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Rethinking the Resonance Frequency (RF) - Part 2: Slow-Paced Breathing

Updated: Dec 30, 2022

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Resonance frequency (RF) assessment identifies the stimulation frequency producing the greatest amplitude heart rate oscillations in one to two sessions. We can assess the RF using slow-paced breathing (SPB) or slow-paced contraction (SPC) methods. This installment focuses on the slow-paced breathing method. We will discuss the equipment used in RF assessment and the importance of breathing assessment and training. Although we recommend using a respirometer, this does not mean you cannot measure the RF or provide effective heart rate variability (HRV) training without one. Instead, we believe that a respirometer offers assessment and training advantages.

In the third installment, we will cover RF assessment using slow-paced contraction. In the final installment, we will describe stepped and sliding protocols and their RF selection criteria. We will suggest ways to simplify RF assessment.

Watch Dr. Inna Khazan's Heart Rate Variability: Harnessing Your Own Personal Superpower

Equipment for RF Assessment Using Slow-Paced Breathing

For publishable research, you will need an electrocardiogram (ECG) with a respirometer. We encourage using an ECG or PPG sensor with a respirometer for clinical work. Smartphone apps like Optimal HRV offer automated RF assessment using a compatible PPG sensor and their proprietary pacing display for personal training.


Clinicians use the PPG and ECG to monitor heart rate (HR) and heart rate variability (HRV). A PPG sensor detects the pulse wave as it travels through the vascular tree.

NeXus BVP sensor

Graphic © paulista/

Red blood cells

ECG sensors detect the R-spike of the QRS complex, measuring the time between successive heartbeats. A wrist placement trades ease of application for vulnerability to movement.

ECG wrist placement

Graphic © arka38/

R-spike of ECG

Note. The interbeat interval (IBI), the time between successive R-spikes, is the basis for HRV time-domain, frequency-domain, and nonlinear measurements. The PPG may be less accurate than the ECG during SPB (Jan et al., 2019) and marked sympathetic nervous system (SNS) activation. Since RF protocols require breathing from 4.5-6.5 bpm and client effort may increase SNS tone, this can reduce PPG sensor accuracy. This problem is compounded if your client experiences chronic anxiety. Disorders such as anxiety are associated with peripheral vasoconstriction can interfere with detecting the pulse from the digits.

In contrast, vasoconstriction does not affect ECG detection of R-spikes from the chest, torso, or wrists (Giardino et al., 2002; Schafer & Vagedes, 2013; Shaffer & Combatalade, 2013).

Be skeptical of claims that ECG and PPG HRV measurements are correlated if the studies don't report their measurement conditions (e.g., paced breathing) and limits of agreement (i.e., allowable error; Shaffer, Meehan, & Zerr, 2020).

The Case for Using a Respirometer

Clinicians use a respirometer, a flexible sensor band that measures abdominal or thoracic movement to detect the respiratory waveform (Moss & Shaffer, 2022). Graphic © BioSource Software LLC.


Respiratory feedback serves multiple functions during RF assessment as adults breathe from 6.5 to 4.5 bpm in 0.5-bpm or smaller steps: pacing, respiration rate (RR) confirmation, and identification of dysfunctional breathing. The blue waveform in the NeXus Biotrace+ screen below represents breathing, and the red is instantaneous HR. This display reveals that the client is breathing at 5.19 bpm (top left).

NeXus breathing display

A respirometer display guides clients to breathe at prescribed rates and confirms their success. Both actions are critical because we cannot evaluate the effects of breathing at 5.5 bpm if a client actually breathed at 6 bpm. This is a serious limitation of pacing displays. Without a respirometer, we cannot immediately confirm their exact breathing rate.

An elephant in the room is that before SPB training, many clients cannot follow an animated pacing display closely at RF breathing rates (e.g., ~ 6 bpm). Graphic © Aleksandr_Kuzmin/

Elephant in the room

Laborde et al. (2021) reported that participants breathed 6.48-6.55 bpm when following a 6-bpm video pacing display based on Thought Technology's EZ-Air software. They did not use a respirometer to measure participants' respiration rate in real-time.

Steffen et al. (2021) cautioned:

It is important to note that the participants 6 breath per minute pacer condition were unable to consistently breathe at that rate, instead achieving an average pace of 12 breaths per minute.

Recommended Practice

At the Truman Center for Applied Psychophysiology, we repeat trials that are +/- 0.25 bpm off from the pacing display, guided by our respirometer display. Otherwise, the calculated RF could be off by +/- 0.5 bpm or worse.

Additional Advantages of Monitoring Breathing

Respiratory monitoring is also essential to identify dysfunctional breathing behaviors like apnea (breath-holding) and overbreathing (excessive exhaling of CO2), which can interfere with HRV biofeedback.

Assess Breathing Before Measuring the RF

RF measurement protocols require that clients breathe consistently at slower-than-typical rates. Breathing assessment can identify rapid breathing and dysfunctional behaviors that could interfere with RF calculation. Asthma, Generalized Anxiety Disorder, low back pain, and dysfunctional breathing behaviors like overbreathing may be associated with rapid breathing. Postpone RF assessment for adults until you successfully train them to breathe effortlessly (Peper et al., 2008) between 4.5 and 6.5 bpm.

Concerns and Contraindications

Clients who find SPB difficult may overbreathe and expel excessive CO2. If they report symptoms of feeling faint or rapid heartbeats, encourage them to take shallower and smoother breaths (Lehrer et al., 2013).

Resonance frequency assessment is contraindicated for clients whose sinus rhythm is continuously driven by a pacemaker because this device externally regulates HRV. Do not assess clients whose overbreathing compensates for increased acidity in the blood due to conditions like kidney disease. SPB could increase CO2 levels in the blood and dangerously increase acidosis (Khazan, 2013).

Practice Breathing Before RF Trials

Before starting RF trials, clinicians should provide their clients with breathing practice because the protocol requires slower-than-normal breathing rates. This precaution is crucial for clinical populations such as clients diagnosed with chronic pain. Although a healthy resting adult breathes from 12 to 20 bpm (Khazan, 2019), a RF assessment protocol instructs adults to breathe at less than half that rate.

Allow clients to practice healthy breathing from 5.5 to 6 bpm before starting RF trials. When a RR is difficult, instruct them to increase or decrease it by 0.5 bpm. For example, if a client typically breathes at 18 bpm, instruct them to decrease their breathing rate every few seconds from 18 bpm to 17.5 bpm to 17 bpm, and so forth. The Optimal HRV app provides an animated pacing display to help clients slow their breathing.

Optimal HRV pacer

Invite your client to experiment with several inhalation-to-exhalation ratios (e.g., 1:1.5 and 1:2). Choose the easiest ratio for your client during RF assessment since slower-than-normal breathing is challenging by itself.

Whether the inhalation-to-exhalation ratio affects RF measurement remains uncertain. Within-subjects studies that utilized a respirometer and measured breathing ratios (Meehan et al., 2017; Shaffer, McCraty, & Zerr, 2014) found no difference between resting HRV metrics (e.g., HR Max-HR Min, pNN50, RMSSD, SDNN, and LF power) when participants breathed at 1:1 and 1:2 inhalation-to-exhalation ratios. Two within-subjects studies that did not directly measure breathing ratios reported increased HRV for longer exhalations than inhalations at 6 bpm (Laborde et al., 2021; Van Diest et al., 2014). During HRV biofeedback training, you can instruct clients to practice "low and slow" breathing with slightly longer exhalations than inhalations (Khazan, 2019). Although longer exhalations may not increase HRV, Lynda Thompson reports that they increase calming sensorimotor rhythm (SMR) activity (Shaffer, 2022).

Measuring the Resonance Frequency Without a Respirometer

Applications like Optimal HRV offer automated RF calculation using an animated pacer. Their 14-minute RF procedure guides clients through seven 2-minute trials with longer exhalations than inhalations: 7.0 bpm - 3.0-s inhale and 4.5-s exhale 6.5 bpm - 3.3-s inhale and 4.9-s exhale

6.0 bpm - 3.6-s inhale and 5.4-s exhale

5.5 bpm - 4.0-s inhale and 5.9-s exhale

5.0 bpm - 4.4-s inhale and 6.6-s exhale

4.5 bpm - 4.9-s inhale and 7.4-s exhale

4.0 bpm - 5.6-s inhale and 8.4-s exhale

Dr. Inna Khazan describes how to determine your RF in the following video.

The app calculates a RF using weighted criteria, including LF power, RMSSD, and HR MinMax.

Resonance Frequency

The bar chart below shows a logarithmic conversion of LF power, RMSSD, and HR MinMax values.