A patient with ventilation-perfusion mismatch may show orthodeoxia and complain of platypnea.
Orthodeoxia is a decrease in oxygenation associated with moving upright from the supine position (ortho + deoxia). The supine measurement should be taken after the person has been in that position for some time, while the upright measurement should be taken soon after the move.
Criteria for othrodeoxia - one of the following:
(1) (PaO2 when supine in mm Hg) - (PaO2 when upright in mm Hg) >= 4 mm Hg (>= 0.5 kPa)
(2) (((PaO2 when supine in mm Hg) - (PaO2 when upright in mm Hg)) / (PaO2 when supine)) >= 5%
This decrease in PaO2 is associated with an increase in the alveolar-arterial oxygen gradient.
Platypnea is the opposite of orthopnea, with worsening dyspnea when moving from supine to an upright position.
In platypnea the person breaths better when flat; in orthopnea the person breaths better when upright.
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