More Than a Number: Implications of Hypoxemia in the Outpatient Management of Bronchiolitis

Column Author: Ron Palmen, MD | Internal Medicine-Pediatrics Resident

Column Editor: Kathleen Berg, MD, FAAP | Hospitalist - Pediatrics; Associate Professor of Pediatrics, University of Missouri-Kansas City School of Medicine; Clinical Assistant Professor of Pediatrics, University of Kansas School of Medicine

Bronchiolitis remains one of the most common diagnoses in ambulatory and acute care pediatric settings. Providers are tasked with recognizing which child with bronchiolitis requires hospitalization and which does not. Admission may be indicated for inadequate oral hydration, significantly increased work of breathing that does not improve with suctioning, or hypoxia. This article reviews evidence surrounding the role of pulse oximetry and implications of hypoxemia in the setting of bronchiolitis.  

The American Academy of Pediatrics recommends supplemental oxygen for consistent oxyhemoglobin saturations below 90%, but this recommendation comes with the caveat of low-level evidence (Quality: D; Recommendation Strength: Weak).¹ What happens when our patient’s saturation is 88% in comparison to 90%? Should this difference impact our decisions about supplemental oxygen or admission?   

Oxygen therapy clearly has a very important role in pediatric care, improving mortality since the use of early oxygen chambers. It has been a staple of care since the 1940s, before the ability to measure artificial arterial oxygen via oxyhemoglobin saturation. In the 1980s, pulse oximetry became routine practice, and the rate of hospitalizations for bronchiolitis increased dramatically, but interestingly without a reduction in mortality.^2,3  

As with any other therapy we provide our patients, we need to consider the risks, costs and benefits of supplemental oxygen. It is helpful to differentiate hypoxemia from hypoxia. Hypoxemia is defined as inadequate arterial oxygen (PaO2); we use oxyhemoglobin saturation (SpO2) as a surrogate. Standard PaO2 ranges from 60 mmHg to 80 mmHg, which correlates to an SpO2 of 90%-95% in a physiologically stable patient. Below 60 mmHg, the slope of the oxyhemoglobin dissociation curve becomes near linear, suggesting ease of oxygen loss from its hemoglobin carrier. Hypoxia, however, is defined as inadequate delivery of oxygen to tissues. The two concepts are intertwined yet separate. A person can have hypoxemia without hypoxia, but a person who has hypoxia will also have a level of hypoxemia.  

Hypoxia is more serious, with the potential to cause localized vasodilation, pulmonary vasoconstriction, metabolic acidosis, and tissue necrosis, and may result in brain injury. It is commonly caused by hypoxemia but also can be secondary to increased metabolic demand such as sepsis, decreased cardiac output, or changes in blood flow. The purpose of supplying additional oxygen is to improve hypoxemia that may lead to tissue hypoxia. Looking mathematically and at the factors at play, we can use this equation for oxygen delivery:  

Delivery of Oxygen = cardiac output x (Hb x SaO2 x 1.34) + (PaO2 x 0.003). 

PaO2 does play a role in the delivery of oxygen, but it’s not the full story. At the bedside, we use the SpO2 as a guide, but we need to respect this as a surrogate for hypoxemia, not hypoxia itself. Additionally, SpO2 is affected by a patient’s temperature, pH, arterial level of carbon dioxide and several other factors. Therefore, in these settings, we lose the accuracy of the surrogate hypoxemia measurements.  

Persistent hypoxemia over time leads to hypoxia and can have negative long-term impacts. Well-documented reviews show the impaired neurodevelopment of children with congenital heart disease who live with a significant level of hypoxemia.⁴ But there are risks with increased oxygen levels in our blood. Oxygen is an oxidizable element and can lead to oxidative stress and increase free radicals, causing further tissue damage and harm.  

How does this information apply to transient desaturations of SpO2 in those with bronchiolitis in the outpatient setting? Two studies from Toronto provide some insight. In one, patients with bronchiolitis were discharged from the emergency department with monitors. Fifty-nine of the 118 patients experienced desaturations with a median time of three minutes. A comparison of desaturation patients with non-desaturation patients showed no differences in hospitalization or clinical worsening.⁵ The other study artificially increased SpO2 readings seen by the providers by 3%. Those in the artificially evaluated SpO2 group had decreased hospitalizations within 72 hours and no increase in unplanned medical visits.⁶ These studies suggest that the decision to admit should not rest on SpO2 alone.  

Hypoxemia is one of many variables to consider when caring for a patient with bronchiolitis, and the limitations of pulse oximetry should factor into clinical decision-making. A child with bronchiolitis who is well hydrated with intermittent tachypnea and self-resolving desaturations to 88% should be triaged differently than an ill-appearing, poorly hydrated child with SpO2 of 91%. Current evidence supports that transient hypoxemia alone does not predict poor outcomes. Hypoxemia should be evaluated with the knowledge that our surrogate for PaO2 is less than perfect.  

References: 

1. Ralston SL, Lieberthal AS, Meissner HC, et al. Clinical practice guideline: the diagnosis, management, and prevention of bronchiolitis [published correction appears in Pediatrics. 2015 Oct;136(4):782]. Pediatrics. 2014;134(5):e1474-e1502. doi:10.1542/peds.2014-2742 
2. Shay DK, Holman RC, Newman RD, Liu LL, Stout JW, Anderson LJ. Bronchiolitis-associated hospitalizations among US children, 1980-1996. JAMA. 1999;282(15):1440-1446. doi:10.1001/jama.282.15.1440 
3. Shay DK, Holman RC, Roosevelt GE, Clarke MJ, Anderson LJ. Bronchiolitis-associated mortality and estimates of respiratory syncytial virus-associated deaths among US children, 1979-1997. J Infect Dis. 2001;183(1):16-22. doi:10.1086/317655 
4. Derridj N, Guedj R, Calderon J, et al. Long-term neurodevelopmental outcomes of children with congenital heart defects. J Pediatr. 2021;237:109-114.e5. doi:10.1016/j.jpeds.2021.06.032 
5. Principi T, Coates AL, Parkin PC, Stephens D, DaSilva Z, Schuh S. Effect of oxygen desaturations on subsequent medical visits in infants discharged from the emergency department with bronchiolitis. JAMA Pediatr. 2016;170(6):602-608. doi:10.1001/jamapediatrics.2016.0114 
6. Schuh S, Freedman S, Coates A, et al. Effect of oximetry on hospitalization in bronchiolitis: a randomized clinical trial. JAMA. 2014;312(7):712-718. doi:10.1001/jama.2014.8637