OBJECTIVE: Different parts of the human spine have to accomplish different functions. But little is known about the exact distribution of forces within the spine and whether this is influenced by bone quality. The purpose of this study was to predict fields and extent of greatest load in compression in a human lumbar spine motion segment using a finite element model. METHODS: A three dimensional isotrophic finite element model was generated using the software ANSYS 5.4. Spinal loading was performed in axial compression (600 N). The model was validated by biomechanical analysis using 12 human spinal segments that were loaded with the same forces. Prediction was done with different E-modulus for cancellous bone, representing a wide range of bone quality between osteoporotic and strong bone quality. RESULTS: Load-sharing was influenced by bone quality: the weaker bone quality is, the higher is the extent of load that is passed through the posterior part of the spine. CONCLUSION: This finite element model predicts that load-sharing in a lumbar spine segment with decreased bone mineral density is different from that in healthy segments. A decrease of bone mineral density is resulting in an increase of load that is passed through the posterior part of the lumbar spine. Keeping in mind the simplifications of this model, the results may influence surgical treatment of patients suffering from osteoporosis or osteolytic destructions of the lumbar spine.

Adult↗

PubMedDate not supplied

Cardiovascular indices of peripheral and central sympathetic activation.

OBJECTIVE: A number of sympathetic nervous system (SNS) parameters have been used in cardiovascular psychophysiology. This study aimed to describe the pattern and redundancy of a set of SNS parameters during peripherally induced changes of cardiac sympathetic activation and reflex modulation of central SNS control. Preejection period (PEP) was assessed as a marker of peripheral sympathetic activation. Low-frequency blood pressure variability (BPV) was assessed as an estimate of central SNS control. METHODS: Peripheral beta-sympathetic stimulation and blockade were achieved with epinephrine and esmolol hydrochloride (beta1-blockade), respectively. Changes in central SNS output were induced by loading and unloading arterial baroreceptors with norepinephrine and nitroprusside sodium, respectively. This single-blinded, crossover study in 24 healthy men also included two placebo control periods. PEP was derived from impedance cardiography and adjusted individually for heart rate. BPV was calculated by power spectral analyses of beat-to-beat heart rate and systolic blood pressure (Finapres system) data. RESULTS: PEP decreased during epinephrine infusion (-40.1 +/- 3.8 ms, p <.0001) and increased during esmolol infusion (+6.6 +/- 3.5 ms, p =.05). PEP was shortened after central SNS activation by nitroprusside (-16.8 +/- 2.9 ms, p < 0.0001). Systolic BPV in the low-frequency range (0.07-0.14 Hz, Mayer waves) increased during nitroprusside infusion (+0.44 +/- 0.19 ln mm Hg(2), p =.03) and decreased during norepinephrine infusion (-0.67 +/- 0.13 ln mm Hg(2), p < 0.0001). Low-frequency BPV did not change significantly during epinephrine or esmolol infusion. CONCLUSIONS: Our data provide empirical evidence of separable peripheral and central sympathetic response components. The combined report of low-frequency BPV and PEP gives distinct information on both central SNS control and the level of sympathetic cardiac activation achieved.

Adrenergic alpha-Agonists↗

PubMedDate not supplied

Hemodynamic and reflex responses to acute and chronic antihypertensive therapy with the calcium entry blocker nifedipine.

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Adult↗

PubMedDate not supplied

Rapid prediction of individual dosage requirements for lignocaine.

The mean and standard deviation of lignocaine (lidocaine) pharmacokinetic parameters in a patient population were determined on the basis of 327 serum concentration measurements obtained in 42 patients treated for ventricular arrhythmias. The application of a Bayesian forecasting method, which uses the estimates of the population parameters and 1 or 2 serum concentration measurements as feedback information, was tested retrospectively in 17 of the 42 patients (group I, 32 levels), and prospectively in 10 additional patients (group II, 20 levels). With 1 individual feedback concentration, sampled 2 to 4 hours after the start of lignocaine infusion, serum concentrations at 12 and 24 hours could be accurately predicted. The prediction error (measured minus predicted concentration) ranged between -1.2 and +1.6 (mean -0.03) mg/L in group I, and from -0.7 to +1.5 mg/L (mean +0.13) mg/L in group II; the correlation coefficient of measured and predicted levels were 0.92 and 0.86, respectively. In contrast, a prediction of lignocaine concentrations in these patients using only population parameters without feedback was poor: range of the prediction error = -3.1 to +3.0 mg/L (mean = +0.001 mg/L, r = 0.63, groups I and II, n = 52). The results demonstrate that with the Bayesian forecasting technique, accurate assessment of individual dosage requirements can be obtained within a few hours after starting lignocaine therapy.

Adult↗

PubMedDate not supplied

Combined therapy of cerebral arteriovenous malformations: histological differences between a non-adhesive liquid embolic agent and n-butyl 2-cyanoacrylate (NBCA).

OBJECTIVE: Based on 2 casuistics, the intraoperative qualities of a new, non-adhesive liquid embolic agent (Onyx, Micro Therapeutics. Inc., Irvine, CA, USA) are to be compared to those of n-butyl 2-cyanoacrylate (NBCA) with regard to the histopathological results after preoperative embolization of a cerebral arteriovenous malformation (AVM). PATIENTS AND METHODS: In a case example, the intraoperative quality of the nidus after embolization of a parieto-occipital AVM with Onyx--a new, non-adhesive liquid embolic agent--consisting of ethylene-vinyl alcohol copolymer (EVOH), dimethyl sulfoxide (DMSO) and tantalum, is described. In the second patient, embolization of a frontal high-flow AVM was performed with NBCA. Both patients underwent surgery with complete resection ofthe AVM. RESULTS: From a neurosurgical point of view, Onyx is suitable for preoperative embolization of AVMs, because the nidus intraoperatively remains elastic and formable and can be dissected from the surrounding brain tissue quite well by microsurgical technique. Inflammatory reactions can be found mainly in the lumina of the vessels. CONCLUSIONS: Onyx promises to be an embolic agent well suitable for subsequent neurosurgical resection. Further studies considering various intervals of time between embolization and resection as well as histopathological and electron microscopical examinations are necessary for evaluation of our first experience with this new embolization agent.

Adult↗