To cite spiro in publications use:

  Simon Nolte (2023). spiro: An R package for analyzing data from
  cardiopulmonary exercise testing. Journal of Open Source Software,
  8(81), 5089, https://doi.org/10.21105/joss.05089

A BibTeX entry for LaTeX users is

  @Article{,
    title = {spiro: An R package for analyzing data from cardiopulmonary exercise testing},
    author = {Simon Nolte},
    year = {2023},
    volume = {8},
    number = {81},
    pages = {5089},
    journal = {Journal of Open Source Software},
    url = {https://joss.theoj.org/papers/10.21105/joss.05089},
    doi = {10.21105/joss.05089},
  }