Yohimbine premedication and 35% CO2 vulnerability in healthy volunteers

Modulation of hyperreactivity to 35% CO2 after one week of treatment with paroxetine and reboxetine: a double-blind, randomized study

The effects of short treatments (7 days) with paroxetine and with reboxetine on the reactivity to inhalations of 35% carbon dioxide (CO2)/65% oxygen (O2) were compared in 28 patients with Panic Disorder who had positive responses to 35% CO2 inhalations. A double-blind, randomized design was applied. Each patient was given the 35% CO2 challenge on days 0 (before starting the treatment) and 7. Anxiety reactivity to CO2 decreased significantly with both drugs but the decrease was significantly stronger in the group treated with paroxetine. The rate of patients whose reactivity has reduced of at least 50% after 7 days was significantly higher in the group treated with paroxetine (10/14, 71.5%) than in the one treated with reboxetine (3/14, 21.5%). These results indicate that the modulation of the serotonergic system is more relevant for CO2 hyperreactivity than the modulation of the noradrenergic one.

Does the brain noradrenaline network mediate the effects of the CO2 challenge?

The inhalation of carbon dioxide (CO2) is commonly used in patients and volunteers as a means of producing anxiety or panic. It is generally believed that patients with panic disorder are more vulnerable to the effects of CO2 than patients with other anxiety disorders or healthy volunteers and there is speculation and debate as to the mechanism for this apparent sensitivity. Recent work from our group has shown that a single inhalation of 35% CO2 activates the hypothalamic-pituitary-adrenocortical (HPA) axis, increases blood pressure (BP) and increases subjective fear responses in healthy volunteers. Correlation analyses reveal a relationship between the changes in BP and the cortisol increase. These findings led us to postulate that a common mechanism may mediate these and the subjective responses to inhalation of CO2. We propose that the noradrenergic system, particularly the locus coeruleus (LC), but including the A1 and A2 cell groups, may be a key mediator of these responses. This article examines the evidence and discusses the results of studies from our laboratory in relation to a neuroanatomical model centring on the LC.

Antipanic drug modulation of 35% CO2 hyperreactivity and short-term treatment outcome

Carbon dioxide (CO2) inhalation induces acute anxiety and panic attacks in patients with Panic Disorder (PD). Anti-panic drugs decrease CO2 reactivity after the first days of treatment; however, the clinical meaning of this finding has not yet been established. This study investigated the effects of treatment with tricyclic antidepressants and selective serotonin re-uptake inhibitors (SSRIs) on CO2 reactivity and compared the relationships between 35% CO2 hyperreactivity modulation and short-term clinical outcome. One hundred twenty-three patients with PD with or without agoraphobia who were hyperreactive to CO2 were randomly assigned to treatment groups with imipramine, clomipramine, paroxetine, sertraline, or fluvoxamine. A double-blind, randomized design was applied. Each patient received the 35% CO2 challenge on days 0, 7, and 30. The severity of clinical symptomatology was measured on days 0 and 30. Decreased hyperreactivity to 35% CO2 in all five treatment groups was already evident after the first week. The decrease in CO2 reactivity at the end of treatment was proportional to the degree of clinical improvement. Multiple regression analyses showed that the decrease in CO2 reactivity after the first week was a significant predictor for good clinical outcome after one month. The results of this study confirm evidence that psychoactive drugs effective in the treatment of PD decrease CO2 hyperreactivity. They also suggest that precocious modulation of CO2 reactivity might fairly reliably predict short-term clinical outcome in patients with "respiratory" PD.

The 35% CO2 hyperreactivity and clinical symptomatology in patients with panic disorder after 1 week of treatment with citalopram: an open study

The effect of a short treatment (7 days) with citalopram on the reactivity to inhalations of 35% CO2 and 65% oxygen and on clinical symptomatology was investigated in 15 patients with panic disorder who had a positive response to 35% CO2 inhalation. An open study design was applied. On day 0, before starting drug treatment, and after 1 week of treatment, each patient underwent the 35% CO2 challenge, and clinical symptomatology was evaluated with psychometric scales. The results showed a significant reduction of CO2 reactivity and of scores on the anticipatory anxiety subscale of Panic Associated Symptoms Scale. These results confirm that the serotonergic system plays an important role in the modulation of CO2 hyperreactivity and suggest an early anxiolytic effect of citalopram in patients who have panic disorder and are hyperreactive to CO2.

Modification of 35% carbon dioxide hypersensitivity across one week of treatment with clomipramine and fluvoxamine: a double-blind, randomized, placebo-controlled study

The effects of short treatments (7 days) with clomipramine and fluvoxamine on the reactivity to inhalations of 35% CO2/65% O2 were compared in 39 panic patients who had positive responses to 35% CO2 inhalations. A double-blind, randomized, placebo-controlled design was applied. Each patient was given the 35% CO2 challenge on days 0 (before starting treatment), 3, and 7. Patients on placebo did not report any significant changes in their reactivity to 35% CO2 across the three sessions, whereas patients on clomipramine and fluvoxamine reported a significant attenuation of the reactivity on day 7. These results indicate that treatments with clomipramine and fluvoxamine decrease hypersensitivity to 35% CO2 after a few days, suggesting a relevant role of the modulation of CO2 sensitivity by the serotonergic system in antipanic properties of these compounds.