When you receive any health assessment it is imperative that the information that is provided to you is as accurate as possible. How else can you make decisions that will shape your future. Human interpretation is always flawed and open to interpretation.

The world of technology races along at a pace that is mind blowing and in line with this the advances that have been made in the area of Cardiovascular Health Technology have also made incredible leaps. In our field we are concerned with Central Blood Pressure and Arterial Hardening in particular.

In the past to accurately assess either Central Aortic Systolic Blood Pressure or Arterial Hardening it was necessary to undergo a surgical procedure

angiogramwhere a catheter was inserted into your femoral artery and then passed up to the opening of your heart. There may have been a pressure sensor at the top of the catheter to take the pressure measurement or there may have been a dye or similar substance injected via the catheter into arteries to show up blockages under X-Ray. No matter what method was used it was and still is a procedure that does not come without some risk.

Thankfully advances in technology now means that we can get these results without the need for invasive surgical procedures. We can achieve over 99% accuracy and you don’t even need to get undressed. At Heart Health Screenings we use the latest in Pulse Wave Analysis Technology to carry our our assessments.

Using a regular blood pressure cuff that you would see in your doctors office we can measure the following;

  • Systolic Blood Pressure (Same as your doctor takes)
  • Diastolic Blood Pressure (Same as your doctor takes)
  • Central Aortic Systolic Blood Pressure (Usually done in hospital)
  • Central Aortic Diastolic Blood Pressure (Usually done in hospital)
  • Arterial Hardening (Using Augmentation Index)
  • Heart Rhythm (Used to detect irregularities in your heart beat in the same way as an ECG)

The incredible aspect of our assessment is that it takes a few minutes, painless and completely non-invasive.

What is central blood pressure?

Central blood pressure 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.

How is central blood pressure different from normal blood pressure?

Normally, blood pressure is measured in the upper arm, which is a ‘peripheral‘ artery. Peripheral blood pressure is usually higher than central blood pressure due to the peripheral site being closer to locations from which echoes reverberate.

The degree to which the peripheral blood pressure is higher than central blood pressure depends partly on the stiffness of the arteries.

Why is central blood pressure important?

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

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 blood pressure measured?

Central blood pressure 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.

How do we measure central blood pressure?

Our technology calculates central blood pressure using a physics-based model of the arteries between the aorta and the cuff. These models relate how pressure waves travel between the aorta and the occluded artery under the suprasystolic cuff, as shown in the diagram that follows.

central_bp

What are the advantages of this way of measuring central blood pressure?

Central pressure is estimated non-invasively by using a mathematical relationship to a peripheral pressure. The degree to which this mathematical relationship matches any individual determines the accuracy of the central pressure estimate in that subject.

Our technique has a number of advantages over empirical transfer function 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, meaning less variability between the arterial geometry of individuals.
  • 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.

When measured at the same height, there is little difference in the mean blood pressure or diastolic blood pressure between the aortic root and the large conduit arteries such as the brachial artery where blood pressure is usually measured. Systolic blood pressure, however, is usually significantly higher at peripheral sites than at the aortic root.1 This is because the pulse pressure (the difference between systolic and diastolic pressure) or amplitude of the pressure wave generated at the aortic root becomes amplified as the pressure wave propagates along the elastic arteries towards the periphery of the circulation. This phenomenon of pressure wave amplification is seen in many wave propagation systems, the most familiar being the approach of waves to a beach where wave height increases in shallower water. It is thought to be due to the summation of backward travelling reflected waves with forward travelling waves.

Importance of central systolic blood pressure

In adults, peripheral systolic blood pressure (pSBP) exceeds aortic root or central systolic blood pressure (cSBP) by about 10 mmHg but differences may exceed 50 mmHg.1 The peripheral amplification is greater in younger subjects, during exercise and increases in heart rate and is affected by drug therapy.1 Because cSBP and central pulse pressure (cPP) are more closely related to the load on the heart and pulsatile stress on the coronary arteries than peripheral pressures, central pressures are likely to be better predictors of cardiovascular events. In studies where this has been tested using non-invasive estimates of central pressures obtained from radial tonometry (see below), this has proved to be the case.2,3

Measuring central systolic blood pressure

cSBP can be measured directly using a pressure sensor placed in the aortic root but this is impractical for most purposes. The approach most widely used at present is to obtain a peripheral blood pressure waveform by applanation tonometry of the radial artery (holding a pressure sensor against the radial artery). This waveform is then calibrated from an oscillometric measurement of peripheral blood pressure at the brachial artery and a mathematical transfer function used to transform the radial artery waveform into a central pressure waveform from which cSBP can then be derived.1,4Concerns as to the applicability of a generalised transfer function to all patients have now largely been dispelled.4 The approach is justified because, for a given waveform morphology, the upper limb arteries (the physical properties of which change little with ageing) exert a relatively predictable influence on waveform amplification. It is now accepted that the accuracy of this approach is limited by the accuracy of the peripheral blood pressure measurement.5 Thus cSBP can only be as accurate as the peripheral blood pressures used to calibrate the waveforms.

Reference List

  1. 1. Nichols WW, O’Rourke MF. McDonald’s blood flow in arteries. Theoretical, experimental and clinical principles. London: Arnold, 1998.
  2. 2. Roman MJ, Devereux RB, Kizer JR, Lee ET, Galloway JM, Ali T, Umans JG, Howard BV. Central pressure more strongly relates to vascular disease and outcome than does brachial pressure: the Strong Heart Study.Hypertension. 2007; 50:197-203.
  3. 3. Williams B, Lacy PS, Thom SM, Cruickshank K, Stanton A, Collier D, Hughes AD, Thurston H, O’Rourke M. Differential impact of blood pressure-lowering drugs on central aortic pressure and clinical outcomes: principal results of the Conduit Artery Function Evaluation (CAFE) study. Circulation. 2006; 113:1213-1225.
  4. 4. O’Rourke MF, Adji A. An updated clinical primer on large artery mechanics: implications of pulse waveform analysis and arterial tonometry. Curr Opin Cardiol. 2005; 20:275-281.
  5. 5. Guilcher A, Brett S, Munir S, Clapp B, Chowienczyk PJ. Estimating central SBP from the peripheral pulse: influence of waveform analysis and calibration error. J Hypertens. 2011.
  6. 6. Guilcher A, Brett S, Clapp B, Chowienczyk PJ. Estimating central systolic blood pressure during oscillometric determination of blood pressure: proof of concept and validation by comparison with intra-aortic pressure recording and arterial tonometry. PubMed. 2012 Jun; 17(3):132-6.

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