Oxyhemoglobin saturation (SaO2) measured by co-oximetry is better than arterial oxygen tension (PaO2) for assessing oxygenation in cyanotic patients, new research suggests.

These two markers, SaO2 (the percentage of oxygen-saturated hemoglobin molecules in the arterial blood) and PaO2 (the oxygen dissolved within the plasma), may influence treatment decisions, including medication, ventilator management, use of extracorporeal life support, and the need for catheterization or reoperation.

SaO2 is directly measured at most hospitals by co-oximetry, and when a co-oximetry SaO2 measurement (not just an estimate) is available, “the SaO2 should be used as the primary metric” for a cyanotic patient, corresponding author Dr. John Kheir, of Boston Children’s Hospital, told Reuters Health by email.

He said he and his colleagues undertook their study because many clinicians look at the PaO2 in isolation when assessing the efficacy of an intervention for a patient, usually because of an incomplete understanding of what SaO2 and PaO2 each measure.

A clear understanding of SaO2 and PaO2 is important, he said, because sometimes these markers of oxygenation provide dissimilar information, particularly in cyanotic patients.

SaO2 is the primary determinant of arterial oxygen content (approximately 90 percent to 95 percent of the oxygen is carried by hemoglobin), and it is also responsive to changes in factors such as temperature and pH, he explained. Instances in which the dissolved oxygen fraction represents an important component of arterial oxygen content are rare, especially in patients with cyanotic congenital heart disease.

“PaO2 is useful when one is trying to examine a diffusion gradient between two sites of known oxygen tension,” most commonly in patients with lung disease and a normal biventricular circulation and no intracardiac shunts, Dr. Kheir explained. “SaO2 is useful when one is trying to understand the oxygen-carrying capacity, particularly in patients with congenital heart disease.”

To study how well PaO2 and SaO2 are correlated in cyanotic patients, Dr. Kheir and colleagues extracted more than 70,000 arterial blood gas (ABG) values from 2,163 patients with univentricular, cyanotic congenital heart disease who were treated at Boston Children’s Hospital between 2006 and 2016.

“Within the clinical range that we studied, the correlation between PaO2 and SaO2 worsened with hypoxemia,” the researchers report in a research letter in JAMA Pediatrics, online August 21. “Narrow ranges of PaO2 (e.g., 30-35 mm Hg) were associated with wide ranges of SaO2 (e.g., 28 percent to 85 percent), with widely disparate prognostic and treatment implications,” they noted.

When the researchers assessed changes in the two measures in sequential ABGs, they found that the magnitude of change in SaO2 in the cyanotic range was substantially larger than the magnitude of change in PaO2. In serial ABGs, PaO2 and SaO2 often changed in opposite directions on the same ABGs.

This means that “a patient believed to be worsening on the basis of PaO2 may frequently have improved based on SaO2, and vice versa,” the researchers noted. Given these findings, “we believe that measured SaO2 provides a superior assessment of oxygenation in cyanotic patients,” the researchers said.

“I think it might be a good idea for experts to write a guideline around this, as this study suggests that there may be best practices in the assessment of oxygenation,” Dr. Kheir said. “One practical implication that we are considering is a change in the ‘critical lab call’ from a PaO2 threshold to an SaO2 threshold.”

The work is a “pretty elegant study” with conclusions that are not overstated, Dr. Jerry J. Zimmerman, a pediatric clinical care expert at Seattle Children’s Hospital and University of Washington School of Medicine, told Reuters Health by email.

“This is an important issue for infants with cyanotic heart disease who are frequently critically ill before surgical intervention,” said Dr. Zimmerman, who was not involved in the study.

He noted it wasn’t clear if the authors adjusted for type of hemoglobin (fetal versus adult) or just concentration of hemoglobin. “Fetal and adult hemoglobin bind oxygen quite differently, and the concentration of both are changing dynamically with normal development as well as with transfusion,” he said.

Ultimately, what is most important in terms of “assessment of oxygen” is determining if oxygen delivery is adequate, Dr. Zimmerman added.

“If oxygen delivery is inadequate, either by low arterial oxygen content or low cardiac output, an oxygen debt ensues that can be measured by increasing lactate (reflecting anaerobic metabolism) or . . . low central venous hemoglobin oxygen saturation (reflecting increased tissue oxygen saturation),” he said.