Venous saturation and the anaerobic threshold in neonates after the Norwood procedure for hypoplastic left heart syndrome

Background. Reduction in oxygen delivery can lead to organ dysfunction and death by cellular hypoxia, detectable by progressive (mixed) venous oxyhemo globin desaturation until extraction is limited at the anaerobic threshold. We sought to determine the critical level of venous oxygen saturation to m aintain aerobic metabolism in neonates after the Norwood procedure (NP) for the hypoplastic left heart syndrome (HLHS). Methods. A prospective perioperative database was maintained for demographi c, hemodynamic, and laboratory data. Invasive arterial and atrial pressures , arterial saturation, oximetric superior vena cava (SVC) saturation, and e nd-tidal CO2 were continuously recorded and logged hourly for the first 48 postoperative hours. Arterial and venous blood gases and cooximetry were ob tained at clinically appropriate intervals. SVC saturation was used as an a pproximation of mixed venous saturation (SvO(2)). A standard base excess (B E) less than -4 mEq/L (BElo), or a change exceeding -2 mEq/L/h (Delta BElo) , were used as indicators of anaerobic metabolism. The relationship between SvO(2) and BE was tested by analysis of variance and covariance for repeat ed measures; the binomial risk of BElo or Delta BElo at SvO(2) strata was t ested by the likelihood ratio test and logistic regression, with cutoff: at p < 0.05. Results. Complete data were available in 48 of 51 consecutive patients unde rgoing NP yielding 2,074 valid separate determinations. BE was strongly rel ated to SvO(2) (model R-2 = 0.40, p < 0.0001) with minimal change after adj ustment for physiologic covariates. The risk of anaerobic metabolism was 4. 8% overall, but rose to 29% when SvO(2) was 30% or below (p < 0.0001). Surv ival was 100% at 1 week and 94% at hospital discharge. Conclusions. Analysis of acid-base changes revealed an apparent anaerobic t hreshold when SvO(2) fell below 30%. Clinical management to maintain SvO(2) above this threshold yielded low mortality. (Ann Thorac Surg 2000;70:1515- 21) (C) 2000 by The Society of Thoracic Surgeons.