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If you're still wondering why you should sign up, here's a sample of one of our 4000+ questions with full explanations.
Sample Question+Explanation
You are called to assess a 4600g newborn who was noted be cyanotic. Upon arrival, you note a bluish
discoloration of the patient's extremities, face, and trunk. An initial arterial blood gas analysis indicates a pAO2
of 40 mm Hg. Physical examination reveals a grade 2/6 systolic murmur and a loud second heart sound. The
chest radiograph reveals a normal sized heart and decreased pulmonary vascular markings. After placing the infant
under a hood containing 100% oxygen, his cyanosis improves, and you obtain the following arterial blood gas
sample:
pH = 7.36
pCO2 = 37 mm Hg
pAO2 = 101 mm Hg
HCO3- = 20
Base excess = -3
Which of the following is the most appropriate interpretation of this infant's diagnostic evaluation?
A. The normal pAO2 on 100% oxygen indicates that cyanosis is likely due to methemoglobinemia
B. The arterial blood gas suggests alveolar hypoventilation is the mechanism for cyanosis
C. There is increased pulmonary vascular resistance, leading to a right-to-left shunt at the atrial or ductal levels
D. There is a right-to-left shunt at the ventricular level, likely due to complex congenital heart disease
E. This is acrocyanosis, and is a normal finding in a young newborn
________________________________________________________________
Explanation
The correct answer is choice C.
This infant has persistent pulmonary hypertension of the newborn (PPHN). Remember that in fetal life, oxygenation occurs in the
placenta - the resistance in the pulmonary vascular bed is high, and blood is shunted away from the fetal lungs through the
ductus arteriosus and the foramen ovale. At birth, however, the pulmonary vascular resistance should drop, beginning with the
infant's first breath. This allows blood to enter the lungs to be oxygenated, and the foramen ovale and ductus arteriosus should
close within hours to days of birth.
In some infants, this process does not occur - the vascular resistance in the pulmonary bed remains high. Because blood follows
the path of least resistance, it is thus "easier" for the blood to pass through the ductus arteriosus or foramen ovale and enter the
systemic circulation, without ever passing through the lungs to be oxygenated. This causes hypoxemia and cyanosis.
PPHN can result from a variety of conditions, most notably meconium aspiration syndrome and congenital diaphragmatic hernia.
Infants of diabetic mothers also have an increased rate of PPHN - and this infant's large size suggests that may be the case in this
vignette.
Even if you didn't recognize that this was PPHN, the clues in the stem should have led you to that physiologic explanation. For
example, the chest x-ray shows decreased pulmonary vascular markings, indicating that blood is not ever entering the lungs. Also,
the single, loud S2 is a marker of pulmonary hypertension.
Methemoglobinemia (choice A) can cause cyanosis, but the pAO2 should improve markedly with the administration of 100%
oxygen. Classically, patients with methemoglobinemia will remain cyanotic even on 100% O2, even though their pAO2 is high.
Hypoventilation (choice B) can cause hypoxemia and cyanosis, but would also cause CO2 retention.
Complex congenital heart disease (choice D) could cause cyanosis. However, the improvement that this patient experienced with
100% oxygen suggests that this is not the case. Even with 100% oxygen, infants with cyanotic heart disease will seldom attain a
pO2 of greater than 100 mm Hg, or have a rise of greater than 10-30 mm Hg from their pre-hyperoxygenated baseline. Also, in
this case, there are clues in the chest x-ray and physical exam that suggest an alternate mechanism of hypoxemia.
Acrocyanosis (choice E) is a bluish discoloration of the extremities that is a normal finding in infants. It is not associated with
systemic hypoxemia. This infant's cyanosis is initially severe and pronounced, and associated with a markedly decreased pAO2.
Suggested References
Tingelstad J. Consultation with the specialist: nonrespiratory cyanosis. Pediatr Rev. 1999; 20:350-352.
MedicalExams.com is giving away 60 Free Challenging USMLE Step 1, 2 and 3 questions with full explanations to everyone who signs up to our site, no obligation!
We are also offering a coupon exclusive to this forum.
The coupon code is: fallspecial15 - It's good for 15% off of all exams!
If you're still wondering why you should sign up, here's a sample of one of our 4000+ questions with full explanations.
Sample Question+Explanation
You are called to assess a 4600g newborn who was noted be cyanotic. Upon arrival, you note a bluish
discoloration of the patient's extremities, face, and trunk. An initial arterial blood gas analysis indicates a pAO2
of 40 mm Hg. Physical examination reveals a grade 2/6 systolic murmur and a loud second heart sound. The
chest radiograph reveals a normal sized heart and decreased pulmonary vascular markings. After placing the infant
under a hood containing 100% oxygen, his cyanosis improves, and you obtain the following arterial blood gas
sample:
pH = 7.36
pCO2 = 37 mm Hg
pAO2 = 101 mm Hg
HCO3- = 20
Base excess = -3
Which of the following is the most appropriate interpretation of this infant's diagnostic evaluation?
A. The normal pAO2 on 100% oxygen indicates that cyanosis is likely due to methemoglobinemia
B. The arterial blood gas suggests alveolar hypoventilation is the mechanism for cyanosis
C. There is increased pulmonary vascular resistance, leading to a right-to-left shunt at the atrial or ductal levels
D. There is a right-to-left shunt at the ventricular level, likely due to complex congenital heart disease
E. This is acrocyanosis, and is a normal finding in a young newborn
________________________________________________________________
Explanation
The correct answer is choice C.
This infant has persistent pulmonary hypertension of the newborn (PPHN). Remember that in fetal life, oxygenation occurs in the
placenta - the resistance in the pulmonary vascular bed is high, and blood is shunted away from the fetal lungs through the
ductus arteriosus and the foramen ovale. At birth, however, the pulmonary vascular resistance should drop, beginning with the
infant's first breath. This allows blood to enter the lungs to be oxygenated, and the foramen ovale and ductus arteriosus should
close within hours to days of birth.
In some infants, this process does not occur - the vascular resistance in the pulmonary bed remains high. Because blood follows
the path of least resistance, it is thus "easier" for the blood to pass through the ductus arteriosus or foramen ovale and enter the
systemic circulation, without ever passing through the lungs to be oxygenated. This causes hypoxemia and cyanosis.
PPHN can result from a variety of conditions, most notably meconium aspiration syndrome and congenital diaphragmatic hernia.
Infants of diabetic mothers also have an increased rate of PPHN - and this infant's large size suggests that may be the case in this
vignette.
Even if you didn't recognize that this was PPHN, the clues in the stem should have led you to that physiologic explanation. For
example, the chest x-ray shows decreased pulmonary vascular markings, indicating that blood is not ever entering the lungs. Also,
the single, loud S2 is a marker of pulmonary hypertension.
Methemoglobinemia (choice A) can cause cyanosis, but the pAO2 should improve markedly with the administration of 100%
oxygen. Classically, patients with methemoglobinemia will remain cyanotic even on 100% O2, even though their pAO2 is high.
Hypoventilation (choice B) can cause hypoxemia and cyanosis, but would also cause CO2 retention.
Complex congenital heart disease (choice D) could cause cyanosis. However, the improvement that this patient experienced with
100% oxygen suggests that this is not the case. Even with 100% oxygen, infants with cyanotic heart disease will seldom attain a
pO2 of greater than 100 mm Hg, or have a rise of greater than 10-30 mm Hg from their pre-hyperoxygenated baseline. Also, in
this case, there are clues in the chest x-ray and physical exam that suggest an alternate mechanism of hypoxemia.
Acrocyanosis (choice E) is a bluish discoloration of the extremities that is a normal finding in infants. It is not associated with
systemic hypoxemia. This infant's cyanosis is initially severe and pronounced, and associated with a markedly decreased pAO2.
Suggested References
Tingelstad J. Consultation with the specialist: nonrespiratory cyanosis. Pediatr Rev. 1999; 20:350-352.
