Nitroglycerin (NTG) and sodium nitroprusside (SNP) are routinely used perioperatively in infants with congenital heart defects. In this study, NTG and SNP were infused in the operating room to increase venous capacitance, reduce systemic and pulmonary afterload, facilitate weaning off cardiopulmonary bypass, stabilize hemodynamics for transport to the intensive care unit (ICU), and reduce the fluid resuscitation needed upon arrival in the ICU. Because of the risk for accumulation of methemoglobin (MetHb) and cyanmethemoglobin (cyan-MetHb) during prolonged continuous infusion of NTG and SNP, it was decided to (1) quantify ICU use, (2) measure % MetHb at 12-hour intervals, and (3) look indirectly for the accumulation of cyan-MetHb by comparing simultaneous pulse oximetry (SpO2) (Nellcor N-100 [Nellcor, Haywood, CO]) and CO-oximetry (SaO2) (Corning 270 [Corning, Medfield, MA]). A total of 69 arterial samples were obtained from 16 infants (median age 4.4 months) following cardiac surgery with bypass. Median doses of NTG, 6.0 mg/kg (range 0.7 to 27.5), and SNP, 3.3 mg/kg (range 0.6 to 33.4), were infused over a median of 64.5 hours (range 12 to 183) (N = 16 patients). The median MetHb was 0.6% (range 0.0 to 1.5) after infusions of NTG, 1.8 micrograms/kg/min (range 0.5 to 4), and SNP, 1.3 micrograms/kg/min (range 0.3 to 8.4) (N = 69 measurements). Regression analysis of oximetry data yielded the equation: SpO2 = 1.04 SaO2 - 3.7%, r = 0.97. The mean difference between SpO2 and SaO2 data pairs was 0.0% (bias) with a SD (precision) of +/- 2.3%.(ABSTRACT TRUNCATED AT 250 WORDS)