From the Guidelines

A mildly decreased INR level implies a higher risk of thromboembolism in patients with mechanical heart valves.

The risk of thromboembolism increases when the INR falls below the therapeutic range, which can lead to serious complications such as stroke or valve thrombosis 1.
For patients with mechanical aortic valve replacements, an INR target of 2.5 (range, 2.0–3.0) provides a reasonable balance between the risks of thromboembolism and bleeding 1.
In patients with mechanical mitral valve replacements, anticoagulation with warfarin is recommended with an INR target of 3.0 (range, 2.5–3.5) to minimize the risk of thromboembolism 1.
The incidence of thromboembolism is higher in patients with mechanical prostheses in the mitral compared with the aortic position, and the rate of thromboembolism is lower in patients with a higher INR goal 1.

If the INR is mildly decreased, the patient's warfarin dose may need to be adjusted to achieve the target INR range 1.
In patients at higher risk of thromboembolism, bridging anticoagulation with either intravenous UFH or subcutaneous LMWH may be recommended during the time interval when the INR is subtherapeutic preoperatively 1.
Excessive anticoagulation (INR ≥5) greatly increases the risk of hemorrhage, while a rapid decrease in the INR that leads to INR falling below the therapeutic level increases the risk of thromboembolism 1. Some key points to consider when managing a mildly decreased INR include:
The patient's individual risk factors for thromboembolism and bleeding
The type and position of the mechanical heart valve
The patient's compliance with warfarin therapy and INR monitoring
The need for bridging anticoagulation during periods of subtherapeutic INR.

From the Research

A mildly decreased INR level may not necessarily lead to an increased risk of thromboembolic events, as suggested by a study that found a target INR range of 2.0-2.5 to be acceptable for preventing thromboembolic events in patients with mechanical mitral valve replacement 2.
However, a lower INR range may be associated with a higher rate of thromboembolism, as found in a systematic review and meta-analysis of randomized controlled trials in atrial fibrillation patients 3.
On the other hand, a lower INR range may also be associated with a lower rate of major bleeding, as found in the same systematic review and meta-analysis 3.
The relationship between INR levels and bleeding risk is complex, and factors such as age, renal failure, and alcohol use may also play a role in determining the risk of bleeding 4.
In patients with a mildly decreased INR level, the risk of bleeding may be lower, but the risk of thromboembolic events may be higher, as suggested by a study that found a negative correlation between the log mean INR and all thromboembolic events 2.

The management of mildly decreased INR levels depends on various factors, including the patient's underlying condition, the presence of bleeding or thromboembolic events, and the patient's overall health status.
In asymptomatic patients with an INR of ≤ 3.3, maintaining the same warfarin dose may be a reasonable approach, as suggested by a study that found no significant difference in outcomes between patients who maintained their warfarin dose and those who reduced their dose 5.
However, in patients with a higher INR or an increased risk of hemorrhage, reducing the warfarin dose may be necessary to minimize the risk of bleeding 5.
The decision to reduce the warfarin dose should be made on a case-by-case basis, taking into account the patient's individual risk factors and health status.