Deep Breathing Test and E/I Ratio: Normal Values by Age and What It Means

How Is the Deep Breathing Test Done?

You sit quietly and breathe deeply and evenly at 6 breaths a minute, 5 seconds in and 5 seconds out, with a guide to keep the pace. The highest and lowest heart rate in each 10 second breath are measured. The average difference over 3 breaths in a row gives the heart rate swing in beats a minute (Barkoula, 2026), and the E/I ratio expresses the same swing as a ratio. Some reports list that difference in beats a minute instead of the ratio, so check which one yours shows. The paced part usually runs about a minute, 6 full breaths (Wiklund, 2018).

Deep breathing belongs to the standard autonomic battery with the Valsalva maneuver and the tilt test. A review calls it "an important part of the battery of cardiovascular autonomic function tests used in clinical autonomic laboratories" (Shields, 2009), and a 2026 study of 223 people with POTS ran heart rate response to deep breathing, Valsalva and a 10 minute head-up tilt as standard reflex testing (Trout, 2026). Our team runs it on the front end of a tilt test, so we have our bearings before the tilt starts. For the rest of the battery, see autonomic testing for POTS.

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What Does the E/I Ratio Measure?

E/I stands for expiration over inspiration. Your heart beats a little faster as you breathe in and a little slower as you breathe out (Novak, 2011), and the E/I ratio turns that swing into 1 number: the average of the longest gap between beats on each breath out divided by the shortest gap on each breath in (May, 2000). That's the same as the fastest heart rate on the breath in divided by the slowest on the breath out, so the ratio sits above 1, and the bigger the swing, the higher it goes.

The swing is run by the vagus nerve. Heart rate variability with deep breathing is "a highly sensitive measure of cardiovagal or parasympathetic cardiac function" (Shields, 2009), and cardiovagal means the vagus nerve's control of the heart.

Our team explains the reflex with a reflex arc drawing from a textbook by autonomic neurologist Peter Novak. Stretch sensors in the lungs, and pressure sensors in the carotid arteries of the neck and in the aortic arch, report each breath and each heartbeat through the glossopharyngeal and vagus nerves to an area of the brainstem called the nucleus tractus solitarius, or NTS. The brainstem uses that steady report to set the heart's rate to the filling of each breath, and the vagus nerve carries the answer back to the heart.

Wearable rings, bands and smartwatches report heart rate variability, often as RMSSD. In a lab study of people with POTS and neurogenic orthostatic hypotension, RMSSD and other time measures correlated with the deep breathing result (Baker, 2018a). That study used clinic ECG recordings, not wearables. In our team's view, the deep breathing test reads the heart and breath relationship on demand, under controlled breathing, in about a minute.

What's a Normal E/I Ratio by Age?

Normal values fall as you get older. A 2026 systematic review of the Ewing battery found "a clear age-related decline" in E/I values, which it ties to "age-related vagal withdrawal", and the cutoffs studies used varied widely. The review found that norms are "influenced by age and sex" and concludes that "each laboratory should establish its own normative data reflective of the specific population it serves" (Barkoula, 2026).

For labs that don't have their own norms, the review proposes these age-based lower limits for the E/I ratio, selected from a study of 90 people with diabetes and 20 controls.

Age	Lowest normal E/I ratio
29 or younger	1.24
30 to 39	1.19
40 to 49	1.15
50 to 60	1.11
61 to 70	1.08
71 to 80	1.05

The clinic's target: our team wants to see an E/I ratio above 1.25. That's our working goal for a strong reflex, kept separate from the published limits above. A result between your age's lower limit and 1.25 is within published norms. In our team's view, a result in that range is a reason to read it alongside your other findings, and on its own it doesn't make a diagnosis.

What Does an Abnormal Deep Breathing Result Mean?

A low ratio means the heart isn't swinging with the breath as much as it should, which points to a weaker cardiovagal reflex. In our team's practice, age, breathing that isn't deep or even, an irregular heart rhythm and some heart rate medications are noted before the result is read, since each can lower it. Shields calls the test "a reliable and sensitive clinical test for early detection of cardiovagal dysfunction in a wide range of autonomic disorders" (Shields, 2009). A single low test doesn't make a diagnosis. The Ewing battery performed best when read with the classic rule of at least 2 abnormal results out of 5 tests, "particularly when age-adjusted cutoffs were applied" (Barkoula, 2026).

A normal number can mislead too. In people with hereditary transthyretin amyloidosis, some recordings had "age-adjusted HRV scores within normal limits but poor regularity", and the authors conclude that autonomic function "cannot be evaluated" from that pattern, because in that group the heart rate swings often reflected a rhythm problem (Wiklund, 2018). An irregular rhythm can inflate the number in anyone, so our team reads the trace itself along with the number.

In a study of 25 people with POTS and 134 healthy young adults, the rise in heart rate during deep breathing didn't differ significantly between the groups, though the healthy group's heart rate variability measures were higher (Baker, 2018b). So a deep breathing result on its own can't confirm or exclude POTS.

In our team's view, an abnormal arc shows that something in the pathway is off, and it doesn't say what. So our team checks other outputs of the same brainstem area around the NTS. When you open wide and say "ah", the soft palate should rise, and rise evenly on both sides. A palate that lifts weakly, or lifts less on 1 side, is a correlate worth noting. Changes in your voice, or in how well you can stick out your tongue or articulate with it, narrow the area down further. These bedside checks are covered in the dysautonomia physical exam.

How Does the Deep Breathing Test Fit With Tilt Testing?

A tilt test shows what your heart rate and blood pressure do when you're brought upright, while deep breathing is a seated test of the vagus nerve's control of the heart (Shields, 2009). In the study of 25 people with POTS and 134 healthy young adults (Baker, 2018b), the heart rate variability time measures were "significantly reduced" during head-up tilt in both groups. For the tilt itself, see the tilt table test guide.

Here's how our team used the pair in 1 patient. This person's E/I ratio was 1.1, below the clinic's 1.25 target. They also lived with constant dizziness and a lot of trouble with motion, from riding in a car to moving through the world. On a subjective visual vertical test, where you set a line to straight up inside dark VR goggles, their average was minus 5 degrees, against the 2 to 3 degree window our team expects. That test shows how well the visual system and the vestibular system work together, and in our team's reading, they meet in the same area of the brainstem as the breathing reflex. In our team's reading, several findings now pointed to 1 region.

This person had already tried treatment to bring the heart rate down, and our team's view is that raising blood pressure alone wouldn't have solved it either. Medications aimed at heart rate and blood pressure can ease symptoms. Their ceiling is that they don't reach a control problem in the brainstem, which is how our team read this case. In our team's view, the high heart rate in POTS is a compensation, the body's rescue for brain blood flow, so our team treated it as a secondary finding and looked for what was driving it. This is 1 patient, shared to show how our team reasons through a result. For more on that idea, see why your heart races when you stand up, and for the type of POTS where norepinephrine runs high, see hyperadrenergic POTS.

What Does the Clinic Add to a Deep Breathing Test?

Our team pairs the deep breathing test with a physical exam of the cranial nerves, sensation, strength, balance and coordination, and with an oculomotor exam that measures how the eyes move. Together they show which pathways are struggling and where. If your symptoms and your results don't seem to line up, see why POTS symptoms don't match your test results.

When several findings cluster in 1 area, our team trains that area with stimulation set at a level that's hard but still doable, the way a good coach picks a weight in the gym. In our team's view, repeating that over time builds the integrity of those neurons. Then our team measures again. Our team's goal in the patient above was to see the visual vertical and the deep breathing result come back toward normal on retesting. That's 1 case, and results vary from person to person.

Deep Breathing Test: FAQ