Methodological trial to investigate the dose-response relationship, test-retest reliability and tolerance to repeated exposure of the 35% carbon dioxide experimental panic challenge in CO2-sensitive healthy volunteers

The CO2 inhalation model for panic has undergone both technical innovation and
relatively extensive validation since the 1980*s.

Panic disorder (PD) patients consistently display the highest sensitivity to
CO2, followed by first degree relatives of PD patients and healthy volunteers,
which show a concentration dependent sensitivity to inhaled CO2. Additionally,
registered anxiolytic drugs administered in clinically effective therapeutic
doses reduce CO2 sensitivity in healthy volunteers and patients over time.

Various CO2 regimens have been applied to induce panic attacks (PAs) in human
populations. Although both single and double vital capacity inhalations of 35%
CO2/65% O2 consistently demonstrate panicogenic effects in healthy volunteers
and panic disorder (PD) patients or their first-degree relatives, the CO2
sensitivity of both similar patient and healthy groups tend to vary between
research groups.
Since varying CO2 administration protocols which include both single and double
vital capacity administrations are being applied across research groups,
differences in CO2 sensitivity could very well be the result of unintended
methodological variability. The lack of a standardized procedure to test
sensitivity to CO2 therefore hampers accurate comparisons between tests
performed under different protocols. A better understanding of whether a single
or double breath 35% CO2/65% O2 is sufficient to induce PAs is expected to
contribute to the validity of acute CO2 inhalation as tool in
pathophysiological research and in early CNS drug development.

The differential response to single versus double 35% CO2 vital capacity
inhalation of CO2 remains to be examined systematically to further validate it
as experimental paradigm for future use. To the best of our knowledge no study
has been previously published that compares single and double vital capacity
35% CO2 inhalation in a single study. Therefore, we aim to investigate the
panicogenic effects of a single vs. a double vital capacity method 35% CO2 in
healthy volunteers. We hypothesize that 35% CO2 double vital capacity
inhalation is associated with a higher percentage of subjects experiencing a
panic attack compared to single vital capacity inhalation.

Additionally, this study also seeks to systematically investigate the
test-retest reliability of the CO2 inhalation challenge in healthy volunteers
who are sensitive to the anxiogenic effects of CO2 inhalation. Previous
research has only examined intervals of up to a few weeks, leaving it unclear
whether individuals who are sensitive to the 35% CO2 inhalation challenge
remain sensitive for longer periods, possibly years later.

Finally, this study seeks to investigate whether tolerance or desensitization
occurs in healthy volunteers who were previously sensitive to the CO2
challenge. While past research has explored this topic in patients with panic
disorders, studies in healthy volunteers who were screened for sensitivity to
the anxiogenic effects of CO2 challenges are non-existent. Previous tolerance
studies have been conducted in healthy volunteers, but these participants were
not selected based on their sensitivity to the anxiogenic effects of CO2
inhalation. If tolerance does not occur after four challenges administered one
week apart over the course of a month, this would justify conducting future
three-way crossover studies.

To investigate the dose-response relationship, test-retest reliability and
tolerance to repeated exposure of the 35% carbon dioxide experimental panic
challenge in CO2-sensitive healthy volunteers as measured with the PSL-IV total
Score, VAS Fear, and VAS Discomfort.

Five visit trial to investigate the dose-response relationship, test-retest
reliability and tolerance to repeated exposure of the 35% carbon dioxide
experimental panic challenge in 20 CO2-sensitive healthy volunteers.
Out of the five scheduled visits:
• single or a double vital capacity inhalation of 35% CO2/65% O2 will be
randomized over a 2-way cross-over part consisting of visit 1 and visit 2.
During the first visit, half of the participants (n=10) will be randomized to
receive a single vital capacity inhalation of CO2 while the other half (n=10)
will be randomized to double vital capacity inhalation of CO2. Approximately a
week later, the groups will switch, with the group originally assigned the
single vital capacity inhalation now receiving the double vital capacity
inhalation, and vice versa.
• visits 3, 4 and 5 will follow a fixed pattern, with all participants
receiving only double vital capacity 35% CO2/65% O2 inhalation roughly a week
apart.
On the first day, prior to the CO2 challenge, the Spielberger State-Trait
Anxiety Inventory (STAI), Dutch Personality Questionnaire (DPQ), and Cloninger
Temperament Character Inventory (TCI) will be administered.
To evaluate the severity of panic symptoms, each subject will complete the
PSL-IV , VAS Fear and VAS Discomfort within five minutes before and as soon as
possible (but no later than 15 minutes) after the CO2 challenge. In addition,
the STAI Y1 will be administered within 20 minutes before and as soon as
possible after the CO2 challenge. Throughout the procedure, vital signs such as
blood pressure and heart rate will be continuously monitored. Lastly,
biomarkers for neuroendocrine autonomic nervous system activation (plasma ACTH,
cortisol and prolactin; saliva alpha-amylase and cortisol) and plasma orexin-1
will be assessed.

The CO2 challenge has previously been established as a safe and effective
method to investigate panicogenic effects in healthy volunteers by multiple
research groups. No serious adverse events have been reported, nor has there
been evidence of increased risk of developing panic disorder (PD) as a result
of these tests. Therefore, the potential CO2-related carry-over effects are
non-existent.

The robust fear-like behavior CO2 induces in preclinical models mirrors the
respiratory and cardiovascular effects observed in humans, making acute CO2
inhalation a valid translational fear challenge. The physiological nature of
CO2 allows for real-time assessment of panic attacks (PAs) in experimental
settings. This model also offers the possibility of demonstrating panicolytic
effects of novel central nervous system (CNS) active compounds in human
subjects. Multiple studies conducted at the Centre for Human Drug Research
(CHDR) have indeed verified these panicolytic effects. In over 300 challenges
with healthy volunteers at CHDR, the CO2 challenge was deemed safe with no
serious adverse events.