Sympathetic and Parasympathetic Coactivation Induces Perturbed Heart Rate Dynamics in Patients with Paroxysmal Atrial Fibrillation

The researchers tested how the heart responds to three different autonomic challenges:

1. **Sympathetic activation alone** – Cold hand immersion (sticking a hand in ice water), which triggers the "fight or flight" system.

2. **Parasympathetic activation alone** – Oxygen inhalation (breathing 100% O₂), which triggers the "rest and digest" system.

3. **Coactivation (CoA)** – Cold face test (applying a cold pack to the forehead), which simultaneously activates both branches of the autonomic nervous system.

They compared heart rate (HR) and heart rate variability (HRV) responses between patients with paroxysmal atrial fibrillation (AF) and healthy controls. In a separate cohort, they repeated the coactivation test after patients underwent pulmonary vein isolation (PVI) – a catheter ablation procedure that electrically isolates the pulmonary veins from the left atrium. In a subset of patients, they performed ultra-high-density endocardial mapping (a detailed electrical map of the inside of the heart) before and during coactivation to see where the heart's electrical signal originates.

**Primary outcome:** Change in heart rate during each autonomic challenge.

**Secondary outcomes:** Changes in HRV parameters (specifically SD1, a non-linear measure of short-term heart rate variability), and shift in the location of the sinoatrial node's earliest activation site.

**AF group:** 26 patients with paroxysmal atrial fibrillation (meaning AF comes and goes, not permanent). Mean age approximately 60 years. No specific exclusion criteria reported beyond the presence of paroxysmal AF.

**Control group:** 10 age-matched healthy individuals (mean age approximately 60 years) with no history of AF or other cardiac conditions.

**Additional PVI cohort:** 7 patients with paroxysmal AF who underwent catheter-based pulmonary vein isolation. These were tested before and after the procedure.

**Mapping cohort:** 6 patients with paroxysmal AF who underwent ultra-high-density endocardial mapping during the coactivation test.

**Setting:** Hospital or clinical research laboratory (likely at the University Heart Center Hamburg or similar institution, given author affiliations).

**Important note:** The sample sizes are very small, especially for the control group (n=10) and the PVI cohort (n=7). This limits the generalizability of the findings.

**Heart rate (HR):** Continuous ECG monitoring, reported in beats per minute (bpm). Values are given as mean ± standard error of the mean (SEM).

**Heart rate variability (HRV):** Non-linear parameter SD1 (standard deviation of instantaneous beat-to-beat variability, measured in milliseconds). SD1 is derived from Poincaré plots and reflects short-term, predominantly parasympathetic modulation of heart rate. Lower SD1 indicates reduced heart rate variability, which is generally associated with worse cardiovascular health.

**Cold hand immersion:** Hand submerged in ice water (temperature not specified, but standard cold pressor test uses ~0–4°C) for a duration not explicitly stated (typically 1–3 minutes in such protocols).

**O₂ inhalation:** Breathing 100% oxygen via a facemask. Duration not specified.

**Cold face test:** Cold pack applied to the forehead. Duration not specified. This is a standard provocation test that simultaneously activates the sympathetic system (via cold stress) and the parasympathetic system (via the trigeminal-cardiac reflex, which slows heart rate in healthy individuals).

**Ultra-high-density endocardial mapping:** A catheter with multiple electrodes is inserted into the heart via a vein in the leg. It records electrical activity at hundreds to thousands of points on the inner surface of the atria. The researchers used an average of 3,442 ± 343 points per map to precisely locate the site where the electrical impulse first appears (the earliest activation site of the sinoatrial node region).

**Pulmonary vein isolation (PVI):** A standard catheter ablation procedure where radiofrequency energy is used to create scar tissue around the openings of the pulmonary veins, electrically isolating them from the left atrium. This is a common treatment for AF.

**Study design:** This is an observational, case-control study with a small pre-post intervention sub-study (PVI cohort) and a mechanistic sub-study (mapping cohort). It is not a randomized controlled trial.

**Randomization:** None. Participants were assigned to groups based on their clinical status (AF vs. control). The order of the three autonomic tests (sympathetic, parasympathetic, coactivation) is not described – it may have been fixed or randomized, but this is not stated.

**Blinding:** No blinding is mentioned. Both participants and researchers likely knew which test was being performed (it's difficult to blind a cold face test or oxygen inhalation). The lack of blinding introduces potential for expectation bias, though objective physiological measures (HR, HRV) are less susceptible to this than subjective reports.

**Duration:** The paper does not specify the duration of each autonomic challenge or the total study session. Based on standard protocols, each test likely lasted 1–5 minutes, with rest periods in between. The PVI sub-study tested patients before and after the ablation procedure (likely within days or weeks, but not specified).

**Statistical approach:** Standard parametric tests (likely paired or unpaired t-tests, or ANOVA) were used to compare HR and HRV changes within and between groups. P-values are reported, but no correction for multiple comparisons is mentioned (they tested three conditions in two groups, plus multiple HRV parameters). This increases the risk of false-positive findings.

**What this design can and cannot prove:**

**Can prove:** That there is an association between AF status and an abnormal heart rate response to coactivation. The mapping data can show that coactivation physically shifts the location of the heart's pacemaker in AF patients.

**Cannot prove:** Causation. This is not a randomized trial, so we cannot conclude that coactivation *causes* AF episodes. The small sample sizes mean the results may not generalize to all AF patients. The lack of blinding and randomization means unknown confounders (e.g., medication use, anxiety levels, baseline heart rate differences) could explain the results. The pre-post PVI comparison is suggestive but lacks a control group (e.g., AF patients who did not undergo PVI but were tested twice).

**Major methodological weaknesses:**

Very small sample sizes (especially controls n=10, PVI cohort n=7, mapping cohort n=6).

No correction for multiple statistical comparisons.

No blinding.

Duration of tests and rest periods not specified.

No information on medication use (many AF patients take beta-blockers or calcium channel blockers that affect heart rate).

No information on whether AF patients were in sinus rhythm or AF during testing (paroxysmal AF means they could be in either state).

**Heart rate changes during autonomic challenges:**

**Sympathetic activation (cold hand immersion):**

- Controls: HR increased from 74 ± 3 to 77 ± 3 bpm (p = 0.0098)

- AF patients: HR increased from 60 ± 2 to 64 ± 2 bpm (p = 0.0076)

- Both groups showed the expected increase, with AF patients starting from a lower baseline.

**Parasympathetic activation (O₂ inhalation):**

- Controls: HR decreased from 71 ± 3 to 69 ± 3 bpm (p = 0.0547 – not statistically significant)

- AF patients: HR decreased from 60 ± 1 to 58 ± 2 bpm (p < 0.0009)

- Only AF patients showed a statistically significant decrease.

**Coactivation (cold face test):**

- Controls: HR decreased from 73 ± 3 to 71 ± 3 bpm (p = 0.084 – not significant, but trending toward the expected decrease)

- AF patients: HR *increased* from 59 ± 2 to 61 ± 2 bpm (p = 0.0006)

- This is the key finding: AF patients showed a paradoxical heart rate increase during coactivation, while controls showed the expected decrease (though not statistically significant in this small sample).

**Heart rate variability (SD1) during coactivation:**

Controls: SD1 increased from 61 ± 7 to 69 ± 6 ms (p = 0.042) – indicating improved short-term HRV

AF patients: SD1 decreased from 44 ± 32 to 32 ± 5 ms (p = 0.3929 – not significant) – indicating impaired HRV

The between-group difference in SD1 change was not directly tested, but the pattern suggests AF patients have blunted or reversed HRV response.

**Effect of pulmonary vein isolation (PVI) on coactivation response:**

Before PVI: Coactivation increased HR in AF patients (consistent with main finding)

After PVI: Coactivation no longer increased HR – the paradoxical response was "abolished"

Exact numbers for this sub-study are not provided in the abstract, only the qualitative statement.

**Ultra-high-density mapping findings:**

During coactivation in AF patients, the site of earliest endocardial activation (where the heart's electrical signal originates) shifted by 18 ± 4 mm from its baseline location.

This shift was documented using an average of 3,442 ± 343 mapping points per patient, providing high spatial resolution.

**Heart rate increase during coactivation in AF patients:** ~2 bpm (from 59 to 61 bpm). This is a small absolute change but represents a reversal of the expected direction (healthy people show a ~2 bpm decrease).

**Heart rate increase during sympathetic activation:** ~3–4 bpm in both groups – a modest but statistically significant effect.

**Heart rate decrease during parasympathetic activation:** ~2 bpm in AF patients, ~2 bpm in controls (not significant in controls).

**Shift in earliest activation site:** 18 mm – this is a substantial shift given that the sinoatrial node region is only a few centimeters across. It suggests the heart's pacemaker effectively "moves" to a different location during coactivation in AF patients.

To put these numbers in perspective: A 2 bpm change in heart rate is roughly equivalent to the effect of standing up from a seated position, or drinking a cup of coffee. It's not clinically dramatic in isolation, but the *direction reversal* (increase instead of decrease) is physiologically meaningful because it indicates a fundamental disruption of normal autonomic control.

**Acknowledged by authors (based on abstract):** The abstract does not explicitly list limitations, but the authors note the small sample sizes and the need for further research.

**Critical reader observations:**

1. **Extremely small sample sizes:** The control group (n=10) is barely adequate for detecting anything but very large effects. The PVI cohort (n=7) and mapping cohort (n=6) are too small for reliable statistical inference. Results from these sub-studies should be considered preliminary.

2. **No correction for multiple comparisons:** The authors report p-values for multiple tests (three conditions × two groups, plus HRV parameters) without adjusting for the increased risk of false positives. Some of the "significant" results may be due to chance.

3. **Baseline heart rate differences:** AF patients had lower baseline heart rates (~59–60 bpm) compared to controls (~71–74 bpm). This could be due to medication (beta-blockers are commonly prescribed for AF) or differences in physical fitness. Lower baseline HR makes it easier to show an increase and harder to show a decrease, potentially biasing the coactivation results.

4. **No information on AF status during testing:** Paroxysmal AF means patients can be in sinus rhythm or AF at any given time. If some patients were in AF during testing, their heart rate dynamics would be fundamentally different. The paper does not clarify this.

5. **No control for medication:** Many AF patients take rate-control drugs (beta-blockers, calcium channel blockers) or antiarrhythmics. These medications directly affect heart rate and autonomic responses. Without reporting medication status, the results are confounded.

6. **Lack of blinding and randomization:** While difficult to blind a cold face test, the lack of any blinding means the researchers' expectations could influence data collection or analysis (e.g., choosing which ECG segments to analyze).

7. **No washout between tests:** If the three autonomic tests were performed in sequence without adequate rest, the responses could be influenced by carryover effects from the previous test.

8. **Limited generalizability:** Results from a small German hospital sample may not apply to all AF patients, especially those with different comorbidities, ages, or ethnicities.

For someone running their own n=1 experiment to explore autonomic function and AF risk:

### What to test

**The cold face test (coactivation):** Apply a cold pack or ice water-soaked cloth to your forehead for 30–60 seconds while monitoring heart rate. This is a safe, non-invasive way to simultaneously activate both sympathetic and parasympathetic systems.

**Compare your response to known norms:** In healthy individuals, the cold face test typically causes a slight heart rate decrease (5–10 bpm) due to the trigeminal-cardiac reflex. A paradoxical increase (or no change) may indicate autonomic dysfunction relevant to AF risk.

### Minimum meaningful duration

**Single session:** 30–60 seconds of cold face test, with 5 minutes of quiet rest before and after.

**For tracking changes over time:** Repeat the test weekly at the same time of day (morning, after fasting, before caffeine) for at least 4–6 weeks to establish your baseline response pattern.

**If you have AF:** Test before and after any intervention (e.g., ablation, medication change, exercise program, vagal maneuvers) to see if the paradoxical response normalizes.

### What to measure

**Heart rate (bpm):** Measure continuously using a chest strap HR monitor (e.g., Polar H10, Garmin HRM-Pro) or a photoplethysmography (PPG) device (e.g., Apple Watch, Oura Ring). Record the average HR during the 30 seconds before the test, during the test, and for 2 minutes after.

**Heart rate variability (HRV):** Specifically the short-term parameter SD1 (or RMSSD, which correlates strongly with SD1). Measure during the test and compare to your resting baseline. A decrease in SD1 during coactivation (instead of the expected increase) may indicate impaired autonomic flexibility.

**Subjective symptoms:** Note any palpitations, dizziness, chest discomfort, or anxiety during the test. These could indicate AF triggers or autonomic instability.

### Key confounds to control for

**Time of day:** Autonomic tone varies diurnally. Test at the same time each day (e.g., 8:00 AM after waking, before breakfast).

**Caffeine, alcohol, nicotine:** Avoid for at least 4 hours before testing. These substances directly affect heart rate and autonomic balance.

**Meals:** Test on an empty stomach or at least 2 hours after eating. Digestion activates the parasympathetic system.

**Hydration:** Dehydration increases sympathetic tone. Drink a glass of water 30 minutes before testing.

**Physical activity:** No exercise for at least 2 hours before testing. Exercise elevates sympathetic tone for hours afterward.

**Medication:** If you take beta-blockers, calcium channel blockers, or antiarrhythmics, note that these will blunt your heart rate response. The test is still useful for tracking relative changes (e.g., before vs. after ablation), but absolute values will differ from unmedicated individuals.

**Respiratory rate:** The cold face test can trigger breath-holding or hyperventilation. Try to breathe normally throughout the test. Consider using a metronome to pace breathing at 6 breaths per minute (0.1 Hz), which maximizes HRV.

**Room temperature:** Cold ambient temperatures can confound the cold face test. Perform