Tissue oxygenation--circulatory aspects.

Abstract

Maintenance of adequate tissue perfusion and oxygenation is one of the major goals of intensive care. Imminent or manifest acute circulatory and respiratory failure are the most common causes of emergency admission to intensive care. Tissue hypoxia due to insufficient blood flow and low arterial oxygen content is common in these patients. The presence and pathogenesis of tissue hypoxia during circulatory shock has been well documented and the importance of tissue oxygenation at this phase of intensive care is not controversial. The presence and relevance of tissue hypoxia in patients without shock is a controversial issue; whether tissue hypoxia is of any importance in patients with stabile haemodynamics remains to be confirmed. Since oxygen delivery to the tissues is the product of blood flow and arterial oxygen content, it is evident that blood flow is a major component of the overall adequacy of tissue oxygen supply. For any given blood flow, the adequacy of the flow depends on the metabolic demands of the tissues and the capability of the tissues to extract the available oxygen. At unchanged blood flow, local and regional blood flow redistribution and changes in local or regional metabolic demand have a considerable impact on the adequacy of tissue perfusion and oxygen supply 11 I. In most clinical disorders of tissue oxygenation, blood volume, cardiac output, and arterial oxygen content are of primary concern. The effects of any therapeutic interventions, such as administration of Vasoadive drugs and mechanical ventilation, will be markedly modified by the volume status of the patient. Two different scenarios of impaired tissue oxygenation due to inadequate perfusion can be distinguished [l]. In low flow states (cardiogenic and hypovolemic shock) both the whole body blood flow and the Various regional blood flows are decreased and the metabolic demands are normal. Under these conditions, various regional circulations are gradually compromised in order to maintain sufficient perfusion of the heart and the brain. In this respect, the splanchnic region has a special role, since splanchnic Vasoconstriction is the first line mechanism in the defense of blood volume and flow [21. Splanchnic vasoconstriction, once established, is maintained even after restoration of the circulating blood volume. This is the most likely explanation for the commonly observed prolonged visceral hypoperfusion after severe hypodynamic shock. The second scenario, which is especially common in sepsis and severe systemic inflammation, includes increased metabolic demand despite normal or even increased blood flow [l]. The main threat for the adequacy of tissue oxygenation here is the substantially increased oxygen demand [3,41. Also in these settings, the splanchnic region appears to have a central role, since the hypermetabolism associated with inflammation is primarily a reflection of splanchnic hypermetabolism. As the result of the regional or local hypermetabolism, splanchnic oxygen extraction tends to be high in severe infections and inflammation, leading to high splanchnic oxygen extraction despite the normal flow. Since peripheral oxygen demands usually do not change, peripheral oxygen extraction is low once the possibly co-existing hypovolemia has been corrected. Assessment of the adequacy of tissue oxygenation in clinical practice is based on indirect indicators, which mainly reflect perfusion and not oxygenation. In addition to the traditional systemic hemodynamic indices, evaluation of mixed venous oxygen saturation, blood lactate levels, the presence of acidosis, signs of end-organ perfusion, and assessment of visceral perfusion by gastrointestinal tonometry help to evaluate the adequacy of tissue perfusion [5,61. Low mixed venous oxygen saturation indicates that the whole body oxygen supply is low as compared to the metabolic demand. Increased blood lactate with or without concomitant acidosis should be considered as a possible sign of inadequate tissue perfusion, although hyperlactatemia may exist due to various other causes than inadequate perfusion. Gastrointestinal tonometry is based on the assumption that the splanchnic perfusion will be easily sacrificed and restored only late, and hence increased local mucosal CO, or decreased pH could serve as an early marker of inadequate tissue perfusion. Gastric mucosal aci-