Small question regarding linear mixed effect model

Thanks so much for reading my post. I've listed key points below. Hopefully once completing reading my problem will become clear.

X is an indicator of arterial stiffness.

I have measured variable X at three repeated times with increasing dose of an intervention.

Typically, I use repeated measures ANOVA to tell the difference over time.

Importantly for this study, it is well known that variable X is influenced by both "the intervention" and heart rate.

Heart rate is also influenced by the intervention.

I want to know the interaction between changes in heart rate and variable X over the repeated measures of the intervention.

I also want to know if there is a change in X when controlling for changes in heart rate.

What is the proper statistical test? I think it is a linear mixed effect model. So far I'm thinking I should run a GLM fixed effect model to get my full model significance. Then I'll have to run a mixed interaction to see if both HR and X are associated over LBNP.

FYI, I have a very small sample size (n=9).

Advice specific to SPSS would be be most useful.

Thanks again!