Respiratory Sinus Arrhythmia

In this experiment, you will be introduced to a technique to evaluate the naturally-occurring variation in heart rate that occurs during respiration.

Written by staff of ADInstruments.

With acknowledgement to Dr. Larry N. Reinking, Biology Department, Millersville University, Millersville, Pennsylvania, USA.

Background

The autonomic nervous system plays a major role in the interaction of the respiratory and circulatory systems, which are intimately related.  Respiratory sinus arrhythmia (RSA) refers to the naturally-occurring synchronous fluctuations in heart rate that are linked to respiration.  Heart rate increases during inspiration and decreases during expiration (Figure 1).

Figure 1. Link between Respiration and Heart Rate

The origin and regulation of RSA is not completely understood, but it is believed RSA improves pulmonary gas exchange efficiency by better matching alveolar ventilation and capillary perfusion throughout the respiratory cycle.  Slowing the heart rate during expiration when alveolar PO₂ is falling and PCO₂ is rising (Figure 2) and increasing it during inspiration when alveolar PO₂ is rising and PCO₂ falling, optimizes delivery of well-oxygenated blood to the tissues.  Lung stretch receptors, stimulated by the increasing tidal volume during inspiration, contribute to this reflex through vagal afferents, as does monitoring of the central PCO₂ in the medulla.  Vagal efferent activity to the heart decreases markedly during inspiration, thus decreasing vagal ‘tone’ and allowing the heart rate to accelerate.

Figure 2. Changes in Blood Gases during the Respiratory Cycle

The decrease in vagal nerve activity during inspiration is best seen in the cardiac cycle of an electrocardiogram, or ECG (the waves of the cardiac cycle are shown in Figure 3. This figure does not depict RSA).  On an ECG, this phenomenon is seen as subtle changes in the R-R interval synchronized with respiration.  The R-R interval is typically shortened during inspiration and lengthened during expiration.  The fluctuation of the R-R interval is a characteristic of healthy individuals. The fluctuation is suppressed when people are stressed or have diseases, such as congestive heart failure.  In people with diabetes, a diminished RSA indicates to clinicians the presence of autonomic neuropathy.

Figure 3. Cardiac Cycle of an Electrocardiogram

In humans, RSA becomes less prominent with age, diabetes, and cardiovascular disease.  There are also differences in people of the same age group, particularly amongst athletes and non-athletes.  Athletes, or people who exercise regularly, have a stronger RSA than non-athletes or sedentary individuals.  Other factors that can change RSA include: gender, body position, resistive breathing, tidal volume, deep breathing associated with meditation, and body mass index and obesity.  It is possible to strengthen RSA through deep and slow breathing patterns developed using such techniques as yoga, meditation, and tai-chi.

Required Equipment

• LabChart software
• PowerLab Data Acquisition Unit
• Respiratory Belt
• 5 Lead Shielded Bio Amp Cable
• Shielded Lead Wires (3 Snap-on)
• Disposable ECG Electrodes
• Electrode Cream or Paste
• Abrasive Gel or Pad
• Alcohol Swabs
• Gauze or cotton ball (or similar material)
• Ballpoint pen
• Reading material

Procedure

Equipment Setup and Electrode Attachment

1. Make sure the PowerLab is turned off and the USB cable is connected to the computer.
2. Connect the Respiratory Belt to Input 1 on the front panel of the PowerLab and the 5 Lead Shielded Bio Amp Cable to the Bio Amp Connector on the front panel (Figure 4). The hardware needs to be connected before you open the settings file.

Figure 4. Equipment Setup for PowerLab 26T

3. Attach the Shielded Lead Wires to the Bio Amp Cable. Channel 1 positive will lead to the left wrist, Channel 1 negative will lead to the right wrist, and the Earth will lead to the right leg.  Attach the Disposable Electrodes to the end of the Channel 1 and Earth wires.  Refer to Figure 5 for proper placement, but do not attach them to the volunteer.  Follow the color scheme on the Bio Amp Cable.
4. Remove any jewelry from the volunteer’s hands, arms, and right leg. Use the ballpoint pen to mark small crosses on the skin on the forearms and right ankle area.  Use Figure 5 as a guide.  Abrade the skin with Abrasive Gel or Pad.  This is important as abrasion helps reduce the skin’s resistance.  After abrasion, clean the area with an Alcohol Swab to remove the dead skin cells.  Wait for the skin to dry, and stick the Disposable Electrodes to the skin (Figure 5).

Note: Do not place the electrodes over the major muscles because muscle activity interferes with the signal recorded from the heart.

5. Check that all three electrodes are properly connected to the volunteer and the Bio Amp Cable before proceeding. Turn on the PowerLab.

Figure 5. Electrode Attachment

Exercise 1: Normal Respiration

In this exercise, you will investigate RSA in normal respiration.

1. Launch LabChart and open the settings file “RSA Settings” from the Experiments tab in the Welcome Center. It will be located in the folder for this experiment.

Note: Channel 2 shows each breath’s duration in seconds.  This is a useful tool to help you visualize where each breath starts and stops.

2. Have the volunteer sit in a relaxed position facing away from the monitor. Have the volunteer read to avoid conscious control of respiration.
3. Select Input Amplifier from the Channel 1 Channel Function pop-up menu. Have the volunteer breathe normally.  Observe the signal (Figure 6) and adjust the range in the dialog so that the maximum respiration occupies about one half to two-thirds of the full scale.

Figure 6. Input Amplifier Dialog

4. Select Bio Amp from the Channel 3 Channel Function pop-up menu. Observe the signal and adjust the range in the dialog so that the maximal electrical response occupies about one half to two-thirds of the full scale.

Note: If the ECG cannot be seen, check that all three electrodes are attached correctly.  If the signal is noisy and indistinct, you may want to use an alternative electrode placement.  Connect the positive electrode to the left upper arm, negative electrode to the right upper arm, and Earth to the right wrist.  Remember to avoid the major muscles of the arm.

5. Start Have the volunteer breathe normally for three minutes, and add a comment with “normal respiration.”  Stop recording.
6. If you cannot see any data for “Period” or “Heart Rate,” you need to adjust the detection settings. Select Cyclic Measurements in the Channel 2 Channel Function pop-up menu for “Period” and in the Channel 4 Channel Function pop-up menu for “Heart Rate.”  Move the Detection Adjustment slider until event markers (the small white circles) appear over each peak (Figure 7).

Figure 7. Cyclic Measurements Dialog for Channel 2

7. Save your data. Do not close the file.  The same volunteer needs to complete the next exercise.

Exercise 2: Forced Breathing

In this exercise, you will examine the effect of forced, slow breathing on RSA.

1. Using the same file, Start Add a comment with “exercise 2.”

2. Have the volunteer sit in a relaxed position facing away from the monitor.
3. Select Input Amplifier from the Channel 1 Channel Function pop-up menu. Have the volunteer take deep, slow breaths.  Observe the signal (Figure 6) and adjust the range in the dialog so that the maximum respiration occupies about one half to two-thirds of the full scale.
4. Start Have the volunteer undergo forced, slow breathing.  The volunteer should breathe slowly while increasing inspiratory and expiratory effort.  Have the volunteer breathe for the same amount of time as in Exercise 1 (approximately three minutes).  Stop recording.
5. Save your data. Do not close the file.

Analysis

Exercise 1: Normal Respiration

1. Examine the data in the Chart View. Autoscale, if necessary.  Use the View Buttons to change the compression to survey the recording for a characteristic number of heart beats per breath.  For your analysis, only use cycles with the characteristic number.
2. Drag across the Time axis to select the data of the representative cycle. Select Zoom View.
3. To analyze your data, designate the first QRS complex (Figure 3) at the start of inhalation as Beat 0, then Beat 1, etc, until you reach the end of exhalation. Use Figure 8 as a guide.  Remember, your number of characteristic heart beats may be different than the example.

Figure 8. Zoom View

4. Select Channel 4 from the Channel Selection buttons in the bottom left corner of Zoom View. Move the Waveform Cursor across the data to determine the heart rate for each beat.  Record these values in Table 1 of the Data Notebook.  The data for this breath will go in the “Breath 1” row.
5. Calculate the mean heart rate for “Breath 1.”
6. Repeat the analysis for eight more breaths that have the characteristic number of heart beats per breath. If you do not have eight more, do the analysis for as many as possible.  Record these values in Table 1 of the Data Notebook.
7. Take all of your “Beat 1” values, and express each value as a percent of the Breath mean. For example, Beat 1/Breath 1 is expressed as a percent of the Breath 1 mean; Beat 1/Breath 2 is expressed as a percent of the Breath 2 mean, etc.  Record these values in Table 2 of the Data Notebook.  Use the following equation:
8. Repeat step 8 for all the heart beat columns in Table 1. Record these values in Table 2.
9. Determine the overall mean percentages for each heart beat of the nine cycles you have analyzed. Record these values in the bottom row of Table 2.

Exercise 2: Forced Breathing

1. Examine the data in the Chart View. Autoscale, if necessary.  Use the View Buttons to change the compression to survey the recording for a characteristic number of heart beats per breath.  For your analysis, only use cycles with the characteristic number.  Repeat the Analysis for Exercise 2.  (The procedure is copied below.)
2. Drag across the Time axis to select the data of the representative cycle. Select Zoom View.
3. To analyze your data, designate the first QRS complex (Figure 3) at the start of inhalation as Beat 0, then Beat 1, etc, until you reach the end of exhalation. Use Figure 8 as a guide.  Remember, your number of characteristic heart beats may be different than the example.
4. Select Channel 4 from the Channel Selection buttons in the bottom left corner of Zoom View. Move the Waveform Cursor across the data to determine the heart rate for each beat.  Record these values in Table 3 of the Data Notebook.  The data for this breath will go in the “Breath 1” row.
5. Calculate the mean heart rate for “Breath 1.”
6. Repeat the analysis for eight more breaths that have the characteristic number of heart beats per breath. If you do not have eight more, do the analysis for as many as possible.  Record these values in Table 3 of the Data Notebook.
7. Take all of your “Beat 1” values, and express each value as a percent of the Breath mean. For example, Beat 1/Breath 1 is expressed as a percent of the Breath 1 mean; Beat 1/Breath 2 is expressed as a percent of the Breath 2 mean, etc.  Record these values in Table 4 of the Data Notebook.  Use the following equation:
8. Repeat step 8 for all the heart beat columns in Table 3. Record these values in Table 4.
9. Determine the overall mean percentages for each heart beat of the nine cycles you have analyzed. Record these values in the bottom row of Table 4.

Data Notebook

Table 1. Heart Rates for Each Heart Beat during the Breath

Table 2. Percents of the Breath Mean BPM

Table 3. Heart Rates for Each Heart Beat during the Breath – Forced Breaths

  Table 4. Percents of the Breath Mean BPM – Forced Breaths

Study Questions

1. What is the apparent physiological function of respiratory sinus arrhythmia?

2. Are the patterns of heart beats observed in the data trace consistent with your answer for Question 1?

3. Would you expect a difference in the magnitude of RSA for normal breathing versus forced, slow breathing? Why would you expect this?

4. In humans, RSA values increase with physical conditioning. Comment on the physiological implications of this observation.

5. Think of ways RSA can be used by clinicians and researchers. Why would studying RSA and understanding it completely be beneficial?

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