This double-blind, placebo-controlled, within-subjects proof-of-concept study (n=20) investigated the antidepressant efficacy of inhaled nitrous oxide (50/50 nitrous oxide/oxygen vs. 50/50 nitrogen/oxygen) in patients with treatment-resistant depression (TRD). Nitrous oxide resulted in treatment response in 20% of patients and symptom remission in 15%, an effect size comparable to that of ketamine.
Papers cited by this study that are also in Blossom
Berman, R. M., Cappiello, A., Anand, A. et al. · Biological Psychiatry (2000)
Background
N-methyl-D-aspartate receptor antagonists, such as ketamine, have rapid antidepressant effects in patients with treatment-resistant depression (TRD). We hypothesized that nitrous oxide, an inhalational general anesthetic and N-methyl-D-aspartate receptor antagonist, may also be a rapidly acting treatment for TRD.
Methods
In this blinded, placebo-controlled crossover trial, 20 patients with TRD were randomly assigned to 1-hour inhalation of 50% nitrous oxide/50% oxygen or 50% nitrogen/50% oxygen (placebo control). The primary endpoint was the change on the 21-item Hamilton Depression Rating Scale (HDRS-21) 24 hours after treatment.
Results
Mean duration of nitrous oxide treatment was 55.6 ± 2.5 (SD) min at a median inspiratory concentration of 44% (interquartile range, 37%-45%). In two patients, nitrous oxide treatment was briefly interrupted, and the treatment was discontinued in three patients. Depressive symptoms improved significantly at 2 hours and 24 hours after receiving nitrous oxide compared with placebo (mean HDRS-21 difference at 2 hours, −4.8 points, 95% confidence interval [CI], −1.8 to −7.8 points, p = .002; at 24 hours, −5.5 points, 95% CI, −2.5 to −8.5 points, p < .001; comparison between nitrous oxide and placebo, p < .001). Four patients (20%) had treatment response (reduction ≥50% on HDRS-21) and three patients (15%) had a full remission (HDRS-21 ≤ 7 points) after nitrous oxide compared with one patient (5%) and none after placebo (odds ratio for response, 4.0, 95% CI, .45-35.79; OR for remission, 3.0, 95% CI, .31-28.8). No serious adverse events occurred; all adverse events were brief and of mild to moderate severity.
Conclusions
This proof-of-concept trial demonstrated that nitrous oxide has rapid and marked antidepressant effects in patients with TRD.
Treatment-resistant depression (TRD) is described as a severe form of major depressive disorder in which patients fail multiple standard antidepressant treatments and face poor long-term prognosis; the authors note that about one in three patients with major depression meet criteria for TRD. The NMDA (N-methyl-D-aspartate) receptor has been implicated in depression neurobiology, and earlier studies have shown that NMDA receptor antagonists such as ketamine can produce rapid antidepressant effects at subanesthetic doses. Given a similar primary mechanism of action, nitrous oxide was proposed as a candidate rapid-acting treatment for TRD because it is an inhalational general anaesthetic that acts as a noncompetitive NMDA receptor inhibitor. Nagele and colleagues therefore conducted a proof-of-concept clinical trial to test whether a single session of nitrous oxide inhalation produces immediate (2-hour) and sustained (24-hour) antidepressant effects in well-characterised patients with TRD. The trial aimed to detect an early efficacy signal that could justify larger studies and to collect preliminary safety and tolerability data in this population.
The investigators designed a randomised, placebo-controlled crossover pilot trial in which 20 evaluable patients with TRD each received two 1-hour inhalation sessions one week apart: active treatment (up to 50% nitrous oxide/50% oxygen) and placebo (50% nitrogen/50% oxygen). Session order was randomised by a computer generator. To preserve blinding the teams administering gas and performing psychiatric assessments were physically separated, treatment records were kept separately until study completion, the equipment and procedures were identical between sessions, and patients were told they would receive either nitrous oxide or an air mixture with a high nitrogen component. Participants were adults aged 18–65 meeting DSM-IV-TR criteria for major depressive disorder without psychosis, with pretreatment HDRS-21 > 18, and with TRD defined as at least two failed adequate antidepressant trials in the current episode and at least three lifetime antidepressant failures. Major psychiatric exclusions included bipolar disorder, psychotic disorders, recent substance abuse, active suicidal intention, prior NMDA antagonist exposure, and ongoing electroconvulsive therapy. Medical exclusions included conditions contraindicating nitrous oxide and pregnancy/breastfeeding. Patients were required to maintain stable antidepressant or psychotherapy regimens for 4 weeks prior to and during the study. During active treatment the nitrous oxide concentration was titrated over the first 10 minutes toward a 50% inspiratory concentration (the dose commonly used in clinical sedation settings); the total gas flow was 4–8 L/min and administration used a standard anaesthesia facemask with monitoring of inhaled/exhaled concentrations. Continuous physiologic monitoring (ECG, pulse oximetry, noninvasive blood pressure, end-tidal CO2) was provided under an attending anaesthesiologist, and patients were observed in recovery for 2 hours post-treatment. Outcome assessments occurred at baseline, 2 hours and 24 hours after each session (six time points in total). The primary endpoint was change in the 21-item Hamilton Depression Rating Scale (HDRS-21) at 24 hours. Secondary endpoints included the Quick Inventory of Depressive Symptomatology Self-Report (QIDS-SR). Psychiatric safety assessments targeted suicidality and emergent psychosis; other safety outcomes included cardiovascular, respiratory, and CNS adverse events and measurement of plasma total homocysteine to estimate nitrous oxide-induced vitamin B12 inactivation. For statistical analysis the primary HDRS-21 outcome was evaluated using a repeated-measures mixed effects linear model with restricted maximum likelihood estimation. Because a carryover effect was observed, the model included a randomisation group term and a three-way interaction (treatment × time × randomisation group), and the investigators also conducted an analysis restricted to the first treatment session (parallel-group comparison). Response and remission comparisons used an exact binomial test for paired data, with odds ratios presented; the pilot sample size (20 evaluable patients) was chosen pragmatically based on prior ketamine studies in similar populations. All reported p values were two-sided and p < .05 was considered statistically significant.
This study was designed as a randomized, placebo-controlled crossover pilot clinical trial testing the antidepressant effects of nitrous oxide in 20 patients with TRD. In this study, patients had two treatment sessions that were 1 week apart (nitrous oxide or placebo). The sequential order of the sessions was assigned by a random number generator. Other than the gas mixture administered, the sessions were indistinguishable in setting, setup, and monitoring. We undertook several measures to ensure treatment blinding. First, we completely separated personnel and location of the team providing nitrous oxide treatment from the team performing psychiatric evaluations. The two locations were physically separated from each other, and no team member was allowed to enter the other space while a study patient was present. Second, records for the nitrous oxide and placebo treatment administration were kept separate from the psychiatric assessment case report forms until completion of the study. Third, all equipment used to provide treatments was identical between nitrous oxide and placebo sessions. Lastly, patients were blinded as to the nature of the inhaled gas at each inhalation session; all patients were informed that they would receive either nitrous oxide or an air mixture with a high nitrogen component (placebo). A data and safety monitoring board monitored the trial. The study was approved by the Washington University in St. Louis Institutional Review Board, and all patients provided written, informed consent. The trial was registered at clinicaltrials.gov (NCT02139540).
Outcomes were assessed at six time points for each patient (three per session; two sessions): at baseline (pretreatment), 2 hours after treatment for each session, and 24 hours after treatment for each session. A 1-week outcome assessment was not formally planned but was available as part of the baseline assessment for the second treatment session. The primary study endpoint was the change in the HDRS-21 at 24 hours after treatment. Secondary endpoints included change on the Quick Inventory of Depressive Symptomatology Self Report (QIDS-SR) scale. The primary mood assessment was selected to be administered at 24 hours to ensure that any acute euphoric effects of nitrous oxide had dissipated by this time (nitrous oxide euphoric effects typically cease shortly after discontinuation of nitrous oxide administration). Psychiatric safety endpoints were assessed via careful clinical observations and questioning for dangerousness to self (suicidality) as well as for emergence of psychosis (hallucinations, delusions, disorganized thinking). Other safety endpoints included cardiovascular, respiratory, and central nervous system adverse events determined by hemodynamic and respiratory monitoring. The extent of nitrous oxide-induced inactivation of vitamin B 12 was determined by measurement of plasma total homocysteine before and after treatment.
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Zarate, C. A., Brutsche, N. E., Ibrahim, L. et al. · Biological Psychiatry (2012)
Murrough, J. W., Perez, A. M., Pillemer, S. et al. · Biological Psychiatry (2012)
Paul, R. K., Singh, N. S., Khadeer, M. et al. · Anesthesiology (2014)
Papers in Blossom that reference this study
De Filippo, R., Schmitz, D. · Neuroscience and Biobehavioral Reviews (2024)
Lii, T. R., Smith, A. E., Flohr, J. R. et al. · Nature Mental Health (2023)
Dai, R., Larkin, T. E., Huang, Z. et al. · NeuroImage (2023)
Quach, D. F., de Leon, V. C., Conway, C. R. · Journal of the Neurological Sciences (2022)
Nagele, P., Palanca, B. J., Gott, B. et al. · Science Translational Medicine (2021)
Kohtala, S., Alitalo, O., Rosenholm, M. et al. · Pharmacology and Therapeutics (2020)
Garcia-Romeu, A., Kersgaard, B., Addy, P. H. · Experimental and Clinical Psychopharmacology (2016)
Between November 2012 and February 2014, 24 patients were enrolled; after three screen failures 21 were randomised, and one patient withdrew before outcome assessment, leaving 20 evaluable patients who completed both sessions (modified intention-to-treat). The cohort had chronic illness: the mean lifetime duration of major depressive disorder was 19 years, the median number of failed adequate antidepressant trials was eight, and patients were taking a median of two antidepressants at study entry. Median baseline HDRS-21 score was 23. Fifteen patients completed the full 60-minute nitrous oxide session. Two patients had brief 5-minute interruptions and three patients discontinued nitrous oxide early (at 55, 28, and 18 minutes) because of emotional discomfort, regurgitation, claustrophobia, or nausea/vomiting. The mean duration of nitrous oxide treatment was 55.6 ± 2.5 minutes at a median inspiratory nitrous oxide concentration of 44% (interquartile range 37%–45%). All placebo sessions were completed for the full 60 minutes. On the primary outcome the extracted text reports significant improvements in depressive symptoms after nitrous oxide compared with placebo. Specifically, the report states a mean HDRS-21 change at 2 hours of 24.8 points and at 24 hours of 25.5 points following nitrous oxide; these figures represent reductions in HDRS-21 score (improvement) although the extraction appears to have omitted explicit negative signs in some places. The corresponding reported comparisons for placebo show smaller, non-significant changes (for example, at 24 hours the placebo change is reported as 22.8 points, p = .07). The between-treatment comparisons for HDRS-21 change favoured nitrous oxide (comparison p < .001). The QIDS-SR also showed a significant reduction at 24 hours after nitrous oxide; the extracted text gives this as a mean change of 23.2 points (95% CI 21.3 to 25.0, p = .001) though the formatting suggests the sign may be missing in extraction. Using conventional thresholds, four patients (20%) met treatment response criteria (≥50% reduction in HDRS-21) 24 hours after nitrous oxide versus one patient (5%) after placebo (odds ratio 4.0, 95% CI 0.45–35.79). Three patients (15%) achieved full remission (HDRS-21 ≤ 7) after nitrous oxide compared with none after placebo (odds ratio 3.0, 95% CI 0.31–28.8). When HDRS-21 severity was collapsed into five levels, 7 of 20 patients (35%) showed at least a two-level improvement 24 hours after nitrous oxide versus 2 patients (10%) after placebo (p = .06). The crossover design exhibited a carryover effect: several patients had markedly lower HDRS-21 scores at the start of their second session, and the statistical test for carryover was significant (p = .02). To address this, the investigators analysed only the first treatment session in a parallel-group manner (10 patients who received nitrous oxide first versus 10 who received placebo first). In that first-session-only analysis, patients who received nitrous oxide first had significant HDRS-21 reductions at 2 hours, 24 hours, and at 1 week compared with those who received placebo first (reported mean reductions for the nitrous-oxide-first group were 27.1 points at 2 hours, 28.6 points at 24 hours, and 25.5 points at 1 week; the extracted text again appears to omit minus signs but these are presented as reductions). Safety data indicated no serious adverse events. Adverse events were described as brief and of mild to moderate severity; a subset of patients required interruption or discontinuation of nitrous oxide for transient symptoms (nausea, anxiety, vomiting). Plasma total homocysteine did not increase after nitrous oxide or placebo in the extracted data, suggesting minimal acute inactivation of vitamin B12-dependent metabolism after a single exposure.
Nagele and colleagues interpret the findings as providing preliminary, proof-of-concept evidence that a single 1-hour inhalation of nitrous oxide can produce rapid antidepressant effects in patients with TRD that are sustained for at least 24 hours and, in some patients, for up to 1 week. In this small pilot cohort nitrous oxide produced treatment response in 20% of patients and remission in 15% by conventional HDRS-21 criteria. The investigators emphasise that adverse events were typically mild to moderate, transient, and reversible, although some patients experienced emotional discomfort or nausea sufficient to interrupt or stop treatment. The discussion places these results in the context of prior work with NMDA receptor antagonists, particularly ketamine, noting a similar rapid onset of antidepressant action but an apparent absence of ketamine-like psychotomimetic side effects with nitrous oxide; the authors suggest that differences in pharmacokinetics and receptor pharmacology may account for some differences among NMDA antagonists. They also acknowledge that other receptor systems (for example, nicotinic acetylcholine receptors) might contribute to rapid antidepressant actions, and that differences between NMDA antagonists (for example, memantine versus ketamine) remain to be clarified. Key limitations acknowledged by the investigators include the carryover effect observed in the crossover design, which altered baseline scores at the second session and complicates interpretation of pooled crossover estimates. Blinding was another concern: nitrous oxide produces noticeable subjective effects (sedation, mild sweet smell/taste) that could have unmasked treatment assignment, and the authors did not formally assess whether patients guessed their allocation. The choice of assessment instruments was also noted as a limitation because HDRS-21 and QIDS-SR include items (sleep, weight) that are designed for changes over days to weeks and may be suboptimal for very rapid antidepressant effects; alternative short-term symptom instruments might be preferable in future studies. Dosing uncertainty was acknowledged — the trial used a conventional 50% inspiratory concentration based on sedation practice in dentistry and obstetrics but the optimal antidepressant dose and duration remain unknown. Safety considerations highlighted by the authors include the potential for adverse effects with repeated dosing, particularly nitrous oxide-induced inactivation of vitamin B12 and related haematologic or neurologic complications when exposures are frequent or in vulnerable patients. The abuse potential of nitrous oxide was also raised as a clinical concern that this pilot trial could not address. The investigators conclude that while the results are promising, the trial was an early Phase II pilot and that larger, controlled studies are required to determine optimal dosing, replicate efficacy, and thoroughly establish the risk–benefit profile before clinical adoption.
This proof-of-concept trial demonstrated that nitrous oxide has rapid antidepressant effects in patients with TRD. These antidepressant effects were sustained for at least 24 hours and in some patients for 1 week. Nitrous oxide resulted in a treatment response in 20% of patients with TRD and remission in 15%. Although a subset of patients experienced adverse events requiring a short interruption or discontinuation of treatment, the mild to moderate nature and immediate Relative change after treatment according to the five levels of depression severity on the 21-item Hamilton Depression Rating Scale (normal, mild, moderate, severe, very severe). Downward arrows indicate improvement; upward arrows indicate worsening of depressive symptoms compared with baseline. For example, a two-level improvement would be from severe depressive symptoms to mild. Nitrous Oxide and Treatment-Resistant Depression reversibility of these events (nausea, anxiety, vomiting) suggest an acceptable risk/benefit ratio for nitrous oxide use in the setting of TRD. The internal validity of our crossover trial was affected by the observed carryover effect (i.e., patients having a different baseline at different treatment sessions). In our study, several patients who returned for their second treatment session had markedly lower depression scores. Typically, carryover effects bias results toward the null hypothesis (i.e., reduce the observable effect size). This was the case in our study: the 10 patients who received nitrous oxide treatment first had a mean reduction in depressive symptoms of 8.6 points on the HDRS-21 compared with 5.5 points for the full cohort. This observation supports the notion that nitrous oxide has true antidepressant efficacy. A second effect that influenced the internal validity of our trial was the presence of a placebo effect. Placebo effects are common in trials of antidepressantsand may introduce bias by masking or exaggerating treatment effects. Pilot studies, such as this early phase II clinical trial, are designed to detect an efficacy signal in a small group of patients and cannot provide robust and definitive measures of effectiveness. Pilot trials should be interpreted with caution because results must be replicated in larger cohorts. Although the antidepressant efficacy results in this trial are promising, several potential limitations should be taken into consideration. First, although our study team went to great lengths to maintain blinding, the euphoric effects of nitrous oxide inhalation are difficult to mask. Nitrous oxide induces sedation and has a slightly sweet smell and taste. It is possible that some patients were able to determine whether they were receiving nitrous oxide or placebo inhalation. We did not test patients to determine if they were aware of their group assignment, and this limits our conclusions. We intentionally selected the 24hour postinhalation mark as the primary measure to minimize acute euphoric effects. However, there remains the possibility that nitrous oxide inhalation may have produced a "masking" of depressive symptoms (i.e., the depressive symptoms were not really altered, but rather "covered up" by other effects). Symptom "masking" has been observed with rapidly acting psychostimulants (methylphenidate and cocaine), which promote a transient alteration in mood but not a true antidepressant effect. Second, although we clinically assessed the presence of euphoria and psychosis at each time point, we did not do standardized testing of either. In general, at 2 hours and 24 hours, the patients did not report euphoric feelings. Third, the use of the HDRS-21 and QIDS-SR scales to measure rapid antidepressant action was a limitation because both scales assess symptom changes occurring over the course of days and weeks rather than hours, including questions related to sleep and weight, and are not ideal for assessing changes in antidepressant action that occur rapidly. Different scales, such as the International Positive and Negative Affect Schedule Short Form or a visual analog scale, might have been superior. Fourth, we had no prior knowledge about dosing in this patient population and opted to use a 50% inspiratory concentration of nitrous oxide, a dose commonly used in dentistry and obstetric analgesia. Subsequent studies may determine that different dosing regimens improve efficacy and tolerance. Compared with ketamine, the most commonly investigated NMDA receptor antagonist drug in major depressive disorder, nitrous oxide had a similarly rapid onset of antidepressant action (within 2 hours) but appeared to be devoid of psychotomimetic side effects seen with ketamine (delusions, illusions, hallucinations), which may result from the more favorable pharmacokinetics of nitrous oxide because its offset occurs on the order of minutes. The fact that both ketamine and nitrous oxide have antidepressant effects in patients with TRD supports the notion that NMDA receptor signaling plays a crucial role in the neurobiology of major depressive disorder. However, recent data indicate that other neurotransmitter receptor systems, including nicotinic acetylcholine receptors, may be important contributors to rapid antidepressant actions. We can only speculate why certain NMDA receptor antagonists (ketamine, nitrous oxide) appear to have rapid antidepressant properties, whereas others, such as memantine, do not. Differences in NMDA receptor channel blocking seem unlikely to contribute because differences between ketamine and memantine are often observable only under extreme depolarization or pathologic receptor activation (simulated ischemia). The presence of extracellular magnesium may distinguish the effects of ketamine and memantine on NMDA receptors, with memantine being relatively ineffective against NMDA receptor-mediated synaptic currents in magnesium. This latter effect also appears to contribute to differences in the ability of the two drugs to promote brain-derived neurotrophic factor production. Differences in mode of administration and pharmacokinetics may also contribute to observed clinical differences between ketamine and memantine. Although nitrous oxide, similar to ketamine, is a noncompetitive NMDA receptor antagonist, it differs from ketamine in lacking use dependence and is not a trapping open channel blocker. Nitrous oxide represents an alternative way to modulate NMDA receptor function clinically. Although a single administration of 50% nitrous oxide/ oxygen has been found to be generally safe (4% nonserious adverse event rate among 25,828 patients receiving sedation (44)), two potential safety concerns exist. First, nitrous oxide administration had to be interrupted or discontinued in a subset of our patients (typically near the end of the 1-hour treatment session), and the adverse event profile indicates that some patients may experience emotional discomfort, paradoxically increased anxiety levels, and nausea during nitrous oxide administration. Although nearly all side effects were limited to the immediate treatment period and disappeared shortly after discontinuation, their nature suggests that perhaps a shorter treatment duration or lower nitrous oxide concentration may be advantageous. A second potential safety concern relates to inactivation of vitamin B 12 by nitrous oxide. Although a single exposure is unlikely to result in clinically relevant hematologic or neurologic complications, the risk for such complications is substantially higher when nitrous oxide administrations are repeated within short periods of time. Hematologic and neurologic complications, such as megaloblastic anemia and myelopathy, have been reported among persons who chronically abuse nitrous oxideand patients with chronic disturbances of folate metabolism. It is likely that for sustained antidepressant effect, nitrous oxide must be administered several times, which would increase the risk for such complications. Nitrous oxide is a drug of abuse, and its abuse potential represents a potential limitation for its clinical utility in major depressive disorder. Our pilot study was not designed to address this safety concern. In conclusion, this preliminary, proof-of-concept clinical trial provides the first evidence that nitrous oxide may have rapid and marked antidepressant effects in patients with TRD. Subsequent studies are required to determine optimal antidepressant dosing strategies and the risk/benefit ratio of nitrous oxide in a larger and more diverse population of patients with TRD.