As already stated, the point of assessment (POA) is 2cm distal to the atrio-caval junction. In a haemodynamically normal, spontaneously ventilating patient, the IVC collapses slightly on inspiration. In fact, in such patients, a pronounced ‘sniff’ results in the walls at the POA just touching, so called ‘kissing contact’.
This is reversed in a mechanically ventilated patient where there is an increased diameter in the abdominal IVC during inspiration.