Identify the etiology of cyanosisin the newborn
Identify the etiology of cyanosisin the newborn: Excerpt from Avoiding Common Pediatric Errors
Author:
Russell Cross, MD
What to Do - Interpret the Data
The detection of persistent cyanosis in the neonate can be difficult because
of the changing physiology in the first several hours following delivery, but
it is an important marker for a number of pathologies. Cyanosis, a blue discoloration of the skin, results from an increased concentration of reduced
(deoxygenated) hemoglobin, typically about 3 g/dL, in the capillary bed.
The amount of reduced hemoglobin is a function of the S-shape of the
hemoglobin-dissociation curve, which is in turn influenced by the concentration of hemoglobin, the amount fetal hemoglobin present, abnormalities
in the hemoglobin itself, and other physiologic parameters such as temperature, pH, and pCo2. The actual percent oxygen saturation at which
cyanosis becomes clinically evident is widely variable, depending on the factors mentioned above, but on average cyanosis is evident when the oxygen
saturation less than low 80s. From a clinical standpoint, cyanosis results
from either a reduction of arterial oxygen saturation, an increased extraction of oxygen at the capillary level, or an abnormality in the hemoglobin
itself.
Focusing on pathologies that result in a reduced arterial oxygen saturation, the causes are straight-forward; either there is diminished oxygen
exchange at the alveolar level or desaturated blood is bypassing the alveolus altogether. Diminished oxygen exchange in the alveolus can result from
inadequate ventilation, obstruction to airway flow, or abnormalities in the
alveolar wall that result in poor exchange of gases. Examples of the latter
include pneumonia, meconium pneumonitis, pulmonary edema, and cystic
fibrosis. Abnormalities that result in desaturated blood bypassing the alveolus include an intracardiac right-to-left shunt or an intrapulmonary shunts.
Intracardiac shunts may result from either from cyanotic congenital heart
disease (CHD) or pulmonary hypertension resulting in a right-to-left shunt
at the ductal or intracardiac level. Intrapulmonary shunts can arise from
an arteriovenous malformation or fistula, for example. The clinician most
frequently is faced with distinguishing whether cyanosis is a result of pulmonary or cardiac disease but should remember that other less causes of
cyanosis also exist.
As with any other diagnosis, details of the history may hold important
clues as to the cause of cyanosis in the newborn. For instance, knowing
whether there was meconium at delivery and the adequacy of airway suctioning in the delivery room can help to raise the suspicion of meconium
aspiration syndromeandresulting pulmonary hypertension. Other perinatal
history such as time of membrane rupture, presence of maternal fever, and
maternal infectious disease history may increase the suspicion for neonatal
pneumonia or sepsis. The onset of cyanosis may also be helpful. For instance, at 12 to 24 hours of life in the absence of other symptoms, cyanosis
is may result from CHD that becomes evident when the patent ductus
closes.
Thoroughpulmonaryand cardiovascularexaminations areimportant in
differentiating the causes of cyanosis. Patients who have signs of respiratory
distress such as tachypnea, along with grunting, nasal flaring, or intercostal
retractionsare morelikelytohave apulmonarycauseforcyanosis.The respiratory distress in these patients is evidence of the body's attempt to increase
alveolar ventilation. Patients with cyanotic CHD may be tachypneic, but
they typically do not have other signs of respiratory distress. Patients with
a pulmonary cause for cyanosis will also be more likely to have abnormal
breath sounds. On cardiac auscultation, attention should be focused on evaluation of the second heart sound in addition to screening for the presence of
a murmur. The second heart sound normally has splitting that varies with
inspiration, and the presence of a single second heart sound may indicate
severe stenosis or absence of either the aortic or pulmonary valve. A loud
second heart sound is also evidence for pulmonary hypertension. Likewise,
presence of a murmur may indicate CHD.
Evaluation of the distribution of the cyanosis is also important. Patients
may have "differential cyanosis" as a result of pulmonary hypertension or
certain cyanotic CHD. In the case of pulmonary hypertension with a patent
ductus and a right-to-left shunt, there may be diminished oxygen saturation
in the lower half of the body as a result of shunting of deoxygenated blood
from the right ventricle being shunted through the ductus into the descending aorta. In contrast, patients with transposition of the great arteries may
have "reverse differential cyanosis" with the upper half of the body having
lower oxygen saturation. This is because the deoxygenated blood leaving
the right ventricle passes into the ascending aorta, first reaching the upper
body vessels. When the aortic blood flow passes the patent ductus, some
mixing occurs with the more fully saturated blood in the pulmonary artery,
increasing the oxygen saturation of the blood delivered to the lower half of
the body.
In clinical practice, the differentiation between pulmonary and cardiac
causes of cyanosis can be difficult. An easy test that can help to make the
differentiation is the hyperoxia challenge. The hyperoxia challenge involves
placingthe patient on 100%oxygen for 20 minutes and observing the oxygen
content (PaO2) before and after. When cyanosis is caused by a fixed shunt
outside the alveolar level of the lungs (e.g., cyanotic CHD), the arterial pO2
will not increase significantly on 100% oxygen because the blood that is
bypassing the lung has no opportunity for increased oxygen absorption. In
contrast, when the cyanosis results from lung disease at the capillary level,
an increased alveolar O2 content will result in increased diffusion across the
alveolar-capillary interface, thereby increasing arterial pO2. Pulmonary hypertension with a patent ductus can complicate the test because the cyanosis
in this setting is created by right-to-left shunt across the patent ductus arteriosus (external to the lung). With the application of 100% O2, there can be
some diminishment in the pulmonary hypertension, which both increases
the amount of blood going to the lungs and diminishes the ductal right-to
left shunt. The effect of both of these is to increase arterial oxygen content
to a level intermediate to that typically seen with cardiac compared to lung
disease.
Cyanotic CHD can be subdivided into two physiologic groups: those
that have obstruction to pulmonary blood flow and those that have normal to increased pulmonary blood flow but with obligate mixing of venous
and arterial blood. Examples of CHD with obstruction to pulmonary blood
flow include tetralogy of Fallot, pulmonary stenosis or atresia, and certain
forms of tricuspid atresia and Ebstein malformation. These types of cyanotic heart disease may be "ductal dependent" if they require ductal patency
in order to provide adequate pulmonary blood flow. Ductal patency is ensured by the use of intravenous prostaglandins. Examples of CHD for which
there is normal to increased pulmonary blood flow with obligate mixing
are transposition of the great arteries and truncus arteriosus. In transposition of the great arteries, the aorta and pulmonary artery are switched and
arise from the incorrect ventricle. The degree of cyanosis in this lesion is
a function of the amount of mixing that occurs between the systemic and
pulmonary circulation. Patients with transposition may also be "ductal dependent" in the sense that they require ductal patency to provide adequate
mixing of venous and arterial blood. In truncus arteriosus, there is a single
great vessel arising from the heart which becomes the aorta and from which
the pulmonary arteries arise. This anatomy creates obligate mixing in the
heart which results in cyanosis. Other examples of cyanotic CHD resulting
from intracardiac mixing include total anomalous pulmonary venous return,
hypoplastic left heart syndrome, and certain forms of double outlet right
ventricle.
Suggested Readings
Kuehl KS, Loffredo CA, Ferencz C. Failure to diagnose congenital heart disease in infancy.
Pediatrics. 1999;103(4 Pt 1):743–747.
Reich JD, Miller S, Brogdon B, et al. The use of pulse oximetry to detect congenital heart
disease. J Pediatr. 2003;142:268–272.
Tingelstad J. Consultation with the specialist: nonrespiratory cyanosis. Pediatr Rev.
1999;20:350–352.
Book Source Details
- Book Title: Avoiding Common Pediatric Errors
- Author(s): Anthony D Slonim MD, DrPH; Lisa Marcucci MD
- Year of Publication: 2008
- Copyright Details: Avoiding Common Pediatric Errors, Copyright © 2008 Lippincott Williams & Wilkins.
More About Neonatal Respiratory Distress Syndrome
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Copyright notice for book excerpts: Copyright © 2008 Lippincott Williams & Wilkins. All rights reserved.
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More About This Book:
Title: Avoiding Common Pediatric Errors
Authors: Anthony D Slonim MD, DrPH; Lisa Marcucci MD
Publisher: Lippincott Williams & Wilkins
Copyright: 2008
ISBN: 0-7817-7489-6
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