Running head: HEART MATH AS A SUBSTANCE ABUSE INPATIENT TREATMENT
Investigating Heart Math as a Treatment for
Substance Abuse in an Inpatient Setting
Edward McMillan
Clinical Mental Health Counseling, Lenoir Rhyne University
EDU 505 Research & Statistics
Richard Trent Codd, III
10/27/2020
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Abstract
The Heart Math system has existed and evolved over the last 25 years and has recently
experienced a significant increase in scientific exploration. Heart Math claims its biofeedback
tools and processes facilitate physical and emotional well-being through the trained increase in
cardiac coherence. The process it teaches includes the practices of active gratitude and heartcentered breathing techniques. The scientific literature is noticeable lacking in the analysis of
these practices and their individual associations with cardiac coherence and resulting increases in
psychological and physiological well-being. This proposed 2x2 factorial analysis is intended to
investigate the interactions and associations among several variables, including gratitude and
heart-centered breathing practices, cardiac coherence, state and trait anxiety and craving in a
sample of individuals who are clients of a 90 day substance use disorder inpatient treatment
system.
Keywords: Heart Math, Cardiac Coherence, Substance Abuse Treatment
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Investigating Heart Math as a Treatment for Substance Abuse in an Inpatient Setting
Heart Math is biofeedback system based on the heart rhythm coherence hypothesis
(McCraty, et al. 2009). The hypothesis holds that the stability and pattern of heart rate activity
from beat-to-beat (Heart Rate Variability) encode biological data over “macroscopic time
scales,” which then effects cognitive performance and emotional experience. (McCraty, et al.
2009). The researchers at the Heart math institute further assert that this encoded information in
heart rate variability can help synchronize the entire physiological system, which they term
“Physiological Coherence” (McCraty & Childre, 2010). It is their thesis that heart-centered
breath, gratitude and other positive emotions in general, as well as those emotions which are selfinduced, shift the heart and the system as a whole into this universally coherent and harmonious
physiological state which is associated with improved performance, increased self-regulation,
and overall sense of well-being (Shaffer et al., 2014).
The Heart Math website boasts that the system “empowers” individuals in the selfregulation of their emotions and behaviors in order to decrease stress, increase resilience, and to
realize their natural intuitive guidance in order to make self-effective choices. They further claim
that the system facilitates the break through to superior levels of “personal balance, creativity,
insight and health” (About Heart Math, 2020). The website also links a google scholar list of 466
citations in text stating that heart math technologies and techniques have been utilized in
over 300 peer-reviewed or independent published studies (About Heart Math, 2020). However,
the vast majority do not seem to be published in peer reviewed journals ion addition to the
criteria for being related to the techniques and technology of Heart Math being very generous.
Heart Math is claimed to have significantly reduced anxiety in nearly half of an 11,000+
sample of users and claims that it is beneficial in treating addiction (About Heart Math, 2020).
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Additionally, substance abuse treatment facilities have begun to advertise the use of the system
with clients (Sanctuary Program Highlights, 2020). This proposed study seeks to investigate the
efficacy of the Heart Math system in increasing cardiac coherence and decreasing both trait and
state anxiety as well as substance craving in a sample of individuals in an inpatient substance
abuse treatment setting. Additional interests of the proposal are the associations between the
primary techniques of the system and the above stated variables.
Literature Review
Heart rate variability refers to the intervals of time between contiguous heartbeats (Lehrer
et al., 2020). An increasing amount of research has shown that structured variability in heart rate
is likely a reliable indication of both physical and psychological general health (Lehrer et al.,
2020). Heart rate variability biofeedback (HRVB), a technique used to increase heart rate
variability, has gained increasing popularity among psychotherapists interested in utilizing
psychophysiology in their practice (Lehrer et al., 2020).
Research has shown that HRVB results in improvements in a variety of psychological
and physiological conditions, such as depression, anxiety, asthma, pain, and hypertension
(Lehrer et al., 2020). HRVB is also related to improvements in numerous types of human
performance, including dance, music, athletics, agility and concentration (Lehrer et al., 2020).
HRVB has recently been put forth as an useful psychotherapy component that focuses on
neurovegetative components of psychological issues and is thought to improve treatment
effectiveness (Caldwell and Steffen 2018; Lehrer 2018; Wheeler 2018).
While there have been clinical and theoretical support and calls for more research into the
usefulness of HRVB and Cardiac Coherence Biofeedback as supplemental treatments in front
line substance abuse disorder programs, very little has been published to date (Eddie et al., 2016;
HEART MATH AS A SUBSTANCE ABUSE INPATIENT TREATMENT
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Leyro et al., 2019). The small amount of research that has been conducted has not included
cardiac coherence or heart math. A 2015 study found that HRVB reduced both anxiety and
cravings in a sample of 48 alcohol dependent individuals undergoing inpatient treatment (Penzlin
et al.) Alayan et al. found that a HRVB practice of more than 12 minutes per day was related to a
decrease in cravings over time in a sample of university students (2019).
Research Methods
Research Questions
1. Will active heart focused breathing increase cardiac coherence?
2. Will active gratitude practice increase cardiac coherence?
3. Will heart focused and gratitude have a combined positive effect on cardiac
coherence?
4. Will heart focused, gratitude, and cardiac coherence be associated with decreased
state and trait anxiety?
5. Will heart focused breathing, gratitude, and cardiac coherence be associated with
decreased drug craving?
Hypothesis
This proposal hypothesizes that gratitude and resonant breathing will have individual and
combined positive effects on cardiac coherence in individuals with substance abuse disorders
who are in a 90 day inpatient treatment facility. Additionally, gratitude, resonant breathing and
cardiac coherence will have individual negative associations with state anxiety as measured by
the state anxiety portion of the Spielberger State-Trait-Anxiety Inventory (STAI-Y) (Spielberger
et al., 1975), general anxiety as measured by the Penn St. Worry Questionnaire (Meyer et al.,
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1990) and drug cravings as measured by the Brief Substance Craving Scale (Somoza et al.,
1995).
Operational Definitions
Heart Focused Breathing: Focusing attention on the heart or center of the chest while
practicing a comfortable and controlled breathing at a rate of 4.5-6.5 breaths per minute
(Schaffer, 2014).
Gratitude Practice: Focusing and refocusing one’s attention on those things or
experiences that an individual has significant personal appreciation for (Heckendorf, 2019).
Cardiac Coherence: “A coherent heart rhythm is defined as a relatively harmonic (sinewave-like) signal with a very narrow, high-amplitude peak in the LF region of the HRV power
spectrum with no major peaks in the VLF or HF regions. Coherence is assessed by identifying
the maximum peak in the 0.04–0.26 Hz range of the HRV power spectrum, calculating the
integral in a window 0.030 Hz wide, centered on the highest peak in that region, and then
calculating the total power of the entire spectrum. The coherence ratio is formulated as: (Peak
Power/[Total Power – Peak Power])” (McCraty, 2011).
`
State Anxiety: Temporary and fluctuating experience of anxiety to a relatively immediate
stressor, in this case the daily experience of early substance abuse recovery within an inpatient
setting (Leal et al., 2017).
Trait Anxiety: A relatively stable personality characteristic describing individual
differences related to the propensity to experience state anxiety or worry (Leal et al., 2017).
Drug Craving: The desire to acquire and use a particular drug or combination of drugs for
the purposes of altering ones state of being (Sayette, 2000).
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Participants and Sampling
Participants for this proposed study will be obtained through a convenience sample in
order to control for those extraneous variables related to therapeutic differences as much as
possible and to facilitate data collection. This sampling decision is discussed in more detail in the
limitations section of this proposal. Participants will be drawn from within a large substance
abuse treatment system (“system”) located in the United States. The system consists of 16
inpatient substance abuse treatment facilities which provide a ninety day inpatient substance
abuse treatment program. Treatment modalities across the system are uniform in both material
and structure. Participants will be recruited during or shortly after the intake process at each
individual system facility. Requirements for inclusion include: a diagnosis of Substance Use
Disorder according to the Diagnostic and Statistical Manual for Mental Disorders, 5th edition
(American Psychiatric Association, 2013), a client of the system, in his or her first week of the
system’s 90 day treatment program, with no serious cardiac issues or illnesses (including but not
limited to arrhythmias), not currently on any medication or supplement regimen that treats or that
may cause symptoms of anxiety, not currently diagnosed with a Personality Disorder, with no
recent history or currently experiencing any form of psychosis, no experience with or working
knowledge of heart math or cardiac coherence, with no metabolic or hormonal disorders, with
full release from system facilities medical staff. The prospective convenient sample will be
largely compiled of white adults aged between 18 and 50, with a mean age of 34 according to the
system’s average clientele. The system accepts both in and out of network insurance pay, as well
as private cash pay. Therefore, the prospective sample will likely be well above average socioeconomic status. This is very comparable to the average population across private inpatient
substance abuse treatment centers and should offer little threat to the external validity for the
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target population. However, substance abuse affects individuals across all socio-economic and
racial lines and any application of the proposed intervention to other subgroups within the
substance abuse treatment domain warrants further investigation.
Experimental Measures and Tools
State-Trait Anxiety Inventory State Scale
The State-Trait Anxiety Inventory (“STAI”) is a relatively short assessment designed to
measure both state anxiety and trait anxiety. The STAI state scale is made up of a total of 20
statements that require respondents to label how they feel at a particular point in time (e.g., calm,
tense) by rating such on a 4 point scale of intensity, which ranges from “not at all” to “very much
so” (Spielberger, 1975; Spielberger 1983). The assessment has been shown to have internal
consistency and reliability estimates that are generally satisfactory for a broad range of studies
across a large variety of populations. (Barnes et al., 2002).
Penn St. Worry Questionnaire
The Penn State Worry Questionnaire for Adults (“PSWQ”) is the most frequently used
measure to assess trait anxiety or pathological worry in adults (Meyer, Miller, Metzger, &
Borcovec, 1990). The PSWQ is a 16-item self-report questionnaire, compiled from a factor
analysis of 161 items believed to be related to worry. The PSWQ is proposed to measure a
respondent’s disposition to worry. Additionally, it measures the intensity, frequency, and
individual tendency for worry to be generalized (Meyer et al., 1990). Individuals are instructed to
rate each item of the assessment on a 5-point Likert scale that ranges from 1 (“not at all typical
of me”) to 5 (“very typical of me”). The first eleven items are positively scored in relation to
pathological worry (e.g. “my worries overwhelm me”), whereas the final five items on the
instrument require reverse scoring and show worry as not being an issue (e.g. “I never worry
HEART MATH AS A SUBSTANCE ABUSE INPATIENT TREATMENT
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about anything”). The scores from each item are added together to yield a total score that ranges
from 16-80, with higher scores representing higher levels of pathological worry (Meyer et al.,
1990).
Brief Substance Craving Scale (2007 edition)
The 2007 edition of the Brief Substance Craving Scale (“BSCS”) is an 8 item (4 items
each for primary and secondary drug of choice), self-report instrument assesses craving for
substances of abuse over a 24 hour period. A five point Likert scale is used to record Intensity
and frequency of craving. The instrument was designed by the Craving Subcommittee of the
NIDA Medications Development Research Units (MDRU) and is an expansion of the Cincinnati
Craving Scale. Reliability data for this instrument has been collected at the Cincinnati MDRU.
The BSCS takes approximately 5 minutes (Somoza et al., 1995).
emWave Pro Desktop System
Heart rate data will be recorded with the emWave pro Desktop system (“EWD”)
(HeartMath, LLC, Boulder Creek California). EWD utilizes a pulse sensor that attaches to the
ear and plugs into a PC or Mac USB port. The heart rate variability monitoring system collects
pulse data, measures and then translates cardiac coherence data into user-friendly biofeedback
information and graphics displayed on a computer screen. The biofeedback data displayed is
utilized by the user to learn how to recognize when he or she is out of coherence and what
thoughts or emotions may have precipitated or caused that loss of coherence. This data is then
used as a prompt to reenter cardiac coherence through the use of active gratitude practice and
heart centered breathing.
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Design and Procedure
The proposed design for this prospective experiment is a 2 x 2 factorial design. The study
will be conducted over a 6 month period, while each individual experiment will last 10 weeks.
The design will be implemented in the following manner so as to minimize threats to internal
validity. Each of the system’s 16 facilities will be assigned as either a Gratitude Treatment,
Breath Treatment, Combined Treatment, or Control Group utilizing a block randomization
method. Each participating intervention center will be randomly assigned to one of the four
treatment conditions in 4 different rounds. Each condition must be filled once in each round
before the next round begins and randomization is reset. This will be done in order to keep
sample sizes as equal as possible and to facilitate the standardization and streamlining of the
training for those system therapists who will be administering the various intervention
conditions.
All groups will be administered the Penn St. Worry questionnaire pre-test, within 48
hours of intake and post-test during discharge. Individual therapists at the system facilities will
receive training in how to coach active gratitude practice and/or heart centered breathing
depending on what group there facility is assigned to. All individual therapists within the system
will receive training on how to use and train clients to use the emWave Pro cardiac coherence
biofeedback system. Individual participation in the study will commence at the beginning of the
participant’s second full week as a client in the facility’s program and conclude 10 full weeks
later.
Gratitude Treatment Group
Those qualifying participants at facilities that are assigned to the Gratitude Treatment
group will undergo 10 minutes of coaching in active gratitude practice at the end of their daily 90
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minute individual therapy session during the first week of the study as a primer. At the end of
the first week, participants will receive a 30 minute training in the use of the emWave Pro.
Beginning the second week participants will begin using the biofeedback system for two 10
minute sessions at the end of daily individual therapy. During the session the participant will be
instructed to engage in active gratitude practice. In between and after biofeedback sessions the
therapist and participant will briefly discuss what the participant noticed regarding the cardiac
coherence biofeedback as it relates to the participant’s gratitude practice, as well as their
emotional and mental processes. The participant will be encouraged to utilize any insights
gained during subsequent sessions.
Breath Treatment Group
Those qualifying participants at facilities that are assigned to the Breath Treatment group
will undergo 10 minutes of coaching in heart focused breathing practice at the end of their daily
90 minute individual therapy session during the first week of the study as a primer. At the end of
the first week, participants will receive a 30 minute training in the use of the emWave Pro.
Beginning the second week participants will begin using the biofeedback system for two 10
minute sessions at the end of daily individual therapy. During the session the participant will be
instructed to engage in heart focused breathing. In between and after biofeedback sessions the
therapist and participant will briefly discuss what the participant noticed regarding the cardiac
coherence biofeedback as it relates to the participant’s breathing and attention on breathing or
lack there-of. The participant will be encouraged to utilize any insights gained during
subsequent sessions.
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Combined Treatment Group
Those qualifying participants at facilities that are assigned to the Combined Treatment
group will undergo 10 minutes of coaching in active gratitude practice and 10 minutes of
coaching in heart focused breathing practice at the end of their daily 90 minute individual
therapy session during the first week of the study as a primer. At the end of the first week,
participants will receive a 30 minute training in the use of the emWave Pro. Beginning the
second week participants will begin using the biofeedback system for two 10 minute sessions at
the end of daily individual therapy. During the session the participant will be instructed to
engage in both active gratitude practice and heart focused breathing. In between and after
biofeedback sessions the therapist and participant will briefly discuss what the participant
noticed regarding the cardiac coherence biofeedback as it relates to the participant’s active
gratitude practice, the participant’s breathing, as well as the participant’s mental and emotional
processes. The participant will be encouraged to utilize any insights gained during subsequent
sessions.
Control Treatment Group
Those qualifying participants at facilities that are assigned to the Control Treatment
group will undergo 10 minutes of psychoeducation in relapse prevention at the end of their daily
90 minute individual therapy session during the first week of the study. At the end of the first
week, participants will receive a 30 minute training in the use of the emWave Pro. Beginning the
second week participants will begin using the biofeedback system for two 10 minute sessions at
the end of daily individual therapy. The participants in the control treatment group will be
instructed to work on a relapse prevention plan during the biofeedback sessions. In between and
after the biofeedback sessions participants in this group will be allowed to briefly stretch, walk
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around or used the bathroom. They will also be allowed to ask the therapist questions regarding
their relapse prevention plans during and after the sessions.
For participants in all groups, the Brief Substance Craving Scale will be administered at
the beginning and the end of the participant’s individual therapy session on Mondays,
Wednesdays, and Fridays. The State Scale portion of the State-Trait Inventory will be
administered at the beginning and end of the participant’s individual therapy sessions on
Tuesdays and Thursdays.
Limitations
Due to the intimate nature of inpatient living and the goal oriented nature of the cardiac
coherence biofeedback software used, treatment diffusion among participants was a primary
concern of this proposal. While the block randomization utilized is intended to control for
treatment diffusion and its threat to internal validity, it along with the convenience sampling
result in a reduction in the randomization of participants to the different treatment groups and
significantly threatens external validity. Additionally, although system programming is
consistently uniform across system facilities, even slight individual differences among therapist
orientation and delivery in group and individual sessions could threaten internal and external
validity. Individual participant needs will likely drive individual sessions and threaten both
internal and external validity. The high frequency and repetition of testing measures also poses a
threat to validity.
Withholding parts of the treatment, relapse prevention work, and/or the entire treatment
from participants is a significant ethical concern. This is especially true due to the serious and
deadly nature of substance abuse disorders. In order to lessen the ethical and therapeutic impact
of missing intervention aspects, participants will be offered free virtual training and therapy
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consisting of those modalities not included in their assigned treatment group intervention. This
will be done by the system’s after care coordinators and after care therapist. Prospective
participants will only be told that they may take part in a study related to heart math and cardiac
coherence. Although participants will not be told of the specific intervention aspects during
intake and the informed consent process in order to protect internal validity, they will be assured
that any treatment aspect that they did not experience will be offered at no extra charge. They
will then be fully debriefed, reminded of this option during the discharge process and
arrangements will be made according to their wishes.
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Reference List
American Psychiatric Association (2013). Diagnostic and statistical manual of mental disorders
(5th ed.). https://doi.org/10.1176/appi.books.9780890425596
About Heart Math. (2020). Heart Math. https://www.heartmath.com/about/
Alayan, N., Eddie, D., Eller, L., Bates, M. E., & Carmody, D. P. (2019). Substance craving
changes in university students receiving heart rate variability biofeedback: A longitudinal
multilevel modeling approach. Addictive Behaviors, 97, 35–41.
https://doi.org/10.1016/j.addbeh.2019.05.005
Barnes, L. L. B., Harp, D., & Jung, W. S. (2002). Reliability generalization of scores on the
Spielberger State-Trait Anxiety Inventory. Educational and Psychological
Measurement, 62(4), 603–618. https://doi.org/10.1177/0013164402062004005
Burch, J. B., Ginsberg, J. P., McLain, A. C., Franco, R., Stokes, S., Susko, K., Hendry, W.,
Crowley, E., Christ, A., Hanna, J., Anderson, A., Hébert, J. R., & O’Rourke, M. A.
(2020). Symptom management among cancer survivors: Randomized pilot intervention
trial of heart rate variability biofeedback. Applied Psychophysiology and
Biofeedback, 45(2), 99–108. https://doi.org/10.1007/s10484-020-09462-3
Caldwell, Y. T., & Steffen, P. R. (2018). Adding HRV biofeedback to psychotherapy increases
heart rate variability and improves the treatment of major depressive
disorder. International Journal of Psychophysiology, 131, 96–101.
https://doi.org/10.1016/j.ijpsycho.2018.01.001
Eddie, D., Vaschillo, E., Vaschillo, B., & Lehrer, P. (2015). Heart rate variability biofeedback:
Theoretical basis, delivery, and its potential for the treatment of substance use
disorders. Addiction Research & Theory, 23(4), 266–272.
https://doi.org/10.3109/16066359.2015.1011625
HEART MATH AS A SUBSTANCE ABUSE INPATIENT TREATMENT
16
Emmons, R. A. (2019). From the science of gratitude to a global gratitude movement. Journal of
Psychology and Christianity, 38(3), 198-205. Retrieved from
https://search.proquest.com/docview/2391221592?accountid=13505
Ginsberg, J. P., PhD., Berry, M. E., M.S., & Powell, D. A., PhD. (2010). CARDIAC
COHERENCE AND POSTTRAUMATIC STRESS DISORDER IN COMBAT
VETERANS. Alternative Therapies in Health and Medicine, 16(4), 52-60. Retrieved
from https://search.proquest.com/docview/670770930?accountid=13505
Leal, P. C., Goes, T. C., da Silva, L. C. F., & Teixeira, S. F. (2017). Trait vs state anxiety in
different threatening situations. Trends in Psychiatry and Psychotherapy, 39(3), 147–157.
https://doi.org/10.1590/2237-6089-2016-0044
Lehrer, P. M. (2018). Heart rate variability biofeedback and other psychophysiological
procedures as important elements in psychotherapy. International Journal of
Psychophysiology, 131, 89–95. https://doi.org/10.1016/j.ijpsycho.2017.09.012
Lehrer, P., Kaur, K., Sharma, A., Shah, K., Huseby, R., Bhavsar, J., & Zhang, Y. (2020). Heart
rate variability biofeedback improves emotional and physical health and performance: A
systematic review and meta analysis. Applied Psychophysiology and Biofeedback, 45(3),
109–129. https://doi.org/10.1007/s10484-020-09466-z
Leyro, T. M., Buckman, J. F., & Bates, M. E. (2019). Theoretical implications and clinical
support for heart rate variability biofeedback for substance use disorders. Current
Opinion in Psychology, 30, 92–97. https://doi.org/10.1016/j.copsyc.2019.03.008
McCraty, R. (n.d.). The science of Heartmath. Retrieved October 21, 2020, from
https://www.heartmath.com/science/
HEART MATH AS A SUBSTANCE ABUSE INPATIENT TREATMENT
17
McCraty, R. (2011). Coherence: Bridging personal, social, and global health. Activitas Nervosa
Superior Rediviva, 53(3), 85– 102
McCraty, R., Atkinson, M., Tomasino, D., and Bradley, R. T. (2009). The coherent heart: heartbrain interactions, psychophysiological coherence, and the emergence of system-wide
order. Integral Rev. 5, 10–115.
McCraty, R., and Childre, D. (2010). Coherence: bridging personal, social, and global
health. Altern. Ther. Health Med. 16, 10–24.
Meyer, T. J., Miller, M. L., Metzger, R. L., & Borkovec, T. D. (1990). Development and
validation of the Penn State Worry Questionnaire. Behaviour Research and
Therapy, 28(6), 487–495. https://doi.org/10.1016/0005-7967(90)90135-6
Penzlin, A. I., Siepmann, T., Illigens, B. M.-W., Weidner, K., & Siepmann, M. (2015). Heart rate
variability biofeedback in patients with alcohol dependence: A randomized controlled
study. Neuropsychiatric Disease and Treatment, 11.
Reyes, F. J. (2014). Implementing heart rate variability biofeedback groups for veterans with
posttraumatic stress disorder. Biofeedback (Online), 42(4), 137-142.
doi:http://dx.doi.org/10.5298/1081-5937-42.4.02
Sanctuary Program Highlights. (2020). The Sanctuary at Sedona. https://www.sanctuary.net/ourapproach/program-highlights/
Shaffer, F., McCraty, R., & Zerr, C. L. (2014). A healthy heart is not a metronome: An
integrative review of the heart’s anatomy and heart rate variability. Frontiers in
Psychology, 5. https://doi.org/10.3389/fpsyg.2014.01040
HEART MATH AS A SUBSTANCE ABUSE INPATIENT TREATMENT
18
Somoza, E., S. Dyrenforth, J. Goldsmith, J. Mezinskis, and M. Cohen (1995). In search of a
universal drug craving scale. Paper presented at the Annual Meeting of the American
Psychiatric Association, Miami, Florida, May 20-25.
Spielberger, C. D. (1975). Anxiety: state-trait process. In C. D. Spielberger & I. G. Sarason
(Eds.), Stress and anxiety (pp. 115–143). New York: Wiley.
Spielberger, C. D. (1983). Manual for state-trait anxiety inventory. Palo Alto, CA: Consulting
Psychologists Press.
Sayette, M. A., Shiffman, S., Tiffany, S. T., Niaura, R. S., Martin, C. S., & Shadel, W. G. (2000).
The measurement of drug craving. Addiction, 95(Suppl2), S189–S210.
https://doi.org/10.1080/09652140050111762
Wheeler, L. F. (2018). Stress and psychotherapy outcome: Implementation of a heart rate
variability biofeedback intervention to improve psychotherapy outcome [ProQuest
Information & Learning]. In Dissertation Abstracts International: Section B: The
Sciences and Engineering (Vol. 79, Issue 3–B(E))