Example calculation of parameters for phenytoin

Combining common factors gives the expression for time to go from an initial concentration of zero to the concentration achieved at a particular fraction of steady state  

Let us assume that we are interested in determining the time required to attain 90% of the true steady-state plasma concentration for phenytoin, which is administered at different rates, where 1 =50L, Vmax=500mgday and FCm = 4pgmL f R 1 = tf (=4mgL ). Using Eq. 15.23, the time required to attain 90% of the steady-state concentration 

What plasma phenytoin concentrations would be achieved at the times and doses above The answer is that slight modifications of the steady-state phenytoin concentration can be made as follows (using Eq. 15.12)  

What if Km was equal to 5.7 rngL instead of 4mgL-i The answer is that, since Km appears only in the numerator of Eq. 15.23, there is a direct proportion between Km and the time to reach 90% of the steady-state phenytoin concentration. Similarly, the value 0.9(Cp)ss is also directly proportional to Km- Therefore, each of the above values can be multiplied by the factor (5.7/4.0), resulting in the figures given in Table 15.2. 

Alternative equation to calculate the time to reach a given fraction of steady state 

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