Monitoring of heparin in vascular surgery

Abstract

In a recent report in the Journal, Quigley et al.1Quigley FG Jamieson GG Lloyd JV Faris IB. Monitoring of heparin in vascular surgery.J Vasc Surg. 1988; 8: 125-127PubMed Scopus (16) Google Scholar described the degree of anticoagulation by heparin in patients undergoing surgery of the infrarenal aorta. We would like to make some comments on this study. First, the degree of heparinization was measured only by the activated partial thromboplastin time (aPTT). The aPTT, however, measures the heparin activity indirectly and recent evidence indicates that this type of assay correlates poorly with heparin levels during cardiovascular surgery.2Culliford AT Sanford NG Starr N et al.Lack of correlation between activated clotting time and plasma heparin during cardiopulmonary bypass.Ann Surg. 1981; 193: 105-111Crossref PubMed Scopus (168) Google Scholar We recently reported a study on the monitoring of heparin during major vascular surgery in which the aPTT, thrombin time, and an antifactor Xa assay were compared.3Teien AN Lie M Abildgaard U. Assay of heparin in plasma using chromogenic substrate for activated factor Xa.Thromb Res. 1976; 8: 413-420Abstract Full Text PDF PubMed Scopus (262) Google Scholar In the latter assay heparin concentration is determined by measuring antifactor Xa activity, with the chromogenic substrate S-2222, bovine factor Xa, and human antithrombin-III concentrate.4Porte RJ de Jong E Knot EAR et al.Monitoring of heparin and haemostasis during reconstruction of the abdominal aorta.Eur J Vasc Surg. 1987; 1: 397-402Abstract Full Text PDF PubMed Scopus (14) Google Scholar Our results demonstrated that the aPTT and the thrombin time are not suitable for an exact determination of heparin concentration during vascular surgery, because changes in heparin concentration, as detected by the anti-Xa assay, were not always reflected by changes in the aPTT and thrombin time. Quigley et al.1Quigley FG Jamieson GG Lloyd JV Faris IB. Monitoring of heparin in vascular surgery.J Vasc Surg. 1988; 8: 125-127PubMed Scopus (16) Google Scholar found an unexplained difference in aPTT levels above and below the aortic clamp. It would have been interesting to know whether measurement of anti-Xa activity might have given more insight into the magnitude of this difference. Second, the authors found no association between decreased aPTT levels in the distal circulation and the occurrence of thrombotic complications. In our study fresh thrombi in both iliac arteries were observed after removal of the clamps in one of the 20 consecutive patients; this patient had low levels of heparin, as measured by the anti-Xa activity. Although the use of heparin in aortoiliac surgery is controversial and some authors do not even use heparin in all their patients, we agree with Quigley et al.1Quigley FG Jamieson GG Lloyd JV Faris IB. Monitoring of heparin in vascular surgery.J Vasc Surg. 1988; 8: 125-127PubMed Scopus (16) Google Scholar that the current methods for administration and monitoring of heparin may not provide an adequate degree of anticoagulation during vascular surgery. A prospective, controlled study, therefore, seems warranted. We think that in such a study the degree of heparinization not only should be measured by “overall” clotting assays such as the aPTT but also include more direct assays for the monitoring of heparin concentration, such as the antifactor Xa assay.