A new scientific statement from the American Heart Association (AHA) provides clinicians with an updated overview of perfusion assessment and imaging tools used in critical limb ischemia (CLI), including the ankle-brachial index (ABI), toe-brachial index (TBI), laser Doppler devices, and implantable devices. The statement, published in Circulation, also covers unmet needs in perfusion assessment and imaging in clinical practice, and discusses the effect that perfusion assessment has on public health and disparities at large.
ABI and TBI for Detecting CLI
The AHA writing committee opened their statement by discussing the ABI value, which is calculated after dividing a limb’s highest posterior tibial and dorsalis pedis artery systolic blood pressure by the other arm’s highest brachial artery systolic blood pressure. Peripheral arterial disease (PAD) is defined by an ABI of ≤0.90, and the value obtained from the index offers prognostic data on risk of death, myocardial infarction, and stroke.
Challenges exist with the use of ABI, especially in patients with diabetes mellitus and chronic kidney disease. Likewise, the ABI is not a highly effective for measuring oxygenation. Despite the predictive value of ABI, the s authors suggest that TBI may offer better prediction for major adverse limb events and tibial disease in patients with CLI.
Although TBI is commonly associated with improved accuracy in detecting limb ischemia, particularly if heavily calcified and poorly compressible vessels are present, no “universally accepted threshold for TBI or toe pressure to confirm the diagnosis” currently exists.
Unmet Needs in Perfusion Assessment
Currently, there is no single vascular test that is important for predicting either wound healing or major amputation for a threatened limb. Angiographic end points can subjectively assess the completeness of procedural perfusion; however, the use of noninvasive diagnostic vascular testing to manage acutely threatened limbs requires additional sophistication.
According to the AHA, a transcutaneous oximetry test (TcPo2) and the quantification of oxygenated vs deoxygenated hemoglobin using standard near-infrared spectroscopy or hyperspectral imaging can evaluate the effects of PAD with diabetic microvascular dysfunction or anemia. These tools, however, are only best helpful for evaluating dermal and immediate subdermal status. Tissue blood flow measurement, compared with oxygenation, is another viable technique that is limited by its lack of ability to measure oxygen or nutrient delivery directly.
Laser Applications for Tissue Perfusion
In the statement, the authors also discuss laser Doppler flowmetry (LDF) for real-time assessment of tissue perfusion. While LDF offers utility during revascularization interventions, it tends to be limited by the small area as well as shallow depth of measurement. Heterogeneity of skin perfusion can also reduce measurement reproducibility. Similar to LDF, laser speckle imaging is also used for real-time perfusion assessment. Motion artifact, device size, and complicated data acquisition analysis can limit the applicability of speckle imaging. The authors note that while laser technologies can be effective for evaluating limb perfusion, studies demonstrating efficacy often include small sample sizes and are limited in their generalizability. Current practice will often rely on ABI, TBI, and TcPo2 before utilizing laser devices.
Perfusion Assessment on Public Health
To prevent major adverse cardiovascular outcomes, including death and amputation, the authors comment that proper diagnosis of CLI is key. Correct diagnoses may rely on perfusion assessment with high sensitivity for CLI vs various hemodynamic measures, such as ABI. Ultimately, correctly identifying CLI may lead to improved medical care and timely revascularization in these patients.
Limb perfusion technologies can vary among patients in terms of hemodynamic cutoffs, suggesting that the combination of hemodynamic variables with other wound healing and limb salvage predictors may offer improved diagnostic and prognostic indicators.
The AHA also suggests that the use of noninvasive perfusion strategies should be integrated into ongoing examinations of patients with CLI, potentially improving patient outcomes and subsequently reducing costs.
The statement also briefly emphasized the promise of telemedicine in this field, adding that “technology through telemedicine provides better access to care, which could help to decrease the healthcare disparities among patients with PAD.”
Disclosure: Several of the study authors declared affiliations with the pharmaceutical industry. Please see the original reference for a full list of authors’ disclosures.
Misra S, Shishehbor MH, Takahashi EA, et al; on behalf of the American Heart Association Council on Peripheral Vascular Disease; Council on Clinical Cardiology; and Council on Cardiovascular and Stroke Nursing. Perfusion assessment in critical limb ischemia: Principles for understanding and the development of evidence and evaluation of devices. A Scientific Statement from the American Heart Association [published online August 12, 2019]. Circulation. doi:10.1161/CIR.0000000000000708