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Is There More to Blood Volume Pulse Than Heart Rate Variability

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Is There More to Blood Volume Pulse Than Heart Rate Variability Biofeedback ©Association for Applied Psychophysiology & Biofeedback Volume 35, Issue 2, pp. 54-61 www.aapb.org SPECIAL TOPICS Is There More to Blood Volume Pulse Than Heart Rate Variability, Respiratory Sinus Arrhythmia, and Cardiorespiratory Synchrony? Erik Peper, PhD,1 Rick Harvey, PhD,1 I-Mei Lin, MA,2 Hana Tylova,1 and Donald Moss, PhD3 1San Francisco State University, San Francisco, CA; 2National Chung Cheng University, Taiwan; 3Saybrook Graduate School, San Francisco, CA Keywords: blood volume pulse, heart rate variability, respiratory sinus arrhythmia, cardiorespiratory synchrony A growing body of research reports the health benefi ts sympathetic-parasympathetic imbalance, new HRV training of training heart rate variability (HRV), and the clinical protocols could be employed as well. use of HRV training protocols has increased dramatically Measuring HRV, the beat-to-beat variability in the sinus in recent years. Many of the home training devices and rhythm over a given period of time, is usually done by many of the sophisticated biofeedback instrumentation calculating the standard deviation of the average of normal- systems rely on the blood volume pulse (BVP) sensor, or to-normal heart beats (SDNN or SDANN). This statistical photoplethysmograph, because it is more user friendly than index has health implications, predicting morbidity and the electrocardiogram used in medical settings. However, mortality. We will elaborate later on this relationship between the BVP signal is valuable in its own right, not merely HRV and health (Kleiger, Miller, Bigger, & Moss, 1987). as a convenient measure of HRV. This article explores the There are many technologies that can estimate the beat- methodology of BVP recording, the underlying physiology, to-beat variability in the sinus rhythm over a given period and the potential benefi ts from BVP treatment and training protocols. For example, the shape of the BVP waveform refl ects arterial changes correlated with hypertension. In Table. Alphabet soup of abbreviations addition, BVP training offers promise for the treatment of migraine and the monitoring of human sexual arousal. BVA Blood volume amplitude BVP Blood volume pulse Studies of methods for training improvement in heart rate variability (HRV), respiratory sinus arrhythmia (RSA), CRS Cardiorespiratory synchrony and cardiorespiratory synchrony (CRS) are promising EKG Electrocardiography areas of current research, expanding our knowledge of new clinical applications and training procedures for improving ECG Electrocardiography sympathetic-parasympathetic balance (see Table for the FFT Fast Fourier transform expansion of the abbrevations in text). For example, research HR Heart rate by Lehrer and colleagues (Lehrer et al., 2006; Lehrer et al., 2004) has demonstrated that HRV training in patients with HRV Heart rate variability asthma reduces their asthma severity. Similarly, research NN Normal-to-normal beat (interbeat interval) by Del Pozo and colleagues (Del Pozo, Del Pozo Scher, & PPG Photoplethysmography Guarneri, 2004) has shown that cardiac patients can improve their HRV. In addition, Giardino, Chan, and Borson (2004) PTT Pulse transit time reported a benefi t for patients with chronic obstructive RSA Respiratory sinus arrhythmia pulmonary disease using HRV training and an exercise protocol. It appears that learning to increase HRV results SDANN Standard deviation of average normal-to-normal in strengthening sympathetic-parasympathetic balance beat (also known as SDNN: standard deviation Biofeedback of the normal-to-normal beat) Ô and offers hope for many chronic illnesses (Gevirtz, 2003). Whereas clinical applications and feedback training of RSA SDAHR Standard deviation of average heart rate or CRS have been used to address illness affected by the Summer 2007 54 Peper, Harvey, Lin, Tylova, Moss Figure 2. Blood volume pulse, blood volume amplitude, heart rate, and Figure 1. Blood volume pulse, blood volume amplitude, heart rate, and respiration during training designed to increase cardiorespiratory synchrony. respiration before training. of time using measures of the volume of blood that passes breaths per minute with a corresponding heart rate of 73 over a photoplethysmographic (PPG) sensor with each beats per minute (SD = 10.1 beats). pulse, also called blood volume pulse (BVP). Recently, HRV Whereas designing training protocols for improving training using BVP measurements has escaped out of the HRV or RSA/CRS is possible using BVP technologies, the laboratory and into the marketplace. BVP training devices BVP signal holds even greater promise as a tool for use in are commercially available as portable units such as the improving other clinical applications and training protocols. StressEraser and the emWave or as units built for use with The remainder of this article explores various components computers such as the FreezeFramer or the Journey to Wild of the BVP technology. Divine. By using these BVP training devices, individuals can become aware of factors that increase or decrease their Background HRV. For example, excessive sensory arousal; shallow, rapid The BVP signal is derived with a PPG sensor that measures breathing; and excessive emotions of fear, worry, anger, changes in blood volume in arteries and capillaries by shining or panic will decrease HRV. On the other hand, reducing an infrared light (a light-emitting diode) through the tissues. sensory arousal, breathing slowly at approximately fi ve to This infrared light is selectively transmitted, backscattered, seven breaths per minute, and experiencing emotions of refl ected, and absorbed. The amount of light that returns appreciation and love will increase HRV. In clinical settings, to a PPG sensor’s photodetector is proportional to the practitioners use sophisticated multichannel biofeedback volume of blood in the tissue. The PPG signal represents systems to train clients to increase HRV and CRS. In most an average of all blood volume in the arteries, capillaries, clinical settings, heart rate information is derived from a and any other tissue through which the light passed. The BVP signal rather than from an electrocardiography (ECG) PPG signal depends on the thickness and composition of the signal. Whereas the ECG signal is more precise (e.g., with tissue beneath the sensor and the position of the source and fewer movement artifacts), applying an ECG sensor is also receiver of the infrared light. more cumbersome and obtrusive compared to applying a Most PPG sensors can be placed anywhere on the body, BVP sensor (e.g., without gel or tape or placement on a chest). from the earlobe to the vaginal wall, with the fi nger as the Using PPG feedback devices alone and/or in conjunction most common location for recording a BVP signal. The PPG with respiratory feedback devices such as respiratory strain sensor measures relative changes in the perfusion of the Biofeedback gauges, clients can learn to increase HRV/RSA, as shown in blood through the tissue underneath the sensor. For example, Figures 1 and 2. changes in the BVP signal can indicate relative changes in The participant data from Figure 1 represent an average the vascular bed due to vasodilation or vasoconstriction Ô respiration rate of 14 breaths per minute, with a corresponding (increase or decrease in blood perfusion) as well as changes Summer 2007 heart rate of 68 beats per minute (SD = 5.0 beats). The data in the elasticity of the vascular walls, refl ecting changes in from Figure 2 represent an average respiration rate of 7 blood pressure. Because the blood volume in the arteries and 55 Blood Volume Pulse Figure 3. Heart rate is derived from measures of blood volume pulse by Figure 4. Recording of the heart rate derived from the raw blood volume pulse measuring the interbeat interval and then transforming this information into during the baseline period. The average heart rate included a segment of very beats per minute. For example, the interbeat interval of 0.80 seconds is equal rapid heart beat refl ecting movement artifact rather than a true increase in to a heart rate of 75 beats per minute, whereas the interbeat interval of 0.93 average heart rate. seconds is equal to a heart rate of 64.5. capillary bed increases with each arterial pulsation, heart rate can be estimated from the BVP signal. Heart rate, or the number of heartbeats per minute, is calculated by estimating the time interval between the heartbeats, called the interbeat interval. The time in seconds of the interbeat interval is divided into 60 seconds to calculate the beat-by-beat heart rate, as shown in Figure 3. The time between each beat can vary; therefore, the estimated beat-by-beat heart rate can also vary. There are two estimates of the variability of the heart rate: variability estimated by the SDANN or variability estimated by the standard deviation of the average beat-by-beat heart rate (SDAHR). A methodological issue includes the fact that the raw BVP signal must be artifact free before meaning is Figure 5. When movement artifact is excluded from the signal shown in Figure assigned to the SDANN and SDAHR measures of variability 4, the actual heart rate is estimated at 61.05 beats per minute. in the heart rate data. The next section address artifacts in the BVP signal. This dialogue illustrates issues related to artifacts in the Inspecting the Raw BVP Signal for Artifacts BVP data. The data included movement artifacts, as shown in When analyzing average heart rates, it is important to Figure 4. After eliminating the movement artifact as shown eliminate artifacts before calculating estimates of HRV. Heart in Figure 5, the heart rate information was nearly fi ve beats rate averages calculated from the data may not be reliable if lower. Note also that in this participant, there was a slight artifacts exist, as illustrated in the following vignette: increase in heart rate during the stressful imagery rehearsal once the segments of data affected by artifact were removed. What happened? He reported increased arousal during Once movement artifacts are removed from BVP the stressful imagery, yet his heart rate was higher during estimates of heart rate, the BVP measure of HRV becomes the initial baseline condition.
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