Posted this in a welcome post but thought it should be shared for everyone who needs it.
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I was told (whilst lying down) to place both hands on the tummy so the fingertips touch eachother then breath through your nose (inhale and exhale) from the diaphram so the upper chest doesnt rise and your fingers part on your tummy instead.
Try to do this 8 times in a minute and breathing and pulse should resume normality within 5-10 mins.
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CONS