume increases with increasing end-diastolic volume, up to a physiological limit
• The first heart sound (S1) corresponds to mitral and tricuspid valve closure at the start of systole
• The second heart sound (S2) corresponds to aortic and pulmonary valve closure at the end of systole
• A third heart sound (S3) is associated with rapid ventricular filling and is pathological in adults over 40 — suggesting heart failure
• A prolonged QTc predisposes to torsades de pointes, a form of polymorphic ventricular tachycardia

How EM Learning Centre Supports Your Revision

Cardiovascular physiology is just one component of a broad and demanding syllabus. To succeed in the MRCEM Primary exam, you need structured, high-quality resources that cover every domain — from anatomy and pharmacology through to pathology and evidence-based medicine. The EM Learning Centre provides a comprehensive MRCEM Primary online revision course with SBA question banks and mock exams designed specifically for emergency medicine trainees. With questions mapped to the current MRCEM Primary syllabus, detailed explanations, and regular content updates, it is the resource trusted by trainees who want to pass first time.