Elisabeth Döderlein
Pharmacodynamic of unfractionated heparin, heparin monitoring using thromboelas-tometry and reference intervals for the ROTEM-analyser in cats
In the first part of this study the precision of thrombelastometry using the ROTEM® delta analyser for measurements of feline citrated blood was assessed. In addition, reference intervals were established for cats with this device and a possible influence of age was investigated. This was also performed with regard to its potential use for monitoring of heparin treatment in the second part of this dissertation.
Analyses with the ROTEM® delta analyser were performed with blood samples of clinically healthy, fasting adult cats after activation with thromboplastin (Ex-tem®), with a contact activator (In-tem®) and with kaolin as well as without activation. Intra-assay-variability was evaluated based on four fold-measurements of blood samples of two cats. Reference intervals were established based on blood samples of 55 animals of different sexes which were assigned to four different age groups (1 [6–12 months]: n=8; 2 [>12–48 months]: n=18; 3 [>48–96 months]: n=14;
4 [>96 months]: n=15), whereby heterogeneous distribution of gender was observed between individual groups.
Overall the ROTEM® delta analyser showed good precision in the present study with coefficients of variation of most of the parameters being less than 10 %. An exception was the maximum lysis (ML) after activation with Ex-tem® (28 %) and without activa-tion (23 %). The reference values (calculated based on the 2.5 and 97.5 %-quantiles) of all parameters of the ROTEM® delta revealed an obvious inter-individual variation (e. g., when using Ex-tem®: coagulation time [CT] 44.0–98.7 s, clot for-mation time [CFT] 34.0–92.7 s, maximum clot firmness [MCF] 53.1–77.7 mm). Ex-tem® produced the shortest CTs and the highest ML. Statistical comparison between different age groups showed only sporadic differences (among others, In-tem®: ML;
Kaolin and native: CT; without activation: CFT). These results reflected that cats of age group 1 showed shorter CTs when compared with older animals and that ani-mals of age group 2 showed a shorter ML in comparison with other age groups. Het-erogeneous distribution of gender in the different age groups in this study may have influenced the statistical analysis of the influence of age, but graphical illustration did not reveal a relevant systematic influence of gender. In addition, the heterogeneous composition of animals may have contributed to the partially wide physiological varia-tion.
The aim of the second part of the present study was to examine the pharmacody-namics of various dosage regimes of unfractionated heparin (UFH) in healthy cats to provide a reliable basis for dosage recommendations for the feline patient. A previ-ous study which developed pharmacokinetic data after a single subcutaneprevi-ous (s.c.) injection of different doses of UFH in cats provided the basis for the calculated dose regimes. The present study examined, amongst other things, the influence on chro-mogenic anti-FXa activity, various coagulation tests, and thrombelastometry. To
ex-amine the dosing regimens of UFH, three different groups of clinically healthy fasting adult cats were treated four times daily at intervals of 6 hours on four consecutive days.
One group received a dosage of 250 international units (I.U.)/kg body weight (group A, n = 2), a second group received a dosage of 200 I.U./kg body weight (group B, n = 4), and a third group a dose of 150 I.U./kg of body weight (group C, n = 6). Blood samples for the subsequent laboratory tests were taken on days 1, 2 and 4 in each case immediately before and two hours after the first and second hep-arin administration.
Groups A and B were not completed and cancelled after two and four animals, be-cause in each of these two groups one of the cats developed major bleeding compli-cations leading to a shock in the cat of group A and unfortunately to death in the cat of group B (necropsy revealed that death was caused by acute cardiovascular failure in the context of hypovolaemic shock as a consequence of acute blood loss from the liver). During the experimental method the affected cat of group A reached a maxi-mum anti-Xa activity of 1.0 I.U./ml with a peak aPTT ratio of 11.1 and the affected animal of group B a maximum anti-Xa activity of 1.2 I.U./ml with a maximal aPTT ra-tio of 6.39.
Overall the results of the second part of this study showed a remarkable interindivid-ual variation of the anti-Xa-activity. For example, two hours after the second heparin injection anti-Xa-activity of group B varied between 0.44 and 0.90 I.U./ml. Median values of group B varied between peak levels of 0.50 and 0.68 I.U./ml and the peak levels of group C between 0.16 and 0.33 I.U./ml.
Median aPTT-ratio when measured with C.K. Prest varied at peak times between 2.06 and 3.16 in group B and between 1.32 and 1.81 in group C. Slightly higher val-ues were calculated for measurements using the second aPTT reagent SynthaFax.
CT ratio values showed that ROTEM analyses were more sensitive than aPTT but less sensitive than the TT. Correlation analysis showed a high correlation between both anti-Xa activity and aPTT ratio (Spearman rank correlation coefficient, rs = 0.851, as measured by CK Prest ®) as well as between anti-Xa activity and CT ratio (rs = 0.863, after activation with kaolin).
It can be concluded that the dosage regimen of group B (200 I.U./kg qid) may be used as a guideline for thrombosis treatment in cats, but inter-individual variation and also the lethal bleeding in one of the cats requires a careful monitoring and dosage adjustment. Apard from anti-Xa-activity aPTT and CT of the koaolin-activated ROTEM analyses seem to be suitable methods for monitoring of heparin treatment in cats. Based on the results of the present study, the dosage regimen of group C (150 I.U./kg qid) is rather suited for a low dose prophylaxis than for treatment of an existing thrombosis, because the majority of the measurement results were in the human target range of low dose prophylaxis.
9. LITERATURVERZEICHNIS