Central Blood Pressure (BP)

Long-term (day-to-day or visit-to-visit) blood pressure variability (BPV) predicts elevated risk of cardiovascular events but represents just one BPV type. Ten-second central systolic blood pressure variability (SBPV) parameters predict first and recurrent cardiovascular events (Sluyter, J.D et al 2019).

Central blood pressure (cBP) is the pressure in the aorta, which is the large artery into which the heart pumps. The term 'central blood pressure' usually refers to the pressure in the aorta near the heart.

Why is central BP important?

Central BP has been shown to more strongly relate to vascular disease and outcome than traditional upper arm blood pressure[1-3]. It also can distinguish between the effects of different hypertension medications when upper arm blood pressure and pulse wave velocity do not [4].

Central blood pressure is the pressure that the heart has to pump against to get blood to flow to the rest of the body. Higher central blood pressures mean that the heart must work harder to do its job. This can eventually lead to heart failure. Central blood pressure also determines the pressure in the blood vessels feeding the brain. If central pressure is too high, it may cause aneurysms and strokes.

How is central BP different from conventional BP?

Conventional BP is measured in the upper arm, which is a 'peripheral' artery. Peripheral BP is usually higher than central BP as it includes the increased pressure associated with more and smaller arteries in the arm. The degree to which the peripheral BP is higher than central BP is determined by the stiffness of the arteries.

How is central BP measured?

Central BP can be directly measured only using a pressure sensor or catheter inserted into the aorta (usually through an artery in the groin or wrist). This procedure is invasive and can lead to complications. Alternately it can be measured non-invasively using Uscom’s patent protected suprasystolic oscillometry - Medi-stats (Uscom) BP+.

How does BP+ measure central BP non-invasively?

BP+ calculates central BP plus pulse pressure waves using a physics-based model of the arteries between the aorta and the cuff. This model relates how pressure waves travel between the aorta and the occluded artery under the suprasystolic cuff, as shown in the diagram that follows. More details are available in [5].

What are the advantages of the BP+ approach to central BP?

BP+ measures BP directly in the brachial artery and the aorta by occluding the brachial artery thereby isolating the complex and variable network of small arteries in the arm and hand (i.e. the peripheral circulation). Other devices measure the arm pressure and use a generalised transfer function from the brachial measurement to create a “central BP” and idealistic pulse wave. The BP+ technique has several advantages over empirical transfer functions and statistical attempts at estimating central blood pressure.

• Upper arm measurement means the distance between the sensor and the heart is much shorter than wrist or finger measurements and the arterial connection more direct.

• Cuff inflated to occlude the brachial artery means individual variations in downstream arteries can be ignored.

• Physics-based model is applicable where statistical and empirical relationships are invalid. This is particularly true for stiff arteries.

Does BP+ require calibration to estimate central BP?

No external calibration is necessary to estimate central blood pressure using BP+. Instead information already collected from the upper-arm blood pressure measurement is used.

Does BP+ assume central mean and diastolic pressures?

BP+ calculates central systolic, diastolic and mean pressures. Other technologies assume central diastolic and/or mean pressures are equal to peripheral blood pressures.

How accurate are BP+ central BP estimates?

Central BP estimates have been independently tested against accepted invasive and non-invasive techniques for central blood pressure measurement and are well within the requirements of American Association for the Advancement of Medical Instrumentation (AAMI) standard SP10.

Does the central BP estimate require patient height, age, sex etc.?

The BP+ central BP estimate utilises a physics based model, and is not statistics-based and therefore does not require the use of additional patient measurements.

Does brachial artery, arm or cuff variation affect accuracy?

The suprasystolic waveform is calibrated from the peripheral BP. Variations in the arm and cuff are therefore not significant. The brachial artery isn't nearly as affected by cardiovascular disease as other major blood vessels and so does not influence the accuracy of the arterial model.

Are cholesterol and central BP related?

Cholesterol is a bio-indicator associated with cardiovascular diseases, but its relationship with central BP is yet to be established. Nevertheless, some research suggests that patients with hypercholesterolemia exhibit increased central BP compared with normocholesterolemic controls [7].

Reference

[4] P. Boutouyrie, A. Achouba, P. Trunet, S. Laurent, and for the EXPLOR Trialist Group, “Amlodipine-Valsartan Combination Decreases Central Systolic Blood Pressure More Effectively Than the Amlodipine-Atenolol Combination: The EXPLOR Study,” Hypertension, vol. 55, Jun. 2010, pp. 1314-1322.

[5] A. Lowe, W. Harrison, E. El-Aklouk, P. Ruygrok, and A.M. Al-Jumaily, “Non-invasive model-based estimation of aortic pulse pressure using suprasystolic brachial pressure waveforms,” Journal of biomechanics, vol. 42, 2009, pp. 2111-2115.

[7] I.B. Wilkinson, K. Prasad, I.R. Hall, A. Thomas, H. MacCallum, D.J. Webb, M.P. Frenneaux, and J.R. Cockcroft, "Increased central pulse pressure and augmentation index in subjects with hypercholesterolemia," J Am Coll Cardiol, vol. 39, Mar. 2002, pp. 1005-1011.