Large Artery Stiffness Assessment Using SphygmoCor Technology

Abstract

Large artery stiffness assessment has been an integral part of the SphygmoCor technology since 1998. Aortic stiffness is approximated with non-invasive measurement of carotid-femoral pulse wave velocity, with improvements made with time to make the assessment procedure quicker and more user independent. Also standard in the devices is the ability to reliably calculate the central aortic waveform shape from a peripheral pressure waveform from either the brachial or radial artery. This waveform contains much information beyond peak and trough (systolic and diastolic pressure). Relative waveform features such as the augmentation index, wave reflection magnitude, reflection time index, and subendocardial viability ratio are parameters that are influenced by the stiffness of systemic arteries. This article briefly describes these parameters related to large artery stiffness and provides reference to validation and repeatability studies relative to the clinical use of the SphygmoCor devices. It is beyond the scope to review here the 424 original research articles that have employed SphygmoCor devices in measuring arterial stiffness. Instead, the method of measurement across the devices is described, including tonometry, volumetric displacement through cuff placement around limbs, and ambulatory monitoring. Key population and subpopulation studies are cited where the average stiffness parameter progression with age and gender, as measured by SphygmoCor devices, is quantified in the healthy and general population. Finally, with reference to guidelines from working groups on arterial stiffness and hypertension, the clinical utility of large artery stiffness measurement is discussed in the context of the arterial stiffness parameters provided by the SphygmoCor systems.

Keywords: Large artery stiffness; SphygmoCor technology.

Figures

Fig. 1

a, b The SphygmoCor CvMS device, which utilizes tonometry (b) for acquisition of the peripheral vascular waveform. c, d The SphygmoCor XCEL device, which utilizes cuff-based volumetric displacement (d) for acquisition of the peripheral vascular waveform.

Fig. 2

SphygmoCor CvMS carotid-femoral pulse wave velocity (cfPWV) measurement method with sequential applanation tonometry of the carotid (c) and femoral (f) sites. Pulse transit times (tt) are calculated from the electrocardiogram (ECG) R-wave to the foot of the applanated waves. Distances (d) are measured from the suprasternal notch (s) to the sites of applanation tonometry.

Fig. 3

SphygmoCor XCEL carotid-femoral pulse wave velocity (cfPWV) measurement simultaneously acquires a carotid (c) pulse by applanation tonometry and a femoral (f) pulse by volumetric displacement within a cuff (C) around the upper thigh and measures the transit time (tt) between the feet of the 2 waves. Distances (d) are measured from the suprasternal notch (s) to the top of the thigh cuff (fC), the site of carotid tonometry (c), and to the site where the femoral artery could be applanated by tonometry (fT). cfPWV is calculated equivalent to the SphygmoCor CvMS calculation (Fig. 2) by subtracting the contribution of the additional femoral segment to both the distance (dfTfC) and the transit time proportional (k2) to that distance. A further correction to the transit time (k1) is made to adjust for the delay in transmission of the pulse from the femoral cuff to the pressure transducer, as opposed to the carotid tonometer where the transducer is placed on the skin above the artery.

Fig. 4

Peripheral (radial) and aortic arterial blood pressure waveforms from 2 sample patients. Whilst “Jan” (solid line) and “Joe” (dashed line) both have a systolic/diastolic blood pressure in the arm of 140/80 mm Hg, the aortic blood pressure magnitude is markedly different, with “Joe” having a blood pressure of 136/80 mm Hg and “Jan” having a blood pressure of 115/82 mm Hg. This difference in magnitude is relayed in the waveform shape of the peripheral waveform.

Fig. 5

Example of an aortic pressure waveform of a nominal pulse height from diastolic pressure (DP) to systolic pressure (SP). The anacrotic shoulder is associated with the commencement of augmentation of the forward-going pressure wave by the reflected pressure wave. The pressure at which this occurs is the inflection pressure (Pi). The incisura, not to be confused with the dicrotic notch seen in more peripheral waves, is associated with the closing of the aortic valve and the end of systole (Pes). The dotted line shows an estimated flow wave of arbitrary units using these points as a guide for the waveform shape. The subendocardial viability ratio can be calculated as the ratio of systolic area to diastolic area.