2. THe answer to your second question first
TACY PHYLAXIS or ACUTE TOLERANCE is when the same dose of drug fails to produce the same effect seen earlier
or
to produce the same earlier effect, you need to give more drug
So if you want to produce the same response as first, you have to increase the dose of drug
Lets say you gave a spinal, and BP fell from 120 systolic to 90 and you gave ephedrine 6 mg and the BP went upto 118 mm
But again the bp fell to 90 systolic, and if you repeat 6 mg and tacypylaxis is on, the BP will go upto only say 100 mm. If you need to go upto 120 mm probably you will have to give 9-12 mg ephedrine.
Just need to remember that not all patients develop tachyphylaxis with the same severity and speed. So you will find a huge amount of variation in patient response.
IF you have a patient crashing BP after spinal and no good response to ephedrine bolus, you have to quickly think of tahyphylaxis and go with a higher dose, failing which patient may become significantly hypotensive for a long period.
This is more of significance, if you are worried about maintaining uteroplacental blood flow and also in patients who have autonomic neuropathy in whom fluid preloading and small dose vasopressors may not be enough to compensate the sympathetic block.
1. Half life got very complicated these days.
If I recall right we have 3 kinds of half life
t1/2 alpha - or the distribution half life
t1/2 beta - or the elimination half life
context sensitive half life - time taken to reach t1/2 beta after cessation of a prolonged infusion of a drug.
HALF LIFE is the time taken for the serum concentration of a drug to be reduced by 50% from the intial peak concentration. It does not define the process how this decrease in concentration occurs
You give 2 mg/kg (100 mg) of propofol for induction. It acheives a serum concentration of 2.2 microg/dl at its peak effect.
Half life is when the total body concentration of Propofol gets to 50 % ie 50 mg
t1/2 alpha
You give 2 mg/kg (100 mg) of propofol for induction. It acheives a serum concentration of 2.2 microg/dl at its peak effect. Patient will wake up say if the concentration goes down below 1 microg/dl. Patient usually wakes up in 5 minutes but Propofol is still in the body as kidney has not yet eliminated it. Then the concentration in serum decereased because the propofol was redistributed to other tissues in the body
t1/2 beta
the kidney has eliminated 50% of the propofol dose given so that the body propofol concentration is reduced to 50 mg
context sensitive half life
you give propofol 2mg/kg and follow it with an infusion of 8 mg/kg/hour for 4 hours.
In such a situation the redistribution sites get saturated with propofol.
Now on stopping infusion, the time taken for patient to wake up and the time taken for serum concentration of propofol to be reduced by 50% takes a much longer time. T
regs
TACY PHYLAXIS or ACUTE TOLERANCE is when the same dose of drug fails to produce the same effect seen earlier
or
to produce the same earlier effect, you need to give more drug
So if you want to produce the same response as first, you have to increase the dose of drug
Lets say you gave a spinal, and BP fell from 120 systolic to 90 and you gave ephedrine 6 mg and the BP went upto 118 mm
But again the bp fell to 90 systolic, and if you repeat 6 mg and tacypylaxis is on, the BP will go upto only say 100 mm. If you need to go upto 120 mm probably you will have to give 9-12 mg ephedrine.
Just need to remember that not all patients develop tachyphylaxis with the same severity and speed. So you will find a huge amount of variation in patient response.
IF you have a patient crashing BP after spinal and no good response to ephedrine bolus, you have to quickly think of tahyphylaxis and go with a higher dose, failing which patient may become significantly hypotensive for a long period.
This is more of significance, if you are worried about maintaining uteroplacental blood flow and also in patients who have autonomic neuropathy in whom fluid preloading and small dose vasopressors may not be enough to compensate the sympathetic block.
1. Half life got very complicated these days.
If I recall right we have 3 kinds of half life
t1/2 alpha - or the distribution half life
t1/2 beta - or the elimination half life
context sensitive half life - time taken to reach t1/2 beta after cessation of a prolonged infusion of a drug.
HALF LIFE is the time taken for the serum concentration of a drug to be reduced by 50% from the intial peak concentration. It does not define the process how this decrease in concentration occurs
You give 2 mg/kg (100 mg) of propofol for induction. It acheives a serum concentration of 2.2 microg/dl at its peak effect.
Half life is when the total body concentration of Propofol gets to 50 % ie 50 mg
t1/2 alpha
You give 2 mg/kg (100 mg) of propofol for induction. It acheives a serum concentration of 2.2 microg/dl at its peak effect. Patient will wake up say if the concentration goes down below 1 microg/dl. Patient usually wakes up in 5 minutes but Propofol is still in the body as kidney has not yet eliminated it. Then the concentration in serum decereased because the propofol was redistributed to other tissues in the body
t1/2 beta
the kidney has eliminated 50% of the propofol dose given so that the body propofol concentration is reduced to 50 mg
context sensitive half life
you give propofol 2mg/kg and follow it with an infusion of 8 mg/kg/hour for 4 hours.
In such a situation the redistribution sites get saturated with propofol.
Now on stopping infusion, the time taken for patient to wake up and the time taken for serum concentration of propofol to be reduced by 50% takes a much longer time. T
regs