Define volume of distribution (15% of marks). Outline the factors affecting volume of distribution (60% of marks) and explain how it may be measured (25% of marks).
The first two parts of the question were reasonably done. Most candidates had well-structured
answers which included drug factors and patient factors. In addition to listing the factors it was
expected candidates state how these factors affect volume of distribution. Explaining how
volume of distribution is determined was not so well done.
"(Vd) is defined as the apparent volume into which a drug disperses in order to produce the observed plasma concentration"
Measurement of the volume of distribution is performed by measuring the concentration of the drug at different points in time following the drugs' administration, and is then calculated using the following formula:
It can be expressed as litres, or indexed to body mass in L/kg.
Plasma concentration can be observed at different times, giving rise to several different possible strategies of calculating the volume of distribution.
Vinitial = Vd of the central compartment (from the rapid distribution phase)
Vextrap = Vd of the tissue compartment (from the elimination phase)
Varea = Vd extrapolated from the AUC of the concentration curve
Vss = Vd in a "steady state" model, the most useful in calculating the loading dose
Factors which influence VD include the following:
|Measurement and pharmacokinetic modelling of Vd|
|Timing of measurements||Depending on when the measurements are taken, the Vd will be different (i.e. it will correspond to Vinitial if the measurements are taken too early, and Vextrap if they are taken during the elimination phase).|
|Pharmacokinetic model||Vinitial, Vextrap, Varea and Vss are various ways to estimate the Vd of a drug from empirical measurements. All of these methods will yield slightly different results - or, occasionally completely different results.|
|Free vs. total drug levels||In highly protein bound drugs, the calculated volume of distribution for the "total" drug levels will be totally different to the Vd calculated for the free drug. Total Vd will correspond to the Vd of the binding protein rather than the drug itself.|
|Properties of the drug|
|Molecule size||The larger the molecule, the harder it will be for it to passively diffuse out of the central compartment, and therefore the smaller the Vd.|
|Molecule charge||Highly ionised charged molecules will have higher water solubility, and may even be trapped in the central compartment by electrostatic factors which keep them bound to proteins with corresponding charge.|
|pKa||pKa determines the degree of ionisation and therefore influences lipid solubility|
|Lipid solubility||Lipid solubility is one of the major determinants of Vd; highly lipid-soluble drugs will have the highest Vd values because of the low fat content of the bloodstream.|
|Water solubility||Highly water-soluble drugs will have difficulty penetrating lipid bilayer membranes and generally tent do have smaller volumes of distribution, essentially being limited to extracellular water.|
|Properties of the patient's body fluids|
|pH||pH interactes with the drug's pKa to influence the degree of lipid solubility. pH also influences the degree of protein binding (a good exmaple of this is ionised calcium)|
|Body water volume||Dehydrated patients will have drug levels concentrated in the plasma just as all dissolved substances are concentrated by loss of water.|
|Protein levels||For highly protein-bound drugs, lower serum protein levels will result in a higher free (unbound) drug fraction. This may have little effect on the Vd as calculated from total drug concentration, but if you are measuring free drug levels it will make the Vd appear smaller.|
|Displacement||Drugs may be displaced from their protein and tissue binding sites by the effects of pH or by competition from other drugs/substances (eg. urea). Displaced drugs mayl redistribute into plasma, decreasing the calculated Vd.|
|Effects of physiology and pathological states|
|Age||As an old professor of mine had put it, babies are grapes and the elderly are raisins. As you age, body water content decreases, shrinking the Vd of water-soluble drugs. Muscle mass also decreases, and so tissue binding diminishes.|
|Gender||Female Vds tend to be lower than male Vds due to the generally lower body water content (Soldin & Mattison, 2009)|
|Pregnancy||Both the body water and the body fat content increases, and therefore the Vd increases for most drugs. Not to speak of the possible distribution into amniotic fluid and foetus.|
|Oedema||Oedema represents increased body water and this influences water-soluble substances; Vd for these will increase|
|Ascites / effusions||Just as in oedema, large fluid collections may sequester water soluble drugs and act as reservoirs.|
|Effects of apparatus|
|Adsorption on to apparatus||Dialysis filters and ECMO circuits tend to adsorb drugs in an unpredictable fashion, resulting in an apparent increase in the volume of distribution.|
|Volume expansion||In the context of bypass circuits and other large extracorporeal machinery, there may be 2000-2500ml of additional extracorporeal fluid, which will change the volume of distribution (particularly for drugs which are largely confined to the central compartment)|
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