Normal pulse oximeter reading in a cyanotic infant

Diagnostic considerations in infants and children with cyanosis

Cyanosis is defined by bluish discoloration of the skin and mucosa. It is a clinical manifestation of desaturation of arterial or capillary blood and may indicate serious hemodynamic abnormality. The goal of this article is to help the reader understand the etiology and pathophysiology of cyanosis and to formulate an approach to its differential diagnosis.

Laboratory assessment of oxygenation in methemoglobinemia

BACKGROUND

This case conference reviews laboratory methods for assessing oxygenation status: arterial blood gases, pulse oximetry, and CO-oximetry. Caveats of these measurements are discussed in the context of two methemoglobinemia cases.

CASES

Case 1 is a woman who presented with increased shortness of breath, productive cough, chest pain, nausea, fever, and cyanosis. CO-oximetry indicated a carboxyhemoglobin (COHb) fraction of 24.9%. She was unresponsive to O(2) therapy, and no source of carbon monoxide could be noted. Case 2 is a man who presented with syncope, chest tightness, and signs of cyanosis. His arterial blood was dark brown, and CO-oximetry showed a methemoglobin (MetHb) fraction of 23%.

ISSUES

Oxygen saturation (So(2)) can be measured by three approaches that are often used interchangeably, although the measured systems are quite different. Pulse oximetry is a noninvasive, spectrophotometric method to determine arterial oxygen saturation (S(a)O(2)). CO-oximetry is a more complex and reliable method that measures the concentration of hemoglobin derivatives in the blood from which various quantities such as hemoglobin derivative fractions, total hemoglobin, and saturation are calculated. Blood gas instruments calculate the estimated O(2) saturation from empirical equations using pH and Po(2) values. In most patients, the results from these methods will be virtually identical, but in cases of increased dyshemoglobin fractions, including methemoglobinemia, it is crucial that the distinctions and limitations of these methods be understood.

CONCLUSIONS

So(2) calculated from pH and Po(2) should be interpreted with caution as the algorithms used assume normal O(2) affinity, normal 2,3-diphosphoglycerate concentrations, and no dyshemoglobins or hemoglobinopathies. CO-oximeter reports should include the dyshemoglobin fractions in addition to the oxyhemoglobin fraction. In cases of increased MetHb fraction, pulse oximeter values trend toward 85%, underestimating the actual oxygen saturation. Hemoglobin M variants may yield normal MetHb and increased COHb or sulfhemoglobin fractions measured by CO-oximetry.

An approach to diagnosis and management of cyanosis and tachypnea in term infants

Tachypnea and cyanosis in the newborn are frequently encountered problems in the nursery. The incidence of respiratory distress ranges from 2.9% to 7.6%, and 4.3% of newborns may require supplemental oxygen therapy. In this article, the pathophysiology,approach to the diagnosis, and management of clinical conditions are discussed. The physiologic mechanisms involved in the pathogenesis of these problems and the clinical assessment and early stabilization of an infant before transport to a tertiary care center, if necessary, are also discussed.