The current concept1 of cyanosis dates back to the work of Lundsgaard,2 in 1919, and the excellent review article by Lundsgaard and Van Slyke,3 in 1923. Lundsgaard defined cyanosis as a blue or bluish color of the skin, of the mucous membranes, and of the other organs usually not visible. Because of this original experimental work, the concept has been evolved that the average capillary oxygen unsaturation was between 6 and 7 vol. % when visible cyanosis was initiated. This value of 6 to 7 vol. %, or approximately 5 gm. of unsaturated hemoglobin, was to be considered as the threshold for cyanosis. Because of advances that have been made since the original papers, a review of cyanosis is indicated.
Patients were selected from the Pediatric Cardiology Service of the Hahnemann Medical College and Hospital and from the cardiac laboratories of The Children's Hospital Research Foundation and the