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Simple brief episodes (possible options):
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Valsalva manoeuvre
rebreathing air in paper bag (as for hyperventilation)
breath holding
sucking ice/swallowing iced water
catheter inserted quickly in and out of nose or nasopharynx
pressure on the eyeballs
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Persistent (assuming exclusion of organic diseases):
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Consider acupuncture, hypnosis or phrenic nerve block.