Two Compartment Model Flashcards
How will the data of a drug that follows the 2CM appear on a graph of log Cp vs. time?
Curved, so can’t do 1CM analysis on this data
Which organs are considered highly perfused?
- Adrenal
- Kidneys
- Thyroid
- Liver
- Heart
- Brain
Which are organs are considered poorly perfused?
- Skin
- Muscle
- Connective tissue
- Fat
What does K12 depict?
First-order constant going from central compartment to tissue compartment
What does K21 depict?
First-order constant going from tissue compartment to central compartment
What does K10 depict?
First-order constant leaving central comparmtent
What is the difference between K0 and Ka?
- K0 depicts the first-order constant entering the central compartment for constant IV infusions
- Ka depicts the first-order constant entering the central compartment for oral, IM, and SC drugs
Which rate constants are involved in the 2CM of an IV bolus?
K12, K21, and K10
Which rate constants are involved in the 2CM of a constant IV infusion?
K12, K21, K10, and K0
Which rate constants are involved in the 2CM of an oral, IM, or SC drug?
K12, K21, K10, and Ka
What does K20 depict?
First-order constant for drug being eliminated from tissue compartment
What are the 3 models of the 2 compartment open model for an IV bolus? Which rate constants are involved in each? Which model do we work w/?
1) Model A - K12, K21, and K10 (model we work w/)
2) Model B - K12, K21, and K20
3) Model C - K12, K21, K10, and K20
What occurs during each phase of a log concentration vs. time graph for a 2CM drug?
- Phase 1 = drug enters central compartment and is filling it up
- Phase 2 = concentration rapidly dropping b/c central compartment is full and filling up peripheral compartment
- Phase 3 = eq’m between central and peripheral compartments
- Phase 4 = elimination has taken over and drug is being eliminated; elimination technically occurs in all phases but only significant during phase 4
What is the relationship between tissue and plasma drug concentration for a 2CM?
- Plasma represents central compartment, so will be highest at Cp0 and will decrease as time increases
- Tissue will be 0 at Cp0 and will peak during phase 2 and decrease at same rate as plasma concentration as time increases
Which equation explain distribution in the central compartment of a 2CM?
Cp = Dp / Vp
Which equation explain distribution in the tissue compartment of a 2CM?
Ct = Dt / Vt
What is b?
Depicts the rate of elimination (similar to k), so slope = -b/2.303 and t1/2 = 0.693/b
What is the first step when doing calculation for 2CM?
Plot the points and use a ruler to see which points fall on a straight line
How are b and B calculated?
- b is similar to k, so can use slope = -b/2.303 or t/12 = 0.693/b
- B similar to Cp0, so use y-int
How are a and A calculated?
- a is the same as b, just for residual data
- A is the same as B, just for residual data instead of observed data
Which formula would you use if you have Cp and B and you want to find b?
log Cp = -bt/2.303 + log B
How do you calculate Cp’ extrapolated?
Using formula from the points you plotted to find b and B, plot in the time points to get Cp’ points
How do you calculate residual points?
Cp minus Cp’
How do you find Cp0 for a drug displaying 2CM?
A + B (must be larger than the largest data point)
How can you calculate Vp (volume of distribution for central compartment)?
Vp = dose/Cp0, so Vp = dose/A+B or dose/K[AUC] infinity
How can you calculate Vt (volume of distribution for tissue compartment)?
Vt = (Vp * k12) / k21
What are the compartments in the 3 compartment open model?
Tissue compartment, central compartment, and deep tissue compartment (central goes to tissue and deep tissue)
What determines the number of compartments or exponentials?
- Route of administration
- Rate of absorption
- Total time for sampling
- Number of samples during collection period
- Assay
How can you tell how many compartments a log concentration vs. time curve represents?
- Straight line = 1 compartment
- Curved line w/ residual points forming a straight line = 2 compartments
- Curved line w/ residual points forming a curved line = over 2 compartments
Which phase does the original data constitute and which data does the extrapolated data constitute?
- Original data = distribution phase
- Extrapolated data = elimination phase
Why does total time of sampling affect the number of compartments?
Stopping too early or starting too late may cause you too miss the other compartments, so make sure to spread samples apart
