Why Do Apnea, Bradycardia, and Desaturation Show Up Together in Preterm Infants?

It's 3am. The monitor alarms. You look up and the oxygen saturation is falling. A second later the heart rate drops too.

We call this an A/B/D event: apnea, bradycardia, desaturation. New nurses often treat those as three separate problems happening to show up at the same time. They're not. They're one event, and once you understand why they're connected, you stop chasing three numbers and start reading what the baby is actually doing.

First, what counts as apnea?

Not every pause in breathing is apnea of prematurity. The working definition most units use is a respiratory pause of 20 seconds or longer, or a shorter pause accompanied by bradycardia or desaturation. That second half matters. A 12-second pause in breathing with a heart rate that never budges is not the same clinical event as a 12-second pause that drags the heart rate into the 80s.

There's also a distinction in apnea type that changes what you do at the bedside:

• Central apnea: no respiratory effort, no airflow. Chest still.

• Obstructive apnea: the baby is trying to breathe, but something is blocking the airway. Chest is moving, air isn't.

• Mixed apnea: both, usually central first, then an obstructive component as the baby tries to recover.

A baby with an obstructed airway and a moving chest needs repositioning, not just stimulation. If you only glance at the monitor and never look at the chest wall, you'll miss which one you're dealing with.

Why do preterm babies do this in the first place?

The brainstem regulates breathing rhythm using chemoreceptors that respond to oxygen and carbon dioxide levels, and in a preterm infant that control system is still developing. The younger the gestational age, the less mature and less consistent that system is, especially during sleep. A large narrative review on this exact topic lays out how that immaturity shows up as apnea, periodic breathing, and intermittent hypoxemia, all on a spectrum tied to gestational age (Erickson, Dobson, & Hunt, 2021).

Why does the oxygen saturation fall next?

When effective breathing stops, fresh air stops reaching the alveoli. Oxygen transfer into the blood drops. Preterm infants desaturate fast because they're working with a smaller margin to begin with: less oxygen reserve, higher oxygen consumption relative to size, and less stable lung volume.

Let's talk a little more about lung volume. Functional residual capacity, the air left in the lungs after you exhale, acts as an oxygen reservoir between breaths. A smaller reservoir means a breathing pause hits the oxygen saturation faster. Research measuring FRC directly in preterm infants found an inverse relationship between FRC and how quickly SpO2 fell during a breathing pause, meaning babies with lower lung volumes desaturated faster during the same length of apnea (Tourneux et al., 2008). Every time you're deciding whether a baby needs a little PEEP or a slightly different position to protect their lung volume, this is the physiology behind that decision.

So, why does the oxygen saturation fall? Because ineffective breathing leads to less oxygen entering the blood, which leads to falling saturation.

Why does the heart rate follow?

If oxygenation falls during the event, the infant may develop bradycardia. That is why we often see the three findings together: a breathing pause, then a fall in oxygenation, then a fall in heart rate.

Most bradycardias are associated with both an apneic pause and a desaturation. When all three occurred, the fall in oxygenation generally begins before the bradycardia.

So, why does the heart rate drop? Pauses in breathing lead to falling oxygenation, which may lead to a falling heart rate.

That word "may" is important. Not every pause causes a meaningful desaturation or bradycardia. Not every bradycardia starts with apnea.

Why doesn't the monitor always show that order?

Sometimes the heart rate alarm goes off before you see the saturation drop on the screen. That does not necessarily mean the heart rate changed first. Pulse oximeters display an averaged value calculated over a window of time, so the settings on your monitor affect how fast a real change actually shows up on the screen. Poor signal from a wiggly baby or a loose probe muddies this further.

There are also events that don't start with breathing at all. Airway obstruction can cut off airflow while the baby still looks like they're trying to breathe. And vagal stimulation, the kind you can get from a deep suction pass, a vagal response to reflux, or even just handling, can drop the heart rate fast without any apnea leading up to it. If you assume every bradycardia means the baby stopped breathing first, you'll miss the ones that didn't.

The order the alarms go off in is a clue. It is not a diagnosis. Go look at the baby.

What should you be assessing?

The monitor tells you something changed. Your assessment tells you what changed and what this particular baby needs right now.

Look at the baby first:

• Is the chest moving? Is air actually moving?

• What's the color, tone, and responsiveness?

• Is head or airway position part of the problem?

• Are the ECG tracing and pulse ox waveform even reliable right now, or is it artifact?

Then look at context:

• Was this during sleep, during cares, during or right after a feed? Reflux-triggered events can be common in growing preemies.

• Did the baby recover on their own, or need stimulation or respiratory support to come back?

• Is this consistent with this baby's usual pattern, or does it look new?

Don't assume events are always due to apnea of prematurity. New, frequent, severe, or worsening events deserve a look at other contributors: infection, intracranial injury, anemia, thermal instability, or simply too much handling too close together.

This is also where caffeine fits into the picture. It's first-line prophylaxis for apnea of prematurity because it stimulates the respiratory center and improves chemoreceptor sensitivity, essentially waking up the part of the system that's still maturing. If a baby is on caffeine and still having frequent events, that's something to discuss with the team.

How do you explain this to parents?

You don't need to teach a family about chemoreceptors. Try something like:

"Since your baby was born early, the part of the brain that keeps breathing steady is still maturing. Sometimes there's a pause in breathing. When that happens, the oxygen level can drop, and the heart rate can slow down too. We watch closely for this and respond based on what your baby needs in the moment."

Then talk about this baby's event specifically: what you saw, what you did, how they recovered, and whether the team is tracking a change in the pattern. That specificity is what actually reassures a parent. "It's normal for preemies" doesn't.

In Summary,

Apnea, desaturation, and bradycardia show up together because breathing, oxygenation, and heart rate are physiologically wired to each other. The usual sequence is a breathing pause, then falling oxygenation, then a slowing heart rate. But it's not identical every time, and the exceptions (obstruction, vagal reflexes, monitor lag) are exactly why a solid assessment is so important. The monitor gives you numbers. Understanding the physiology, and actually looking at the baby, tells you what those numbers mean.

Want to get more comfortable connecting neonatal physiology to what you're seeing at the bedside, instead of just memorizing facts for the test? That's exactly what my RNC-NIC/CCRN-Neonatal certification review course is built to do. Check it out today!

References

Erickson, G., Dobson, N. R., & Hunt, C. E. (2021). Immature control of breathing and apnea of prematurity: The known and unknown. Journal of Perinatology, 41(9), 2111–2123. https://doi.org/10.1038/s41372-021-01010-z

Tourneux, P., Léké, A., Kongolo, G., Cardot, V., Dégrugilliers, L., Chardon, K., Storme, L., Krim, G., Libert, J. P., & Bach, V. (2008). Relationship between functional residual capacity and oxygen desaturation during short central apneic events during sleep in "late preterm" infants.Pediatric research,64(2), 171–176. https://doi.org/10.1203/PDR.0b013e318179951d