![]() Normal ScvO 2 = 70-75%, reflecting 25-30% extraction.Global impairment in oxygen delivery can thus be determined by monitoring central venous oxygen saturation (measured at SVC-RA junction with a central venous line) or mixed venous oxygen saturation (measured with a Swan Ganz catheter at the pulmonary artery).Enter values and press calculate button to calculate the gradient between the. PaO2 can be estimated from SaO2 via the hemoglobin oxygen dissociation curve: The Alveolar-arterial gradient is used to evaluate causes of hypoxemia.S/F ratio: SaO 2/FiO 2= similar to P/F ratio but used when no PaO 2 not available (roughly 253 for ALI and 212 for ARDS)- technically requires oxygen saturation OSI: Oxygen saturation index= P aw*FiO 2 /SpO2 : Similar to OI but used when PaO 2 not available (roughly 6.5 and 7.8 for ALI and ARDS, respectively).No formal guidelines exist for transition from conventional ventilation to HFOV to ECMO in the pediatric population.OI 40= consideration of ECMO (in neonatal population).Current Berlin Criteria use the P/F ratio to define and classify ARDS in adults.The lower the number the more severe the impairment in oxygenation. Diffusion impairment (very rare to see, especially in children, can sometimes be seen during exercise and higher cardiac output as not enough time for oxygen loading to occur at the avleolar/capillary interface.VQ mismatch (on the same continuum as shunt).Shunt (can be intracardiac as in right to left shunt or intrapulmonary).due to altitude or subambient oxygen delivery) The A-a gradient increases with age, as well as, higher FiO 2 ’s. The normal Aa gradient is due to intrinsic shunt existing in healthy human beings (ie Thebesian veins, bronchial veins which drain venous blood into the left side of the heart as well as likely microatelectasis and thus natural intrapulmonary shunt) What is a normal A-a gradient It is generally accepted that a healthy person breathing room air will have an A-a gradient of 5 15 mmHg due to ventilation and perfusion (V/Q) mismatching, and right-to-left pulmonary shunting of blood.The normal value for the Aa gradient is 5-10 and increases with age Aa gradient: The difference between the alveolar and arterial partial pressure of oxygen= PAO 2-PaO 2 .Figure 1 :Relationship between altitude, barometric pressure and inspired PO2 (IJOEM2018) ![]()
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