Monthly Archives: January 2017

Reflection Coefficient

Movement of solutes across the blood brain barrier depends on the reflection coefficient of the solute.  Reflection coefficient is defined as the selectivity of the blood brain barrier to a particular substance.  Molecules with a reflection coefficient of 1 are excluded by the blood brain barrier.  As shown in the table below, sodium chloride is effectively “reflected back” or excluded by the blood brain barrier compared to the other osmotic compounds listed.

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Reference:

Qureshi, Adnan I. and Jose I. Suarez. “Use Of Hypertonic Saline Solutions In Treatment Of Cerebral Edema And Intracranial Hypertension”. Critical Care Medicine 28.9 (2000): 3301-3313.

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Empiric Antibiotics for Brain Abscess

Base empiric therapy on presumed source of abscess and Gram stain results

Source: mouth, ear, sinus

  • [Penicillin (mouth) OR cefotaxime (ear/sinus)] + [metronidazole]

Source:  neurosurgery / post-op

  • [ceftazidime OR cefepime OR meropenem OR imipenem] + [vancomycin]

Source: penetrating trauma

  • [ceftriaxone or cefotaxime] + [vancomycin] +/- [metronidazole  (sinus involvement)]

Source:  hematogenous spread (IE, multiple abscess)

  • [ceftriaxone or cefotaxime] + [vancomycin] + [metronidazole]

Source: unknown

  • [ceftriaxone or cefotaxime] + [vancomycin] + [metronidazole]

 

Dosages:

  • Cefepime 2g IV q8h
  • Cefotaxime 2g IV q4-6h
  • Ceftriaxone 2g IV q12h
  • Ceftazidime 2g IV q8h
  • Imipenem 500-1000mg q6h
  • Meropenem 2g IV q8h
  • Metronidazole 15mg/kg IV load then 7.5mg/kg IV q8h;  usually 1G load & 500mg q8h
  • Nafcililn 2g IV q4h
  • Oxacillin 2g IV q4h
  • Penicillin G 20-24 M units / day IV in 6 divided doses
  • Vancomycin 15-20mg/Kg IV q8-12h

 

Reference:

Uptodate: Treatment and prognosis of bacterial brain abscess, accessed 01/10/2017.

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Definitions for ICU Infections

Bloodstream infections – (+) blood cultures without evidence of a separate primary source

UTI

  • ≥10,000 CFUs, or
  • (+) urinalysis (i.e. (+) leukocyte esterase or nitrate or >5 WBC/mL) and ≥1000 CFU.

Pneumonia

  • new infiltrate on CXR and (+) sputum culture or change in respiratory status (increased ventilator support or oxygen requirements)

Sinusitis – purulent nasal discharge with corresponding CT imaging

Ventriculitis – (+) CSF culture

CDAD – diarrhea with (+) C. difficile toxin PCR

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Reference:

Halvorson, Karin et al. “Procalcitonin Is A Poor Predictor Of Non-Infectious Fever In The Neurocritical Care Unit”. Neurocritical Care (2017).  Epublished.

Hemodynamic Parameters

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Reference:

Parrillo, Joseph E and R. Phillip Dellinger. Critical Care Medicine. 4th ed. 3, 31-46.e2

Hemodynamic Monitoring Devices

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*CCO, continuous cardiac output; CVP, central venous pressure; EVLW, extravascular lung water index; GEDV, global end-diastolic volume index; PPV, pulse pressure variation; PVI, plethysmographic variability index; SVV, stroke volume variation.

 

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Reference:

Parrillo, Joseph E and R. Phillip Dellinger. Critical Care Medicine. 4th ed. 3, 31-46.e2

Pulsion Medical Systems reference card.

Steroid Comparison

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<click here for pdf version>

(Marino, 2014)

Classification of Hydrocephalus

 

The first level of classification of hydrocephalus should be based on the point where flow of CSF is restricted.  These potential sites of restriction would include:

  • the foramen of Monro
  • the aqueduct of Sylvius
  • the basal cisterns
  • the arachnoid granulations
  • theoutflow of venous blood from dural venous sinuses

Hydrocephalus without a point of obstruction or increased resistance to flow would be communicating hydrocephalus.

After the point of obstruction has been determined, the classification should then include the etiology of the condition, chronicity and age of the person.

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[ABOVE] Illustration derived from applying engineering principles to the study of ventricular volume regulation.  The CSF system is illustrated using a compartmental model, with each compartment having its own pressure and volume related to CSF flow.  Circuit Diagram of the CSF pathway as a hydraulic analog of an electrical circuit.

[BELOW] Six compartment model of the CSF pathways.

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[BELOW] Artist rendering of the circuit diagram.

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Based on their studies, there were no pressure differential anywhere within the system.  Pulse wave was transmitted undiminished and instantaneously to all transducers intracranialy.  This means that the brain is a viscoelastic substance, acting as a fluid chamber where changes in pressure are transmitted instantaneously and fully to all areas.

Exception to the rule:  If one of the lateral ventricles was drained to subatmospheric pressure, a pressure differential of 12mm Hg can be measured.  This is seen in “post-shunt ventricular asymmetry” – in children who are shunted, septum pellucidum is drawn toward shunted ventricle and rests on head of caudate nucleus leading to a functional and reversible obstruction of flow.

NPH – likely obstruction between spinal and cortical subarachnoid spaces, dense arachnoidal thickening around brainstem in posterior fossa, blockage results from either SAH or infection, frequently involves area around brainstem selectively, amenable to endoscopic III ventriculostomy

Increased pressure in dural venous sinus results in pseudotumor cerebri and not HCP.  Drainage of CSF into venous sinuses requires a gradient between ICP and sagital sinus pressure of 5-7mm Hg.  If pressure in sagittal sinus is elevated, ICP must elevated in order for CSF to be absorbed.  ICP slowly goes up until CSF can be absorbed (if skull volume is fixed).  In cases of large craniectomies, ICP is in communication with atmospheric pressure.  ICP cannot go above atmospheric pressure and patient develops hydrocephalus.

Study done in rabbits where SSS was occluded.  Rabbits whose skull was intact developed intracranial hypertension without ventriculomegaly.  Craniectomized rabbits developed hydrocephalus.

Table below shows the treatment options for each point of obstruction in the CSF pathway.

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Reference

Rekate, Harold L. “A Consensus On The Classification Of Hydrocephalus: Its Utility In The Assessment Of Abnormalities Of Cerebrospinal Fluid Dynamics”. Child’s Nervous System 27.10 (2011): 1535-1541.