The study aimed to find clinical predictors of early vs. late ECT response. The authors conducted a retrospective chart review of 1799 patients treated mostly with ultra-brief unilateral ECT due to MDD or bipolar depression (most participants). The main finding is that 32.6% of the patients responded by the 5th ECT session (early response). In those who did not respond by the 5th session, the switch from ultra-brief to brief pulse ECT increased the odds of a response. The main strength is the large cohort. Despite some limitations (mainly related to the retrospective design and adequately acknowledged by the authors), this well-written report has significant clinical implications. It underscores the role of early response as a predictor of a favorable outcome. It also indicates that switching to more effective ECT forms (brief vs. ultra-brief) may increase the chance of response in patients without early response.

I hope the authors find the comments below helpful. They are divided into general comments and comments referring to a specific part of the manuscript.

General comments:

1. The overall response rate of 56.4% was relatively low. Could the authors discuss the reasons for this? May they be, for example, using mainly ultra-brief ECT, or not severely ill population, or including non-ECT responsive diagnoses (“Others”)? What does this response rate say about the external validity of the study? In other words, do the results apply to a general ECT population?

2. The Quick Inventory of Depressive Symptomatology (QIDS) is a self-report instrument. Is this tool a reasonable/established way to evaluate treatment response?

3. What was the rationale for including patients referred to ECT because of non-affective diseases (non-MDD and non-Bipolar disorder) when assessing response as a change in depressive symptoms? According to Table 1, the number of individuals with the clinical diagnosis "Others" is 89 (4.95%). Are these subjects included in the overall analyses? If Yes, could this confound the results? For example, if a patient was referred to ECT because of treatment-resistant positive symptoms of schizophrenia, should one expect an effect on depressive symptoms?

4. As noted by the authors, psychotic depression is one of the best predictors of ECT response. Furthermore, two well-established predictors of non-response are longer duration of the current depressive episode and higher treatment resistance. Since they were not analyzed in the study, I assume these variables were unavailable. Please clarify.

5. There is agreement that remission should be the goal of the treatment. Did the author consider including remission as an outcome?

Specific comments:

1. In the Abstract/Methods, there is an incomplete sentence: ”Response was.”

2. The "Significant Outcomes" section states, "Older age was associated with higher odds of ECT response by treatment #5." Although the result was statistically significant, the aOR was 1.01 (95% CI 1,00 – 1.01). Is the difference clinically relevant? Should this outcome be part of the "Significant Outcomes" section?

3. The Methods/ECT treatment section states, "For all analyses, patients with a diagnosis other than MDD or BPAD were grouped together ."
   Please clarify what this means.

4. In the Methods/Outcome measure – It would be informative to report ranges/cuts off of QIDS indicating the severity of depressive symptoms. If possible, it would also be helpful to write whether the different QIDS cuts off correspond to objective depression scales such as the 17-item Hamilton Rating Scale for Depression or others.

5. According to the Results section, the baseline QIDS was not related to the odds of early response. How can this finding be interpreted considering the well-established relationship between higher severity of depression and better outcomes?

6. In the discussion section, the authors discuss reasons for an unexpected finding: bilateral ECT had lower odds of response than unilateral ECT. How many patients were treated with bitemporal vs. bifrontal ECT among the bilateral ECT patients? Could this have an impact on the outcome?

The authors have adequately addressed all the comments. One minor issue needs authors’ attention:

The bilateral ECT seems to be used as a synonym of bitemporal ECT. In fact, the bilateral placement can be either bitemporal or bifrontal. For clarity reasons, please consider checking for this issue through the entire manuscript. For instance, in the following sentence in the Results section: “Looking across all treatment timepoints 30.0% of bilateral/bifrontal ECT treatments were specifically had bifrontal electrode placement.” Wouldn't this sentence be clearer if the first “/bifrontal” was omitted?