The patient with acute neurological problems
Peate Ian, Dutton Helen in Acute Nursing Care, 2020
A good cerebral blood flow and oxygen delivery is essential for normal cerebral function. Although the brain only weighs about 1.5kg, or 2% of body weight, it receives approximately 20% of the cardiac output and 20% of available oxygen. The brain is able to autoregulate to control cerebral blood flow. Autoregulation describes the brain’s ability to maintain a constant blood flow irrespective of changes to blood pressure. This phenomenon only functions correctly when mean arterial pressure (MAP) is between 60 and 150mmHg. Below 60mmHg, cerebral blood flow diminishes, and above 150mmHg, it increases. Hypotension causes cerebral ischaemia and brain injury. Neurological management focuses on the maintenance of MAP by the administration of fluids and vasopressors.
Functions of the Cardiovascular System
Peter Kam, Ian Power, Michael J. Cousins, Philip J. Siddal in Principles of Physiology for the Anaesthetist, 2020
Metabolic autoregulation (‘metabolic-flow coupling’). Local myogenic control is also important in the heart, and coronary blood flow is held constant between aortic diastolic pressures of 60 and 180 mmHg. Myocardial oxygen consumption is the most important determinant of coronary blood flow. During heavy exercise, the increased cardiac oxygen demand is met mainly by an increase in coronary blood flow (from the resting value of 80 mL/min/100 g up to 300–400 mL/min/100 g), although there is also a rise in coronary oxygen extraction to 90%. Coronary blood flow rises in proportion to cardiac metabolic activity, through the release of local vasodilator substances from the working myocardial cells. Many factors have been proposed as being responsible for this metabolic control of arteriolar tone in the coronary circulation, including reduced oxygen tension, increased carbon dioxide tension, hydrogen ions, potassium, lactic acid, pyruvate, inorganic phosphate, interstitial fluid osmolarity, NO and adenosine. Decreased Po2 is the most potent stimulus of all to coronary blood flow. Adenosine is the chief mediator in hypoxic vasodilation of the coronary vascular bed.
Neural Control of the Intestinal Circulation and its Interaction With Autoregulation
Irving H. Zucker, Joseph P. Gilmore in Reflex Control of the Circulation, 2020
By definition autoregulation is the ability of an organ to maintain a relatively constant blood flow in the presence of changes in perfusion pressure. This is accomplished by intrinsic vascular responses that probably involve a combination of metabolic, myogenic and/or endothelial mechanisms. Whatever the exact cause, evidence has indicated that the mechanisms responsible for intestinal autoregulation of blood flow continue to operate in the presence of neural and humoral vasoconstriction (Meininger and Trzeciakowski, 1988). A consequence of this observation is that a component of the vascular resistance residing in a vascular bed at any given moment will be a function of the autoregulation in that vascular bed. The autoregulatory ability of a vascular bed can be defined by its pressure-flow relationship. From knowledge of the pressure-flow relationship a very simplified view of the interaction is to assume that any change in arterial pressure brought about by the pressor response would cause the normal operating point on the pressure-flow curve to shift along that curve until the point defined by the new arterial pressure is reached. As such a component of the change in vascular resistance will be due to an autoregulatory adjustment to the change in arterial pressure.
Mechanistic links between systemic hypertension and open angle glaucoma
Published in Clinical and Experimental Optometry, 2022
Ying-kun Cui, Li Pan, Tim Lam, Chun-yi Wen, Chi-wai Do
However, under physiological conditions, there is a lack of a linear relationship between ocular perfusion pressure and ocular blood flow.43 This is attributed to the ability of maintaining a relatively constant ocular blood flow despite fluctuating ocular perfusion pressure, which is known as autoregulation.42 Autoregulation is a complicated process and refers to the intrinsic property of organs to maintain a constant blood flow in response to changes in perfusion pressure. It is controlled by both myogenic and metabolic mechanisms. Since the retina has no autonomic innervation, the blood supply to the inner retina is regulated by local vascular mechanisms. In the myogenic mechanism, the smooth muscle cells in the blood vessels contract when being stretched. This process is possibly mediated by activating voltage-gated Ca2+ channels, resulting in an increased vascular resistance due to vasoconstriction.44
Kidney physiology and pathophysiology during heat stress and the modification by exercise, dehydration, heat acclimation and aging
Published in Temperature, 2021
Christopher L. Chapman, Blair D. Johnson, Mark D. Parker, David Hostler, Riana R. Pryor, Zachary Schlader
There is great interest in accurately quantifying changes in renal blood flow because it is a highly controlled variable that has implications for the regulation of blood pressure and water and electrolytes. Thus, it is also important to note that the kidneys have an intrinsic ability to maintain blood flow at varying arterial pressures (i.e., autoregulate). Renal blood flow autoregulation is mediated by actions of the afferent arterioles and interlobular arteries and their myogenic response to constrict or relax in response to changes in perfusion pressure [173-175]. Approximately, 50% of the total autoregulatory response [176,177] rapidly occurs within 3-10 seconds [178,179], which is contributed to by unloading of the renal baroreceptors and tubuloglomerular feedback provided by the juxtaglomerular apparatus [180,181]. Tubuloglomerular feedback also results in renin release by the afferent arterioles in response to sensation of decreased NaCl delivery to the macula densa in the distal tubule [182], which ultimately ensures a relatively stable renal blood flow and glomerular filtration rate (see Glomerular filtration rate). These neural (discussed previously in Autonomic control of kidney function), hormonal (discussed previously in PHYSIOLOGY AND ASSESSMENT OF BODY WATER REGULATION), and autoregulatory mechanisms offer a complex and highly redundant control of renal blood flow to maintain homeostasis utilizing many systems.
A randomized controlled study comparing high-dose insulin to vasopressors or combination therapy in a porcine model of refractory propranolol-induced cardiogenic shock
Published in Clinical Toxicology, 2019
Katherine G. Katzung, Jenna M. Leroy, Sean P. Boley, Samuel J. Stellpflug, Joel S. Holger, Kristin M. Engebretsen
Knowing this, we were surprised to find that by adding NE (a medication known to cause vasoconstriction in end-organ capillary beds) to HDI therapy led to improved brain tissue oxygenation. The reason for this is unclear. The answer may be related to balancing increased perfusion via cerebral microcapillary vasodilation with the autoregulation phenomenon of cerebral perfusion. Autoregulation of cerebral blood flow is the ability of the brain to maintain relatively constant blood flow and as a result, maintain cerebral oxygenation despite changes in cerebral perfusion pressure [22,23]. In humans, literature suggests that cerebral blood flow is maintained provided that the cerebral perfusion pressure is within 50–150 mmHg [24,25]. In refractory PICS, the MAP falls below the lower limit of autoregulation and cerebral ischemia occurs [26]. While insulin may maintain the vasodilation in the brain to avoid microcapillary ischemia, the peripheral vasoconstriction and resultant increased MAP affected by the addition of the vasopressor may better maintain the animal in this range of autoregulation and stave off a drop in cerebral perfusion pressure.