Although increasing global D o dos may improve blood flow to regionally hypoxic tissues by raising blood flow through all capillary beds, this is an inefficient process and, if achieved using vasoactive drugs, may adversely affect regional distribution, particularly to the kidneys and splanchnic beds. The potent ? receptor agonist noradrenaline is frequently used to counteract sepsis induced vasodilation and hypotension. The increase in blood pressure may improve perfusion to certain hypoxia sensitive vital organs but may also compromise blood flow to other organs, particularly the splanchnic bed. The role of vasodilators is less well defined: tissue perfusion is frequently already compromised by systemic hypotension and a reduced systemic vascular resistance, and their effect on regional distribution is unpredictable and may impair blood flow to vital organs despite increasing global D o 2. In a group of critically ill patients prostacyclin increased both D o 2 and V o 2 and this was interpreted as indicating that there was a previously unidentified oxygen debt. However, there is no convincing evidence that vasodilators improve outcome in critically ill patients. An alternative strategy that attempts to redirect blood flow from overperfused non-essential tissues such as skin and muscle tissues to underperfused “vital” organs by exploiting the differences in receptor population and density between different arteries is theoretically attractive. While dobutamine ine hydrochloride has dopaminergic and ?-adrenergic but no ?-adrenergic effects and may selectively increase renal and splanchnic blood flow. 25
Diagram showing the importance of local capillary oxygen tension and diffusion distance in determining the rate of oxygen delivery and the intracellular P o 2. On the left there is a low capillary P o 2 and pressure gradient for oxygen diffusion with an increased diffusion distance resulting in low intracellular and mitochondrial P o 2. On the right the higher P o 2 pressure gradient and the shorter diffusion distance result in significantly higher intracellular P o 2 values.
This changed relationship is not seen when D o
The sigmoid oxygen-haemoglobin dissociation relationship is influenced by various physicochemical factors and its position is defined by the Pa o 2 at which 50% of the Hb is saturated (P50), normally 3.5 kPa. An increase in P50 or rightward shift in this relationship reduces the Hb saturation (Sa o 2) for any given Pa o 2, thereby increasing tissue oxygen availability. This is caused by pyrexia, acidosis, and an increase in intracellular phosphate, notably 2,3-diphosphoglycerate (2,3-DPG). The importance of correcting hypophosphataemia, often found in diabetic ketoacidosis and sepsis, is frequently overlooked. 26
Determine away from intercapillary distance into outcomes of hypoxia, anaemia, and low flow-on the fresh new clean air delivery-usage relationship
Mathematical models of tissue hypoxia show that the fall in cellular oxygen resulting from an increase in intercapillary distance is more severe if the reduction in tissue D o 2 is caused by “hypoxic” hypoxia (a fall in Pa o 2) rather than “stagnant” (a fall in flow) or “anaemic” hypoxia (fig 5 ? ). 27 Studies in patients with hypoxaemic respiratory failure have also shown that it is Pa o https://datingranking.net/cs/nostringsattached-recenze/ 2 rather than D o 2-that is, diffusion rather than convection-that has the major influence on outcome. 9
With a normal intercapillary distance illustrated in the top panels the D o 2/V o 2 relationship is the same for all interventions. However, in the lower panels an increased intercapillary distance, as would occur with tissue oedema, reducing D o 2 by progressive falls in arterial oxygen tension results in a change in the D o 2/V o 2 relationship with V o 2 falling at much higher levels of global D o 2. 2 is reduced by anaemia or low blood flow.