Vitamin K antagonists (VKAs)
VKAs such as acenocoumarol, fenprocoumon, and warfarin, inhibit the
carboxylation of the vitamin K-dependent coagulation factors II, VII,
IX, and X in the liver necessary for coagulation and thus indirectly
inhibit the coagulation process. VKAs are used for the prophylaxis and
treatment of VTE and stroke prevention in atrial fibrillation. As VKAs
are absorbed in the proximal intestine, bariatric surgery could have an
impact on the absorption of VKAs.
Several studies investigated the effect of bariatric surgery on the
daily dose of warfarin
121–124. All studies demonstrated that the warfarin
dose was significantly reduced after RYGB surgery, especially in the six
month postoperatively. After the direct postoperative phase, the
required dose tended to gradually go back up to pre-surgical levels at
six months to one year after surgery 121–124. It is
not yet entirely clear why a lower dose is needed shortly after the
operation. While it can be hypothesized that absorption may be increased
because of crushing the tablets by patients directly after surgery,
another explanation may lie in the changes that are associated with
bariatric surgery that are to date not fully understood, such as
metabolic changes 4. Also changes in diet during the
first months after surgery may play a role. Similar to the results of
platelet aggregation inhibitors, it seems that bariatric surgery may
improve the response on VKAs compared to obese subjects, especially in
the first months after surgery 121,122. It is known
that compared to normal weight patients, obese patients require a higher
average daily dose and also require more time to achieve therapeutic
international normalized ratio (INR) 125. While no
studies have been conducted on acenocoumarol and fenprocoumon after
bariatric surgery, it may seem that these results also apply to these
drugs. Overall, more frequent monitoring of the INR seems appropriate in
the first year after bariatric surgery.