Persistent Hypoxemia in a Teen With Down Syndrome

CITATION

Upchurch A, Sharma S, Hadigal S, Wagner M, Berry R. Persistent hypoxemia in a teen with down syndrome. J Clin Sleep Med. 2019;15(10):1547–1549.

A 15-year-old female with Down syndrome and congenital heart disease (atrial septal defect, ventricular septal defect) presents for a diagnostic sleep study for evaluation of loud snoring. Patient has not had an adenotonsillectomy. The sleep technologist calls concerning a low arterial oxygen saturation (SpO₂) of 86% to 87% on 2 L per minute (lpm) of supplemental oxygen. Pertinent examination in clinic showed no cardiac murmur and mild digital clubbing. The technologist is very concerned that increasing the oxygen liter flow as high as 6 lpm resulted in no change in the patient’s SpO₂ (Figure 1). The patient was asymptomatic with respiratory rate of 16 breaths per minute. Apnea-hypopnea index measured during polysomnography was AHI 1.8 events/h overall.

Figure 1. Thirty second segment of patient’s sleep study demonstrating hypoxemia with good plethysmographic tracing from the pulse oximeter.

QUESTION:

What is the most likely cause of the low SpO₂?

ANSWER:

Right to left intracardiac shunt

DISCUSSION

Detailed review of the patient’s medical history found that she has an atrial septal defect with evidence of a bidirectional shunt along with severe pulmonary hypertension. A right to left shunt due to the atrial septal defect is the most likely cause of low SpO₂. Hypoxemia that does not respond to increases in supplemental oxygen is indicative of shunt physiology.¹

Approximately 45% to 50% of all people born with Down syndrome have some form of congenital heart disease (CHD).² CHD seen in Down syndrome typically consists of atrioventricular canal defects and ventricular septal defects.^2,3 However, other abnormalities such as atrial septal defects, patent ductus arteriosus, and Tetralogy of Fallot can also be seen. Uncorrected, these defects can lead to pulmonary hypertension and the potential for right to left intracardiac shunting as seen in this patient.

A right to left shunt occurs when blood flows from the right side of the heart to the left side without participating in gas exchange.¹ Shunt pathology can be secondary to an anatomic shunt, such as an intracardiac shunt or an arteriovenous malformation. However, there are also pulmonary shunts due to severe lung disease when blood flows past nonventilated alveoli as in the adult respiratory distress syndrome.¹ The defining feature of shunt pathology is failure for oxygenation to improve with increases in supplemental oxygen therapy.¹ Other causes of hypoxemia including low fraction of inspired oxygen (high altitude), hypoventilation, and ventilation-perfusion mismatch, show improvement with increased supplemental oxygen. If a low SpO₂ is due to lung disease (ventilation-perfusion mismatch) or hypoventilation this should be suspected from the patient’s medical history (chronic obstructive pulmonary disease, potent opioids). Hypoventilation can be evaluated using a measurement of CO₂ (end-tidal PCO₂, transcutaneous PCO₂).

Because the arterial oxygen saturation (SpO₂) rather than the partial pressure of oxygen (PaO₂) is measured in the sleep center, one must also consider situations with a low SpO₂ but normal PaO₂ or an inaccurate oximetry reading. Abnormal hemoglobins with a rightward shift of the oxyhemoglobin dissociation curve (sickle cell disease) can result in an abnormal SpO₂ when the PaO₂ is normal.⁴ Methemoglobinemia may result in an inaccurate SpO₂ reading.⁵ Evaluation of the accuracy of the pulse oximeter measurement can be assessed by observing an adequate plethysmographic waveform (Figure 2) and making certain the patient does not have nail coverings that interfere with the function of the probe. Troubleshooting by the sleep technologist might include moving the oximetry probe to another finger, changing the pulse oximeter probe, or checking their own saturation with the current probe.

Figure 2. Two eight second segments are shown from the same patient.

The segment on the left demonstrates hypoxemia with poor plethysmographic oximeter tracing and the segment on the right demonstrates normal oxygenation in the same patient with a robust plethysmographic oximeter tracing.

The patient reported here had persistent hypoxemia due to an intracardiac right to left shunt through an atrial septal defect along with significant pulmonary hypertension. Increasing the amount of supplemental oxygen did not result in improvement in her SpO₂ because blood flow through the atrial septal defect by passed oxygenation the lungs.

SLEEP MEDICINE PEARLS

• 1.Hypoxemia that fails to respond to supplemental oxygen is consistent with shunt pathology.
• 2.When evaluating an asymptomatic patient with a decreased SpO₂, troubleshooting measures include:

  • ensuring there is no nail polish on the patient’s finger

  • switching location of the probe

  • testing the pulse oximetry device on another to verify the device’s accuracy

  • evaluating the plethysmographic waveform to ensure a good reading

  • replacing oximetry probe

• 3.Ensure accurate capnography reading and add transcutaneous CO₂ monitoring if end tidal wave form is not accurate.
• 4.In patients with hypoxemia review the patient’s history for additional causes including lung disease, heart disease with intracardiac shunt, or medications causing methemoglobinemia.

DISCLOSURE STATEMENT

All authors have seen and approved the manuscript. Work for this study was performed at the University of Florida in Gainesville, Florida. Dr. Berry reports financial support in the form of research grants from Phillips Respironics and ResMed and serves as a research consultant for ABio Clinical Research Partners, LLC. The other authors report no conflicts of interest.