Many vascular biomarkers have been proposed as an aid for clinicians to refine cardiovascular risk stratification and decision-making . One such biomarker is augmentation index (AIx), a variable calculated from the first and second peaks of arterial pressure waveforms, usually recorded at peripheral sites such as the radial, brachial or carotid arteries. An estimation of central aortic AIx can also be synthesized from radial or brachial waveforms using mathematical transfer functions, and this derived AIx signal is indicative of the magnitude of late systolic left ventricular loading, mostly attributed to arterial compliance and wave reflection .
Although there is incomplete evidence to currently recommend AIx for general clinical use , several factors underlie an interest in AIx as a potentially useful cardiovascular risk variable. Firstly, observational studies among different patient groups and the general community are fairly consistent in reporting an association between increased AIx and adverse cardiovascular events independent from blood pressure . Secondly, there is no theoretical necessity to synthesize a central AIx using a transfer function because virtually identical AIx pressure waveform information is contained within the nontransformed peripheral (radial) artery waveform . The difference in morphology being that the augmented component is generally responsible for the peak (systolic) pressure in the central aorta, whereas it modulates the shoulder in the peripheral artery waveform and does not contribute significantly to the magnitude of the systolic peak. Thirdly, therapeutic targeted lowering of AIx is possible with antihypertensive medications [5–7]. Lastly, even though AIx tends to be higher in people with high blood pressure, the index itself is calculated independent of blood pressure, thus, an indication of vascular risk can be identified even when cuff blood pressure measurement may be inaccurate . That is, added value beyond the specific conventional measurement of systolic and diastolic pressure may be gained by the analysis of the arterial pulse waveform .
In this issue of the Journal, García-Ortiz et al. present data on testing the performance of AIx measured using a wrist-mounted device incorporating a triangular array of three pressure sensors , compared with a conventional approach of radial applanation tonometry using a single sensor mounted at the tip of a hand-held probe. Measures were compared under resting conditions and, as expected for any sensors measuring a tonometric signal at the same location, there was strong concordance between radial AIx measured with the wrist-mounted and hand-held devices. Confirmation of the clinical relevance of the radial AIx measures was also evaluated by association with cardiovascular risk markers and, again as expected, there were moderate associations with endpoints such as Framingham risk, carotid–intima media thickness and estimated glomerular filtration rate.
The use of a wrist-mounted device tested in this study offers the advantage of continuous recording and, thus, a theoretical opportunity to evaluate ambulatory arterial waveforms. Indeed, the device manufacturer asserts storage capacity of up to 2 weeks of continual monitoring, which could produce information of significant value beyond a single ‘snapshot’ assessment of AIx, or even beyond assessment over a 24-h sleep–wake cycle. Ambulatory assessment was not performed in the current study, but is an area of future research need to better understand feasibility in terms of subject acceptability and comfort, as well as susceptibility of waveform signal quality to movement artefact.
Achieving high-quality recording of radial tonometric waveforms under ambulatory conditions, and with wrist position changes expected during activities of daily living, represents a considerable technical challenge. In the absence of automated repositioning of tonometric sensors, or some other method to achieve good quality control of waveform recordings, it would seem difficult to achieve. Nonetheless, similar technology, in the form of a fixed wristwatch-like device taking intermittent readings, is already commercially available and has been used to demonstrate circadian effects on central versus peripheral blood pressures after drug treatment . However, all wrist-worn devices using applanation tonometry are limited by the obligatory application of external pressure to obtain the arterial waveform to abide by the principle of applanation for reliable detection of the transmitted force because of arterial pressure. With long-term use, this can lead to potential superficial microcirculatory damage and bruising of the skin. To mitigate this limitation, progress is being made with other sensor modalities that do not rely on external applied pressure, such as bioimpedance  and continuous wave radar .
In any case, beyond pure focus on AIx, if we can record a peripheral arterial signal of good quality using any number of different methods, including the wrist-mounted device in this current study , it also opens opportunities to test the value of new conceptual models to explain cardiovascular mechanics from analysis of peripheral waveforms. A recent example of this comes from the derivation of reservoir-wave parameters from radial tonometric signals [15,16]. Even though this conceptual model is neither widely accepted nor fully understood , reservoir-wave parameters have shown to predict cardiovascular outcomes more strongly than standard cuff blood pressure or AIx among different patient populations and ethnic groups [18–21]. Thus, through trial and error, the analysis of untransformed radial waveforms provides a possible platform for new discovery, problem solving and learning in the field of cardiovascular haemodynamics. The work by García-Ortiz et al. is a worthwhile step in the direction towards achieving new understanding in the area.
Conflicts of interest
There are no conflicts of interest.
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