Hemoglobin Is a Vital Determinant of Arterial Oxygen Content in Hypoxemic Patients with Pulmonary Arteriovenous Malformations

Pa and Sa are commonly measured in respiratory practice, but arterial oxygen content (Ca ) refers to the volume of oxygen delivered to the tissues per unit blood volume. Ca is calculated from Sa and the hemoglobin concentration in blood, recognizing that each gram of hemoglobin can transport approximately 1.34 ml of oxygen when fully saturated. To prospectively evaluate serial changes in Ca in humans, incorporating and excluding dynamic changes to oxygenation and hemoglobin parameters that may occur during life. A cohort of 497 consecutive patients at risk of both hypoxemia and anemia were recruited. The patients had radiologically proven pulmonary arteriovenous malformations (PAVMs), which result in hypoxemia due to right-to-left shunting, and concurrent hereditary hemorrhagic telangiectasia, which placed them at risk of iron deficiency anemia due to recurrent hemorrhagic iron losses. Presentation Sa (breathing room air, by pulse oximetry), hemoglobin, red cell and iron indices were measured, and Ca calculated as Sa × hemoglobin × 1.34 ml/g. Serial measurements were evaluated in 100 cases spanning up to 32.1 (median, 10.5) years. Presentation Ca ranged from 7.6 to 27.5 (median, 17.6) ml/dl. Ca did not change appreciably across the Sa quartiles. In contrast, hemoglobin ranged from 5.9 to 21.8 g/dl (median, 14.1 g/dl), with a linear increase in Ca across hemoglobin quartiles. After PAVM embolization and an immediate increase in Sa , hemoglobin fell and Ca was unchanged 1.6-12 (median, 4) months later. When hemoglobin fell because of iron deficiency, there was no change in Sa . Similarly, when hemoglobin rose after iron treatment, there was no change in Sa , and the expected Ca increment was observed. These relationships were not evident during pregnancy when hemoglobin fell, and PAVMs usually deteriorated: in pregnancy Sa commonly increased, and serial Ca values (incorporating hemodilution/anemia) more accurately reflected deteriorating PAVM status. An apparent fall in Ca with age in females was attributable to the development of iron deficiency. There was an unexplained increase in Ca with age in follow-up of males after embolization. Hemoglobin/Ca should be further incorporated into oxygenation considerations. More attention should be given to modest changes in hemoglobin that substantially modify Ca .