The best replacement for capnography is pulse oximetry.

The best alternatives to capnography are rated as following:
★★★★★  pulse oximetry
★★★★★  capnometry
★★★  oximetry
★★  end tidal co2
★  etco2
★  capnography
★  end tidal co2 monitoring
  capnometer
  pulse ox

Your alternative for capnography is not listed? Suggest!
Speaking about capnography, one probably shouldn't forget about nasal cannula.

Other search terms related to capnography and their ratings are:
★★★★★ nasal cannula
★★ zoll
You have in mind a companion for capnography and it is not yet here? Propose!



Google Trends

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capnography
pulse oximetry
capnometry