I'm trying to understand how to perform meta-regression treating each arm of studies in my data as an individual study with a single proportion as the outcome. I have read through the documentation for the R package metafor, but have only found the option to estimate using relative risk, odds ratio or other similar measures for two-arm studies. I have also found this function metaprop in the package meta (https://rdrr.io/cran/meta/man/metaprop.html) that seems it might be a good option, but I cannot see how to perform the regression against a predictor. Any other R package suggestions?
EDIT: Thanks to guidance from mdewey and dbwilson, I have come to understand the possible models and code used to create said models. In the analyses below I am using the function rma.mv.
As the interest in my analysis is in the effect of the variable plateau pressure (PP from now on), I would like to include it in a meta-regression model with mortality rate as the outcome. Data come from 25 studies each with 2 arms, but as there is a strongly significant difference in PP between the two arms, I do not believe both variables should be included in the model. However, as the results of each arm of a given study are clearly not independent it seems to me that a way to account for this would be to include the Study id variable (1,1,2,2,...25,25) as a random effect. However, when I first had run the model with Overall id (1,2....50) as the random effect, the effect estimate of PP was approx. 0.01, p<0.0001, and when changing the random effect to Study id, the effect of PP was 0.0038, p = 0.119. I am unable to understand the reason for this change; under the model with Overall id as a random effect each observation would have a different estimated random effect, but in the model with Study id as a random effect the estimated random effect would be the same within each study for the two arms. I do not understand then why there would be a dramatic change in the significance of the fixed effect PP, due to the random effect being estimated on a per study base, rather than an individual basis. If anyone has insight on why this change may be occurring it would be very helpful and appreciated.