Blood autoregulation

Explain how the myogenic mechanism causes autoregulation of blood flow... 

Interestingly, RBF remains relatively constant when MAP changes in the range of 85 to 180 mmHg. This ability to maintain a constant blood flow in spite of changes in MAP is referred to as autoregulation. The mechanism of... 

Blood pressure lowering drugs reduce risk of stroke (and myocardial infarction and death) in middle aged patients and even better in the elderly (NNT 86 vs 29 over 5 years) (Pearce 1998). However in the elderly the dysfunction in the autoregulation of brain blood flow, salt and fluids, and increased sensitivity to adverse effects and symptoms may change the picture. 

Gebremedhin D, Lange AR, Lowry TF, Taheri MR, Birks EK, et al. 2000. Production of 20-HETE and its role in autoregulation of cerebral blood flow. Circ Res 87 60-65. 

Captopril is the fastest of the oral ACE inhibitors. It can also be used sublingually in patients who cannot swallow. Captopril shifts the entire curve of cerebral autoregulation in such a way that cerebral blood flow is maintained as the systemic pressure falls. Caution is needed in patients with significant renal... 

A primary advantage of etomidate is its ability to preserve cardiovascular and respiratory stability both cardiac output and diastolic pressure are well maintained. Use of etomidate may offer some advantage to the patient with compromised myocardial oxygen or blood supply or both, since it produces mild coronary vasodilation. Thus, coronary vascular resistance decreases with no change in perfusion pressure. Preservation of diastolic perfusion pressure may be particularly important when myocardial blood supply cannot be increased by autoregulation. 

Subsequent to the ingestion of iodine in various forms, I is absorbed by the small intestine and enters the blood. Two competing pathways are involved in the clearance of I from the blood renal filtration into urine and thyroidal uptake. The renal clearance rate for I (30-50 mL/minute) varies only with the glomerular filtration rate. However, the thyroidal 1 clearance rate is autoregulated to maintain an absolute thyroidal I uptake rate of approximately 100 jig I each day. To accomphsh this, the thyroidal I clearance rate may vary (3 to 100 mL/minute) depending on the concentration of I in the blood. 

Sevoflurane has a dose-dependent effect on cerebral blood flow and intracranial pressure cerebral autoregulation is preserved (this is not the case with isoflurane). During hypocarbia, in the absence of nitrous oxide, 1 MAC does not increase intracranial pressure (ICP). It reduces the cerebral metabolic rate for oxygen (CMR02) by approximately 50% at concentrations approaching 2 MAC. This is similar to the reduction observed during isoflurane anaesthesia. 

Depending on the concentration, volatile anesthetics decrease the glomerular filtration rate and renal blood flow, and increase the filtration fraction. Since renal blood flow decreases during general anesthesia in spite of well-maintained or even increased perfusion pressures (due to increased renal vascular resistance), autoregulation of renal flow may be impaired by these drugs. 

A Dynamic Model of Renal Blood Flow Autoregulation, Bull. Math. Biol. 56, 411-430 (1994). 

Dirnagl U, Pulsinelli W (1990) Autoregulation of cerebral blood flow in experimental focal brain ischemia. J Cereb Blood Flow Metab 10 327-336... 

Waltz AG, Sundt TM Jr (1968) Influence of systemic blood pressure on blood flow and microcirculation of ischemic cerebral cortex a failure of autoregulation. Prog Brain Res 30 107-112... 

Jalan R., Olde Damink S. W., Deutz N. E., Hayes P. C., and Lee A. (2001) Restoration of cerebral blood flow autoregulation and reactivity to carbon dioxide in acute liver failure by moderate hypothermia. Hepatology 34,50-54. 

The caliber of arteriolar resistance vessels controls blood flow through the coronary bed. Arteriolar caliber is determined by myocardial 02 tension and local concentrations of metabolic products, and is automatically adjusted to the required blood flow (B, healthy subject). This metabolic autoregulation explains why anginal attacks in coronary sclerosis occur only during exercise (B, patient). At rest, the pathologically elevated flow resistance is... 

Q3 The major control in the coronary circuit is autoregulation, predominantly hypoxia. A small degree of hypoxia dilates coronary vessels and produces a large increase in blood flow increased arterial PC02 and decreased pH also dilate coronary arteries and increase blood flow. 

Flow to the brain tissue is precisely regulated by a process of autoregulation, according to local chemical conditions. Cerebral blood vessels dilate and so increase blood flow in response to decreased pH and arterial PO2 and to increased arterial PCO2, conditions associated with increased metabolic activity. The neurones are very sensitive to changes in cerebral blood flow interruption of flow for a few seconds causes unconsciousness. 

The brain uses a substantial proportion of body oxygen and there is a generous blood supply to the brain from the carotid and vertebral arteries. Interruption of brain blood flow for more than a very short time causes neuronal damage and ultimately cell death. Cerebral blood flow is normally controlled by autoregulation. 

Cerebral blood flow, perfusion pressure and autoregulation... 

Cerebral blood flow depends on cerebral perfusion pressure and cerebrovascular resistance. The perfusion pressure is the difference between systemic arterial pressure at the base of the brain when in the recumbent position and the venous pressure at exit from the subarachnoid space, the latter being approximated by the intracranial pressure. Cerebral perfusion pressure divided by cerebral blood flow gives the cerebrovascular resistance. In normal humans, cerebral blood flow remains almost constant when the mean systemic blood pressure is between approximately 50 and 170mmHg, which, under normal circumstances when the intracranial venous pressure is negligible, is the same as the cerebral perfusion pressure. This homeostatic mechanism to maintain a constant cerebral blood flow in the face of changes in cerebral perfusion pressure is known as autoregulation (Reed and Devous 1985 Powers 1993). Autoregulation is less effective in the elderly, and so postural hypotension is more likely to be symptomatic (Wollner et al. 1979 Parry et al. 2006). 

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