105348.fig.001a
(a)
105348.fig.001b
(b)
Figure 1: The key determinants of organ perfusion are depicted. In (a), the relationship between oxygen consumption (VO2) and delivery (DO2) is indicated. Patients usually function on the rightward side of the curve where an excess of oxygen is supplied relative to demand. As delivery decreases relative to consumption, the patient moves left on the curve. A decrease in central venous oxygen saturation accompanies leftward movement on the curve. In severe cases where delivery is unable to meet metabolic demands, the patient slips beneath the critical oxygen delivery threshold. Organ dysfunction and lactic acidosis are regarded as evidence of pathologic oxygen supply [6]. In (b), the autoregulatory curve (describing constancy of organ blood flow over a broad range or pressures) is shown. Some patients with chronic hypertension have curves shifted to the right relative to the normotensive curve shown here [5]. For both relationships shown, the flat horizontal portions indicate safe ranges, indicative of adequate organ blood flow and intact homeostatic mechanisms. Movement to the down sloping portions on the left indicates decompensation, placing the patient at risk for organ failure. Abbreviations: VO2, oxygen uptake/per minute; CaO2, oxygen content of arterial blood (mainly hemoglobin); CO, cardiac output; MAP, mean arterial pressure; SVR, systemic vascular resistance.