Review for 10.1186/s12876-020-01341-9 (2020)

Reviewer ’s report  
 
Title: CO2 or air cholangiography reduces the risk of po st-ERCP cholangitis in patients with 
Bismuth type IV hilar biliary obstruction  
 
Version: 1 Date: 28 Mar 2020  
 
Reviewer's report:  
 
The authors concluded that ERCP with CO2/air cholangiography is feasible and effective in 
patients with type IV hilar obstructions. This is  interesting study, but there are several points to 
be clarified in the manuscript.  
 
1. In group B and C, there were more patients who inserted two stents than group A, although the 
difference was not statistically significant. Can you perform multivariate  analysis and analyze 
whether the number of stents and the type of contrast agent affect the frequency of cholangitis?  
 
2. In chronological order, contrast was used first. Next, air and carbon dioxide were used 
sequentially. Is there any possibility that ch olangitis was less frequent in group B and C than 
group A due to the improvement in the skill of the endoscopist during the study period?  
 
3. How do you perform carbon dioxide cholangiography technically? Do you use a syringe filled 
with carbon dioxide? The n, how do you fill the syringe with carbon dioxide? Please describe.  
 
4. In Results, you commented there was no significant difference in types of tumor etiology 
between 3 groups. However, there is no data about tumor etiology in table 1.  
 
5. The first sent ence is awkward in Backgrounds.  
In patients with malignant hilar biliary obstruction is associated with a poor prognosis  
   
Are the methods appropriate and well described?  
If not, please specify what is requir ed in your comments to the authors.  
 
Yes 
 
Does the work include the necessary controls?  
If not, please specify which controls are required in your comments to the authors.  
 
Yes 
 
Are the conclusions drawn adequately supported by the data shown?  
If not, please e xplain in your comments to the authors.  
 
Yes 
 
 
 
Are you able to assess any statistics in the manus cript or would you recommend an 
additional statistical review?  
If an additional statistical review is recommended, please speci fy what aspects require further 
assessment in your comments to the editors.  
 
I am able to assess the statistics  
 
Quality of written English  
Please indicate the quality of language in the manuscript:  
 
Acceptable  
 
Declaration of competing interes ts 
Please complete a declaration of competing interests, considering the following questions:  
 
1. Have you in the past five years received reimbursements, f ees, funding, or salary from an 
organisation that may in any way gain or lose financially from the pu blication of this 
manuscript, either now or in the future?  
2. Do you hold any stocks or shares in an organisation t hat may in any way gain or lose 
financially from the publication of this manuscript, either now or in the future?  
3. Do you hold or are you c urrently applying for any patents  relating to the content of the 
manuscript?  
4. Have you received reimbursements, fees, funding, or salary from an organization that 
holds or has applied for patents relating to the content of the manuscript?  
5. Do you have any other financial competing interests?  
6. Do you have any non -financial competing interests in relation to this paper?  
 
If you can answer no to all of the above, write 'I declare that I have  no competing interests' 
below. If your reply is y es to any, please give details below.  
 
I declare that I have no competing interests.  
 
I agree to the open peer review policy of the journal. I understan d that my name will be included 
on my report to the authors and, if the manuscript is accepted for publicati on, my named report 
including any attachments I upload will be posted on the website along with the authors'  
responses. I agree for my report to be made available under an Open Access Creative Commons  
CC-BY license ( http://creativecommons.org/licenses/by/4 .0/). I understand that any comments 
which I do not wish to be included in my named report can be in cluded as confidential comments 
to the editors, which will not be published.  
 
I agree to the open peer review policy of the journal