Respiratory Instrumentation

What You Will Learn

When you train a client to breathe slowly and rhythmically, how do you know it is actually working? The answer lies in the instruments you choose. This unit walks you through the respiratory hardware that clinicians use in biofeedback-assisted relaxation training and HRV interventions—from the flexible respirometer band around the abdomen to the capnometer that reveals whether your client is overbreathing.

You will learn how each instrument works, where to place sensors, how to spot artifacts that can mislead you, and what normal values look like. Along the way, you will encounter a sobering example of racial bias built into pulse oximetry and explore complementary tools like the incentive inspirometer and surface EMG. By the end, you will be able to select, attach, troubleshoot, and interpret the instruments that make respiratory biofeedback both precise and clinically meaningful.

Overview of Respiratory Instruments

This section introduces the core instruments you will use to monitor and train breathing. Slow-paced breathing is a frequent component of biofeedback-assisted relaxation training (BART)—a structured approach that uses real-time physiological feedback to teach self-regulation—and interventions to increase heart rate variability. Two instruments form the foundation of breathing assessment and training: the respirometer, a flexible sensor band placed around the abdomen or chest that measures respiration rate and pattern, and the electromyograph, which detects the electrical activity of accessory muscles recruited during dysfunctional breathing.

Respirometer photograph

Three additional instruments provide complementary information. A capnometer measures end-tidal CO2—the concentration of carbon dioxide at the end of exhalation—to detect overbreathing. An oximeter monitors blood oxygen saturation from a digit or earlobe. An incentive inspirometer calculates the volume of air inhaled in a single breath, helping patients strengthen their diaphragm after surgery or during chronic lung disease management.

For the most comprehensive picture, Gilbert (2019) recommends monitoring five channels simultaneously: scalene SEMG, exhalation temperature from the nostril, a chest respirometer, heart rate, and end-tidal CO2. This multi-channel approach lets you cross-check each signal against the others, so a normal-looking respiration rate cannot mask excessive effort that is quietly reducing HRV and end-tidal CO2. Graphic © Association for Applied Psychophysiology and Biofeedback.

Five breathing channels display

Five-channel breathing assessment showing scalene SEMG, nostril temperature, chest respirometer, heart rate, and end-tidal CO2. The vertical grid lines delineate the phases of inhalation and exhalation, revealing how the scalene muscles activate and the chest cavity expands during the inspiratory phase. Concurrently, the Heart rate (RSA) tracing demonstrates respiratory sinus arrhythmia, characterized by vagal withdrawal (heart rate acceleration) during inhalation and vagal reactivation (heart rate deceleration) during exhalation. The End-tidal CO2 and Exhalation temperature channels provide distinct markers for the expiratory phase, tracking the release of warm, carbon dioxide-rich air from the lungs. Graphic © Association for Applied Psychophysiology and Biofeedback.

A Thermistor Can Detect Temperature Shifts Between Inhalation and Exhalation

A thermistor—a temperature-sensitive resistor—can detect the 2°-5° F temperature shift from inhalation to exhalation when positioned in a nostril's airstream in a 70°-75° F room (Gilbert, 2019). This simple, low-cost sensor is especially useful for confirming whether a client is breathing through the nose or mouth during training.

BCIA Blueprint Coverage

This unit addresses III. HRV Instrumentation: C. Respirometer and D. Accessory SEMG.

Topics covered include the respirometer, surface electromyograph, capnometer, oximeter, incentive inspirometer, normal values, and drug effects on respiratory measurements.

🎧 Chapter Lecture: Respiration Hardware

The Respirometer: Your Window into Breathing Patterns

A respirometer is a flexible sensor band, placed around the chest, abdomen, or both, that monitors respiration rate, depth, inhalation-to-exhalation ratio, inhalation and exhalation slopes, pauses, smoothness, and effort (Gilbert, 2019). Think of it as a sophisticated measuring tape that translates the mechanical expansion and contraction of your client's torso into electrical signals your biofeedback software can display and analyze.

Two respirometers

Placement of Respiratory Sensor Bands. To accurately capture breathing mechanics, two respirometer bands are secured horizontally around the torso. The upper thoracic band is positioned across the chest, directly beneath the axillae, to measure rib cage excursion. The lower abdominal band is placed at the level of the umbilicus to detect abdominal expansion driven by diaphragmatic contraction. Comparing signals from both sites allows for the assessment of breathing synchrony and the differentiation between thoracic and diaphragmatic respiration.

Using Respirometers to Measure Abdominal or Thoracic Dominance

When respirometers placed around the chest and abdomen share equal sensitivity to stretch, we can calculate the ratio of their standard deviations to determine whether a client is breathing abdominally or thoracically (Peper et al., 2016). Clinicians may prefer two respirometers when training clients to shift from thoracic to abdominal breathing (Gilbert, 2019).

Respirometer standard deviation comparison

Dual-Channel Respirometer Monitoring. This graph illustrates simultaneous respiratory tracings obtained from thoracic (upper trace, blue) and abdominal (lower trace, red) strain gauges over a one-minute interval. The vertical axis represents relative expansion, where upward deflections indicate inhalation and downward slopes represent exhalation. The synchronous alignment of peaks and troughs across both channels demonstrates coordinated respiratory mechanics, indicating a normative breathing pattern without paradoxical movement. The accompanying table quantifies the physiological data, providing statistical metrics such as minimum and maximum amplitude, average excursion, and signal variability, which are critical for assessing respiratory rate, depth, and stability in psychophysiological analysis. Graphic © Association for Applied Psychophysiology and Biofeedback.

Consider Maria, a 35-year-old marketing executive who came to your practice complaining of chronic neck tension and frequent headaches. When you place respirometers around both her chest and abdomen, you notice the thoracic channel shows much larger excursions than the abdominal channel, with a ratio of 0.68. This pattern, combined with visible shoulder elevation during each breath, confirms your suspicion: Maria is a chronic thoracic breather. Training her to shift toward abdominal dominance, with a goal ratio above 1.0, becomes a primary focus of her treatment plan.

Identifying Dysfunctional Breathing with a Respirometer

A respirometer can help identify and retrain dysfunctional breathing behaviors like apnea, clavicular breathing, gasping, reverse breathing, sighing, and thoracic breathing (Peper et al., 2016). Each of these patterns leaves a characteristic signature in the respiratory waveform. Apnea appears as flat-line periods, sighing shows up as sudden large excursions, and reverse breathing reveals itself when the abdominal and thoracic channels move in opposite directions.

Measuring HR Max - HR Min and Resonance Frequency

In heart rate variability (HRV) biofeedback, a respirometer enables us to measure HRV and determine a client's resonance frequency. Since a respirometer monitors the respiratory waveform, the software can calculate when each breathing cycle starts and stops. This allows us to measure the HRV time-domain metric HR Max - HR Min, the mean difference between the highest and lowest heart rates during each respiratory cycle.

A respirometer is also indispensable in determining a client's resonance frequency, which is the rate that best stimulates their cardiovascular system (Shaffer & Meehan, 2020). Most adults find their resonance frequency between 4.5 and 7 breaths per minute, but individual variation is substantial, which is precisely why measurement matters.

How Respirometers Work

Current is run through a respirometer, resistance changes as the band stretches, and the output voltage is proportional to length changes. The amplified voltage is displayed in relative units of stretch (RUS) from 0-100 (Peper et al., 2016; Shaffer & Combatalade, 2013).

We can only compare two waveforms when their respirometers share a similar range (e.g., 3 units of signal change per 5 mm of stretch) and are displayed using the same lower and upper values on the vertical axis. Peper et al. (2016) provide a detailed description of sensor calibration.

Clinicians should be cautious when comparing measurements across sessions using the same abdominal and thoracic sensors. Since peak values depend on the exact placement of the band, the constrictiveness and thickness of the clothing under the band, and band tension, reliability can be very low (Shaffer & Combatalade, 2013).

Mind Media respirometer

NeXus Respirometer. This figure shows a MindMedia NeXus respirometer, a device used for the non-invasive measurement of respiration. The assembly includes an adjustable elastic strap, designed to be worn around the thorax or abdomen, which contains a sensor that detects expansion and contraction with each breath. The blue and black transmitter module converts these mechanical movements into electrical signals, which are then conveyed through the attached cable and connector to a data acquisition system for the recording and analysis of respiratory rate and pattern.

Below is a BioGraph ® Infiniti display of abdominal movement. Note the slow rhythmic breathing pattern.

BioGraph ® Infiniti display showing rhythmic abdominal breathing.

Sensor Placement: Getting It Right

Since constrictive clothing can impede abdominal and chest expansion, clinicians should instruct clients to avoid wearing a "tight belt, slimming underwear, constrictive pants, or a very tight bra" (MacHose & Peper, 1991; Peper et al., 2016). This conversation can feel awkward, but it is essential for accurate measurement. Consider providing clients with a pre-session checklist that includes clothing recommendations.

Avoiding Respirometer Artifacts

Avoid thick clothing (e.g., sweaters) because it can compromise accuracy and promote upward or downward slippage. Tape or pin the respirometer to your client's clothing to prevent slippage artifacts (Gilbert, 2019).

Since belly contours vary greatly, place the abdominal respirometer above the navel within a horizontal zone that expands during inhalation (Gilbert, 2019).

To ensure adequate sensor responsiveness, ask your client to exhale completely and then slightly tighten the sensor band during the post-expiratory pause. Avoid tightness extremes. A respirometer that is too tight can curtail spontaneous breathing, while one that is too loose can overlook small abdominal or chest excursions (Gilbert, 2019).

When the respiratory sensor is poorly attached and loosens or comes off during the middle of an inhalation, this can result in signal clipping and sharply change the signal's offset on the screen (Shaffer & Combatalade, 2013).

Signal clipping artifact

Respiration Signal Clipping Due To a Loose Sensor. This trace illustrates the artifact created when a respiratory belt lacks sufficient tension. During the "Loose sensor" phase, the lack of tension prevents the waveform from completing its full downward excursion during exhalation, resulting in a flattened signal floor known as clipping (highlighted in red). Tightening the sensor restores the full dynamic range of the waveform, allowing for accurate measurement of the respiratory cycle. Graphic © Association for Applied Psychophysiology and Biofeedback.

You want to see a sinusoidal signal pattern with sufficient amplitude to detect the peaks and valleys.

Ideal sinusoidal breathing pattern

Cardiorespiratory Synchronization and Respiratory Sinus Arrhythmia (RSA). This biofeedback display illustrates the physiological coupling between the respiratory and cardiovascular systems. The blue sinusoidal waveform tracks the respiratory cycle, where the ascending slope represents inhalation and the descending slope represents exhalation. The red, stepped waveform tracks the instantaneous heart rate in beats per minute, calculated on a beat-to-beat basis. The graph demonstrates Respiratory Sinus Arrhythmia (RSA), a naturally occurring variation in heart rate that occurs during a breathing cycle. Through the vagus nerve, the parasympathetic nervous system is withdrawn during inhalation, causing the heart rate to accelerate (the red line rises), and reactivated during exhalation, causing the heart rate to decelerate (the red line falls). In this example, the subject is breathing at a controlled rate of 4.1 breaths per minute—known as the resonance frequency—which maximizes the amplitude of these heart rate oscillations and induces a state of high Heart Rate Variability (HRV).

Understanding the Limitations of Respirometer Displays

Review the raw respirometer waveforms for responsiveness to abdominal and thoracic excursion, dysfunctional breathing behaviors, and smoothness. Here is a sobering realization: numeric displays of abdominal movement and respiration rate may look correct, while end-tidal CO2 and HRV are reduced by excessive effort (Shaffer et al., 1998). In other words, a client can appear to breathe at the "correct" rate while still breathing dysfunctionally. This is why multimodal assessment matters.

The Tracking Test: Verifying Your Setup

By performing a tracking test, you can determine whether a respirometer display mirrors your client's abdominal movement. Instruct your client to inhale and then exhale, and check whether the abdominal waveform mirrors stomach movement.

If you question the accuracy of respiration rate measurements, visually count the number of abdominal or chest excursions over one minute.

Below is a BioGraph ® Infiniti display of an abdominal respiratory band tracking test.

BioGraph ® Infiniti display of a respiratory tracking test.

Normal Values for Respiration Rate

Typical resting adult respiration rates are between 12-20 bpm (Saatchi et al., 2025). Following a stressor, clinicians may measure elevations above baseline and the time required to recover to baseline values (Khazan, 2013, p. 46).

The respirometer is your primary tool for visualizing breathing patterns. When properly calibrated and positioned, it reveals rate, depth, rhythm, and effort. Two-channel recordings allow you to calculate abdominal-to-thoracic ratios and identify dysfunctional patterns. Always verify your setup with a tracking test and remember that numeric displays can mask problems that only the raw waveform reveals.

Check Your Understanding

  1. Why might a respirometer show "normal" respiration rate values while end-tidal CO2 remains low?
  2. What does an abdominal-to-thoracic ratio below 1.0 indicate about a client's breathing pattern?
  3. How would you explain the importance of clothing choices to a client before a respiratory biofeedback session?
  4. What are three types of artifacts that can compromise respirometer accuracy, and how can you prevent each?
  5. Why is the tracking test an essential first step before beginning respiratory training?

Surface EMG: Detecting Hidden Breathing Effort

Clinicians use the surface EMG (SEMG) to monitor accessory breathing muscles, including the sternocleidomastoid, scalenes, pectoralis major and minor, serratus anterior, and latissimus dorsi, that may be recruited during overbreathing. When these muscles activate during quiet breathing, it signals that your client is working harder than necessary to breathe, a pattern that contributes to fatigue, neck pain, and anxiety.

Sternocleidomastoid muscle Scalene muscle Trapezius muscle

Accessory breathing muscles: sternocleidomastoid, middle scalene, and trapezius. Graphics adapted © SciePro/Shutterstock.com.

Trapezius and facial muscle placements using Mind Media EXG sensors are shown below. Active sensors are red and black, and the reference is white.

Trapezius electrode placement

Trapezius Placement. This figure illustrates the bilateral placement of surface electromyography (sEMG) electrodes designed to record the electrical activity of the upper trapezius muscles. The active recording electrodes (paired red and black leads) are positioned over the belly of the upper trapezius on both the left and right sides, while a single common ground electrode (white lead) is placed centrally over the spinous processes of the thoracic spine. Although primarily involved in scapular elevation and neck movement, the upper trapezius muscles function as accessory muscles of inspiration. Monitoring their activation levels is crucial in assessing dysfunctional breathing patterns, as they are often inappropriately recruited during quiet breathing in individuals who overbreathe.

Facial muscle electrode placement

Masseter Placement. This graphic illustrates the positioning of surface EMG sensors over the masseter muscle located in the cheek. While the masseter is primarily involved in mastication (chewing) rather than respiration, monitoring its electrical activity is valuable when assessing dysfunctional breathing patterns. Individuals experiencing overbreathing or hyperventilation often exhibit increased systemic stress and sympathetic nervous system arousal, which frequently manifests as subconscious jaw clenching or elevated tension in facial muscles. Therefore, increased signal activity in this region can serve as a secondary indicator of the overall muscular tension associated with disordered breathing habits.

The photograph below depicts an SEMG placement to monitor the trapezius (yellow sensor) and scalene (blue sensor). The reference (black sensor) is located over the spine. As you can see, the color codes for active and reference electrodes differ across manufacturers.

Trapezius and scalene electrode placement

Posterior Neck Muscles. This figure shows the bilateral placement of surface EMG electrodes over the posterior cervical muscles, which include the upper trapezius and splenius capitis. The active recording leads (black and blue) are positioned over the bellies of these muscles on the left and right sides of the neck, respectively. A ground electrode (yellow) is placed on the spinous processes of the cervical spine to serve as a reference point. While these muscles primarily extend and stabilize the head and neck, they are often recruited as accessory inspiratory muscles during overbreathing to help elevate the upper chest and fixate the neck. Increased EMG activity in this region during resting breathing is a common indicator of a dysfunctional, apical breathing pattern.

James, a 42-year-old software developer, reported persistent neck and shoulder tension despite regular massage therapy. During your assessment, you placed SEMG sensors over his upper trapezius and scalene muscles while monitoring his breathing. What you discovered was revealing: every time James inhaled, his trapezius activity spiked to 8-12 microvolts, well above the 2-3 microvolts you would expect during effortless breathing. James was essentially doing "mini-shrugs" with every breath, hundreds of times per day. Once he saw this pattern on the screen, he understood why massage provided only temporary relief. The real issue was his breathing mechanics.

ECG Artifact in SEMG Recordings

Evaluate the raw signal for ECG artifact, which may be superimposed on the SEMG signal. You can minimize the risk of this artifact by using a narrower bandpass and closer electrode spacing.

ECG artifact in SEMG signal

ECG Contamination of the Electromyogram (EMG). This waveform illustrates a raw EMG signal recorded from the upper trapezius muscle that has been compromised by cardiac electrical activity. The sharp, rhythmic spikes indicated by the arrows represent the R-wave of the heart's QRS complex, appearing as distinct artifacts within the muscle data. This type of signal contamination frequently occurs when recording from the torso or neck, particularly if the recording electrodes are placed with a wide horizontal separation. Because the heart generates a strong electrical field, the signal can volume-conduct through body tissues and be detected by the EMG sensors, potentially inflating integrated EMG values and complicating the analysis of true muscle tension.

The BioGraph ® Infiniti screen below provides respiratory and SEMG biofeedback to teach rhythmic breathing while maintaining relaxed accessory muscles. Note the overuse of accessory muscles during clavicular breathing.

BioGraph ® Infiniti display showing respiratory and SEMG biofeedback during clavicular breathing, demonstrating excessive accessory muscle recruitment.

Surface EMG reveals the muscular effort behind breathing that respirometry alone cannot detect. Monitoring accessory muscles like the trapezius and scalenes helps identify clients who are working too hard to breathe, even when their rate and rhythm appear normal. Watch for ECG artifact in your recordings and use appropriate filtering to maintain signal clarity.

Check Your Understanding

  1. What does elevated trapezius or scalene activity during quiet breathing indicate about a client's respiratory pattern?
  2. How can you distinguish ECG artifact from genuine muscle activity in an SEMG recording?
  3. Why is combining respirometry with SEMG more informative than using either modality alone?
  4. What technical adjustments can minimize ECG artifact in chest and neck SEMG recordings?

Oximetry: Measuring Blood Oxygen Saturation

A pulse oximeter utilizes a photoplethysmograph (PPG) sensor to measure blood oxygen saturation from a finger or earlobe. Oximeters compare the ratio of red and blue wavelengths in the blood to measure hemoglobin oxygen transport. During hyperventilation or overbreathing, oxygen saturation (PaO2) may approach 100%. PaO2 values over 98% signal that less O2 and nitric oxide are available for body tissues (Gilbert, 2019).

Mind Media oximetry sensor

Pulse Oximetry. This image shows a standard finger-clip sensor connected to a MindMedia NeXus data acquisition system. This setup non-invasively measures peripheral blood oxygen saturation (SpO₂) and pulse rate by using optical sensors to detect changes in blood flow through the fingertip.

Oximetry can reassure clients anxious about suffocating that they are receiving sufficient oxygen. Oximetry can also detect abnormally low saturation (≤ 85%) due to compromised circulation or emphysema (Gilbert, 2019).

Racial Bias in Pulse Oximetry: A Critical Concern

Physicians use oximetry to identify patients who need supplemental oxygen. Since this technology was developed using a racially non-diverse population, it should not be surprising that occult hypoxemia (undetected blood oxygen levels below 88%) was three times more likely in Black than White patients in two large cohorts.

Comic illustrating racial bias in oximeters

Racial Bias in Oxygen Monitoring. This illustration depicts the clinical implications of skin pigmentation on pulse oximetry accuracy. Because early device standardization protocols primarily utilized subjects with lighter skin tones, the algorithms often fail to account for the light-absorbing properties of melanin in patients with darker skin. This calibration error creates a significant risk of "occult hypoxemia," a phenomenon where the oximeter displays a normal saturation level despite the patient suffering from dangerously low arterial oxygenation. As highlighted in the scenario, this discrepancy led to delayed interventions during the COVID-19 pandemic, demonstrating why clinicians must triangulate device data with patient-reported symptoms and why inclusive engineering is a prerequisite for equitable healthcare.

The COVID-19 pandemic exacerbated this problem since this racial bias may increase Black patients' risk of hypoxemia and denial of supplemental oxygen. These findings imply that oximetry measurements exaggerate Black arterial oxygen saturation. Researchers should develop race-appropriate norms and address racial bias in other technology used in diagnosis and decision-making (Sjoding et al., 2020).

Accuracy of Pulse Oximetry in Measuring Arterial Oxygen Saturation, According to Race. Shown is a comparison of paired measurements of pulse oximetry readings of oxygen saturation and time-matched directly measured arterial oxygen saturation among hospitalized patients who were stratified according to race. The shaded area indicates an arterial oxygen saturation of less than 88%. In the box plot, the horizontal line within each box represents the median. The top and bottom of each box represent the upper and lower limits of the interquartile range. The whiskers represent 1.5 times the interquartile range. Data points indicate outliers outside this range.

Oximetry Artifacts

Finger vasoconstriction due to cold exposure or sympathetic activation can prevent accurate measurement. Review the unit on Cardiovascular Instrumentation for possible solutions. Excessive variability in the time between heartbeats can also compromise oximeter measurements due to irregular pulse pressure (Gilbert, 2019).

Normal Values for Oxygen Saturation

A range of 95%-98% is ideal. Levels below 90% may indicate dysfunctional breathing or life-threatening medical conditions like emphysema (Gilbert, 2012).

Pulse oximetry provides valuable information about blood oxygen levels and can reassure anxious clients. However, clinicians must be aware of the significant racial bias in oximeter readings, which can underestimate hypoxemia in Black patients by approximately 3 percentage points. Cold fingers and irregular heart rhythms can also compromise accuracy.

Check Your Understanding

  1. Why might a pulse oximeter reading of 100% actually indicate a problem rather than optimal oxygenation?
  2. How does racial bias in pulse oximetry affect clinical decision-making, and what should clinicians do about it?
  3. What physiological factors can cause inaccurate pulse oximetry readings?
  4. When would oximetry be particularly useful for reassuring an anxious client during breathing training?

Capnometry: The Gold Standard for Detecting Overbreathing

A capnometer monitors end-tidal CO2, the percentage of CO2 in exhaled air at the end of exhalation.

Capnometer device

Portable Capnometer. This device connects directly to a patient's airway circuit to provide real-time monitoring of ventilation. The digital display indicates an end-tidal carbon dioxide (EtCO2) partial pressure of 38 mmHg, which falls within the normal physiological range of 35–45 mmHg. The device also reports a respiratory rate of 12 breaths per minute. By utilizing infrared absorption spectroscopy directly at the sampling site, mainstream capnometers allow for immediate assessment of gas exchange and are critical for confirming endotracheal tube placement and detecting respiratory depression in emergency or intensive care settings.

A capnometer draws exhaled air into its gas composition analysis unit to calculate the percentage of CO2 in each sample.

Capnometer tracing

Respiratory Patterns. This waveform traces the partial pressure of exhaled carbon dioxide (PCO2) in millimeters of mercury (mmHg) over a duration of five minutes. The recurring peaks represent the end-tidal CO2 (EtCO2), which approximates the alveolar CO2 concentration at the end of exhalation. The recording begins with a period of rapid, rhythmic ventilation (00:00–01:00), followed by a section of marked irregularity (01:40–03:20) characterized by inconsistent amplitudes and intervals, likely resulting from vocalization or movement artifacts. The final segment demonstrates a return to a stable rhythm with a noticeably decreased respiratory rate, illustrating how physiological demand or conscious control can alter ventilation frequency while maintaining normocapnic EtCO2 levels. Graphic © Inna Khazan.

Capnometer Artifacts

Check the raw waveform for false readings due to nasal cannula obstruction or withdrawal from the nostril, or a shift to mouth breathing.

Normal capnometer waveform

Capnograph Display. This clinical display integrates several critical physiological metrics to provide a comprehensive view of a patient's respiratory and cardiovascular status. The most prominent feature is the capnogram in the upper left, which plots the partial pressure of exhaled carbon dioxide over time; the waveform exhibits a classic "boxy" shape, where the sharp upstroke represents the beginning of exhalation, the plateau reflects the expulsion of alveolar gas, and the rapid drop-off marks the start of inhalation. Directly correlated to this waveform is the End-Tidal CO₂ (EtCO₂) reading of 39 mmHg in the upper right, which falls well within the standard range of 35–45 mmHg, indicating effective ventilation. Below this, the Respiratory Rate (RR) of 18 breaths per minute confirms a normal breathing frequency. The bottom right panel addresses oxygenation rather than ventilation, showing a Pulse Oximetry (SpO₂) level of 100% and a Pulse Rate of 77 beats per minute, signifying that the patient's blood is fully saturated with oxygen. Finally, the bottom left quadrant synthesizes these variables into an Integrated Pulmonary Index (IPI) of 8 out of 10, a calculated score that—along with the one-hour trend graph—suggests this adult patient is maintaining stable and healthy respiratory function without signs of distress or apnea. Graphic © Association for Applied Psychophysiology and Biofeedback.

When using a nasal cannula, a capnometer requires exhalation through the nose to collect a valid sample (Gilbert, 2019).

Nasal cannula placement

Nasal Cannula. This figure illustrates the correct placement of a nasal cannula designed for capnometric monitoring, a non-invasive method for measuring end-tidal carbon dioxide (EtCO2) levels. The device features twin prongs positioned within the nares to directly sample exhaled air from the upper airway during the respiratory cycle. As the subject breathes, the sampling tube continuously transports a portion of the exhaled gas to a connected capnograph or gas analyzer, allowing for real-time assessment of ventilatory status, respiratory rate, and breath-by-breath carbon dioxide concentration. This setup is widely utilized in clinical and psychophysiological settings to monitor respiratory mechanics without significantly impeding normal airflow or speech.

The Tracking Test for Capnometry

Using math or a mild emotional stressor, you can determine whether a capnometer display mirrors your client's respiratory activity. This maneuver can markedly decrease end-tidal CO2.

Capnometer response to emotional stressor

ETCO2 Changes During Psychophysiological States. The waveform displays the partial pressure of carbon dioxide in exhaled air (mmHg) over a nine-minute session. During the initial phase of controlled slower breathing, the subject demonstrates high-amplitude, low-frequency waveforms with stable ETCO2 levels, reflecting optimal gas exchange. The onset of an emotional stressor precipitates a hyperventilatory response, characterized by a rapid, erratic respiratory rate and a significant drop in ETCO2 concentration (hypocapnia) as carbon dioxide is exhaled faster than it is produced. Following the stressor, the resumption of normal breathing allows for the stabilization of respiratory frequency and the recovery of ETCO2 to a homeostatic baseline of approximately 40 mmHg.. Graphic © Inna Khazan.

Normal Values for End-Tidal CO2

A reading of 36 mmHg corresponds to about 5% CO2 in exhaled air. Resting end-tidal CO2 values between 35-45 mmHg are normal, 30-35 mmHg indicate mild-to-moderate overbreathing, 25-30 mmHg show moderate-to-severe overbreathing, and below 25 mmHg signal severe overbreathing (Khazan, 2013, pp. 46, 80).

Overbreathing severity

Dysfunctional breathing manifests in various patterns, starting with apnea, a brief suspension of breathing that decreases ventilation and may raise blood pressure. Other inefficient patterns include clavicular breathing, where the chest rises and accessory muscles consume more oxygen than provided, and reverse breathing, which expends excessive energy as the abdomen contracts during inspiration. Similarly, thoracic breathing utilizes upward rib movement that expends excessive energy to incompletely ventilate the lungs. Conditions like hyperventilation and overbreathing—characterized by subtle behaviors like gasps and sighs—reduce end-tidal CO2 below 5%, exceeding the body's elimination needs. While healthy breathing maintains end-tidal CO2 levels between 35 and 45 mmHg, overbreathing pushes these levels lower, ranging from 30-35 mmHg in mild-to-moderate cases to less than 25 mmHg in severe instances.

Sarah, a 28-year-old graduate student, came to your office with complaints of dizziness, tingling in her hands, and difficulty concentrating during exams. Her respirometer showed a normal rate of 14 breaths per minute with smooth waveforms. However, when you added capnometry to your assessment, you discovered her end-tidal CO2 was only 28 mmHg, well into the moderate-to-severe overbreathing range. Sarah was taking quick, shallow breaths that looked rhythmic on the respirometer but were washing too much CO2 out of her bloodstream. Without capnometry, this pattern would have been invisible.

Capnometry provides the most direct measure of overbreathing by tracking end-tidal CO2. While respirometry shows the mechanics of breathing and SEMG reveals muscular effort, capnometry answers the critical question: is your client maintaining healthy CO2 levels? Values below 35 mmHg indicate overbreathing that may cause symptoms even when rate and rhythm appear normal.

Check Your Understanding

  1. Why is capnometry sometimes called the "gold standard" for detecting overbreathing?
  2. What end-tidal CO2 value would indicate moderate-to-severe overbreathing?
  3. How does mouth breathing affect capnometry accuracy when using a nasal cannula?
  4. Why might a client show normal respirometer values while their capnometer reveals overbreathing?
  5. What is the clinical significance of an emotional stressor causing a sudden drop in end-tidal CO2?

Incentive Inspirometer: Training Deeper Breaths

An incentive inspirometer consists of a calibrated cylinder with a piston that moves upward as the patient inhales and indicates the volume of air inhaled in a single breath (the incentive).

Incentive Inspirometer. This volumetric incentive spirometer serves as a foundational tool for respiratory training and post-operative care by utilizing a simple biofeedback loop. Unlike devices that measure expiratory force, this instrument requires the user to perform a slow, deep inhalation to lift the piston within the graduated chamber, providing immediate visual confirmation of the volume of air inspired. This sustained maximal inspiration is crucial for recruiting alveoli, preventing lung collapse, and enhancing overall pulmonary compliance, making it an excellent practical example of how physiological monitoring can drive behavioral conditioning in a clinical context. Graphic © SciePro/Shutterstock.com.

Training with an incentive inspirometer can increase the strength of the diaphragm muscle and improve ventilation. Watch the Kaiser Permanente training video. This device is used to help surgical patient recovery by preventing fluid and mucous accumulation and subsequent pneumonia. Patients diagnosed with asthma and emphysema can also benefit from increasing suction during inhalation and more effectively exchanging air with the alveoli in the lungs (Gilbert, 2019).

Inspirometer Practice. This close-up photograph in a hospital setting shows a patient intently using an incentive spirometer, a medical device used for respiratory rehabilitation. He holds the clear plastic unit firmly, sealing his lips around the blue mouthpiece to take a slow, deep inhalation. This specific action creates negative pressure, causing the blue indicator balls inside the calibrated chambers to rise, providing him with immediate visual feedback on the volume and flow of his breath. By repeatedly performing these controlled, deep breathing exercises, he is working to expand his lung capacity, strengthen respiratory muscles, and keep air sacs open, a vital process for preventing post-surgical complications such as pneumonia or atelectasis during his recovery.

Summary of Respiratory Instrumentation

The table below summarizes the key features, measurements, and clinical applications of each respiratory instrument covered in this unit. Use it as a quick-reference guide when selecting the right tools for your assessment or training session.

Respiratory instrumentation summary table

Respiration Instruments. This infographic summarizes four common instruments used to monitor breathing-related physiology in clinical and biofeedback settings, pairing each device with the physiological signal it captures and a typical resting reference range. A capnometer (CAP) measures end-tidal CO₂ (EtCO₂), the partial pressure (or percent) of carbon dioxide at the end of exhalation, which reflects how effectively ventilation is clearing CO₂ from the lungs; in healthy resting adults, EtCO₂ is commonly ~35–45 mmHg (≈5–6%), and sustained deviations can suggest hypoventilation or hyperventilation depending on context. An electromyograph (EMG) records muscle electrical activity from surface electrodes, often used over neck and shoulder muscles to quantify resting tension and teach down-training during relaxation or breathing work. A pulse oximeter (PPG) uses photoplethysmography to estimate arterial oxygen saturation (SpO₂) noninvasively, with typical sea-level resting values in the mid- to high-90% range for healthy individuals, while reminding you that accuracy varies with motion, perfusion, skin temperature, and measurement site. Finally, a respiratory belt (RESP) tracks thoracic and/or abdominal movement to estimate breathing rate and pattern; the graphic’s normal resting reference (12–20 breaths/min) reflects the common adult resting range, and the belt is especially useful for visualizing breathing variability, breath holds, and shifts between chest-dominant and abdominal-dominant breathing during training. Overall, the figure emphasizes that these tools do not merely “measure breathing,” they capture different parts of respiratory physiology—ventilation (CO₂ removal), muscle activation, oxygenation, and mechanical movement—so interpreting them together provides a more complete picture than any single device alone.

Drug Effects on Respiratory Measurements

Many of the clients you see will be taking medications that affect respiratory parameters, making it essential to account for pharmacological influences when interpreting biofeedback data. The table below summarizes how common drug classes alter respiration rate, depth, and chemistry. Always ask about current medications during your intake interview and consider their effects when setting baselines and training goals.

Drug effects on respiration rate

Differential Effects of Psychoactive Substances on Respiration Rate. This infographic delineates the physiological impact of various drug classes on human respiration rate (RR), categorized by their influence on the central nervous system (CNS). Depressants, including alcohol and narcotic pain medications, function by inhibiting CNS activity, which typically results in a suppressed respiratory rate; this effect is particularly pronounced and potentially dangerous at high dosages due to the depression of the brainstem's respiratory centers. In contrast, stimulants such as amphetamines, cocaine, and methamphetamine act as powerful CNS excitants, leading to a marked increase in respiration rate. The visualized waveforms represent the relative frequency of breathing cycles, contrasting the slow respiration associated with depressants against the rapid patterns induced by stimulant use.

Since John's SpO2 is 96%, he plans to increase it to 99% because he believes higher values are better. What did John forget? When SpO2 exceeds 98%, the additional oxygen is primarily dissolved in the plasma, where it can produce oxidative stress, vasoconstriction, and tissue injury—especially in critically ill patients. While 100% SpO2 is not harmful in brief or healthy contexts (such as post-exercise recovery), in many clinical settings the ideal range is 95-98%.

Pulse oximeter display
A pulse oximeter display showing oxygen saturation.

Comprehension Questions: Additional Instruments and Drug Effects

  1. What does an incentive inspirometer measure, and which patients benefit from its use?
  2. Why is John's plan to increase his SpO2 from 96% to 99% misguided?
  3. How might common medications affect respiratory measurements during a biofeedback session?

Cutting Edge Topics

Wearable Respiratory Monitoring

Advances in wearable technology are bringing continuous respiratory monitoring out of the clinic and into daily life. Flexible strain sensors, piezoelectric textiles, and smart garments can now track respiration rate and pattern throughout the day, including during sleep, exercise, and high-stress situations. For HRV biofeedback practitioners, these devices hold particular promise: they could allow you to review a client's breathing data from the hours and days between sessions, providing insights into how well training skills are transferring to real-world conditions. As sensor accuracy improves, wearable respirometers may become standard tools for longitudinal assessment in both clinical rehabilitation and optimal performance programs.

Smartphone-Based Capnography

Portable capnography has traditionally required expensive, dedicated hardware that limits access outside the clinic. Emerging approaches pair compact, low-cost CO2 sensors with smartphone applications to provide real-time end-tidal CO2 feedback in the client's own environment. These tools could make capnometric biofeedback significantly more accessible for clients managing chronic hyperventilation or panic disorder, particularly those in rural or underserved areas where frequent clinic visits are impractical.

Addressing Equity in Pulse Oximetry

Following research documenting racial bias in pulse oximeters, the FDA and professional organizations have called for re-evaluation of oximetry accuracy across diverse populations. New sensor calibration protocols are being developed that account for varying melanin levels, and some manufacturers are testing multi-wavelength systems designed to reduce measurement discrepancies across skin tones. These efforts reflect a broader movement in medical technology toward ensuring that diagnostic tools perform equitably for all patients—a priority that directly affects clinical decision-making in the diverse populations many of you serve.

Essential Skills

1. Explain the respiration signal, healthy breathing, and biofeedback to a client.

2. Explain sensor attachment to a client and obtain permission to monitor her.

3. Explain how to select a placement site and demonstrate how to attach a respiration sensor to the chest and abdomen. Show how to monitor the accessory muscles to measure breathing effort.

4. Perform a tracking test asking your client to take a slow deep breath.

5. Identify breath-holding, gasping, and movement artifacts in the respiration signal and how to remove them from the raw data.

6. Explain how to identify clavicular breathing, excessive breathing effort, reverse breathing, and thoracic breathing.

7. Explain how posture and clothing can affect breathing.

8. Demonstrate how to find your client's resonance frequency and explain why this is important.

9. Demonstrate how to instruct a client to utilize a breathing pacer and the feedback display.

10. Discuss strategies for slowing your client's breathing toward 5-7 breaths per minute.

11. Demonstrate a respiratory biofeedback training session, including record keeping, goal setting, site selection, baseline measurement, display and threshold setting, coaching, and debriefing at the end of the session.

12. Demonstrate how to select and assign a practice assignment based on training session results.

13. Evaluate and summarize client progress during a training session.

Assignment

Now that you have completed this module, describe the method you use to monitor client breathing and the types of measurements you record. If you use a strain gauge to measure breathing, where do you place the strain gauge and why? What precautions do you take when attaching the strain gauge to female clients?

Glossary

capnometric biofeedback: the display of end-tidal CO2 back to the monitored individual.

ECG artifact: electrocardiographic signal superimposed on the SEMG recording, which can be minimized with narrower bandpass and closer electrode spacing.

end-tidal CO2: the percentage of CO2 in exhaled air at the end of exhalation.

HR Max-HR Min: a heart rate variability time-domain metric that calculates the average difference between the highest and lowest heart rates during each respiratory cycle.

incentive inspirometer: a calibrated cylinder with a piston that moves upward as the patient inhales and indicates the volume of air inhaled in a single breath (the incentive).

occult hypoxemia: undetected arterial oxygen saturation below 88%.

photoplethysmographic (PPG) sensor: a photoelectric transducer that transmits and detects infrared light that passes through or is reflected off tissue to measure brief changes in blood volume and detect the pulse wave.

pulse oximeter: a device that measures dissolved oxygen in the bloodstream using a photoplethysmograph sensor placed against a finger or earlobe.

relative units of stretch (RUS): the unit of measurement used by respirometers to display amplified voltage proportional to band stretch, on a 0-100 scale.

resonance frequency: the frequency at which a system, like the cardiovascular system, can be activated or stimulated.

respirometer: a sensor that changes resistance to a current as it expands and contracts during the respiratory cycle.

slippage artifact: the upward or downward movement of a respirometer, often due to thick underlying clothing.

surface EMG (SEMG): the recording of muscle action potentials detected by surface electrodes placed over skeletal muscles.

thermistor: a temperature-sensitive resistor.

torr: a unit of atmospheric pressure, named after Torricelli, which equals 1 millimeter of mercury (mm Hg) and is used to measure end-tidal CO2.

References

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Fried, R. (1987). The hyperventilation syndrome: Research and clinical treatment. Johns Hopkins University Press.

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