A woman with coronary artery disease is starting to go for a walk. As she begins, her heart rate accelerates from a resting pulse of 60 bpm until it reaches a rate of 120 bpm, at which point she begins to feel a tightening in her chest. She stops walking to rest and the tightening resolves. This has been happening to her consistently for the last 6 months. Which of the following is a true statement?

Increasing the heart rate decreases the relative amount of time spent during diastole

This patient's chest pain is indicative of transmural ischemia

Perfusion of the myocardium takes place equally throughout the cardiac cycle

Increasing the heart rate increases the amount of time spent during each cardiac cycle

Perfusion of the myocardium takes place primarily during systole

A 37-year-old man is brought to the emergency department by ambulance after a motor vehicle accident. He suffered multiple deep lacerations and experienced significant blood loss during transport. In the emergency department, his temperature is 98.6°F (37°C), blood pressure is 102/68 mmHg, pulse is 112/min, and respirations are 22/min. His lacerations are sutured and he is given 2 liters of saline by large bore intravenous lines. Which of the following changes will occur in this patient's cardiac physiology due to this intervention?

Increased cardiac output and increased right atrial pressure

Increased cardiac output and unchanged right atrial pressure

Decreased cardiac output and increased right atrial pressure

Increased cardiac output and decreased right atrial pressure

Decreased cardiac output and decreased right atrial pressure

A 67-year-old woman comes to the physician because of intermittent chest pain and dizziness on exertion for 6 months. Her pulse is 76/min and blood pressure is 125/82 mm Hg. Cardiac examination shows a grade 3/6, late-peaking, crescendo-decrescendo murmur heard best at the right upper sternal border. An echocardiogram confirms the diagnosis. Three months later, the patient returns to the physician with worsening shortness of breath for 2 weeks. An ECG is shown. Which of the following changes is most likely responsible for this patient's acute exacerbation of symptoms?

Decreased left ventricular preload

Impaired pulmonary artery outflow

Increased systemic vascular resistance

Impaired contractility of the left ventricle

Decreased impulse conduction across the AV node

A 27-year-old man is running on the treadmill at his gym. His blood pressure prior to beginning his workout was 110/72. Which of the following changes in his cardiovascular system may be seen in this man now that he is exercising?

Decreased systemic vascular resistance

Increased systemic vascular resistance

A 35-year-old man presents to the physician’s clinic due to episodic chest pain over the last couple of months. He is currently pain-free. His chest pain occurs soon after he starts to exercise, and it is rapidly relieved by rest. He recently started training for a marathon after a decade of a fairly sedentary lifestyle. He was a competitive runner during his college years, but he has only had occasional exercise since then. He is concerned that he might be developing some heart disease. He has no prior medical issues and takes no medications. The family history is significant for hypertension and myocardial infarction in his father. His vital signs include: pulse 74/min, respirations 10/min, and blood pressure 120/74 mm Hg. The ECG test is normal. The physician orders an exercise tolerance test that has to be stopped after 5 minutes due to the onset of chest pain. Which of the following contributes most to the decreasing cardiac perfusion in this patient's heart?

Force of myocardial contraction

A 28-year-old research assistant is brought to the emergency department for severe chemical burns 30 minutes after accidentally spilling hydrochloric acid on himself. The burns cover both hands and forearms. His temperature is 37°C (98.6°F), pulse is 112/min, respirations are 20/min, and blood pressure is 108/82 mm Hg. Initial stabilization and resuscitation is begun, including respiratory support, fluid resuscitation, and cardiovascular stabilization. The burned skin is irrigated with saline water to remove the chemical agent. Which of the following is the most appropriate method to verify adequate fluid infusion in this patient?

Pulmonary capillary wedge pressure

Effects of heart rate on cardiac cycle for USMLE Step 2 CK: High-Yield Physiology Lessons

Cardiac Cycle & HR - The Basic Beat

Inverse Relationship: As Heart Rate (HR) ↑, the duration of the cardiac cycle ↓.
- Cycle Length (sec) = $60 / HR (bpm)$.
Diastolic Shortening: ↑HR primarily shortens diastole, particularly the diastasis phase. Systole's duration is less affected.
Physiological Limits: At very high rates (e.g., >180 bpm), diastolic filling time becomes so short that stroke volume and cardiac output may fall.

⭐ Since coronary arteries perfuse the myocardium primarily during diastole, tachycardia can ↓ myocardial oxygen supply, precipitating ischemia.

Wiggers Diagram: Cardiac Cycle at Normal vs. Fast Heart Rate

Rate Effects on Phases - Systole vs. Diastole

As heart rate (HR) ↑, the total cardiac cycle duration ↓.
Diastole shortens significantly more than systole.
- At very high HR, the diastolic filling time becomes the primary limiting factor for cardiac output.

⭐ High-Yield: The majority of coronary blood flow to the left ventricle occurs during diastole. A significant reduction in diastolic time at high heart rates can precipitate or worsen myocardial ischemia, especially in patients with pre-existing coronary artery disease (CAD).

Clinical Consequences - The Extremes

Tachycardia (HR > 180 bpm):
- Drastically ↓ diastolic filling time.
- Leads to ↓ stroke volume & ↓ cardiac output ($CO = HR \times SV$).
- Myocardial perfusion suffers as coronary arteries fill primarily during diastole. This can precipitate ischemia.
Bradycardia (Severe):
- Extremely low heart rates lead to ↓ cardiac output, despite increased filling time and stroke volume.
- Can cause inadequate cerebral perfusion, leading to syncope (e.g., Stokes-Adams attacks).

⭐ At high heart rates, diastole shortens disproportionately more than systole, compromising ventricular filling and coronary blood flow.

High‑Yield Points - ⚡ Biggest Takeaways

Increased heart rate primarily and disproportionately shortens diastole, especially the diastolic filling period.

This leads to reduced ventricular filling time, causing a decrease in end-diastolic volume (preload).

Consequently, stroke volume may decrease, and at very high rates, cardiac output can paradoxically fall.

Coronary artery perfusion is also significantly reduced as it occurs almost exclusively during diastole.

On an ECG, the T-P interval is the most abbreviated segment.

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