Sympathetic modulation of hypercapnic cerebral vasodilation in dogs

Interaction between the respiratory system and cerebral blood flow regulation

This review summarizes the interaction between the regulatory system of respiration and cerebral vasculature. Some clinical reports provide evidence for the association between these two physiological regulatory systems. Physiologically, arterial carbon dioxide concentration is mainly regulated by two feedback control systems: respiration and cerebral blood flow. In other words, both of these systems are sensitive to the same mediator, i.e., carbon dioxide, at a set point. In addition, respiratory dysfunction alters various physiological factors that affect the cerebral vasculature. Therefore, it is physiologically plausible that these systems are closely linked. The regulation of arterial carbon dioxide concentration affected by respiration and cerebral blood flow may be a key factor for a rise in the risk of brain disease in the patients with respiratory dysfunction. For example, the management of respiratory disease (e.g., patients with chronic obstructive pulmonary disease) and the use of prophylactic therapy are essential to reduce the risk of stroke.

Manipulation of central blood volume and implications for respiratory control function

The respiratory operating point (ventilatory or arterial Pco2 response) is determined by the intersection point between the controller and plant subsystem elements within the respiratory control system. However, to what extent changes in central blood volume (CBV) influence these two elements and the corresponding implications for the respiratory operating point remain unclear. To examine this, 17 apparently healthy male participants were exposed to water immersion (WI) or lower body negative pressure (LBNP) challenges to manipulate CBV and determine the corresponding changes. The respiratory controller was characterized by determining the linear relationship between end-tidal Pco2 (PetCO2) and minute ventilation (V̇e) [V̇e = S × (PetCO2 − B)], whereas the plant was determined by the hyperbolic relationship between V̇e and PetCO2 (PetCO2 = A/V̇e + C). Changes in V̇e at the operating point were not observed under either WI or LBNP conditions despite altered PetCO2 ( P < 0.01), indicating a moving respiratory operating point. An increase (WI) and a decrease (LBNP) in CBV were shown to reset the controller element (PetCO2 intercept B) rightward and leftward, respectively ( P < 0.05), without any change in S, whereas the plant curve remained unaltered at the operating point. Collectively, these findings indicate that modification of the controller element rather than the plant element is the major factor that contributes toward an alteration of the respiratory operating point during CBV shifts.

Cerebral blood flow during exercise: mechanisms of regulation

The response of cerebral vasculature to exercise is different from other peripheral vasculature; it has a small vascular bed and is strongly regulated by cerebral autoregulation and the partial pressure of arterial carbon dioxide (Pa(CO(2))). In contrast to other organs, the traditional thinking is that total cerebral blood flow (CBF) remains relatively constant and is largely unaffected by a variety of conditions, including those imposed during exercise. Recent research, however, indicates that cerebral neuronal activity and metabolism drive an increase in CBF during exercise. Increases in exercise intensity up to approximately 60% of maximal oxygen uptake produce elevations in CBF, after which CBF decreases toward baseline values because of lower Pa(CO(2)) via hyperventilation-induced cerebral vasoconstriction. This finding indicates that, during heavy exercise, CBF decreases despite the cerebral metabolic demand. In contrast, this reduced CBF during heavy exercise lowers cerebral oxygenation and therefore may act as an independent influence on central fatigue. In this review, we highlight methodological considerations relevant for the assessment of CBF and then summarize the integrative mechanisms underlying the regulation of CBF at rest and during exercise. In addition, we examine how CBF regulation during exercise is altered by exercise training, hypoxia, and aging and suggest avenues for future research.

Integration of cerebrovascular CO2 reactivity and chemoreflex control of breathing: mechanisms of regulation, measurement, and interpretation

Cerebral blood flow (CBF) and its distribution are highly sensitive to changes in the partial pressure of arterial CO(2) (Pa(CO(2))). This physiological response, termed cerebrovascular CO(2) reactivity, is a vital homeostatic function that helps regulate and maintain central pH and, therefore, affects the respiratory central chemoreceptor stimulus. CBF increases with hypercapnia to wash out CO(2) from brain tissue, thereby attenuating the rise in central Pco(2), whereas hypocapnia causes cerebral vasoconstriction, which reduces CBF and attenuates the fall of brain tissue Pco(2). Cerebrovascular reactivity and ventilatory response to Pa(CO(2)) are therefore tightly linked, so that the regulation of CBF has an important role in stabilizing breathing during fluctuating levels of chemical stimuli. Indeed, recent reports indicate that cerebrovascular responsiveness to CO(2), primarily via its effects at the level of the central chemoreceptors, is an important determinant of eupneic and hypercapnic ventilatory responsiveness in otherwise healthy humans during wakefulness, sleep, and exercise and at high altitude. In particular, reductions in cerebrovascular responsiveness to CO(2) that provoke an increase in the gain of the chemoreflex control of breathing may underpin breathing instability during central sleep apnea in patients with congestive heart failure and on ascent to high altitude. In this review, we summarize the major factors that regulate CBF to emphasize the integrated mechanisms, in addition to Pa(CO(2)), that control CBF. We discuss in detail the assessment and interpretation of cerebrovascular reactivity to CO(2). Next, we provide a detailed update on the integration of the role of cerebrovascular CO(2) reactivity and CBF in regulation of chemoreflex control of breathing in health and disease. Finally, we describe the use of a newly developed steady-state modeling approach to examine the effects of changes in CBF on the chemoreflex control of breathing and suggest avenues for future research.

Cerebral Autoregulation and Syncope

Whatever the pathogenesis of syncope is, the ultimate common cause leading to loss of consciousness is insufficient cerebral perfusion with a critical reduction of blood flow to the reticular activating system. Brain circulation has an autoregulation system that keeps cerebral blood flow constant over a wide range of systemic blood pressures. Normally, if blood pressure decreases, autoregulation reacts with a reduction in cerebral vascular resistance, in an attempt to prevent cerebral hypoperfusion. However, in some cases, particularly in neurally mediated syncope, it can also be harmful, being actively implicated in a paradox reflex that induces an increase in cerebrovascular resistance and contributes to the critical reduction of cerebral blood flow. This review outlines the anatomic structures involved in cerebral autoregulation, its mechanisms, in normal and pathologic conditions, and the noninvasive neuroimaging techniques used in the study of cerebral circulation and autoregulation. An emphasis is placed on the description of autoregulation pathophysiology in orthostatic and neurally mediated syncope.

Interaction among autoregulation, CO2 reactivity, and intracranial pressure: a mathematical model

The relationships among cerebral blood flow, cerebral blood volume, intracranial pressure (ICP), and the action of cerebrovascular regulatory mechanisms (autoregulation and CO2 reactivity) were investigated by means of a mathematical model. The model incorporates the cerebrospinal fluid (CSF) circulation, the intracranial pressure-volume relationship, and cerebral hemodynamics. The latter is based on the following main assumptions: the middle cerebral arteries behave passively following transmural pressure changes; the pial arterial circulation includes two segments (large and small pial arteries) subject to different autoregulation mechanisms; and the venous cerebrovascular bed behaves as a Starling resistor. A new aspect of the model exists in the description of CO2 reactivity in the pial arterial circulation and in the analysis of its nonlinear interaction with autoregulation. Simulation results, obtained at constant ICP using various combinations of mean arterial pressure and CO2 pressure, substantially support data on cerebral blood flow and velocity reported in the physiological literature concerning both the separate effects of CO2 and autoregulation and their nonlinear interaction. Simulations performed in dynamic conditions with varying ICP underline the existence of a significant correlation between ICP dynamics and cerebral hemodynamics in response to CO2 changes. This correlation may significantly increase in pathological subjects with poor intracranial compliance and reduced CSF outflow. In perspective, the model can be used to study ICP and blood velocity time patterns in neurosurgical patients in order to gain a deeper insight into the pathophysiological mechanisms leading to intracranial hypertension and secondary brain damage.

Intracranial and extracranial blood flow during acute anxiety

While large numbers of studies are available on anxiety and cerebral blood flow (CBF), little is known about their relationship to extracranial (forehead) flow. The participants were 24 generalized anxiety disorder (GAD) patients and 26 normal volunteers. A randomized, between groups, repeated measures design was used to evaluate changes in cerebral blood flow. Measurements of CBF, forehead skin perfusion and ratings of anxiety and physiologic indices were made under resting conditions and during anxiety induction with epinephrine or saline infusions, given under double-blind conditions while subjects inhaled room air or 5% CO2. These subjects were divided into three groups; those with decreased anxiety, those with mild anxiety, and those with more severe anxiety increase. Subjects with severe anxiety showed least hypercarbic CBF increase (indicating cerebral vasoconstriction) and maximal increase in forehead skin perfusion. Those with minimal anxiety had most hypercarbic cerebral vasodilation and least increase in forehead skin perfusion. Forehead skin perfusion correlated positively with anxiety levels, and negatively with hypercarbic cerebral vasodilation. In animals, sympathetic activation limits hypercapnic cerebral vasodilation. Thus, the restricted hypercapnic cerebral vasodilation during severe anxiety may be mediated through cervical sympathetic fibers which innervate cerebral vessels.

Cerebral vasodilation and vasoconstriction associated with acute anxiety

A randomized, between-groups, repeated measures design was used to evaluate changes in cerebral blood flow (CBF), rating scales, and physiologic indices under resting conditions, during 5% CO2 inhalation in combination with epinephrine or saline infusions, in generalized anxiety disorder patients and controls. Subjects were divided into those with decreased anxiety and mild and more severe anxiety increase. The first group was found to have most pronounced CBF increase during CO2 inhalation, with the second group showing less marked increase, and the last group the least increase. In animals, sympathetic activation limits hypercapnic cerebral vasodilation. Thus, the restricted hypercapnic cerebral vasodilation during severe anxiety may be mediated through cervical sympathetic fibers, which innervate cerebral vessels.

Effect of hypoxic-hypocapnia on cerebral regional oxygen consumption and supply

The effect of hypoxic-hypocapnia (PaO2, 35 mm Hg, PaCO2, 22 mm Hg) on regional cerebral O2 consumption and supply was determined in 18 alpha-chloralose-anesthetized open-chest adult cats. Regional arterial and venous O2 saturation measurements determined by a microspectrophotometric technique were combined with regional flow measurements by the radioactive microsphere method to calculate cerebral regional O2 consumption. In a control group, after blood flow was determined with 15 +/- 3-microns-diameter microspheres, the heads were quick-frozen and measurements of flow and arterial-venous O2 differences were obtained from nine brain regions. In the experimental group, similar measurements were obtained after the induction of hypoxic-hypocapnia. Cerebral blood flow was significantly lower than the control group during hypocapnia. Cerebral blood flow increased with the induction of hypoxia. Arterial and venous O2 saturation decreased uniformly and to the same extent in the nine examined brain regions. This maintained the arterial-venous O2 saturation difference. O2 extraction and consumption were unaffected. The brain O2 supply/consumption ratio was maintained during hypoxic-hypocapnia indicating adequate and uniform protection in this condition throughout the brain.

The electroencephalogram, blood flow, and oxygen uptake in rabbit cerebrum

In the present study, the relationships among electroencephalographic (EEG) amplitude shifts, cerebral blood flow (CBF), and cerebral oxygen uptake (CMRO2) have been characterized in halothane-anesthetized rabbits. CBF was measured by timed collection of venous effluent from the superior sagittal sinus. CMRO2 was calculated as the product of CBF and the arteriovenous difference in oxygen content. The depth of anesthesia in the first series of experiments was maintained at a constant level that was characterized by spontaneous EEG shifts from high- to low-voltage states (HV-LV shifts). These shifts were associated with transient decreases in mean arterial pressure (MAP), which averaged 23 +/- 2 mm Hg (n = 17). Ninety seconds after spontaneous HV-LV shifts, MAP had returned to its original value, CBF had increased by 26 +/- 7% (n = 8), and CMRO2 had increased 22 +/- 4% (n = 7). In a second series of experiments, HV-LV shifts were induced by a 90-s application of a standardized nociceptive stimulus (n = 13). Following these stimulation-induced HV-LV shifts, CBF increased 28 +/- 5% and CMRO2 increased 27 +/- 4%. Under scopolamine (0.35 mg/kg, i.v., n = 8), no change in CBF was observed following HV-LV shifts induced by 90-s of stimulation, although CMRO2 increased significantly by 14 +/- 3%. After 300 s of post-scopolamine stimulation, however, both CBF and CMRO2 had significantly increased by 12 +/- 3 and 15 +/- 3% (n = 8) of control, respectively. These results demonstrate that HV-LV shifts, whether spontaneous or stimulation-induced, are associated with significant increases in both CBF and CMRO2. Because the early (90-s) increases in CBF but not those in CMRO2 could be blocked by scopolamine, we suggest that the cerebral vasodilatation that occurs during the early phase of HV-LV shifts involves cholinergic mechanisms. Because scopolamine could not block the increase in CBF observed after 300 s of stimulation, we suggest that the final value of CBF obtained after an HV-LV shift is determined by a combination of both cholinergic and noncholinergic factors.