IV ANESTHETICS- barbs Flashcards
What is the MOA of barbiturates?
Binds the GABA receptor and potentials the inhibitory effect through increasing the duration of opening the chloride channel
Depresses the RAS
What determines the potency of a barb?
The length of the chain at C5
Longer = more potent = more lipid soluble
What is the phenyl group for on phenobarbital?
Anticonvulsant
Why are thiopental and thiamyal more potent?
Because their sulfur makes them more lipid soluble: more rapid onset and shorter duration of action
Why do induction doses need to be lower in the elderly?
Slower redistribution
How are barbiturates principally biotransformed?
By phase I metabolism in the liver
What mechanism is responsible for the awakening from a single sleep doses of barbs?
Redistribution
What decreases the GFR of barbs?
Increased protein binding
What increases renal absorption about the barb?
Increased lipid solubility
What are the properties of methohexital?
It is highly lipid soluble
Cleared by the liver more rapidly
Excreted in the feces
Why are thiopental, thiamyal and methohexital fast onset?
They are highly lipid soluble
Thiopental is also highly protein bound
What are the CV effects of barbiturates?
Decrease in BP due to vasodilation of the capacitance vessels (depresses the medullary vasomotor center)
Tachycardia
Maintenance of cardiac output
Increased myocardial contractility (compensatory responses)
In what situations will the cardiac output and bp fall using barbs?
When someone has blunted barorerceptor response: CHF! Beta blockade, hypovolemia, HTN
How do you avoid a drop in BP and HR when using barbs?
Adequate prep hydration
Slow induction
What are the respiratory effects of barbs?
Depress the medullary vent center
Cause airway obstruction
Depress the airway reflexes –> laryngospasm (light anesthesia) or bronchospasm
What are the cerebral effects of barbiturates?
Constrict the cerebral vessels:
Decrease CBF and ICP
INCREASE CPP (because arterial blood pressure does not decrease as much as ICP)
What does CPP equal?
Cerebral artery pressure - greater of jugular venous pressure or ICP
Map - ICP
It represents the pressure gradient driving cerebral blood flow
How much decline do barbs cause in cerebral oxygen consumption?
Up to 50% of normal
What can you use to see the effects of barb-induced oxygen consumption pattern?
It is reflected in the EEG
Low voltage fast activity –> high voltage slow activity
What might the patient taste when using thiopental?
Garlic, onions or pizza
Are barbs analgesic?
No
Can actually decrease the pain threshold
Do barbs cause muscle relaxation?
No
Some cause involuntary muscle contraction (methohexital)
What are the renal effects of barbs?
Reduce RBF and GFR in proportion to the fall in blood pressure
What are the hepatic effects of barbs?
Decreases hepatic blood flow
Reduces metabolism of TCAs and other drugs
Induces the production of aminolevulinic acid synthetase which stimulates the formation of porphyria
