Cardiac: MRCEM Primary questions with answers

Cardiac physiology is part of the physiology section of MRCEM Primary, which carries 60 of the 180 questions. The topic follows the heart from the myocyte and conducting system through the ECG, action potentials, the cardiac cycle, heart volumes and heart sounds. It then covers reflex control, blood pressure regulation, the vasculature, microcirculation and lymph, and hormonal control of blood flow. This is the groundwork for understanding shock and arrhythmia.

5 free questions below, each with the answer and the full explanation · 163 in the full bank · Last updated after the 27 October 2026 MRCEM Primary sitting.

1. Question 1 · Ventricular Myocyte vs Pacemaker Action Potential

   A male aged 50 with SVT is treated with adenosine; following the administration, the patient’s heart rate begins to slow down. During which phase of the cardiac action potential does the AV nodal delay primarily occur?

   1. APhase 0
   2. BPhase 1
   3. CPhase 2
   4. DPhase 4
   5. EPhase 3
   Show answer and explanation

   Answer: D. The question keys the AV nodal delay to phase 4, the resting phase of the nodal cell action potential.

   • A: Upstroke phase; not the phase this question links to AV delay.
   • B: Early repolarisation phase, absent in nodal cells.
   • C: Plateau phase, not the phase keyed here.
   • E: Repolarisation phase, not the keyed answer.

   Full explanation: Ventricular Myocyte vs Pacemaker Action Potential

   Ventricular myocyte action potential

   • RMP of Ventricular myocyte is -90mv depolarization initiates when RMP reaches threshold of –65mv 

   • Fast Na channels activate and a rapid Na+ influx depolarizes membrane potential. 

   Pacemaker action potential: 

   • Action potential of SA node differs from ventricular muscle. 
   • RMP of SA node is –60mv depolarization initiates when RMP reaches threshold of –40mv. 
   • Slow calcium channels activation and calcium influx initiates depolarization unlike of other ventricular muscle Na influx initiates depolarization 

    

2. Question 2 · Cardiac Cycle

   A woman aged 53 presents to the ED with intermittent palpitations and a loud first heart sound (S1) on physical exam. What is the most likely cause of the loud first heart sound?

   1. APericarditis
   2. BAortic Stenosis
   3. CMitral stenosis
   4. DPulmonary hypertension
   5. EAortic regurgitation
   Show answer and explanation

   Answer: C. Mitral stenosis causes a loud S1 and is associated with palpitations from atrial fibrillation.

   • A: Does not typically give a loud S1.
   • B: Gives an ejection systolic murmur and soft or absent S2.
   • D: Loud S2, not loud S1.
   • E: Causes an early diastolic murmur, not a loud S1.

   Full explanation: Cardiac Cycle

   Heart as pump

   •   The heart is a → chambered muscular organ that contracts rhythmically to pump blood into arteries and receive it back through veins, thereby keeping blood in constant circulation. 
   • Composed of Two separate pumps, right and left, each consisting of an atrium and a ventricle.   

   Cardiac Cycle 

   The events that occur at the beginning of a heartbeat and last until the beginning of the next heartbeat are called the cardiac cycle.  

   • Each beat of the heart begins with a spontaneous action potential that is initiated in the sinus node of the right atrium near the opening of the superior vena cava. 
   • The action potential travels through both atria and the A-V node and bundle into the ventricles. A delay of more than 1/10 of a second occurs in the A-V node and bundle, which allows the atria to contract before the ventricles contract. 

   Phases of Cardiac Cycle 

   Two, 

   Diastole 

   Period of relaxation or filling is called diastole. Can be divided into: 

   1. Period of Isovolumic or Isometric Relaxation 

   • During this, ventricles begin to relax (after systole ended), 
   •  Intra-ventricular pressure falls aortic and pulmonary valves close.  
   •  But ventricular vol. does not change. 

   2.  Period of Rapid Filling of Ventricles (First 1/3): During this, atrial pressure becomes greater than    intra-ventricular pressure (just after systole is over), AV valves (tricuspid and mitral) open, and blood flows rapidly into respective ventricles.  

   3. Diastasis (Middle 1/3): During this, blood continues to empty into atria from veins and passes on thru atria directly into ventricles

   4.Atrial Systole (Late 1/3): During this, atria contracts and pump 20-30% blood into ventricles. Systolic pressure of right atrium rises to 4-6 mm Hg. and that of left atrium to 7-8 mm Hg. 

     

    Systole:

   Period of Contraction can be divided into 

   1. Period of isovolumic or isometric contraction During this, ventricles begin to contract.  

   • ventricular pressure rise  
   • AV valves are closed.  
   • semilunar (aortic & pulmonary) valves do not open. So, there is no emptying and no volume change.   

   2. Period of Ejection: During this, left ventricular pressure rises slightly above 80 mm Hg and right ventricular pressure slightly above 8 mm Hg. These pressures push aortic and pulmonary valves open and empty blood into the aorta and pulmonary trunk. Divided into. 

   • Period of Rapid Ejection: During first 1/3, 70% emptying occurs. 
   •  Period of Slow Ejection During last 2/3, 30% emptying occur. 

   Relationship of the Electrocardiogram to the Cardiac Cycle 

   • The P wave is caused by the spread of depolarization across the atria, which causes atrial contraction. Atrial pressure increases just after the P wave. 
   • The QRS waves appear as a result of ventricular depolarization about 0.16 second after the onset of the P wave, and this initiates ventricular contraction; then the ventricular pressure begins to increase. 
   • The ventricular T wave is caused by repolarization of the ventricle. 

   Functions of Heart Valves 

   •  AV (tricuspid and mitral valves) prevent backflow of blood from ventricles to atria during systole.  
   •  Semilunar (aortic and pulmonary) valves prevent backflow of blood from aorta and pulmonary trunk into ventricles during diastole. 

    

3. Question 3 · Electrocardiography (Ecg)

   A male aged 40 years presents to the ED with chest pain and the following ECG is taken. Which artery is most likely to be occluded? 

   1. Aleft main coronary artery
   2. BRight coronary artery
   3. CObtuse marginal artery
   4. DLeft circumflex
   5. ELeft anterior descending
   Show answer and explanation

   Answer: B. Inferior ST elevation with reciprocal changes in I and aVL indicates right coronary artery occlusion.

   • A: Would cause widespread anterolateral changes.
   • C: Supplies lateral wall; would give lateral changes.
   • D: Lateral or posterior changes more typical.
   • E: Causes anterior ST elevation.

   Full explanation: Electrocardiography (Ecg)

   ECG

   • 12 Conventional ECG leads record the difference in potential. 
   • Six extremity limb leads and six chest pericardial leads. 
   • Six Extremity leads: Three bipolar leads (I, II, III) and three unipolar leads aVR, aVL, aVF 

   Six chest leads: 

   • V1:4TH intercostal space, just to right the sternum 
   • V2:4TH intercostals space, just to left of the sternum. 
   • V3: Midway between V2 and V4 
   • V4: Midclavicular line, fifth intercostal space 
   • V5: Anterior axillary line, same level as V4 
   • V5: Midaxillary line, same level as V4 and V5  

   The three limb electrodes, I, II and III form a triangle (Einthoven’s Triangle), at the right arm (RA), left arm (LA) and left leg (LL). 

   Lead type	Positive input	Negative input
   Lead 1	Left arm.	right arm
   Lead 2	left leg.	Right arm
   Lead 3	Left leg.	left arm.

4. Question 4 · Heart Volumes

   A woman aged 60 attends the emergency department with severe epistaxis that has lasted 30 minutes. She is dizzy, with a heart rate of 115 beats per minute and a blood pressure of 92/58 mmHg. What is the most likely cardiovascular consequence of this condition?

   1. ADecreased stroke volume
   2. BIncreased stroke volume
   3. CBradycardia
   4. DIncreased preload
   5. EIncreased cardiac output
   Show answer and explanation

   Answer: A. Blood loss reduces venous return and preload, decreasing stroke volume with compensatory tachycardia.

   • B: Reduced filling lowers stroke volume.
   • C: Hypovolaemia causes tachycardia, as seen here.
   • D: Venous return and preload fall with blood loss.
   • E: Cardiac output falls as stroke volume falls.

   Full explanation: Heart Volumes

   Heart volume

   • The total blood volume in the circulatory system of a healthy adult is about 5 L. 
   • Stroke Volume Output: The volume of blood that is ejected by each ventricle during systole is called stroke volume output (Value:70 ml/beat) 
   • Heart rate= 70bpm 
   • Cardiac output (CO) = Stroke volume x Heart rate = 70 mL/beat x 70 beats/min = 4900 mL/min 
   • Mean arterial pressure: Average arterial pressure during single cardiac cycle. MAP can be calculated by:  Diastolic pressure + 1/3 of pulse pressure (SBP-DBP) OR 80+ 1/3 (120-80) = 93mmhg where BP 120/80 
   • End-Diastolic Volume: The volume of blood in each ventricle at the end of the diastole is called end-diastolic volume. (Value:120-130ml) 
   • End-Systolic Volume: The volume of blood in each ventricle at end of systole is called end-systolic volume. (Value: 50-60ml) 
   • Ejection Fraction: Fraction of end-diastolic volume that is ejected during systole, is called ejection fraction. (Value: 60%) 

5. Question 5 · Components of the Vasculature

   Which of the following type of blood vessels contributes the largest share of systemic vascular resistance?

   1. ACapillaries
   2. BArteriole
   3. CArteries
   4. DVeins
   5. EVenules
   Show answer and explanation

   Answer: B. Arterioles are the main resistance vessels, with muscular walls and narrow lumens.

   • A: Capillaries are numerous in parallel, so contribute less resistance.
   • C: Large arteries offer less resistance than arterioles.
   • D: Veins are capacitance vessels with low resistance.
   • E: Venules have little resistance.

   Full explanation: Components of the Vasculature

   Components of the vasculature 

   1: Arteries:

   • deliver oxygenated blood to the tissues. are thick walled, with extensive elastic tissue and smooth muscle. 
   • are under high pressure. The blood volume contained in the arteries is called the stressed volume. 

   1. Arterioles:  

   •  Are the smallest branches of the arteries.
   •  Are the site of highest resistance in the cardiovascular system. have a smooth muscle wall that is extensively innervated by autonomic nerve fibers. 
   • The greater the change in resistance at any point in the vasculature, the greater the loss of pressure at that point. Arterioles have the most increase in resistance and cause the largest decrease in blood pressure.
   •  Arteriolar resistance is regulated by the autonomic nervous system (ANS). 
   •  Alpha-1Adrenergic receptors are found on the arterioles of the skin, splanchnic, and renal circulations. 
   •  Beta-2 Adrenergic receptors are found on arterioles of skeletal muscle. 

   1. Capillaries: 

   • Have the largest total cross-sectional and surface area.
   • Consist of a single layer of endothelial cells surrounded by basal lamina. 
   • Are thin walled. 
   • Are the site of exchange of nutrients, water, and gases. 

   Capillary permeability: 

   Capillaries throughout the body vary in their permeability based on the size of their pores. There are three basic types: 

   • Continuous capillaries, found in the skin, lungs, muscles and CNS, are the most selective with low permeability, as junctions between the endothelial cells are very tight, restricting the flow of molecules with MW > 10,000. 
   • Fenestrated capillaries, found in renal glomeruli, endocrine glands and intestinal villi, are more permeable with less tight junctions, and the endothelial cells are also punctured by pores which allow large amounts of fluids or metabolites to pass. 
   • Discontinuous capillaries, found in the reticuloendothelial system (bone marrow, liver and spleen), have large gaps between endothelial cells and are permeable to red blood cells. 

   4:  Venules

   • are formed from merged capillaries.

   5: Veins: 

   • progressively merge to form larger veins. The largest vein, the vena cava, returns blood to the heart.
   • Are thin walled. 
   • are under low pressure. 
   • contain the highest proportion of blood in the cardiovascular system. The blood volume contained in the veins is called the unstressed volume.  
   • Have alpha 1 adrenergic receptors. 

     

   Pressure profile in blood vessels:

   As blood flows through the systemic circulation, pressure decreases progressively because of the resistance to blood flow.

   Thus, pressure is highest in the aorta and large arteries and lowest in the venae cavae. The largest decrease in pressure occurs across the arterioles because they are the site of highest resistance. 

   Mean pressures in the systemic circulation are as follows: 

   •  Aorta, 100 mm Hg 
   • Arterioles, 50 mm Hg 
   • Capillaries, 20 mm Hg 
   • Vena cava, 4 mm Hg