Oxygenation in patients with a functionally univentricular circulation andcomplete mixing of blood - Are saturation and flow interchangeable?

Background-Perioperative management of patients with complete mixing of pul monary and systemic blood centers on approximately equating pulmonary (Qp) and systemic (Qs) blood flow (Qp/Qs approximate to 1). This empirically der ived target is opposed by theoretical studies advocating a target Qp/Qs wel l below 1. We studied the cause of this persistent discrepancy. Methods and Results-Classic theoretical studies have concentrated on maximi zing 1 of many potential combination parameters of arterial oxygen content (Cao(2)) and systemic blood flow: total oxygen delivery (Do(2))=Cao(2) X Qs . We defined "useful" oxygen delivery as the amount of oxygen above a notio nal saturation threshold (Sat(Thresh)): D(u)o(2) = carrying capacityX(Sao(2 ) - Sat(Thresh)) X Qs, Whereas Do(2) peaks at Qp/Qs ratios <1, D(u)o(2) pea ks at higher Qp/Qs ratios, nearer to (or exceeding) 1. Systemic venous satu ration (which mirrors tissue oxygen tension) peaks at Qp/Qs=1. Conclusions-First, the standard model of single-ventricle physiology can be reexpressed in a form allowing analysis by differential calculus, which al lows broader conclusions to be drawn than does computer modeling alone. Sec ond, the classic measure Dot fails to reflect the fact that proportional ch anges in saturation and flow are not clinically equivalent, Recognizing thi s asymmetry by using D(u)O-2 can give a target Qp:Qs balance that better re presents clinical experience. Finally, to avoid an arbitrary choice of Sat( Thresh), systemic venous oxygen saturation (Ssvo(2)) may be a useful parame ter to maximize: this occurs at a Qp/Qs ratio of 1. Attempts to increase Do , by altering Qp/Qs away from this value will inevitably reduce Ssvo(2) and therefore tissue oxygenation. Oxygen delivery is far from synonymous with tissue oxygen status.