Cardiac cycle

Cardiac cycle US Medical PG Question 1: A 75-year-old man presents to the emergency department after an episode of syncope while walking outside with his wife. His wife states that he suddenly appeared pale and collapsed to the ground. She says he remained unconscious for 1 minute. He says he noticed a fluttering in his chest and excessive sweating before the episode. He has type 2 diabetes mellitus, essential hypertension, and chronic stable angina. He has not started any new medications in the past few months. Vital signs reveal: temperature 37.0°C (98.6°F), blood pressure 135/72 mm Hg, and pulse 72/min. Physical examination is unremarkable. ECG shows an old bifascicular block. Echocardiogram and 24-hour Holter monitoring are normal. Which of the following is the best next step in the evaluation of this patient's condition?

• A. Cardiac enzymes
• B. Continuous loop recorder (Correct Answer)
• C. Valsalva maneuver
• D. Electroencephalography (EEG)
• E. Tilt-table test

Cardiac cycle Explanation: ***Continuous loop recorder*** - This patient's syncope is preceded by **palpitations (fluttering in chest)** and **sweating**, suggesting a cardiac etiology, specifically a **transient arrhythmia** not captured on a standard ECG or 24-hour Holter. - A continuous loop recorder provides prolonged monitoring (months to years), increasing the likelihood of detecting intermittent arrhythmias responsible for syncopal episodes. *Cardiac enzymes* - While cardiac enzymes (e.g., troponin) are crucial for evaluating **acute myocardial ischemia** or infarction, the patient presents with syncope and no new chest pain, and his stable angina suggests chronic disease rather than an acute event leading to syncope in this specific instance. - An **ECG showing an old bifascicular block** and an **unremarkable physical exam** make an acute cardiac event less likely as the primary cause of syncope when an arrhythmia is suspected. *Valsalva maneuver* - The Valsalva maneuver is a diagnostic tool often used to differentiate between certain types of **tachyarrhythmias** or to evaluate for **autonomic dysfunction**, but it is not an evaluative step for a patient presenting with unexplained syncope where an arrhythmia has not yet been documented. - It would not help in identifying the cause of intermittent syncope in a patient whose standard workup has been unremarkable, as it's a test for immediate physiological response, not prolonged cardiac rhythm monitoring. *Electroencephalography (EEG)* - EEG is indicated when **seizure disorder** is suspected as the cause of loss of consciousness, often characterized by tonic-clonic movements, post-ictal confusion, or focal neurologic signs, which are absent in this patient's presentation. - The patient's pre-syncopal symptoms of **palpitations and sweating** point away from a seizure and towards a cardiac cause. *Tilt-table test* - A tilt-table test is used to evaluate for **vasovagal syncope** or **postural orthostatic tachycardia syndrome (POTS)**, often considered when other cardiac causes are ruled out or when syncope is typically triggered by prolonged standing. - Given the patient's pre-syncopal **palpitations**, a **cardiac arrhythmia** remains a higher suspicion than vasovagal syncope at this stage, especially after normal echocardiogram and Holter monitoring, necessitating further arrhythmia investigation.

Cardiac cycle US Medical PG Question 2: A 62-year-old man comes to the physician for decreased exercise tolerance. Over the past four months, he has noticed progressively worsening shortness of breath while walking his dog. He also becomes short of breath when lying in bed at night. His temperature is 36.4°C (97.5°F), pulse is 82/min, respirations are 19/min, and blood pressure is 155/53 mm Hg. Cardiac examination shows a high-pitch, decrescendo murmur that occurs immediately after S2 and is heard best along the left sternal border. There is an S3 gallop. Carotid pulses are strong. Which of the following is the most likely diagnosis?

• A. Aortic valve regurgitation (Correct Answer)
• B. Tricuspid valve regurgitation
• C. Mitral valve prolapse
• D. Mitral valve regurgitation
• E. Mitral valve stenosis

Cardiac cycle Explanation: ***Aortic valve regurgitation*** - A **high-pitch, decrescendo murmur immediately after S2** and heard best along the **left sternal border** is characteristic of **aortic regurgitation**. - Symptoms like **dyspnea on exertion** and **orthopnea**, an **S3 gallop**, and a **wide pulse pressure** (155/53 mmHg) further support heart failure due to chronic aortic regurgitation. *Tricuspid valve regurgitation* - This typically presents with a **holosystolic murmur** best heard at the **left lower sternal border** that increases with inspiration. - Clinical signs often include **jugular venous distension** and **peripheral edema**, not primarily a decrescendo diastolic murmur. *Mitral valve prolapse* - Characterized by a **mid-systolic click** followed by a **late systolic murmur**, and symptomatically may be asymptomatic or cause palpitations. - The described diastolic murmur and symptoms of heart failure do not align with mitral valve prolapse. *Mitral valve regurgitation* - Typically presents as a **holosystolic murmur** heard best at the **apex** and often radiating to the axilla. - While it can cause dyspnea and an S3, the character and timing of the murmur reported (decrescendo, immediately after S2) are inconsistent with mitral regurgitation. *Mitral valve stenosis* - This condition presents with a **diastolic rumble** heard best at the **apex** with an opening snap. - The murmur described is a high-pitch decrescendo murmur, which is distinct from the low-pitched rumble of mitral stenosis.

Cardiac cycle US Medical PG Question 3: During exercise, what is the primary mechanism for increased oxygen delivery to active muscles?

• A. Decreased blood viscosity
• B. Increased cardiac output (Correct Answer)
• C. Increased hemoglobin affinity
• D. Enhanced oxygen diffusion

Cardiac cycle Explanation: ***Increased cardiac output*** - During exercise, **cardiac output** increases significantly due to both an elevated **heart rate** and increased **stroke volume**, directly pushing more oxygenated blood to the active muscles. - This augmentation in blood flow is the primary factor ensuring a sufficient supply of oxygen and nutrients to meet the heightened metabolic demands of exercising muscles. *Decreased blood viscosity* - While factors like **hemodilution** can decrease blood viscosity during prolonged exercise, this effect is relatively minor and not the primary mechanism for acute increases in oxygen delivery compared to the dramatic increase in cardiac output. - A decrease in blood viscosity can slightly improve flow efficiency, but it doesn't fundamentally change the amount of blood pumped per minute to the muscles. *Increased hemoglobin affinity* - An *increased* hemoglobin affinity for oxygen would actually make it *harder* for oxygen to unload from hemoglobin to the tissues, which is counterproductive for oxygen delivery during exercise. - In fact, during exercise, local conditions like increased temperature, decreased pH (**Bohr effect**), and increased 2,3-BPG tend to *decrease* hemoglobin's affinity for oxygen, facilitating oxygen release to active muscles. *Enhanced oxygen diffusion* - While exercise does improve the efficiency of oxygen extraction at the tissue level due to a steeper partial pressure gradient and increased capillary recruitment, the *rate* of oxygen diffusion across the capillary membrane isn't the primary modulator of overall oxygen delivery. - The main determinant is the *amount* of oxygenated blood reaching the muscle, which is governed by cardiac output and local blood flow regulation.

Cardiac cycle US Medical PG Question 4: A 27-year-old man presents to the clinic for his annual physical examination. He was diagnosed with a rare arrhythmia a couple of years ago following an episode of dizziness. A mutation in the gene encoding for the L-type calcium channel protein was identified by genetic testing. He feels fine today. His vitals include: blood pressure 122/89 mm Hg, pulse 90/min, respiratory rate 14/min, and temperature 36.7°C (98.0°F). The cardiac examination is unremarkable. The patient has been conducting some internet research on how the heart works and specifically asks you about his own “ventricular action potential”. Which of the following would you expect to see in this patient?

• A. Abnormal phase 2 (Correct Answer)
• B. Abnormal phase 4
• C. Abnormal phase 0
• D. Abnormal phase 3
• E. Abnormal phase 1

Cardiac cycle Explanation: ***Abnormal phase 2*** - Phase 2 of the ventricular action potential, also known as the **plateau phase**, is primarily maintained by the influx of **L-type calcium channels** and the efflux of potassium. - A mutation in the gene encoding for the L-type calcium channel protein would directly affect phase 2 and likely **result in an abnormal plateau phase** of the action potential. *Abnormal phase 4* - Phase 4 represents the **resting membrane potential** in ventricular myocytes and is maintained by **inward-rectifier potassium channels**. - Mutations affecting L-type calcium channels would not directly or primarily cause an abnormality in the resting potential. *Abnormal phase 0* - Phase 0, the **depolarization phase**, is driven by the rapid influx of **sodium ions** through fast voltage-gated sodium channels. - While calcium channels play a minor role, their primary impact is not on the initial rapid upstroke of phase 0. *Abnormal phase 3* - Phase 3, the **repolarization phase**, is primarily mediated by the **efflux of potassium ions** through various potassium channels (e.g., delayed rectifier potassium channels). - Although calcium channel inactivation contributes to the end of the plateau, the **dominant ion flux** determining phase 3 is potassium efflux. *Abnormal phase 1* - Phase 1, the **initial repolarization phase**, is characterized by the **inactivation of sodium channels** and a brief efflux of potassium ions through transient outward potassium channels. - L-type calcium channel activity is just beginning during this phase and is not the primary determinant of its shape.

Cardiac cycle US Medical PG Question 5: A 60-year-old male engineer who complains of shortness of breath when walking a few blocks undergoes a cardiac stress test because of concern for coronary artery disease. During the test he asks his cardiologist about what variables are usually used to quantify the functioning of the heart. He learns that one of these variables is stroke volume. Which of the following scenarios would be most likely to lead to a decrease in stroke volume?

• A. Anxiety
• B. Heart failure (Correct Answer)
• C. Exercise
• D. Pregnancy
• E. Digitalis

Cardiac cycle Explanation: ***Heart failure*** - In **heart failure**, the heart's pumping ability is impaired, leading to a reduced **ejection fraction** and thus a decreased **stroke volume**. - The weakened myocardium cannot effectively contract to expel the normal volume of blood, resulting in lower blood output per beat. *Anxiety* - **Anxiety** typically causes an increase in **sympathetic nervous system** activity, leading to increased heart rate and myocardial contractility. - This often results in a temporary **increase in stroke volume** due to enhanced cardiac performance, not a decrease. *Exercise* - During **exercise**, there is a significant **increase in venous return** and sympathetic stimulation, leading to increased **end-diastolic volume** and contractility. - This physiological response causes a substantial **increase in stroke volume** to meet the body's higher oxygen demands. *Pregnancy* - **Pregnancy** leads to significant **physiological adaptations** to accommodate the growing fetus, including a substantial increase in **blood volume**. - This increased blood volume and cardiac output result in an **increase in stroke volume** to maintain adequate perfusion for both mother and fetus. *Digitalis* - **Digitalis** is a cardiac glycoside that **increases intracellular calcium** in myocardial cells, enhancing the **force of contraction**. - This positive inotropic effect leads to an **increased stroke volume** by improving the heart's pumping efficiency.

Cardiac cycle Flashcard 1 of 5

Question: _____ is the fourth phase of a cardiac cycle and represents the period just after mitral valve opening

Answer: Rapid ventricular filling

Cardiac cycle Flashcard 2 of 5

Question: What type of action potential occurs in the ventricles, atria, and Purkinje system?_____

Answer: Myocardial action potentials (phase 0 through phase 4)

Cardiac cycle Flashcard 3 of 5

Question: What phase of the cardiac cycle is most affected by changes in heart rate?_____

Answer: Reduced ventricular filling (diastasis)

Cardiac cycle Flashcard 4 of 5

Question: Cardiac contractility correlates directly with intracellular _____ concentration

Answer: Ca2+

Cardiac cycle Flashcard 5 of 5

Question: Blood flows continuously through the cardiac cycle in all organs except the _____

Answer: heart