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doctor explaining x-ray result to women patient CHS/SCHTA pregnancy : Report of the Canadian Hypertension Society Consensus Conference: 3. Pharmacologic treatment of hypertensive disorders in pregnancy. November 18, 2020) – Two Canadian medical journals are retracting columns about a seminal Toronto case that prompted health agencies around the world to caution against giving codeine to nursing mothers for pain relief. Hypertension Canada’s 2017 guidelines for diagnosis, risk assessment, prevention, and treatment of hypertension in adults. In other words, although the studies mentioned above are of significant importance to improve our physiological knowledge, they risk being insufficiently utilized for the solution of real clinical problems because of their excessive mathematical and formal intricacy. Figure 6 shows two examples of bifurcation diagrams obtained with the present model (see appendix for mathematical details). As clearly shown by the bifurcation diagram in Fig. 6, self-sustained plateau waves can occur only if Ro is further increased with respect to the value used in Fig. 7.Fig. 7.Time pattern of ICP and CBF computed with model in response to a moderate decrease in SAP (from 100 to 90 mmHg) between 10 and 20 s. Among the different kinds of bifurcation described in mathematical textbooks, a particular role is played by the so-called Hopf bifurcation, which represents the condition where an equilibrium point loses its stability and a periodic oscillation appears (11). Vaisseau éclaté dans l'oeil et hypertension . This is the bifurcation leading to the emergence of ICP plateau waves in patients with severe brain disease.

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The term “bifurcation” is currently used in the mathematical literature to denote those particular values of parameters at which a model exhibits a qualitative change in the topological structure of its trajectories. Hence, as shown inappendix , computation of equilibrium points and stability properties (eigenvalues) can be carried out using rather simple algorithms, and model dynamics can be presented as trajectories in the phase plane. In particular, in a second-order system with constant input quantities, the trajectories can converge toward a stable equilibrium point or approach a closed curve (the “limit cycle”). In the model, dural sinus pressure (Pvs) is an input quantity.

We require the model to settle at a stable equilibrium level when all model parameters and input quantities are at their basal value. Because in all simulations of Fig.8 the injection was carried out between 10 and 12 s, we consider paradoxical the ICP increases occurring after 12 s. Because CSF is not augmenting in this period, but rather lessening because of CSF reabsorption, the paradoxical response is imputable to a progressive rise in CBV induced by the action of cerebral autoregulation mechanisms. Model assumes that, because of collapse of last section, cerebral venous pressure (Pv) is always approximately equal to intracranial pressure (Pic). About 80% is contained in the venular and venous segments, whereas the remaining 20% (9-20 ml) is in arterial-arteriolar vessels. Limitations must be clearly specified and recognized to avoid the inappropriate use of the model beyond its actual capacity. Finally, we must assign a value to the parameters describing the action of the feedback autoregulation mechanisms. Recently, however, we suggested that the response to PVI tests may also contain information on the status and the dynamics of cerebral autoregulation (41). The existence of a strict correlation between PVI and autoregulation has been documented by other authors through clinical and experimental works (7, 12, 13), but the deep nature of this relationship is insufficiently understood.

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The model incorporates a simplified biomechanical description of the arterial-arteriolar cerebrovascular bed, the large cerebral veins, CSF production and reabsorption processes, and the craniospinal storage capacity (Fig. 1).Fig. 1.Electric analog (A) and corresponding mechanical analog (B) of intracranial dynamics according to present model. The model does not incorporate an explicit description of the venous circulation downstream of the bridge veins (i.e., resistance Rdv is not used in the computation); according to the Starling resistor hypothesis, these veins are assumed to collapse or to narrow at their entrance into the dural sinuses. Then a qualitative description of the model is presented. This cascade of events, or self-sustained cycle, can then continue until vasodilation is maximal.Fig. Comment diminuer la tension artérielle . CBF then passes through venous cerebrovascular bed, mimicked as series arrangement of proximal venous resistance (Rpv) and resistance of collapsing lateral lacunae and bridge veins (Rdv). The basal value of arterial and venous resistances was computed using the pressure distribution reported in Table1 (see also Refs. Cerebral blood flow was assessed under two different conditions: LBNP and a CO2 challenge using the two-breath test of Edwards et al. In this condition, cerebral blood flow (CBF) depends on the difference between arterial pressure and ICP; i.e., it is independent of the downstream pressure at the venous sinuses.

3.Main feedback loops included in model.

SAP − ICP, where SAP is systemic arterial pressure) is reduced, vasodilation occurs and is accompanied by an increase in cerebral blood volume (CBV). Conditions of clinical relevance, characterized by paradoxical responses, may occur, among others, during acute hypotension (see ICP response to acute SAP changes) and PVI tests (seeSensitivity analysis to parameter changes during PVI tests). Previously, using a more complex model, we were able to demonstrate that intracranial dynamics may become unstable in pathological conditions (40). The main alterations favoring intracranial instability were a decrease in intracranial compliance and an increase in the Ro, provided these changes occur in patients with preserved autoregulation mechanisms. All the model equations are presented in appendixes a-c, together with parameter numerical values and some guidelines to facilitate practical implementation of the model using a computer. 1) The model does not distinguish between proximal and distal segments of the arterial-arteriolar cerebrovascular bed; i.e., only one arterial-arteriolar segment, extending from large intracranial arteries down to cerebral capillaries, is included. 3.Main feedback loops included in model. Finally, the model, despite its simplicity, includes several distinct feedback loops. Three feedback loops are negative, and so they tend to stabilize ICP. Ro, cerebrospinal fluid outflow resistance; Rpv, proximal venous resistance; Rf, cerebrospinal fluid formation resistance; ΔCa 1 and ΔCa 2 , amplitude of sigmoidal curve; Ca n, basal arterial compliance;kE, elastance coefficient;kR, resistance coefficient; τ, time constant; qn, basal cerebrospinal fluid; G, gain; Pa, systemic arterial pressure; Pic, intracranial pressure; Pvs, dural sinus pressure; Ca, arterial compliance.

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Karahalios DGRekate HLKhayata MHApostolides PJ1996Elevated intracranial venous pressure as a universal mechanism in pseudotumor cerebri of varying etiologiesNeurology46198202.Karahalios DG, Rekate HL, Khayata MH, and Apostolides PJ. 6.32 × 103mmHg ⋅ s ⋅ ml−1), and a normal autoregulation gain. Emphasis is given to feedback IV, which is associated with active cerebral blood volume (CBV) alterations induced by autoregulation. According to Fig. 8, the conditions that may favor the occurrence of paradoxical responses are a highkE value (i.e., a steeper pressure-volume relationship, Fig.8A), a high G value (Fig.8B), and a high basal value of arterial-arteriolar compliance (i.e., a high basal value of the arterial-arteriolar blood volume, Fig.8D).

Another significant consequence of the positive-feedback loop is the possible occurrence of paradoxical responses, i.e., responses characterized by a delayed amplification of a small initial perturbation. Another study suggests that consumption of a tian ma concoction helps regulate nitric-oxide and antioxidative stress that can prevent renal damage due to hypertension. In the model the variations in arterial resistance are strictly related to the variations in compliance and blood volume. Hypertension systolique . Arterial-arteriolar cerebrovascular bed consists of a regulated capacity (Ca), which stores a certain amount of blood volume, and a regulated resistance (Ra), which accounts for pressure drop to capillary pressure (Pc). First, it exhibits a small number of parameters, each able to account for an entire physiological and clinical phenomenon in a concise way.

Thus there is also a need for some simplified models that are able to describe certain clinical aspects of ICP dynamics with sufficient accuracy and, at the same time, incorporate a minimum of mathematics. In contrast, if intracranial compliance worsens, the system moves toward its instability boundary and the positive-feedback loop becomes more influential in intracranial dynamics. To build a simplified model of ICP dynamics aimed at clinical purposes, a few simplifications were introduced with respect to the more accurate mathematical model presented elsewhere (40, 41). The two main simplifications are discussed below. Methods: We randomly assigned 3845 patients from Europe, China, Australasia, and Tunisia who were 80 years of age or older and had a sustained systolic blood pressure of 160 mm Hg or more to receive either the diuretic indapamide (sustained release, 1.5 mg) or matching placebo. By roughly considering the arterial-arteriolar cerebrovascular bed as the parallel arrangement of several equal microvessels and applying the Hagen-Poiseuille law (25), resistance is inversely proportional to the fourth power of inner radius, hence, to the second power of blood volume. This means that the equilibrium point is located high on the exponential intracranial pressure-volume relationship, which corresponds to a zone of reduced intracranial compliance and to a high risk of paradoxical response (Fig.8C).

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Despite its limitations, the use of a reduced model presents some significant advantages. According to its original version, this test is normally used to estimate the PVI (defined as the volume, in ml, that should be added to the CSF space to produce a 10-fold increase in ICP), Ro, and CSF production rate. The Antihypertensive and Lipid-Lowering Treatment to Prevent Heart Attack Trial. Furthermore, according to Fig.8C, increasing Ro causes an increase in the ICP equilibrium level (i.e., the level before the maneuver).

Furthermore, the reduced model is of the second order, with only two state (or memory) variables. This is a plane describing the mutual dependence of the two memory variables: the first is plotted as an independent variable in the x-axis and the second as the dependent variable in they-axis. The final model is of the second order; i.e., it contains only two state (or memory) variables: ICP, which reflects the volume in the craniospinal pressure-volume curve, and the arterial-arteriolar compliance, which is influenced by the action of cerebrovascular control mechanisms. All PVI tests were carried out with a 2-ml bolus injection between 10 and 12 s (vertical dotted lines, injection period).