Hemodynamic responses to oxygen breathing in children with severe anemia.

Hemodynamic responses to 100% oxygen inhalation were examined in seven anemic children (hemoglobin 1.9 to 3.8 g/100 ml blood; hematocrit 9 to 14%). During air breathing mean values for cardiac index were 7.71 L/min/m2, for heart rate 129/min, for stroke index 61.7 ml/m2, and for systemic vascular resistance 11.5 R.U. Oxygen administration significantly decreased mean values of cardiac index to 6.61 L/min/m2, of heart rate to 119/min and of stroke index to 56.6 ml/m2, and increased mean systemic vascular resistance to 14.2 R.U. Oxygen breathing raised systemic oxygen content 23% and oxygen transport 5%, despite decreases in cardiac ouput. Since oxygen breathing did not change the hematocrit (and thus viscosity), the increase in vascular resistance indicates active vasoconstriction. It is postulated that in anemia tissue hypoxia causes vasodilation of systemic resistance vessels. When anemia is not severe, baroreceptor reflexes prevent significant reductions in blood pressure and systemic vascular resistance by compensatory, neurogenic vasoconstriction. When anemia becomes severe and vasodilation extreme, blood pressure is maintained by an increase in cardiac output. Cardiac output is elevated by decreased vagal inhibition and increased myocardial contractility, which cause tachycardia and increased stroke volumes respectively.