Oxygen extraction

In a third study the time course of the effects of intravenous and intracoronary injections of cysteinyl leukotrienes on metabolic parameters and systemic and coronary hemodynamics was examined in patients with normal coronary arteries [32]. LTD4 (3 nmol, injected into the left coronary artery) induced an early (20 s), transient fall in mean arterial pressure paralleled by rises in heart rate and plasma levels of epinephrine and norepinephrine, all of which had returned to baseline by 10 min. CVR rose at 10 and 15 min and myocardial oxygen extraction at 15 min. Thus, small doses of cysteinyl leukotrienes may induce both an early, transient fall in mean arterial pressure, with secondary sympathoadrenergic activation, and a later increase in small coronary arteriolar resistance. 

The oxygen concentration in cerebral venous blood is substantially lower than in the cerebral arterial blood because of the high oxygen extraction 536... 

The Vo2 Do2 ratio (oxygen extraction ratio) can be used to assess adequacy of perfusion and metabolic response. Patients who are able to increase Vo2 when Do2 is increased are more likely to survive. However, low Vo2 and 02 extraction ratio values are indicative of poor 02 utilization and lead to greater mortality. 

Effect on coronary blood of flow Myocardial oxygen extraction is nearly maximum at rest, so, there is little reserve to meet increased demands. So, increased myocardial demand for O2 can only be met by increased coronary blood flow which depends on diastolic pressure and duration of diastole because coronary blood flow is negligible during systole. 

Thus, some sort of an adsorption complex is readily formed, but its nature apparently depends on the surface state of the solid it appears most likely that at least two types of complex are involved, one of which may be mainly responsible for the heat effects measured by Dell and Stone, while the other, which we believe is confined to a few very reactive sites, is concerned in the CO-oxidation by an oxygen extraction process ... 

Neuroimaging technique for measuring cerebral blood flow, cerebral blood volume, metabolic rate, oxygen utilization and the oxygen extraction volume. 

Q6 Cyanosis refers to the bluish colour of reduced haemoglobin in the tissues. When blood flow through tissues is slowed, blood remains in the tissues for a longer time and more oxygen than normal can be extracted. Increased oxygen extraction results in an increased concentration of reduced haemoglobin and makes the skin appear bluish in colour. The blue colour is most easily seen in the lips and mucous membranes. 

CMRO2 with age, but they remain coupled so that the oxygen extraction fraction remains more or less constant (Blesa et al. 1997). 

It follows from the above that the ratio of cerebral blood flow to cerebral blood volume is a measure of cerebral perfusion reserve (Schumann et al. 1998). Below a ratio of approximately 6.0, even if cerebral blood flow is still normal, vasodilatation and cerebral blood volume are maximal and the reserve is exhausted, as shown by a rising oxygen extraction fraction on PET. 

Around acutely infarcted brain, there is an ischemic penumbra (Astrup et al. 1981). Here the blood flow is low, function depressed and the oxygen extraction fraction high. In other words, there is viable tissue with misery perfusion where the needs of the tissue are not being met The tissue may die or recover, depending on the speed and extent of restoration of blood flow. This concept opens up the possibility of a therapeutic time window during which restoration of flow or neuronal protection from ischemic damage might prevent both immediate cell death and the recruitment of neurons for apoptosis (see Ch. 21). 

During or following a cerebrovascular event, some brain areas may show relative or absolute hyperemia owing to good collateral flow, reperfusion after an occluded artery has been reopened and/or inflammation and vasodilatation in response to hypercapnia. In hyperemic areas, oxygen extraction fraction is low and there is luxury perfusion, indicating that flow is in excess of metabolic requirements, perhaps because the tissue has been irreversibly damaged. 

Wise RJ S, Bernardi S, Frackowiak RS J et al. (1983). Serial observations on the pathophysiology of acute stroke. The transition from ischaemia to infarction as reflected in regional oxygen extraction. Brain 106 197-222... 

It is conceivable that patients with impaired cerebral reactivity and raised oxygen extraction fraction are at particular risk of stroke without surgery, and that this impairment can be corrected by carotid endarterectomy, but the studies have been too small to be sure (Schroeder 1988 Naylor et al. 1993b Yonas et al. 1993 Hartl et al. 1994 Yamauchi et al. 1996 Visser et al. 1997 Silvestrini et al. 2000 Markus and Cullinane 2001). Also, we do not know what proportion of strokes in patients with recently symptomatic severe carotid stenosis are actually caused by impaired cerebral reactivity, either as a direct result of low flow or perhaps indirectly as a result of an inadequate collateral circulation to compensate for acute arterial occlusion if it should occur. Nor do we know whether the risk of surgery is higher in these patients and so whether, on balance, carotid endarterectomy will indeed reduce stroke risk any more than in those without impaired reactivity. 

The risk-benefit relationship has been evaluated in only one completed randomized trial and this failed to show any benefit from routine surgery (EC-IC Bypass Study Group 1985). However, it has been argued that patients with impaired cerebrovascular reactivity, or with maximal oxygen extraction, were not identified and perhaps it is these patients who might benefit from surgery (Warlow 1986 Derdeyn et at 2005), but proof of this hypothesis would require a further randomized trial in this specific subgroup (Karnik et at 1992). 

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