PEDIATRIC PUZZLER

PEDIATRIC PUZZLER

Cyanosis in a newborn babe: Using the blue ox

By Sean Devine, MD, Stanford G. Ewing, MD, Bradley W. Robinson, MD,and Walter W. Tunnessen, Jr., MD

A newborn infant has been transferred to your hospital for consultation.On arrival he is cyanotic, hypoxemic, tachypneic, and grunting; not a greatstart to life. Despite endotracheal intubation with 100% oxygen delivery,his percent oxygen saturation by pulse oximetry is only in the twenties.Normal saline is administered by bolus injection, and dopamine, 20 mcg/kg/min,is begun. The infusion of prostaglandin E1, which had been started beforetransfer, is increased to 0.1 mcg/kg/min. A portable chest X-ray is orderedand shows bilateral pneumothoraces, right greater than left (Figure 1).A chest tube is immediately placed with improvement in the infant's condition.The oxygen saturation improves to between 80% and 90%.

With the infant stable for the time being, you begin to consider thenext steps in diagnosis and management. As you prepare to speak to the fatherand look over the transfer note more thoroughly, the pulse oximeters catchyour eye. There seems to be a difference in oxygenation. The pulse oximeteron the right upper extremity is reading 76%, while the right lower extremityP_ox is 92%! Curious.

The history is of little help. The infant's mother is 23 years old, andthis was her first pregnancy. She had prenatal care since the eighth weekof gestation. Tobacco, alcohol, and intravenous drug use during pregnancyare denied. During the pregnancy the mother had two bladder infections andone bout of streptococcal pharyngitis, all treated successfully withoutcomplication. Family history is negative for diabetes and cardiac problems.The infant was born at 41 weeks gestation, weighing 4.94 kg. Chorionic membranesruptured six hours before delivery, and the fluid was meconium stained.The Apgar scores were 2 at 1 minute, 5 at 5 minutes, and 7 at 10 minutesafter birth.

An endotracheal tube was placed and then removed, since no meconium waspresent below the vocal cords. Despite an FiO₂ of 85%, the pulseoximeter reading was never more than 73%.

An arterial blood gas on 88% inspired oxygen showed a pH of 7.30, pCO₂53 mmHg, pO₂26mm Hg, bicarbonate 26 mmol/L, and oxygen saturation42%. The infant was immediately reintubated and prostaglandin E1, 0.05 mcg/kg/min,was started. A blood culture was obtained and antibiotics were begun. Arrangementswere made for the transfer to your hospital.

With the infant hooked up to a ventilator, you carefully perform a physicalexamination. His temperature is 36.5° C, heart rate 160 beats per minute,and blood pressure 73/51 mm Hg. The ventilator is set at 60 breaths perminute. The infant's chest appears symmetrical, and good air entry is heardon both sides. On examination of the heart, the first sound seems normal,but the second heart sound is single and loud. No murmurs are heard. Theonly other findings are weak femoral pulses and warm, dry, and dusky skin.

The initial laboratory studies, obtained on arrival, are called in shortlyafter you finish your examination. The complete blood count shows a WBCof 19,000/mL with a differential count containing 30% band forms. The hemoglobinis 15.6 g/dL, hematocrit 47.3%, and platelet count 174,000/mL. Serum electrolytesare normal. A repeat arterial blood gas on 100% inspired oxygen revealsa pH of 7.47, pCO₂ 26 mm Hg, pO₂ 36 mm Hg, and a bicarbonateof 19 mmol/L. An electrocardiogram shows a normal sinus rhythm, a normalQRS axis, and right ventricular hypertrophy. The repeat chest X-ray showsreexpansion of the lung fields with increased pulmonary vascularity anda normal-sized heart (Figure 2).

Axing out the possibilities

The next step is to consider the possible explanations for this infant'sdire circumstances. Three broad categories of possibilities come to mind:infectious, pulmonary, and cardiac. An infection could account for the significantleft shift in the white blood cell count and the cyanosis, but one wouldexpect more acidosis if circulatory collapse and tissue hypoperfusion wereresponsible. Nevertheless, blood cultures had been obtained and antibioticsstarted.

Pulmonary disorders certainly could be responsible for the infant's condition.Persistent pulmonary hypertension of the newborn (PPHN), pneumothorax, diaphragmatichernia, respiratory distress syndrome, and pulmonary hypoplasia all arepossibilities. A diaphragmatic hernia should have shown up on the chestX-ray, however, and the pneumothorax was corrected with the chest tube.The lungs do not appear to be hypoplastic. What about PPHN? Right-to-leftshunting through a patent ductus arteriosus may result from this problem.But postductal oxygenation should be lower than preductal oxygenation, andin this infant the postductal (lower extremity) oxygen saturation by pulseoximetry was higher than the preductal (upper extremity) saturation.

That leaves us with cyanotic congenital heart disease as the most likelyexplanation of the infant's clinical picture. The hyperoxic or oxygen challengetest, a key way to hone in on cyanotic congenital heart disease, has alreadybeen done. While breathing 100% oxygen, if the pO₂ rises above150 torr, cyanotic congenital heart disease may be excluded.¹This infant's pO₂ never rises above 36 torr on 100% oxygen. Cyanoticcongenital heart disease seems likely.

Now we need to concentrate on the possible causes of cyanotic congenitalheart disease. The chest X-ray shows increased pulmonary blood flow, thedifferential diagnosis of which includes d-transposition of the great arteries(d-TGA), truncus arteriosus, total anomalous pulmonary venous return (TAPVR),and hypoplastic left heart syndrome.²

• D-transposition usually presents with a single S2 on physical examination and an "egg on a string" appearance of the heart on chest radiograph.
• Truncus arteriosus usually presents with an ejection click and a diastolic murmur from truncal valve regurgitation.
• Obstructed TAPVR usually presents with right ventricle hypertension, a small heart, and pulmonary edema on chest X-ray.
• Hypoplastic left heart syndrome often presents with shock when the patent ductus arteriosus closes.

All of these cardiac lesions may show right ventricular hypertrophy onECG, so this finding is not helpful. Sorting through the possibilities maybe difficult.

The Blue Ox Special

The differential pulse oximetry readings between the upper and lowerextremities again comes to mind. As you recall, there are two physiologicconditions that cause higher oxygenation in the descending aorta than inthe ascending aorta: transposition of the great vessels and supracardiacTAPVR. In transposition, pulmonary venous return recirculates from the leftventricle back to the pulmonary artery, while systemic venous return circulatesfrom the right ventricle back to the aorta. To achieve adequate oxygen saturationfor tissue survival, blood must mix through an atrial septal defect, ventricularseptal defect, or patent ductus arteriosus. In this infant, prostaglandinE1 may be keeping the patent ductus arteriosus open, resulting in shuntingof blood with higher oxygen saturation from the pulmonary artery to thedescending aorta. This would explain the paradoxically higher oxygen saturationin the lower extremities (Figure 3). The right-to-left shunt through thepatent ductus arteriosus may be explained by persistent pulmonary hypertensionor a juxtaductal coarctation.

In supracardiac TAPVR, the higher oxygen saturation in the descendingaorta is the result of oxygenated blood streaming from the vertical veinto the superior vena cava. It moves through the tricuspid valve and rightventricle into the main pulmonary artery and then through the patent ductusarteriosus into the descending aorta.

Fortunately, echocardiography offers a noninvasive method of sortingout the various congenital heart abnormalities. The echocardiogram confirmsthe diagnosis of d-transposition of the great arteries with an intact interventricularseptum and bidirectional shunting through a large patent ductus arteriosus.

Now for management

Once the diagnosis of the congenital heart lesion is made, managementof the infant can be more directed. Prostaglandin E1 is used to maintainductal patency and to promote pulmonary venous return, which in turn increasesleft-to-right mixing at the atrial level. In order to maximize adequatemixing, a balloon atrial septostomy is performed. Without a septostomy,an infant may not survive because bidirectional shunting often does notpersist across the ductus.³ Unfortunately, these two therapeuticinterventions do not resolve the problem of elevated pulmonary vascularresistance. Hyperventilation, sedation, alkalization, and the administrationof nitric oxide may be used to reduce pulmonary vascular resistance priorto definitive surgical intervention.^4,5 The surgical treatmentof choice of simple transposition of the great arteries is an arterial switchprocedure or Jatene procedure. The aorta and pulmonary arteries are transectedcephalad to their valves and then anastomosed to the opposite stumps. Thecoronary arteries are also moved from the original aorta to the "neoaorta"(near the pulmonary valve).

D-transposition with an intact interventricular septum is found in approximately5% of infants born with congenital heart disease.⁶ This lesionhas not been associated with chromosomal abnormalities or other congenitalmalformations, although it is found more often in siblings of infants whohave the lesion than in the general population, suggesting heritability.There is a 2:1 male predominance. The lesion occurs more frequently in large-for-gestational-ageinfants, infants of diabetic mothers, and, possibly, in infants of mothersexposed to amphetamines, trimethadione, and sex hormones.⁵

Infants with d-TGA typically present in the first 24 to 48 hours of life,usually with profound cyanosis not associated with respiratory distress.This infant had marked cyanosis from birth because of persistent pulmonaryhypertension associated with the sharply increased pulmonary vascular resistanceand decreased pulmonary venous return. Persistent pulmonary hypertensionis an uncommon complicating factor in d-TGA, occurring in only 1% to 2%of cases with an intact ventricular septum.⁴ The characteristicclinical findings in d-TGA with pulmonary hypertension are a loud, singleS2--the result of a loud P2--and a right-to-left shunt at the ductal level.Radiologically, as noted earlier, the heart may give the appearance of an"egg on a string," and the pulmonary vascularity may be normalto increased.

The take-home message

Preductal and postductal oximetry can provide extremely helpful cluesin the diagnosis of cyanotic congenital heart disease. Some have advocatedpulse oximetry as the "fifth vital sign."⁷ The paradoxicaldifference in oxygenation of the upper and lower extremities in this infanthelped focus on the diagnosis of cyanotic congenital heart disease, and,in fact, suggested the diagnostic possibilities. In an emergent situation,before echocardiography can be performed, this observation may be helpfulin life-saving management. Applying this clue to diagnosis may make youa person of great stature and reputation among your peers--a Paul Bunyonwith the blue ox, so to speak!

DR. DEVINE is a first-year resident in medicine/ pediatrics at AlbertEinstein Medical Center, Philadelphia, PA.

DR. EWING is with the Division of Cardiology, Children's Hospital ofPhiladelphia.

DR. ROBINSON is with the Division of Pediatric Cardiology at St. Christopher'sHospital for Children, Philadelphia.

DR. TUNNESSEN, who serves as Section Editor for Pediatric Puzzler, isSenior Vice President, American Board of Pediatrics, Chapel Hill, NC, anda member of the Contemporary Pediatrics Editorial Board.

REFERENCES

1. Jones RWA, Baumer JH, Joseph MC, et al: Arterial oxygen tension andresponse to oxygen breathing in differential diagnosis of congenital heartdisease in infancy. Arch Dis Child 1976;51:667

2. Devine S, Anisman PC, Robinson BW: A basic guide to cyanotic congenitalheart disease. Contemporary Pediatrics 1998;15(10):133

3. Bricker JT: Transposition physiology, in Garson A Jr, Bricker JT,McNamara DG (eds): The Science and Practice of Pediatric Cardiology, Philadelphia,PA, Lea & Febiger, 1990, p 1173

4. Lucianai GB, Chang AC, Starnes VA: Surgical repair of transpositionof the great arteries in neonates with persistent pulmonary hypertension.Ann Thorac Surg 1996;61:800

5. Chang AC, Wernovsky G, Kulik TJ, et al: Management of the neonatewith transposition of the great arteries and persistent pulmonary hypertension.Am J Cardiol 1991;68:1253

6. Park MK: Cyanotic congenital heart disease, in Pediatric Cardiologyfor Practitioners, ed 3. St. Louis, MO, Mosby Year Book, 1996, p 176

7. Mower WR, Sachs C, Nicklin EL, et al: Pulse oximetry as a fifth pediatricvital sign. Pediatrics 1997;99:681