Oxygen uptake-to-delivery relationship: a way to assess adequate flow

Abstract

Invasive and noninvasive monitoring facilitates clinical evaluation when resuscitating patients with complex haemodynamic disorders. If the macrocirculation is to be stable, then it must adapt to blood flow or blood flow must be optimized. The objective of flow monitoring is to assist with matching observed oxygen consumption (VO2) to pathophysiological needs. If an adequate balance cannot be maintained then dysoxia occurs. In this review we propose a simple schema for global reasoning; we discuss the limitations of VO2 and arterial oxygen delivery (DaO2) assessment; and we address concerns about increasing DaO2 to supranormal values or targeting pre-established levels of DaO2, cardiac output, or mixed venous oxygen saturation. All of these haemodynamic variables are interrelated and limited by physiological and/or pathological processes. A unique global challenge, and one that is of great prognostic interest, is to achieve rapid matching between observed and needed VO2--no more and no less. However, measuring or calculating these two variables at the bedside remains difficult. In practice, we propose a distinction between three situations. Clinical and blood lactate clearance improvements can limit investigations in simple cases. Intermediate cases may be managed by continuous monitoring of VO2-related variables such as DaO2, cardiac output, or mixed venous oxygen saturation. In more complex cases, three methods can help to estimate the needed VO2 level: comparison with expected values from past physiological studies; analysis of the relationship between VO2 and oxygen delivery; and use of computer software to integrate the preceding two methods.

Figure 1

Pathophysiological changes in the VO₂/DO₂ relationship. Normal relationship is shown in a solid black line, and abnormal relationships in dotted lines. 1: Increased VO₂ needs; 2: impaired EO₂; 3: other mechanisms (see text). The grey curves are the corresponding EO₂/DO₂ relationships. DO₂, oxygen delivery; EO₂, oxygen extraction ratio; VO₂, oxygen consumption. Reproduced with permission from Squara [4].

Figure 2

Comparison between the VO₂/DO₂ relationship and the VO₂/time relationship in one hypothetical example. For this figure, DO₂ increased linearly with time. This helps to identify the critical VO₂ point and eliminates the possible effect of mathematical coupling of error. DO₂, oxygen delivery; VO₂, oxygen consumption. Reproduced with permission from Squara [4].

Figure 3

Nomogram showing CI, SvO₂ and VO₂ isopleths (dotted black lines). The green point shows the expected values for a 59-year-old woman at basal metabolism, and the green dotted arrow indicates the expected normal variations in case of hypo- or hypermetabolism. The real position of the patient in the nomogram can be continuously monitored. This is adequate for diagnostic purposes. The patient's position can move from the normal profile to a characterized area of septic or cardiogenic shock (grey dotted arrows and areas). However, for therapeutic objectives, this nomogram gives no idea of needs. If we made the hypothesis that the ⁿVO₂ (red isopleth) is higher than the ^oVO₂, then the red double arrow indicates the difference between ⁿVO₂ and ^oVO₂. Units for CI are l/min per m² and for VO₂ they are ml/min per m². CI, cardiac index; ^(n/o)VO₂, (needed/observed) oxygen consumption; SvO₂, mixed venous oxygen saturation.