A PFO, or Patent Foramen Ovale, is a heart condition where a small hole remains open after birth. This can increase the risk of decompression illness (DCI) for divers. Divers with a PFO may face a 2.5 times higher risk of developing DCI. They should consider diving conservatively or get a procedure to close the PFO. Staying shallow and limiting dives helps reduce DCI risk for those with a PFO.