Cardiac output determinants US Medical PG Practice Questions and MCQs
Practice US Medical PG questions for Cardiac output determinants. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Cardiac output determinants US Medical PG Question 1: A 25-year-old male athlete undergoes a cardiopulmonary exercise test. As exercise intensity increases from rest to moderate levels, which of the following best describes the relationship between oxygen consumption and cardiac output?
- A. Linear increase until anaerobic threshold (Correct Answer)
- B. Exponential increase throughout exercise
- C. Plateau at low exercise intensities
- D. No change until anaerobic threshold
Cardiac output determinants Explanation: ***Linear increase until anaerobic threshold***
- During incremental exercise, both **oxygen consumption (VO2)** and **cardiac output (CO)** increase proportionally with work rate.
- This **linear relationship** continues until the body reaches the **anaerobic threshold**, beyond which other physiological responses begin to dominate.
*Exponential increase throughout exercise*
- An **exponential increase** would imply a disproportionately rapid rise in oxygen consumption and cardiac output even at low-to-moderate exercise intensities, which is not physiologically accurate.
- While both parameters do increase, the initial increase is typically linear, reflecting the immediate physiological demands.
*Plateau at low exercise intensities*
- A **plateau** would suggest that the body's demand for oxygen and the heart's pumping capacity stabilize despite an increase in exercise intensity, which contradicts the need for increased energy supply during exercise.
- The cardiovascular system actively responds to even low-intensity exercise to meet metabolic demands.
*No change until anaerobic threshold*
- **No change** would mean that the cardiovascular system is not responding to the increased metabolic demands of exercise, which is incorrect.
- Both VO2 and CO begin to rise almost immediately upon starting exercise to meet the muscles' increasing oxygen requirements.
Cardiac output determinants US Medical PG Question 2: 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 output determinants 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 output determinants US Medical PG Question 3: A 45-year-old unconscious man is brought to the emergency department by a friend who witnessed him collapse. They were working in a greenhouse spraying the vegetables when the man started to complain of blurred vision and nausea. On the way to the hospital, the man lost consciousness and lost bladder continence. The patient’s vital signs are as follows: blood pressure 95/60 mm Hg; heart rate 59/min; respiratory rate 22/min; and temperature 36.0℃ (96.8℉). On examination, he is unconscious with a GCS score of 7. His pupils are contracted and react poorly to light. Lung auscultation reveals diffuse wheezing. Cardiac auscultation is significant for bradycardia. Abdominal auscultation reveals increased bowel sounds. A cardiac monitor shows bradycardia with grade 2 AV-block. Which of the following leads to the cardiac manifestations seen in this patient?
- A. Activation of M1-cholinergic receptors
- B. Activation of β2-adrenergic receptors
- C. Activation of M2-cholinergic receptors (Correct Answer)
- D. Inhibition of β1-adrenergic receptors
- E. Inhibition of M2-cholinergic receptors
Cardiac output determinants Explanation: ***Activation of M2-cholinergic receptors***
- The patient's symptoms (blurred vision, nausea, unconsciousness, incontinence, miosis, wheezing, bradycardia, increased bowel sounds) are classic for **organophosphate poisoning**, which involves excessive activation of the **parasympathetic nervous system**.
- **M2 receptors** are primarily located in the heart and, when activated by acetylcholine, lead to slowed heart rate (bradycardia) and decreased conduction (AV block), characteristic of the cardiac manifestations observed.
*Activation of M1-cholinergic receptors*
- **M1 receptors** are primarily found in neural tissue and glands, contributing to increased GI motility and glandular secretions but not directly to cardiac slowing.
- Their activation does not directly cause the observed **bradycardia** and **AV block**.
*Activation of β2-adrenergic receptors*
- **β2-adrenergic receptors** are part of the sympathetic nervous system and are present in the smooth muscle of the bronchioles and blood vessels.
- Their activation typically causes **bronchodilation** and **vasodilation**, which would counteract the patient's wheezing and hypotension, and are not involved in bradycardia.
*Inhibition of β1-adrenergic receptors*
- **β1-adrenergic receptors** are found in the heart and their inhibition would lead to bradycardia, but organophosphate poisoning causes **cholinergic excess**, not adrenergic inhibition.
- This mechanism would not explain the other widespread **parasympathetic activation** symptoms like miosis, wheezing, and increased GI motility.
*Inhibition of M2-cholinergic receptors*
- **Inhibition of M2-cholinergic receptors** would lead to an increase in heart rate and improved AV conduction, rather than the severe **bradycardia** and **AV block** observed.
- This mechanism is characteristic of **anticholinergic toxidrome**, which presents with opposing symptoms.
Cardiac output determinants US Medical PG Question 4: An 83-year-old male presents with dyspnea, orthopnea, and a chest radiograph demonstrating pulmonary edema. A diagnosis of congestive heart failure is considered. The following clinical measurements are obtained: 100 bpm heart rate, 0.2 mL O2/mL systemic blood arterial oxygen content, 0.1 mL O2/mL pulmonary arterial oxygen content, and 400 mL O2/min oxygen consumption. Using the above information, which of the following values represents this patient's cardiac stroke volume?
- A. 30 mL/beat
- B. 70 mL/beat
- C. 40 mL/beat (Correct Answer)
- D. 60 mL/beat
- E. 50 mL/beat
Cardiac output determinants Explanation: ***40 mL/beat***
- First, calculate cardiac output (CO) using the **Fick principle**: CO = Oxygen Consumption / (Arterial O2 content - Venous O2 content). Here, CO = 400 mL O2/min / (0.2 mL O2/mL - 0.1 mL O2/mL) = 400 mL O2/min / 0.1 mL O2/mL = **4000 mL/min**.
- Next, calculate stroke volume (SV) using the formula: SV = CO / Heart Rate. Given a heart rate of 100 bpm, SV = 4000 mL/min / 100 beats/min = **40 mL/beat**.
*30 mL/beat*
- This answer would result if there was an error in calculating either the **cardiac output** or if the **arteriovenous oxygen difference** was overestimated.
- A stroke volume of 30 mL/beat with a heart rate of 100 bpm would yield a cardiac output of 3 L/min, which is sub-physiologic for an oxygen consumption of 400 mL/min given the provided oxygen content values.
*70 mL/beat*
- This stroke volume is higher than calculated and would imply either a significantly **lower heart rate** or a much **higher cardiac output** than derived from the Fick principle with the given values.
- A stroke volume of 70 mL/beat at a heart rate of 100 bpm would mean a cardiac output of 7 L/min, which is inconsistent with the provided oxygen consumption and arteriovenous oxygen difference.
*60 mL/beat*
- This value is higher than the correct calculation, suggesting an error in the initial calculation of **cardiac output** or the **avO2 difference**.
- To get 60 mL/beat, the cardiac output would need to be 6000 mL/min, which would mean an avO2 difference of 0.067 mL O2/mL, not 0.1 mL O2/mL.
*50 mL/beat*
- This stroke volume would result from an incorrect calculation of the **cardiac output**, potentially from a slight miscalculation of the **arteriovenous oxygen difference**.
- A stroke volume of 50 mL/beat at 100 bpm would mean a cardiac output of 5 L/min, requiring an avO2 difference of 0.08 mL O2/mL, which is not consistent with the given values.
Cardiac output determinants US Medical PG Question 5: Which factor most strongly influences coronary blood flow during exercise?
- A. Endothelin release
- B. Metabolic demand (Correct Answer)
- C. Myogenic response
- D. Neural regulation
- E. Baroreceptor reflex
Cardiac output determinants Explanation: **Metabolic demand**
- During exercise, increased **myocardial activity** leads to a higher demand for oxygen and nutrients, prompting a significant increase in coronary blood flow.
- Local release of **metabolites** such as adenosine, nitric oxide, and hydrogen ions causes powerful vasodilation of coronary arteries, closely matching blood supply to demand.
*Endothelin release*
- **Endothelin** is a potent vasoconstrictor and plays a role in regulating vascular tone, but its primary influence is not the immediate or strongest factor dictating increased coronary flow during exercise.
- While it can modulate flow, metabolic changes are the dominant driver for the rapid and substantial increases needed during exertion.
*Myogenic response*
- The **myogenic response** is an intrinsic property of vascular smooth muscle cells to contract when stretched (due to increased pressure) and relax when pressure decreases, helping to maintain relatively constant blood flow.
- This mechanism primarily contributes to **autoregulation** and flow stability, but it does not account for the massive increase in flow required by the heart during exercise.
*Neural regulation*
- **Neural regulation**, primarily sympathetic stimulation, increases heart rate and contractility, which indirectly increases metabolic demand.
- However, direct neural effects on coronary arteries can be complex (both vasodilation and vasoconstriction depending on receptor type), and the overriding control during exercise is typically metabolic.
Cardiac output determinants US Medical PG Question 6: A 70-year-old male presents for an annual exam. His past medical history is notable for shortness of breath when he sleeps, and upon exertion. Recently he has experienced dyspnea and lower extremity edema that seems to be worsening. Both of these symptoms have resolved since he was started on several medications and instructed to weigh himself daily. Which of the following is most likely a component of his medical management?
- A. Lidocaine
- B. Verapamil
- C. Carvedilol (Correct Answer)
- D. Aspirin
- E. Ibutilide
Cardiac output determinants Explanation: ***Carvedilol***
- The patient exhibits classic symptoms of **heart failure**, such as **dyspnea on exertion**, **orthopnea** (shortness of breath when he sleeps), and **lower extremity edema**.
- **Beta-blockers** like carvedilol are essential for managing **chronic heart failure** by reducing myocardial oxygen demand and improving cardiac function.
*Lidocaine*
- **Lidocaine** is primarily an **antiarrhythmic drug** used for acute treatment of **ventricular arrhythmias**, not for chronic heart failure management.
- It works by blocking sodium channels and has no direct benefit in addressing the underlying pathophysiology of heart failure.
*Verapamil*
- **Verapamil** is a **non-dihydropyridine calcium channel blocker** typically used for hypertension, angina, and supraventricular tachyarrhythmias.
- It can have **negative inotropic effects**, which are generally contraindicated or used with extreme caution in patients with **systolic heart failure** due to its potential to worsen cardiac function.
*Aspirin*
- **Aspirin** is an **antiplatelet agent** used for primary or secondary prevention of **atherosclerotic cardiovascular disease** (e.g., in patients with coronary artery disease).
- It does not directly manage the symptoms or pathophysiology of **heart failure** unless there is a coexisting ischemic etiology.
*Ibutilide*
- **Ibutilide** is an **antiarrhythmic drug** specifically used for the rapid conversion of **atrial flutter and atrial fibrillation** of recent onset to sinus rhythm.
- It is not a medication used for the long-term management of **heart failure** symptoms described in the patient.
Cardiac output determinants US Medical PG Question 7: Which neurotransmitter is primarily responsible for parasympathetic effects on heart rate?
- A. Norepinephrine
- B. Dopamine
- C. Acetylcholine (Correct Answer)
- D. Epinephrine
Cardiac output determinants Explanation: ***Acetylcholine***
- **Acetylcholine** is the primary neurotransmitter released by postganglionic parasympathetic neurons.
- It acts on **muscarinic receptors** (M2 receptors) in the heart to decrease heart rate.
*Norepinephrine*
- **Norepinephrine** is primarily associated with the **sympathetic nervous system**, increasing heart rate and contractility.
- It acts on **beta-1 adrenergic receptors** in the heart.
*Dopamine*
- **Dopamine** is a precursor to norepinephrine and epinephrine, and primarily functions as a neurotransmitter in the **central nervous system** and in regulating renal blood flow.
- While it can have cardiac effects, it is not the primary neurotransmitter for parasympathetic actions on heart rate.
*Epinephrine*
- **Epinephrine** (adrenaline) is a hormone released by the adrenal medulla and a neurotransmitter in the sympathetic nervous system, causing an **increase in heart rate** and contractility.
- It works through **beta-1 adrenergic receptors**, antagonistic to parasympathetic effects.
Cardiac output determinants US Medical PG Question 8: A 73-year-old woman presents to clinic with a week of fatigue, headache, and swelling of her ankles bilaterally. She reports that she can no longer go on her daily walk around her neighborhood without stopping frequently to catch her breath. At night she gets short of breath and has found that she can only sleep well in her recliner. Her past medical history is significant for hypertension and a myocardial infarction three years ago for which she had a stent placed. She is currently on hydrochlorothiazide, aspirin, and clopidogrel. She smoked 1 pack per day for 30 years before quitting 10 years ago and socially drinks around 1 drink per month. She denies any illicit drug use. Her temperature is 99.0°F (37.2°C), pulse is 115/min, respirations are 18/min, and blood pressure is 108/78 mmHg. On physical exam there is marked elevations of her neck veins, bilateral pitting edema in the lower extremities, and a 3/6 holosystolic ejection murmur over the right sternal border. Echocardiography shows the following findings:
End systolic volume (ESV): 100 mL
End diastolic volume (EDV): 160 mL
How would cardiac output be determined in this patient?
- A. 108/3 + (2 * 78)/3
- B. (160 - 100) / 160
- C. 160 - 100
- D. (160 - 100) * 115 (Correct Answer)
- E. (100 - 160) * 115
Cardiac output determinants Explanation: ***(160 - 100) * 115***
- **Cardiac output (CO)** is calculated as **stroke volume (SV) multiplied by heart rate (HR)**.
- **Stroke volume** is determined by subtracting the **end-systolic volume (ESV)** from the **end-diastolic volume (EDV)** (SV = EDV - ESV).
*(108/3 + (2 * 78)/3)*
- This formula represents the calculation for **mean arterial pressure (MAP)**, which is not directly used to determine cardiac output.
- **MAP** is approximated as (Systolic BP + 2 * Diastolic BP) / 3.
*(160 - 100) / 160*
- This formula calculates the **ejection fraction (EF)**, which is the fraction of blood pumped out of the ventricle with each beat.
- While **ejection fraction** is a crucial measure of cardiac function, it does not directly determine cardiac output.
*160 - 100*
- This calculation represents the **stroke volume (SV)** (EDV - ESV), which is the amount of blood ejected from the ventricle per beat.
- However, to get the **cardiac output**, stroke volume must be multiplied by the heart rate.
*(100 – 160) * 115*
- This calculation would result in a **negative stroke volume**, which is physiologically incorrect as stroke volume must be a positive value.
- **Stroke volume** is always calculated as the **end-diastolic volume minus the end-systolic volume**.
Cardiac output determinants US Medical PG Question 9: A 34-year-old male is brought to the emergency department by fire and rescue following a motor vehicle accident in which the patient was an unrestrained driver. The paramedics report that the patient was struck from behind by a drunk driver. He was mentating well at the scene but complained of pain in his abdomen. The patient has no known past medical history. In the trauma bay, his temperature is 98.9°F (37.2°C), blood pressure is 86/51 mmHg, pulse is 138/min, and respirations are 18/min. The patient is somnolent but arousable to voice and pain. His lungs are clear to auscultation bilaterally. He is diffusely tender to palpation on abdominal exam with bruising over the left upper abdomen. His distal pulses are thready, and capillary refill is delayed bilaterally. Two large-bore peripheral intravenous lines are placed to bolus him with intravenous 0.9% saline. Chest radiograph shows multiple left lower rib fractures.
Which of the following parameters is most likely to be seen in this patient?
- A. Increased cardiac output
- B. Increased mixed venous oxygen saturation
- C. Decreased pulmonary capillary wedge pressure (Correct Answer)
- D. Decreased systemic vascular resistance
- E. Increased right atrial pressure
Cardiac output determinants Explanation: ***Decreased pulmonary capillary wedge pressure***
- The patient presents with classic signs of **hemorrhagic shock** (hypotension, tachycardia, somnolence, abdominal bruising, thready pulses) due to trauma, likely involving the spleen or kidney given the left upper abdominal bruising and rib fractures.
- **Decreased pulmonary capillary wedge pressure (PCWP)** is expected in hypovolemic shock because it reflects left atrial and left ventricular end-diastolic pressure, which will be low due to reduced venous return and intravascular volume.
*Increased cardiac output*
- In **hemorrhagic shock**, the body attempts to compensate by increasing heart rate, but overall **cardiac output is typically decreased** due to profound reduction in preload (venous return) from blood loss.
- While heart rate is elevated, the stroke volume is severely diminished, leading to a net decrease in cardiac output despite compensatory efforts.
*Increased mixed venous oxygen saturation*
- **Mixed venous oxygen saturation (SvO2)** is generally **decreased in hemorrhagic shock** due to increased oxygen extraction by tissues.
- Inadequate oxygen delivery to the tissues forces them to extract more oxygen from the blood, leading to a lower SvO2.
*Decreased systemic vascular resistance*
- In **hemorrhagic shock**, the body activates compensatory mechanisms, including generalized **vasoconstriction**, to maintain blood pressure and prioritize blood flow to vital organs.
- This leads to an **increased systemic vascular resistance (SVR)**, not decreased, as reflected by the thready distal pulses and delayed capillary refill.
*Increased right atrial pressure*
- **Right atrial pressure (RAP)**, representing CVP, is typically **decreased in hemorrhagic shock** due to reduced circulating blood volume.
- A lower RAP indicates decreased venous return to the heart, a hallmark of hypovolemia.
Cardiac output determinants US Medical PG Question 10: A 21-year-old man presents to his physician for a routine checkup. His doctor asks him if he has had any particular concerns since his last visit and if he has taken any new medications. He says that he has not been ill over the past year, except for one episode of the flu. He has been training excessively for his intercollege football tournament, which is supposed to be a huge event. His blood pressure is 110/70 mm Hg, pulse is 69/min, and respirations are 17/min. He has a heart sound coinciding with the rapid filling of the ventricles and no murmurs. He does not have any other significant physical findings. Which of the following best describes the heart sound heard in this patient?
- A. Fourth heart sound (S4)
- B. Opening snap
- C. Third heart sound (S3) (Correct Answer)
- D. Second heart sound (S2)
- E. Mid-systolic click
Cardiac output determinants Explanation: ***Third heart sound (S3)***
- An **S3 heart sound** is a low-pitched sound heard during **rapid ventricular filling** in early diastole, immediately after S2.
- In young, healthy individuals, especially athletes, an S3 can be a normal physiological finding, representing rapid filling of a **compliant ventricle**.
*Fourth heart sound (S4)*
- An **S4 heart sound** occurs during **atrial contraction** against a stiff or non-compliant ventricle, just before S1.
- It is typically associated with conditions like **ventricular hypertrophy** or **ischemia** and is less likely to be a normal finding in a young, healthy individual.
*Opening snap*
- An **opening snap** is a high-pitched, crisp sound heard after S2, caused by the sudden opening of a **stenotic mitral** or **tricuspid valve**.
- It indicates valvular pathology, specifically **mitral stenosis**, and is not related to ventricular filling in a healthy heart.
*Second heart sound (S2)*
- The **S2 heart sound** represents the **closure of the aortic and pulmonary valves** at the end of ventricular systole.
- While it marks the beginning of diastole, it does not coincide with the rapid filling of the ventricles itself.
*Mid-systolic click*
- A **mid-systolic click** is typically associated with **mitral valve prolapse**, caused by the sudden tensing of the chordae tendineae or valve leaflets.
- It occurs during systole, not diastole, and is not related to ventricular filling.
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