Heparin Concentration Measurement
Protamine titration is at present the most widely used method
for determining heparin concentration in the perioperative setting. Protamine is
a strongly basic polycationic protein that directly inhibits heparin in a stoichiometric
manner. In other words, 1 mg of protamine will inhibit 1 mg (approximately 100 U)
of heparin. This reaction forms the basis for the protamine titration method of
measurement of heparin concentration. As increasing concentrations of protamine
are added to a sample of heparin-containing blood, the time to clot formation decreases
until the point at which the protamine concentration exceeds the heparin concentration
in the blood sample, and then the time to clot formation increases. If a series
of blood samples with incremental doses of protamine are analyzed, the sample in
which the protamine and heparin concentrations are most closely matched will be the
first one to generate a clot. Thus, by using protamine titration methodology, it
is possible to estimate the heparin concentration present in a given sample of blood.
[834]
Assuming that the heparin-protamine titration curve of an individual
patient remains constant throughout the
operative period, it is possible to use the protamine titration method to estimate
the dose of heparin required to achieve a desired plasma heparin concentration.
In a similar manner, determination of the heparin concentration at the conclusion
of surgery allows a more precise estimate of the amount of protamine required to
neutralize the circulating heparin. More precise administration of heparin and protamine
may have a number of clinical advantages. Some investigations have suggested that
thrombin activity during cardiopulmonary bypass may be reduced when stable heparin
concentrations are maintained.[835]
More effective
thrombin inhibition during bypass should benefit patients by decreasing consumption
of coagulation factors and thereby reducing postoperative bleeding.[836]
[837]
Although protamine titration was initially
performed manually, automated methods are currently available (Hepcon HMS; Medtronic
Blood Management, Parker, CO). These monitors use cassettes of four to six protamine-containing
vials to perform the titration assay. Different measurement cassettes are chosen,
depending on the expected range of heparin concentration.
Protamine titration methodology is relatively sensitive to low
concentrations of heparin and is limited only by the range of protamine concentrations
provided for testing. For this reason, protamine titration has proved particularly
useful for verifying heparin neutralization after protamine administration. This
method for determining heparin concentration is relatively resistant to the influences
of hypothermia and hemodilution. Furthermore, heparin concentration monitoring by
protamine titration is not altered during aprotinin therapy, in contrast to celite
ACT measurements. The major limitation of heparin concentration monitoring is failure
to assess functional coagulation or the intrinsic clotting potential of the blood.
To use an extreme example, consider a patient with a homozygous deficiency of antithrombin
III. Although functional measures of coagulation, such as the ACT, would clearly
identify these patients by failure to achieve the desired prolongation of the ACT
after heparin administration, measures of heparin concentration alone would demonstrate
the expected blood heparin level but would fail to identify the lack of anticoagulant
effect.
Preliminary reports have described another method for measuring
heparin concentration in whole blood with electrochemical sensors. These heparin
sensors use a polyvinyl chloride membrane impregnated with triiododecylmethylammonium
chloride (TDMAC) to produce an electric potential in response to heparin, which correlates
well with laboratory-based measures of heparin concentration.[838]
The heparin sensor offers several advantages over protamine titration methods for
determination of the heparin concentration. First, the heparin sensor is not dependent
on clot formation, which allows determination of heparin concentrations in patients
with underlying coagulopathies, as well as in blood samples containing anticoagulants
in addition to heparin, such as citrate or ethylenediaminetetraacetic acid (EDTA).
In addition, the heparin sensor produces a linear response over a greater heparin
concentration range than current protamine titration methods do. Rather than reporting
heparin concentration discontinuously, as necessitated by protamine titration cartridges,
heparin concentrations determined with ion-selective membranes are reported over
a continuous range. Despite the apparent advantages of these electrochemical sensors,
their accuracy may be altered by high plasma concentrations of salicylate, nitrate,
iodide, or bromide.[839]
The clinical usefulness
of these ion-selective electrodes awaits further development of these sensors into
commercially applicable forms.
 |
