Cardiac Anatomy and Physiology

Cardiac Anatomy and Physiology

Brittany M. Graham

Joseph J. Maleszewski

Jonathan N. Johnson


1. Left ventricular (LV) isovolumic contraction continues until what cardiac event occurs?

A. Mitral valve opens

B. Passive atrial filling

C. Increased ventricular volume

D. Aortic valve opens

E. Aortic pressure greater than left ventricular

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1. (D) Flow-volume loops are highly likely to be tested on the board examination (see the ICU section of this book for further questions on flow-volume loops). At end diastole, LV filling is complete, and the mitral valve closes. There is a period of isovolumic contraction, after which the aortic valve opens secondary to a lower pressure in the aorta compared to the LV.

2. Many metabolic factors are responsible for regulating coronary arterial blood flow. Which of the following metabolic factors is derived from the breakdown of high-energy phosphates?

A. Prostaglandin

B. Nitric oxide

C. Endothelin-1

D. Adenosine

E. Vascular endothelial growth factor

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2. (D) Adenosine is produced from the breakdown of ATP, which cannot be regenerated at times of low oxygen tension. Therefore, at times of low oxygen tension, AMP is made and then further broken down into adenosine, which causes coronary artery vasodilation. Nitric oxide induces the cyclic guanosine monophosphate which causes muscle relaxation. Endothelin-1 causes tonic vasoconstriction. Prostaglandin induces smooth muscle relaxation.

3. Figure 1.1 was taken from which cardiac chamber?

A. Right atrium

B. Right ventricle

C. Left atrium

D. Left ventricle

E. Coronary Sinus

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3. (D) Fibrous continuity of the semilunar and atrioventricular valves is a feature of a morphologic left ventricle. The aortic valve cusps can be seen in the top of Figure 1.1 with mitral valve and associated chordae seen at the bottom of the image. Conversely, the morphologic right ventricle exhibits muscular separation of the semilunar and atrioventricular valves.

4. You are evaluating a toddler admitted to the pediatric service due to failure to thrive. There is concern that a vascular ring may be present. Upon completing an echocardiogram, you note that there is a right-sided arch. In combination with a right-sided aortic arch, which of the following would be concerning for a symptomatic vascular ring?

A. Mirror image branching

B. An aberrant right subclavian and diverticulum of Kommerell

C. An aberrant left subclavian and left-sided patent ductus arteriosus

D. Aberrant right subclavian with a left-sided diverticulum of Kommerell

E. An aberrant left subclavian with a right-sided patent ductus arteriosus

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4. (C) A vascular ring occurs when there is abnormal regression of the embryologic aortic arches. In the setting of a right-sided aortic arch, the combination of an aberrant left subclavian artery and left-sided patent ductus arteriosus will lead to a vascular ring. A diverticulum of Kommerell is specific to a bulbous outpouching of the aorta that appears at the origin of an aberrant subclavian artery. To determine whether there is a vascular ring present, you must know arch sidedness, aortic arch branching, and side/presence of PDA.

5. You diagnose a neonate with polysplenia. Which of the following is most likely to be true regarding this patient?

A. Limb-length abnormalities are present

B. There are multiple spleens on both the left and right sides

C. Multiple gallbladders are common

D. The SVC is interrupted

E. The IVC is interrupted, with azygos continuation to SVC

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5. (E) In polysplenia, the multiple spleens are typically located all on the same side of the vertebral column as the stomach. The gallbladder is typically single, but patients may have concurrent biliary atresia. The abdominal situs is variable and can be normal, mirror image, or indeterminate. The IVC commonly is interrupted with azygos continuation to the SVC.

6. In the mature cardiac myocytes, the majority of calcium involved in the binding of troponin C and thus the initiation of myocyte contraction is stored in which cellular space?

A. Extracellular space

B. T-tubule

C. Mitochondria

D. Lysosomes

E. Sarcoplasmic reticulum

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6. (E) In mature myocytes, the sarcoplasmic reticulum stores the most important source of calcium involved in the initiation of myocyte contraction. Calcium enters the myocyte during the action potential through L-type voltage-gated calcium channels. This calcium then activates the calcium release channel (also called the ryanodine receptor), causing release of calcium from the sarcoplasmic reticulum. In immature cardiac myocytes, the function and organization of the sarcoplasmic reticulum is not yet mature, and activation is more dependent on flow through the L-type calcium channels.

7. The term straddling may be applied to which of the following?

A. Semilunar valves

B. Atrioventricular valves

C. Both semilunar and atrioventricular valves

D. Neither semilunar nor atrioventricular valves

E. Valve of the fossa ovalis

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7. (B) Straddling is a feature of a valve’s insertion into the ventricles, which is only a feature of atrioventricular valves because of the tensor apparatus (tendinous cords and papillary muscles) necessary for their function. Overriding, a feature of the valve annulus, can be seen in both atrioventricular and semilunar valves.

8. A 15-year-old patient with hypoplastic left heart syndrome had a total cavopulmonary anastomosis (Fontan). In clinic, O2 saturations are much lower than anticipated. On imaging, there is no obstruction to flow at the level of the Glenn, Fontan, or Branch PAs. Which of the following is the most likely cause of the desaturations?

A. Venovenous collateral circulation

B. Aortopulmonary collateral circulation

C. Venoarterial collateral circulation

D. Aneurysmal arteriovenous malformation

E. Coronary fistula

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8. (A) Fontan or stage III palliation includes connection of IVC flow to the pulmonary artery, allowing all pulmonary arterial flow to be passive. Venovenous collaterals lead to direct flow of deoxygenated systemic blood into the pulmonary venous circulation.

Aortopulmonary collaterals develop when there is no sufficient pulmonary blood flow present, and do not result in hypoxemia.

9. A 12-year-old boy is diagnosed with mild aortic stenosis. You suspect that he has an abnormal aortic valve. Echocardiographic imaging demonstrates that his valve is similar to the following pathology image (Figure 1.2). Which of the following is the aortic cusp pattern in this patient?

A. Bicuspid valve with fusion of the right and noncoronary cusps

B. Quadricuspid valve with a cleft of the left coronary cusp

C. Unicuspid valve with fusion of more than one cusp

D. Bicuspid valve with fusion of the right and left cusps

E. Bicuspid valve with fusion of the left and noncoronary cusps

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9. (D) Of patients with a bicuspid aortic valve, by far the most common form is fusion of the right and left cusps (75%). The next most common are patients with fusion of the right and noncoronary cusps, followed by those with left and noncoronary cusp fusion. Fusion of more than one cusp can result in a unicuspid valve.

10. Which one of the following pairs is mismatched?

A. Right coronary artery near tricuspid annulus

B. Tricuspid valve direct septal cordal insertions

C. Right ventricle crista terminalis

D. Left ventricle fine apical trabeculations

E. Right atrium limbus of fossa ovalis

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10. (C) The crista terminalis is a right atrial structure, which represents the interface between the sinus portion of the atrium and the embryologic (trabeculated) atrium.

11. You are assessing a patient with coarctation of the aorta. The coarctation is discrete with no aortic arch hypoplasia. Surgical repair is planned via a lateral thoracotomy instead of median sternotomy. Which of the following would most accurately determine arch sidedness prior to surgical intervention?

A. Suprasternal notch echo view of the third branch of the aorta

B. Subcostal echo view demonstrating the relationship of the spine to the aorta

C. CT angiogram (CTA) demonstrating the branching pattern of the arch when compared to the branch pulmonary artery

D. Suprasternal notch echo view demonstrating the relationship between the aortic arch and left pulmonary artery

E. CT angiogram (CTA) demonstrating the relationship between the aortic arch and the bronchus

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11. (E) While arch sidedness can be determined by echocardiography, CTA allows for a more detailed assessment of the anatomy. Arch sidedness is critical to determine prior to surgical repair via lateral thoracotomy. This is because depending on the side of the arch, surgical planning will change. Whether or not an aortic arch is right or left sided is determined by which bronchus the aortic arch travels over. When assessing by echo, the suprasternal notch will demonstrate the direction the first aortic branch travels. The subcostal views do not allow you to determine arch sidedness.

12. Which of the following factors has the greatest impact on the pressure change across two points in a vessel?

A. Vessel radius

B. Blood viscosity

C. Vessel length

D. Maximum velocity of blood flow

E. Hemoglobin concentration

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12. (A) This question refers to the Poiseuille-Hagen relationship, where the resistance R between two points is a function of pressure and flow. This is ultimately described by the equation R = (8 × L × ε)/(π × r4), where the radius is raised to the fourth power (L = length of the vessel, ε = viscosity).

13. On gross inspection of a heart specimen, there is a defect adjacent to the interventricular component of the membranous septum. You note that there is aortic-tricuspid continuity. Which of the following is the most likely diagnosis?

A. Perimembranous VSD with inlet extension

B. Outlet muscular VSD

C. Anterior muscular VSD

D. Apical muscular VSD

E. Doubly committed VSD

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13. (A) There are different names for ventricular septal defects based on their anatomic location. Central perimembranous VSDs can have inlet or outlet components. If there is tricuspid-aortic continuity, then the VSD is a perimembranous inlet VSD. If the lesion extends toward the aortic valve, then it is a perimembranous outlet VSD.

Overall, think of VSDs as perimembranous, inlet, outlet, and muscular. “Central” is another name given in some textbooks to describe perimembranous defects. (See Figure 1.11.)

14. The normal left aortic arch is primarily derived from which embryologic aortic arch?

A. Fourth (IV) aortic arch

B. First (I) aortic arch

C. Second (II) aortic arch

D. Third (III) aortic arch

E. Sixth (VI) aortic arch

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14. (A) The majority of the aortic arch arises from the left fourth aortic arch, while the right fourth aortic arch gives rise to the proximal portion of the right subclavian artery. The pulmonary arteries and ductus arteriosus arise from the left sixth aortic arch.

15. Embryologically, the ductus arteriosus and the left pulmonary artery arise from the:

A. Left fourth (IV) aortic arch

B. Left sixth (VI) and fourth (IV) aortic arches, respectively

C. Left fourth (IV) and sixth (VI) aortic arches, respectively

D. Left sixth (VI) aortic arch

E. Right fifth (V) arch

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15. (D) The pulmonary arteries and ductus arteriosus arise from the left sixth aortic arch. The majority of the aortic arch arises from the left fourth aortic arch, while the right fourth aortic arch gives rise to the proximal portion of the right subclavian artery. The fifth aortic arch most typically involutes.

16. You diagnose a 5-day-old female infant with tetralogy of Fallot. Of the following defects, which is most likely to be seen concurrently on echocardiogram?

A. Right aortic arch

B. Aortic stenosis

C. Atrial septal defect or patent foramen ovale

D. Coarctation of the aorta

E. Mitral valve prolapse

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16. (C) An atrial septal defect or a patent foramen ovale is present in over 80% of patients with tetralogy of Fallot (occasionally termed the “pentalogy of Fallot”). Abnormalities of the left side of the heart are rare in patients with tetralogy. A right aortic arch occurs in around 25% of patients.

17. In contrast to a left-sided aortic arch, a right-sided aortic arch travels over the:

A. Left atrium

B. Right pulmonary artery

C. Right bronchus

D. Left pulmonary artery

E. Left bronchus

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17. (C) The laterality of the aortic arch is determined by the bronchus it travels over. Both left and right aortic arches travel over the right pulmonary artery. A left aortic arch will travel over the left bronchus and a right aortic arch over a right bronchus.

18. A 4-year-old patient is postoperative day 2 from surgical repair of a moderate to large-sized secundum atrial septal defect. There is a distinct high-frequency “scratching” sound heard throughout the cardiac cycle and diffuse ST elevation on EKG. Which of the following structures is responsible for these findings?

A. Parietal pleura

B. Pericardial reflection

C. Epicardium

D. Myocardium

E. Visceral pericardium

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18. (E) Postoperative atrial septal defect patients are at high risk of developing a friction rub (heard throughout the cardiac cycle). This is secondary to fibrinous pericarditis where fibrin is replaced by fibrovascular granulation tissue. This leads to adherence of the parietal and visceral layers of the heart. The visceral pericardium (epicardium) covers the heart and intrapericardial portions of the great vessels. The parietal pericardium is a strong sac that surrounds the heart.

19. In the patient mentioned in question 18, the fibrinous pericarditis goes untreated. Several months later the patient returns with difficulty breathing, lower extremity edema, and hepatomegaly. What is the underlying physiology leading to this clinical presentation?

A. Decreased systolic function due to fibrous deposition within the myocardium

B. Decreased diastolic function due to excessive strain

C. Increased afterload secondary to reduced stroke volume

D. Decreased diastolic function due to constriction

E. Increased afterload secondary to restriction

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19. (D) The parietal pericardium is a strong, flask-shaped sac that surrounds the heart. This sac limits diastolic dimensions.

In the setting of chronic pericarditis, there can be further stiffening of the pericardium leading to constriction and elevated ventricular end diastolic pressures.

20. A newborn female infant presents with cyanosis. The following echocardiogram is obtained (Figure 1.3). In patients with this anatomy, which of the following is the most common coronary arterial abnormality?

A. Intramural left coronary

B. Left anterior descending from the RCA

C. Single right coronary artery

D. Single left coronary artery

E. Left circumflex from the RCA

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20. (E) The echocardiographic image (see Figure 1.3) represents a patient with d-transposition of the great arteries. Note the immediate posterior course of the great vessel arising from the left ventricle, indicative of the vessel being a pulmonary artery. There is a ventricular septal defect also present in the image. Patients with transposition of the great arteries most commonly have normal coronary anatomy (67%), but approximately 16% of patients will have an anomalous circumflex coronary artery arising from the right coronary. The next most common abnormalities are a single right coronary artery and an inverted right coronary and left circumflex (inverted origins of the RCA and LCx but normal origin of the LAD from the anterior facing sinus).

21. How does atrial natriuretic peptide (ANP) work on the kidney?

A. Decreases tubular resorption of sodium

B. Activates vasopressin receptors

C. Inhibits ion exchange in the ascending loop of Henle

D. Inhibition of sodium resorption in the proximal tubule

E. Dilation of the efferent arteriole

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21. (A) ANP is released in response to stretch from either atrium. ANP works on the kidney by dilating the afferent arteriole and constricting the efferent arteriole, effectively increasing GFR. It also works on the distal tubules to decrease sodium resorption. ANP also has vasodilator and cardioinhibitory effects.

22. In a normal adolescent heart, the average ratio of ventricular septal thickness to left ventricular free wall thickness is:

A. 1.1

B. 2.9

C. 1.9

D. 0.6

E. 2.4

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22. (A) In normal hearts in the first two decades of life, the thickness of the ventricular septum and left free wall are similar (mean = 1.1, range: 0.8 to 1.4). This ratio increases slowly in adulthood and averages greater than 1.2 by age 70. It can also be affected in diseases of asymmetric hypertrophy such as hypertrophic cardiomyopathy. The average ratio between left and right ventricular thickness is 3 (range: 2 to 5). Due to high right-sided pressure in utero, this ratio is lower in fetuses and neonates.

23. Cardiac situs is determined by position of which structure?

A. Cardiac apex

B. Left atrium

C. Left ventricle

D. Right atrium

E. Right ventricle

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23. (D) The pathologic definition of cardiac sidedness (situs) is the position of the right atrium.

24. A 3-year-old boy with a history of a ventricular septal defect (VSD) presents to your office for follow-up. On examination, you hear a diastolic murmur at the apex. Echocardiography is performed (Figure 1.4). Prolapse of which aortic cusp is most likely causing the diastolic murmur?

A. Left

B. Anterior

C. Septal

D. Right

E. Noncoronary

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24. (D) The subarterial type of VSD (also called supracristal or infundibular) comprises 5% of VSDs at autopsy, but is significantly more common in Asian populations. Due to the location of the VSD, there is deficiency of the support structure below the aortic valve, with subsequent herniation of the right coronary leaflet through the defect. This may also occur in some patients with perimembranous defects.

25. You diagnose a neonate with asplenia. Which of the following is most likely to be present in this patient?

A. The liver is midline with two mirror-image left lobes

B. Descending aorta and IVC on the same side of vertebral column

C. The biliary tree is patent with multiple gallbladders

D. Stomach position is fixed to the right side

E. Normal rotation of the bowels

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25. (B) In most asplenia patients, the descending aorta and IVC will travel on the same side of the vertebral column. There is a high incidence of bowel malrotation, and the stomach can be located on the left, right, or midline. There is typically only one gallbladder, but it can be variable in position, depending on the site of the liver. Biliary atresia may occur. The liver is most commonly midline with two mirror-image right lobes.

26. A 16-year-old boy is a long-distance runner for his high school. He has been training all year preparing for the state track meet. How did his resting hemodynamics change from pre- to posttrained state?

A. Increased stroke volume

B. Increased heart rate

C. Decreased blood volume

D. Increased myocardial oxygen demand

E. Increased resting arterial blood pressure

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26. (A) Repetitive exercise for prolonged periods of time results in benefits on one’s cardiovascular health and increases an individual’s work capacity. Changes include increased blood volume and stroke volume and decreased heart rate, resting arterial blood pressure, and myocardial oxygen demand.

27. Which of these sites contains contractile cardiac myocytes?

A. Proximal aorta

B. Epicardium

C. Proximal main pulmonary artery

D. Proximal pulmonary veins

E. Distal inferior vena cava

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27. (D) The pulmonary veins contain cardiac myocytes instead of smooth muscle cells in the last 1 to 3 cm before insertion to the left atrium. This allows them to minimize retrograde flow during atrial systole. This can also be a source of atrial fibrillation, which is why pulmonary vein isolation procedures are often used in adults.

28. Identify the arrowed structure in Figure 1.5.

A. Levoatrial cardinal vein

B. Ligament of Marshall

C. Thebesian vein

D. Eustachian valve

E. Ligamentum arteriosum

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28. (B) The structure shown, a small fibrous ridge traveling anterior to the pulmonary veins and the left pulmonary artery, represents the vestige of the left-sided superior vena cava—otherwise known as the ligament of Marshall.

29. The valve of the fossa ovalis represents the remnant of which embryologic structure?

A. Septum primum

B. Septum secundum

C. Ostium primum

D. Ostium secundum

E. Septum spurium

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29. (A) The valve of the fossa ovalis is derived from septum primum, while the limbus of the fossa ovalis is derived from septum secundum.

30. A trauma patient is rushed to the OR due to concern for cardiac tamponade. He was in a rapid deceleration accident. Initially at the scene he was lucid, but coded upon arrival to the ED.

Which of the following is the most likely area of injury?

A. Main pulmonary artery

B. Terminal SVC

C. Ascending aorta

D. Descending aorta

E. Distal IVC

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30. (C) The ascending aorta, main pulmonary artery, and terminal SVC are considered “intrapericardial” structures. Injury to the proximal aorta would lead to a pericardial effusion due to the location of the vessel within the pericardium and not within the mediastinum. In deceleration injuries, the aorta is the most likely structure to be injured. IVC and descending Ao injuries would not lead to cardiac tamponade as these structures do not lie within the pericardium.

31. Which of the following factors would shift the O2 dissociation curve to the left?

A. Increased temperature

B. Increased pCO2

C. Increased 2,3-DPG

D. Increased pH

E. Increased fetal hemoglobin concentration

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31. (D) The hemoglobin-oxygen dissociation curve helps to understand the relationship between pO2 and oxygen saturations. Increasing the pH (alkalosis), decreasing temperature, and decreasing 2,3-DPG will shift the curve to the left; likewise, acidosis, increased temperature, and increasing 2,3-DPG shift the curve to the right.

32. A single sinoatrial node in a normal position is typically found in which of the following?

A. Left juxtaposition of the atrial appendages

B. Right atrial isomerism

C. Right juxtaposition of the atrial appendages

D. Left atrial isomerism

E. Situs inversus of the atria

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32. (C) In patients with right atrial isomerism, bilateral sinus nodes can be encountered. In left atrial isomerism, the sinus node can be absent or malpositioned. In left-sided juxtaposition of the atrial appendages, the sinus node is often displaced anteriorly or inferiorly. Left juxtaposition is associated more with abnormal ventriculoarterial connections, while right-sided juxtaposition is more commonly associated with simpler lesions, like atrial septal defects.

33. Systemic arteriolar vasodilation occurs in response to:

A. Decreased pCO2

B. Decreased H+

C. Decreased pO2

D. Decreased K+

E. Decreased Mg+

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33. (C) Specific tissues are able to regulate local blood flow in response to changing metabolic demands. A decrease in the pO2 causes a systemic arteriolar vasodilation, as the local tissues attempt to get more oxygen delivery through increased volume of flow. Similarly, increasing pCO2, increasing H+ (acidosis), or increasing K+ will cause local vasodilation. Some tissues will also release adenosine as a vasodilator in response to increased oxygen demand.

34. Which term best describes the type of defect that is characterized by large atrial septal and ventricular septal defects as well as a common atrioventricular valve, but with separate left and right orifices?

A. Partial AVSD

B. Intermediate AVSD

C. Complete AVSD

D. Transitional AVSD

E. Membranous VSD

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34. (B) An intermediate AVSD is a rare subtype of a complete defect where the common AV valve has separate left and right orifices. This is accompanied by a large primum ASD and inlet VSD, and the clinical picture is similar to complete AVSD. Partial AVSD consists of a septum primum ASD and cleft left AV valve anterior leaflet, but the left and right AV valves are separate. Transitional defects are a subtype of partial defects and include a small inlet VSD. See Figure 1.12.

35. Which chamber is shown in Figure 1.6?

A. Right atrium

B. Right ventricle

C. Left atrium

D. Left ventricle

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35. (B) Three characteristic features of a morphologic right ventricle are shown in this example: (1) direct septal cordal insertions onto the ventricular septum; (2) course apical trabeculations; and (3) muscular separation of the atrioventricular and semilunar valves (tricuspid and pulmonary, respectively, in this case).

36. Which fetal venous structure has the lowest oxygen saturation?

A. Ductus venosus


C. Left hepatic vein

D. Coronary sinus

E. Right pulmonary vein

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36. (D) The least saturated blood in the fetus is in the coronary sinus and the superior vena cava, the oxygen having been used by the head/brain or the myocardium. The inferior vena cava, left hepatic vein, and ductus venosus receive some or all of their flow from the umbilical vein, and thus will have higher oxygen saturations.

37. The direction in which blood flows through an ASD primarily is related to the:

A. Pulmonary vascular resistance

B. Systemic vascular resistance

C. Relative compliances of the left and right ventricles

D. Morphology of the eustachian valve

E. Redundancy of the atrial septum

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37. (C) The primary determinant of the direction of blood flow through an atrial septal defect is the relative compliances of the left and right ventricles. In the otherwise normal patient, the right ventricle will be more compliant than the left ventricle, with less resistance to filling from the right atrium, and thus left-to-right shunting across the ASD. The vast majority of patients with ASDs have a relatively normal pulmonary resistance.

38. A newborn male infant is diagnosed with hypoplastic left heart syndrome (HLHS). Of the following, which anatomic form of HLHS will most likely be seen in this patient?

A. Aortic valve atresia with patent mitral valve

B. Aortic valve stenosis with patent mitral valve

C. Aortic valve stenosis with mitral valve stenosis

D. Aortic valve atresia with mitral valve atresia

E. Aortic valve atresia with mitral valve regurgitation

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38. (D) Aortic atresia with mitral valve atresia occurs in 36% to 46% of patients with HLHS, compared to 13% to 26% of patients with aortic stenosis with mitral stenosis. About 20% to 29% of patients have aortic atresia with a patent mitral valve.

39. In the cardiac sarcomere, which of the following named features includes the entirety of the myosin contractile elements?

A. E-line

B. A-band

C. I-band

D. H-zone

E. Z-disk

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39. (B) The A-band, bisected by the M-line, contains all the myosin contractile elements of the sarcomere. The I-band, bisected by the Z-disk, contains purely actin elements of the sarcomere. The H-zone is a central subsection of the A-band that does not include the areas of myosin-actin overlap.

40. A patient with pulmonary atresia is undergoing an operation. While operating, the surgeon wants to find out if there is a remnant of the hypoplastic or atretic main pulmonary artery. Which of the following would be the most helpful landmark?

A. Coronary sinus

B. Transverse sinus

C. Fossa ovalis


E. Pericardial reflection

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40. (B) During surgery, the transverse sinus can be used to identify the ascending aorta which can then be used to help identify the remnant of the pulmonary artery.

The transverse sinus is a tunnel-shaped structure that runs between the anterior/superior walls of the atria and posterior/inferior to the great vessels.

41. Truncus arteriosus is diagnosed in a newborn male infant. Of the following, which truncal valve morphology are you most likely to find?

A. Unicuspid

B. Bicuspid

C. Quadricuspid

D. Pentacuspid

E. Sextacuspid

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41. (C) The truncal valve in truncus arteriosus is most commonly tricuspid (˜70%). The next most common form of truncal valve is quadricuspid (˜20%), followed by bicuspid (˜10%), pentacuspid (<1%), and unicommissural (<1%). The valve is in fibrous continuity with the mitral valve in all patients, but can also rarely be in fibrous continuity with the tricuspid valve.

42. The resting potential of which ion is primarily responsible for the baseline (phase 4) resting conductance of cardiac myocytes?

A. Calcium

B. Sodium

C. Potassium

D. Chloride

E. Magnesium

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42. (C) The IK1 potassium current is the dominant resting conductance of the myocyte. It keeps the myocyte negatively polarized around -85 mV, until an action potential arrives to activate the cell into phase 0. See Figure 1.13.

43. What is the most common location of the pulmonary artery in patients with truncus arteriosus?

A. Branch pulmonary arteries arise from posterior sides of the truncus

B. Branch pulmonary arteries arise from the lateral sides of the truncus

C. Main pulmonary artery arises from the truncus

D. Branch pulmonary arteries arise from the descending aorta

E. Main pulmonary artery arises from the innominate artery

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43. (C) 48% to 68% of patients with truncus have type I, with the main pulmonary artery arising from the left posterolateral aspect of the truncus just above the valve. 29% to 48% of patients have type II, with branch pulmonary arteries arising from the posterior surface of the truncus. 6% to 10% of patients have type III truncus, with branch pulmonary arteries arising from the lateral sides of the truncus. In type IV truncus, the branch pulmonary arteries arise from descending aorta.