Reference: Ann Surg. 2021 Jul 1;274(1):63-69
Surgical patients are at risk for postoperative deep vein thrombosis (DVT). Prophylaxis options include low-molecular-weight heparin (LMWH), graduated compression stockings (GCS), intermittent pneumatic compression (IPC), or various combinations of these methods. Prediction rules can assess the individual risk and guide prophylactic strategies. One example is the ACCP recommendation for patients at high-risk (Caprini score ≥ 5). It suggests either LMWH or low-dose unfractionated heparin for this group, with consideration of mechanical methods such as GCS or IPC. A recent randomized trial compared patients at very high risk (Caprini score ≥ 11) who had prophylaxis with LMWH, GCS, and IPC to those treated only with LWMH and GCS. The primary outcome assessed was venous thrombosis of the lower extremities, whether symptomatic or not.
The trial included 407 patients assigned to their respective groups based on the last digit of their medical record number, with even numbers having all 3 interventions and the odd numbers getting just the LMWH and GCS. Venous thrombosis was assessed by ultrasound every 3-5 days postoperatively until discharge. There was an attempt to blind the outcome assessors by not having them see the medical record number and, if possible, perform the US in a different room from the patient’s hospital room. Patients were also assessed at 30 days and 180 days for the identification of DVTs found post hospital discharge. Other outcomes that were noted included symptomatic DVT, pulmonary embolism, and skin injury (to assess for problems from the IPC). Of the 204 patients with all 3 prophylactic interventions, only one had an asymptomatic DVT (0.5%), and this was a calf muscle DVT. In the control group, 34 of 203 patients had asymptomatic DVT (16.7%). Of the DVTs observed in the hospital in the control group, 15 were calf DVTs and 5 were thigh DVTs. A total of 5 patients experienced pulmonary embolism in the control group, 3 of which were fatal (none in the intervention group), one of whom had no detectable DVT. Leg skin injury occurred in 12.3% in the group treated with IPC vs. 7.4% in the control group.
This study addresses the important question of how to reduce DVTs in high-risk patients undergoing surgery. Unfortunately, there are serious methodologic flaws. First, the use of the last digit of the medical record for assignment is considered quasi-randomization and thus inherently offers less certainty about the validity of the outcome because the groups were less likely to be prognostically equal at the outset. Secondly, this method of assignment makes allocation concealment virtually impossible. Finally, the primary outcome of asymptomatic lower leg DVT raises questions about the usefulness of the information. The real goal is to prevent pulmonary emboli and symptomatic DVTs, not asymptomatic calf DVTs. Asymptomatic calf DVTs often resolve on their own and the clinical benefit in preventing them is not clear. In this trial, asymptomatic DVTs represented a significant percentage of the DVTs found. The most important outcome may be the difference in pulmonary embolism rates, but its significance is limited by the trial design flaws. This trial is yet another example of the frustration that arises when important clinical questions are investigated with poorly designed trials.
For more information, see the topic Venous Thromboembolism (VTE) Prophylaxis for Nonorthopedic Surgical Patients in DynaMed.
DynaMed EBM Focus Editorial Team
This EBM Focus was written by Alan Ehrlich, MD, Executive Editor at DynaMed and Associate Professor in Family Medicine at the University of Massachusetts Medical School. Edited by, Dan Randall, MD, Deputy Editor at DynaMed; Carina Brown, MD, Assistant Professor at Cone Health Family Medicine Residency; Katharine DeGeorge, MD, MS, Associate Professor of Family Medicine at the University of Virginia and Clinical Editor at DynaMed; Nicole Jensen, MD, family physician at WholeHealth Medical; Tanya Tupper, RT(N), CNMT, PET, Senior Medical Writer at DynaMed, Vincent Lemaitre, PhD, Medical Writer at DynaMed, and Christine Fessenden, Editorial Operations Assistant at DynaMed.