Chemical Description of Hydroxyethyl Starch (HES) Solutions

 

Source of HES: amylopectin

 

Descriptors:

(1) monodispersed vs polydispersed

(2) molecular weight

(3) degree of hydroxyethyl substitution (fraction of glucose units that are hydroxyethylated)

(4) C2/C6 ratio of hydroxyethylation

(5) concentration of solution in grams per 100 mL (percent)

(6) duration of action

 

Most forms of HES are polydispersed, indicating that it is composed of molecules with a range of sizes and different molecular weights. A monodispersed solution contains molecules all of the same molecular weight.

 

The molecular weight for the individual molecules in a polydispersed solution show a wide range, which has resulted in a variety of terms:

(1) weight averaged molecular weight

(2) number average molecular weight

(3) average molecular weights

 

weight averaged molecular weight =

= SUM(((weight of molecule type in mixture) / (total weight of mixture in grams)) * (molecular weight of molecule type))

 

number average molecular weight -

= (total dry weight in grams) / (number of molecules in solution)

 

average molecular weight =

= (SUM(all molecular weights)) / (number of different molecular weights present)

 

Parameter	Finding	Term
molecular weight, weight averaged	40,000 to 70,000 Dalton	low
	130,000 to 200,000	medium
	450,000 to 480,000	high
degree of hydroxyethyl substitution	0.4 to 0.5	low
	0.6 to 0.7	high
C2/C6 ratio	< 8	low
	> 8	high
concentration	6%	low
	10%	high
duration of action	less than 12 hours	short acting
	up to 36 hours	long acting

 

The rate of metabolism is affected by the degree of hydroxyethyl substitution and the C2/C6 ratio of hydroxyethylation. A high degree of substitution and a high C2/C6 ratio is associated with a high in vivo molecular weight and slow metabolism.

 

A HES with a substitution of 0.45 to 0.5 is termed a pentastarch.