Effect of alpha-adrenergic blockade on the cerebrovascular response to increased intracranial pressure after hemorrhage

Regulation of cerebrovascular compliance compared with forearm vascular compliance in humans: a pharmacological study

Increasing evidence indicates that cerebrovascular compliance contributes to the dynamic regulation of cerebral blood flow but the mechanisms regulating cerebrovascular compliance in humans are unknown. This retrospective study investigated the impact of neural, endothelial, and myogenic mechanisms on the regulation of vascular compliance in the cerebral vascular bed compared with the forearm vascular bed. An index of vascular compliance (C_i) was assessed using a Windkessel model applied to blood pressure waveforms (finger photoplethysmography) and corresponding middle cerebral artery blood velocity or brachial artery blood velocity waveforms (Doppler ultrasound). Data were analyzed during a 5-min baseline period (10 waveforms) under control conditions and during distinct sympathetic blockade (experiment 1, phentolamine; 10 adults), cholinergic blockade (experiment 2, glycopyrrolate; 9 adults), and myogenic blockade (experiment 3, nicardipine; 14 adults). In experiment 1, phentolamine increased C_i similarly in the cerebral vascular bed (131 ± 135%) and forearm vascular bed (93 ± 75%; P = 0.45). In experiment 2, glycopyrrolate increased cerebrovascular C_i (72 ± 61%) and forearm vascular C_i (74 ± 64%) to a similar extent (P = 0.88). In experiment 3, nicardipine increased C_i but to a greater extent in the cerebral vascular bed (88 ± 88%) than forearm vascular bed (20 ± 45%; P = 0.01). Therefore, adrenergic, cholinergic, and myogenic mechanisms contribute to the regulation of cerebrovascular and forearm vascular compliance. However, myogenic mechanisms appear to exert more specific control over vascular compliance in the brain relative to the forearm.NEW & NOTEWORTHY Vascular compliance represents an important determinant in the dynamics and regulation of blood flow through a vascular bed. However, the mechanisms that regulate vascular compliance remain poorly understood. This study examined the impact of neural, endothelial, and myogenic mechanisms on cerebrovascular compliance compared with forearm vascular compliance. Distinct pharmacological blockade of α-adrenergic, endothelial muscarinic, and myogenic inputs altered cerebrovascular and forearm vascular compliance. These results further our understanding of vascular control and blood flow regulation in the brain.

Acute Management of Hypertension Following Intracerebral Hemorrhage

Intracerebral hemorrhage (ICH) is responsible for approximately 15% of strokes annually in the United States, with nearly 1 in 3 of these patients dying without ever leaving the hospital. Because this disproportionate mortality risk has been stagnant for nearly 3 decades, a main area of research has been focused on the optimal strategies to reduce mortality and improve functional outcomes. The acute hypertensive response following ICH has been shown to facilitate ICH expansion and is a strong predictor of mortality. Rapidly reducing blood pressure was once thought to induce cerebral ischemia, though has been found to be safe in certain patient populations. Clinicians must work quickly to determine whether specific patient populations may benefit from acute lowering of systolic blood pressure (SBP) following ICH. This review provides nurses with a summary of the available literature on blood pressure control following ICH. It focuses on intravenous and oral antihypertensive medications available in the United States that may be utilized to acutely lower SBP, as well as medications outside of the antihypertensive class used during the acute setting that may reduce SBP.

The Lund concept for the treatment of patients with severe traumatic brain injury

Two different main concepts for the treatment of a severe traumatic brain injury have been established during the last 15 years, namely the more conventional concept recommended in well-established guidelines (eg, U.S. Guideline, European Guideline, Addelbrook's Guideline from Cambridge), on the one hand, and the Lund concept from the University Hospital of Lund, Sweden, on the other. Owing to the lack of well-controlled randomized outcome studies comparing these 2 main therapeutic approaches, we cannot conclude that one is better than the other. This paper is the PRO part in a PRO-CON debate in this journal on the Lund concept. Although the Lund concept is based on a physiology-oriented approach dealing with the hemodynamic principles of brain volume and brain perfusion regulation, traditional treatments are primarily based on a meta-analytic approach from clinical studies. High cerebral perfusion pressure has been an essential goal in the conventional treatments (the cerebral perfusion pressure-guided approach), even though it has been modified in a recent up date of U.S. guidelines. The Lund concept has instead concentrated on management of brain edema and intracranial pressure, along with improvement of cerebral perfusion and oxygenation (the intracranial pressure and perfusion-guided approach). Although conventional guidelines are restricted to clinical data from meta-analytic surveys, the physiological approach of Lund therapy finds support in both experimental and clinical studies. It offers a wider base and can also provide recommendations regarding fluid therapy, lung protection, optimal hemoglobin concentration, temperature control, the use of decompressive craniotomy, and ventricular drainage. This paper puts forward arguments in support of Lund therapy.

Intracranial pressure following resuscitation with albumin or saline in a cat model of meningitis

OBJECTIVE

To compare the intracranial pressure after resuscitation to normovolemia by using 20% albumin or normal saline in a cat model of meningitis.

DESIGN

Prospective, randomized animal study.

SETTING

University hospital laboratory.

SUBJECTS

Twenty adult, male cats.

INTERVENTIONS

Meningitis was induced by intrathecal injection of Escherichia coli-derived lipopolysaccharide (0.8 × 10 units/kg). Four hours after the lipopolysaccharide injection, the animals were randomized to intravenous treatment with 0.4 mL/kg/hr of 20% albumin or 7.5 mL/kg/hr of 0.9% sodium chloride for 6 hrs (n = 7 per group). A control group receiving lipopolysaccharide but no fluid was also studied (n = 6).

MEASUREMENTS AND MAIN RESULTS

Effects on intracranial pressure, mean arterial pressure, plasma volume (I-albumin technique), plasma oncotic pressure, and brain metabolism via cerebral interstitial lactate/pyruvate ratio and glycerol and glucose levels (microdialysis technique) were evaluated. Plasma volume decreased by approximately 20% and intracranial pressure increased from 10 to approximately 20 mm Hg at 4 hrs after the lipopolysaccharide injection. Six hours later, plasma volume had returned to baseline in both fluid groups while there was a further reduction in the control group. Intracranial pressure was higher in the saline group than in the albumin and control groups and was 25.8 ± 2.8 mm Hg, 18.3 ± 0.6 mm Hg, and 20.4 ± 1.7 mm Hg, respectively. Plasma oncotic pressure was higher in the albumin group than in the saline and control groups. Mean arterial pressure and microdialysis data were within normal range and did not differ among the groups.

CONCLUSIONS

The results showed that the choice of resuscitation fluid may influence intracranial pressure in meningitis. The lower intracranial pressure in the colloid group may be explained by a higher plasma oncotic pressure and less fluid distribution to the brain interstitium.

Prostacyclin reduces elevation of intracranial pressure and plasma volume loss in lipopolysaccharide-induced meningitis in the cat

BACKGROUND

Severe meningitis may compromise cerebral perfusion through increases in intracranial pressure (ICP) and through hypovolemia caused by a general inflammation with systemic plasma leakage. From its antiaggregative/antiadhesive and permeability-reducing properties, prostacyclin (PGI2) is a potential adjuvant treatment in meningitis, but previously published data have been ambiguous. The objective of this study was to evaluate the effects of PGI2 on meningitis on ICP, plasma volume, blood pressure, and cerebral oxidative metabolism.

METHODS

Meningitis was induced by intrathecal injection of lipopolysaccharide (LPS, 0.8 x 10 units/kg) in cats. Four hours after the injection, the animals were randomized to intravenous treatment with either low-dose PGI2 (1 ng/kg/min) or the vehicle for 6 hours (n = 7 in each group). No LPS and no PGI2 or vehicle was given to three cats (sham group). Effects of treatment on ICP, mean arterial pressure, plasma volume (I-albumin technique), and brain tissue lactate/pyruvate ratio (microdialysis technique) were evaluated.

RESULTS

ICP increased from 10.0 mm Hg +/- 1.3 mm Hg and 10.8 mm Hg +/- 1.7 mm Hg to 19.9 mm Hg +/- 1.7 mm Hg and 19.6 mm Hg +/- 3.3 mm Hg in the PGI2 and the vehicle group, respectively, 4 hours after the LPS injection (not significant). ICP increased further to 21.8 mm Hg +/- 4.5 mm Hg and to 25.8 mm Hg +/- 6.0 mm Hg after treatment for 6 hours with PGI2 or vehicle, respectively (p < 0.05). There was no significant difference in arterial pressure between groups. Plasma volume loss was less in the PGI2 group than in the vehicle group at the end of the experiment and urine production and arterial oxygenation was higher in the PGI2 group. Lactate/pyruvate ratio was within the normal range in all groups.

CONCLUSION

Low-dose PGI2 may be a beneficial adjuvant therapy for meningitis by reducing elevation of ICP and plasma volume loss.

Circulating norepinephrine and cerebrovascular control in conscious humans

BACKGROUND

Cerebral vasoconstriction without concurrent changes in systemic arterial blood pressure has been observed in both normal individuals and those with idiopathic orthostatic intolerance following several minutes of postural stress when circulating catecholamines are elevated. Therefore, we tested the hypothesis that alpha-adrenergic activation with and without elevated circulating norepinephrine (NE) directly affects cerebrovascular tone in healthy individuals.

METHODS

Mean arterial pressure (MAP; tonometry) and cerebral blood flow velocity (MFV) in the middle cerebral artery (transcranial Doppler) were measured in seven healthy individuals during 15 min periods of saline and of 50 (low NE) and 100 (high NE) ng kg(-1) min(-1) infusions of NE. Following this, phentolamine (PHO) was administered to return MAP back to baseline while high NE infusion continued (high NE+PHO). Finally, NE infusion was stopped allowing the persistent effects of PHO to dominate.

RESULTS

Circulating NE caused a dose-dependent increase in MAP (P<0.05). During combined high NE+PHO, blood pressure was initially reduced to baseline levels but then increased a second time (P<0.05) during the final approximately 5 min of this phase. MFV remained constant during both low NE and high NE. In contrast, the secondary increase in BP during the late high NE+PHO phase was associated with elevated MFV. Cerebral vascular resistance (CVR) increased during high NE but was reduced to baseline during both early and late portions of the combined high NE+PHO phase (i.e. despite the late-phase increase in blood pressure).

CONCLUSIONS

The increase in CVR during NE infusion was explained by an autoregulatory response to the increased blood pressure and not an alpha-mediated constriction. However, PHO appeared to interfere with the normal autoregulatory response to increasing blood pressure.

Evaluation of functional response of cerebral arteries by a new morphometric technique

The contractile response of the rabbit basilar artery under four conditions was determined: (1) response in a resting condition without exclusion of the sympathetic nervous system (control group I); (2) response in a resting condition with alpha-adrenoceptor blockade (group II); (3) response to subarachnoid haemorrhage (SAH) (group III); and (4) response to SAH with alpha-adrenoceptor blockade (group IV). It was also ascertained whether it was possible to measure contractile response using a new morphometric method. Vessels were prepared by intracardial perfusion fixation, stained by haematoxylin and eosin, and the length of the intimal corrugations were measured by computer image analysis. Two procedures were followed in order to express the intensity of intimal corrugation, indicating the contractibility of the basilar arteries: (1) the corrugation coefficient (CC) of the basilar artery intima was estimated by dividing the precisely measured length of the intimal corrugations by the length of the measured vessel wall section of the vessel cross-sections (obtained histologically); (2) the lumen reduction coefficient (LRC) of the basilar artery was determined by dividing the "ideal" luminal area (calculated from the total length of the intimal circumference) by the real luminal cross-section area. The results of CC measurements revealed the smoothest intima (mean CC = 1.146, P = 0.00) and the least reduction of lumen (mean LRC = 0.26, P = 0.000) in group II (rabbits without SAH but with alpha-blocker phenoxybenzamine), and in group IV (SAH group of rabbits with alpha-blocker phenoxybenzamine) where the mean CC was 1.141 (P = 0.001) and the mean LRC was 0.33 (P = 0.002) in comparison with the SAH-only group III, pointing out the effectiveness of alpha-blockade even against SAH vasospastic stimuli. Control group I (without SAH and without treatment) showed a greater degree of corrugation in the intima and an increased reduction in the lumina than in groups II and IV, but still significantly less than in group III (mean CC = 1.197, P = 0.001, and mean LRC = 0.40, P = 0.028), thus demonstrating a certain resting tone of the basilar arteries (in an ideal situation, without any tone at all, the CC and LRC would be equal one). The highest degree of intimal corrugation and the greatest lumina reduction were discovered in the SAH-only group III (mean CC = 1.374 and mean LRC = 0.60). The differences among groups I, II and IV were insignificant. The results of this study suggest four conclusions: (i) the possibility of evaluating the functional response of rabbit cerebral arteries using this new morphometric technique; (ii) the adrenergic influence on resting tone of these arteries; (iii) the likely preventive role of an alpha-blockade on post-SAH vasospasm of basilar arteries in rabbits; and (iv) good comparability of the results of CC and LRC measurements with the angiographically estimated vessel diameters of other similar studies.

The neglected prehospital phase of head injury: apnea and catecholamine surge

The prehospital phase of head injury, also called the critical phase, consists of trauma-induced apnea and stress catecholamine release. This immediate period after head injury remains poorly summarized in the literature and essentially ignored with respect to treatment. A MEDLINE search of the literature on apneustic response and catecholamine surge after head injury and a review of literature from my acquired references revealed 116 references (from more than 600) that were pertinent. Apnea induced by head injury produces hypoxia, hypercarbia, and subsequent cardiac failure and hypotension, which, along with substantially elevated catecholamine values, promote secondary mechanisms of organ injury. Treatment for this immediate period after head injury requires a rapid response to the scene of trauma and development of treatment options that can be instituted at the scene of injury.

The neuroprotective effect of DL-tetrahydropalmatine in rat heatstroke

After the onset of heatstroke, rats with saline injection displayed hyperthermia, decreased mean arterial pressure, decreased cerebral blood flow, increased brain monoamine release, and increased neuronal damage score compared with those of normothermia, control rats. The heatstroke-induced hyperthermia, arterial hypotension, cerebral ischemia, brain monoamine overload, and cerebral neuronal injury were attenuated by pretreatment with dl-tetrahydropalmatine. The data indicate that DL-tetrahydropalmatine pretreatment provides neuroprotective effect in heatstroke.