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29. Accurate assessment of haemodynamics after Norwood procedure: importance of systemic oxygen consumption
Jia Li, Gencheng Zhang, Helen Holtby, Tilman Humpl, William Williams, Chris Caldarone, Andrew N Redington, Glen S Van Arsdell; Toronto, ON, Canada

Objective:
Management strategy for the postoperative Norwood neonate has been based on experimental and theoretic models that have intrinsic limitations - particularly a key assumption of oxygen consumption (VO₂). Indicators such as systemic venous saturation (SvO₂), systemic arterial and venous oxygen saturation difference (Sa-vO₂), and oxygen excess factor (Ω = SaO₂/Sa-vO₂) have been used to estimate systemic blood flow (Qs) and oxygen delivery (DO₂). These are indirect reflectors of what would be ideal to know: systemic (Qs) and pulmonary (Qp) blood flow. We examined the correlation of indirect measures to VO₂ derived measures.
Methods:
Respiratory mass spectrometry was used in 13 patients to obtain continuous VO₂ measurements for 72 hours following the Norwood procedure. Arterial, superior vena caval, and pulmonary venous blood gases were measured at 2-4 hour intervals. Qp, Qs, DO₂, and oxygen extraction ratio (ERO₂) were calculated. Multiple regression analysis was used to test whether SvO₂, Sa-vO₂ and Ω accurately predict Qs and DO₂, and other contributors to oxygen transport.
Results:
We obtained 323 sets of measurements of Qs, Qp, VO₂, DO₂ and ERO₂. There were wide inter- and intra-individual variations in all values during the study. SvO₂, Sa-
VO₂ and Ω poorly correlated with Qp (r =0.18, r=0.1, r=0.08); they were variably correlated with Qs and DO₂ (r = 0.39 to 0.72) and poorly correlated with VO₂ (r = 0.21 to 0.31). See figures. Inclusion of VO₂ improved correlation (r = 0.64 to 0.90, p < 0.01) particularly in the 1st 24 hours (r = 0.68 to 0.93, p < 0.01) when correlation was the least. SvO₂, Sa-vO₂, and Ω closely correlated with ERO₂ (r =0.81 to 0.95, p < 0.01).
Conclusions:
Changes in SvO₂, Sa-vO₂ and Ω are closely correlated with ERO₂ and independent of Qp. DO₂ and Qs were not reliably correlated with conventional measures because of the variability of VO₂. Accurate knowledge of DO₂ and Qs may alter management strategy, particularly in the 1^st 24 hours, and thereby potentially improve outcomes.

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