Evaluating the use of a portable ultrasound machine to quantify intima-media thickness and flow-mediated dilation: Agreement between measurements from two ultrasound machines

Abstract

Flow-mediated dilation (FMD) and common carotid intima-media thickness (CIMT) are intermediate endpoints for cardiovascular disease. The purpose of this study was to determine whether a portable ultrasound machine was capable of valid measurements of FMD and CIMT compared with a clinic-based machine under similar conditions. Vascular images were taken on 23 apparently healthy young adults with the portable type and clinic type instruments. The analyses revealed a high level of agreement between the two machines for measurements of maximum CIMT (mean difference [MD] = –0.025 mm, limits of agreement [LOA] = –0.080, 0.029 mm), mean CIMT (MD = 0.001 mm, LOA = –0.065, 0.065 mm) and FMD measures of brachial diameter (baseline MD = 0.199 mm, LOA = –0.210, 0.608 mm, maximum MD = 0.218 mm, LOA = –0.162, 0.597 mm). Reasonable agreement was found for %FMD measurements (MD = 0.27%, LOA = –4.91, 5.44%). The within-machine coefficient of variation results for maximum CIMT (5.0%), mean CIMT (4.3%), baseline (6.3%) and maximum (5.4%) brachial diameter and %FMD (30.1%) were comparable with normal within-subject variation. We conclude that the portable ultrasound machine can provide measurements of FMD and CIMT that are highly comparable with measurements obtained from a clinic-based machine under similar clinical conditions. (E-mail: [email protected])

Introduction

The major clinical complications of atherosclerosis, including ischaemic syndromes that affect the heart, brain and extremities develop from an asymptomatic phase to a manifest disease over decades (Ross 1999). Injury to the endothelium causing dysfunction is hypothesised to be the initial phase in atherogenesis, whereas the gradual diffuse thickening of the inner-most layer of the artery wall, the intima, follows endothelial damage (Ross 1993, Ross 1999). These early changes in arterial function and structure precede clinical diagnosis of vascular diseases and are detectable by various imaging techniques.

Current techniques utilise noninvasive high-resolution, B-mode ultrasound to measure brachial artery flow-mediated dilation (FMD) and common carotid artery intima-media thickness (CIMT) to determine the effects of cardiovascular risk factors on endothelial function in peripheral arteries (Adams et al 1996, Celermajer et al 1992, Urbina et al 2002) and structural changes in the carotid wall (Baldassarre et al 2000, Bots et al 1997, Davis et al 2001). Assessment of these vascular properties provides an opportunity for presymptomatic detection of the disease and may allow an increased understanding of the atherogenic process.

Both CIMT and brachial FMD have become accepted as surrogate endpoints for ischaemic disease in case-control, cross-sectional, longitudinal and intervention studies (Azen et al 1996, Blankenhorn et al 1993, Clarkson et al 1996, Gokce et al 2003, Jarvisalo et al 2004, Plotnick et al 1997, Simons et al 1998). The requirement of clinic-based equipment to image these vascular properties, however, may limit its application as a research tool in some settings. For example, selective bias may be introduced in several instances: if the population is sampled with favourable logistical settings to attend a clinic, if specific subpopulations of a representative sample are unable to attend a clinic due to residential setting or if loss to follow-up affects a certain subpopulation of the sample. The potential to reduce selection bias would be improved if the technology used to assess such vascular parameters were easily transportable and provided comparable results with established machines used in clinical practice.

The Acuson Cypress^™ (Siemens Medical Solutions USA Inc., Mountainview, CA, USA) is a portable ultrasound machine capable of producing clear and precise images. No study has previously assessed the validity of CIMT and brachial FMD measurements made by a portable device. Before validation in a fieldwork setting, it is essential to assess the validity of measurements owing to machine and not situational differences. Conducting this approach will 1) allow the agreement between machines to be assessed directly as a function of the between-machine difference in technology and 2) if validity is demonstrated, the reliability of the portable machine can be further examined under different fieldwork conditions.

The purpose of this study was to compare the between-instrument agreement of mean and maximum CIMT and brachial FMD measurements between a portable ultrasound machine (Acuson Cypress^™) and an established clinic-based ultrasound machine (Acuson Sequoia^™) under similar conditions.