Accessing CloudWatch Event Variables in CodePipeline

In my project I'm using Lambda functions in order to trigger other CodePipelines (SubPipeline1, SubPipeline2, ...) when necessary (based on this approach described here in a blogpost). Other than this example the Lambda functions are actually part of a CodePipeline (OrchestratingPipeline) themselfes.

CodeCommitCloudWatchEvent -> OrchestratingPipeline
                             |                  |
                             v                  v
                      SelectiveTriggerFnct1 SelectiveTriggerFnct2
                             |                  |
                             v                  v
                        SubPipeline1        SubPipeline2

In these Lambda functions I now want to access the original CloudWatch Event which contains an attribute "OldCommitId". How can I get this done?

I already checked/tried:

• When starting the pipeline I can not pass any additional parameters to the pipeline
• The CodeCommit source stage does not provide the OldCommitId but just the current one