BP+

• Overview
• Applications
• Features
• Evidence
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Central Blood Pressure

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 arteriels 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 - the 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 a number of 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].

Arterial Stiffness

What is arterial stiffness?

Arteries are composed of three layers and function normally by varying in diameter in response to changes in the muscular tone in the smooth muscle layer of the artery to regulate the flow of blood in the artery. Arterial elasticity is an important requirement for normal arterial function and haemostatic control.

The muscles in a normal artery is compliant and changes diameter freely, however if the artery narrows then the BP rises. This contributes to hypertension and requires specific medication for successful treatment.

Why is arterial stiffness important?

Arterial stiffness in the large arteries has been shown to predict the likelihood of future cardiovascular events, such as heart attacks, heart failure and strokes, which may lead to death. Additionally, arterial stiffness has been shown to better predictor these complications than other commonly measured parameters such as conventional upper arm BP.

How does BP+ measure arterial stiffness?

BP+ technology allows rapid, simple and accurate measurement of arterial pressure wave forms, and these pressure waves provide simples measures including augmentation index and central BP which indicate large artery stiffness.

How is arterial stiffness related to BP?

Narrowed and stiff arteries increase the resistance against which the heart pumps during each beat and increases BP and cardiovascular risk.

When the large arteries become stiff, resistance and BP increases. Conversely, when arteries are compliant and elastic, the heart does not need to work so hard to deliver blood to the rest of the body.

What can be done about high arterial stiffness?

There are many ways to reduce arterial stiffness.

• Exercise helps to prevent arteries from becoming stiff and can reduce levels of stiffness.
• Some (but not all) hypertension drugs reduce arterial stiffness by relaxing the muscles in the artery wall.
• New drugs are being developed to target the arterial stiffness.
• Your doctor can advise about the best treatment and management for vascular health.

 

Augmentation Index

What is augmentation index?

The augmentation index is a measure of wave reflection and arterial stiffness and calculated as a ratio from the BP waveform.

Augmentation index is commonly measured as the ratio of the central Pulse Pressure (cPP) and the reflected pulse pressure which augments the central BP, called central Augmentation Pressure (cAP) shown in blue in the first graph below.

Why is augmentation index important?

Augmentation index is a proposed marker of arterial status:

• Augmentation index is a predictor of adverse cardiovascular events in a variety of patient populations, and higher augmentation index is associated with target organ damage^[8].
• Augmentation index can distinguish between different vasoactive medications when upper arm BP and pulse wave velocity do not^[9].

 

Why are there different ways of calculating augmentation index?

Central Augmentation Index (cAIx) is defined for a central pressure waveform as the ratio of Augmentation Pressure (cAP) to Pulse Pressure (cPP):

cAIx = cAP / cPP

The wave shape is different for a pressure waveform obtained at the upper arm or wrist. In some cases, late systolic pressure may be higher than early systolic pressure. In other cases, as shown below, a small pressure augmentation is visible in late systole and is lower than early systolic pressure.

Peripheral augmentation index (pAI) is defined as the ratio of late systolic pressure (P2) to early systolic pressure (P1):

pAI = P2 / P1

How does BP+ measure augmentation index?

BP+ augmentation index is calculated using the peripheral augmentation index formula above.

What is the accuracy of BP+ measurement?

Accuracy of BP+ algorithms has been checked against expert analysis of 50 measurements taken from a cross-sectional clinical study. The maximum likely error is just 4% of the typical measurement range.

Is augmentation index related to age?

Both augmentation index and central BP are known to increase with the age of the subject and be related to cardiovascular outcome.

Is augmentation index related to gender?

On average, women have a higher augmentation index than men due in part to their smaller arteries.

Is augmentation index related to central BP?

Augmentation index and central BP are thought to measure different aspects of arterial stiffness.

Is augmentation index related to height?

Some evidence suggests that the timing of the reflected wave is related to the dimensions of the body. In shorter individuals, a shortened return time for reflected waves leads to an increase in central pressure augmentation^[10].

Does BP+ correct for heart rate when calculating augmentation index?

The BP+ algorithm does not correct for heart rate as the relationship between heart rate and peripheral augmentation index has not been established.

Low heart rates tend to be related to increased central augmentation index. It has been proposed that higher heart rates shorten systole (the ejection phase) and therefore the reflected wave will arrive at a relatively later moment in the cardiac cycle, lowering central augmentation index^[11].

Advanced BP+ analysis of PWA components can provide direct insight into these effects.

Is cholesterol related to augmentation index?

Cholesterol is a bio-indicator associated with cardiovascular diseases, but its relationship with central BP is yet to be established.

Nevertheless, central augmentation index has been shown to positively correlate with cholesterol levels^[12,13].

Does brachial artery, arm or cuff variation affect the measurement?

As augmentation index is a ratio, variations in the arm, cuff or artery do not significantly affect the measurement.

How do variations in BMI affect augmentation index?

Body mass index (BMI) does not affect the measurement of augmentation index.

However, studies have found that that increased body fat content, but not BMI, is strongly associated with increased large artery stiffness and wave reflection^[14].

Does brachial artery variation affect the readings?

As augmentation index is a ratio, variations in the brachial artery produce minimal changes for the measurement. Additionally the brachial artery isn't nearly as affected by disease as other major blood vessels and thus still provides the gold standard site for measurement. 

Vascular Age

What is vascular age?

Vascular age is the apparent age of the blood vessels, particularly the arteries when compared to what is normal for the healthy population. Vascular age is affected by genetic predisposition, lifestyle choices and other factors.

Why is vascular age important?

A person whose vascular age is older than his or her chronological age may be at increased risk of developing cardiovascular disease later in life.

The effects of early vascular aging may best be managed by early intervention.

The graph above shows early vascular ageing along the red arrow, compared to normal vascular ageing along the blue arrow. Early vascular ageing may lead to premature cardiovascular complications. Early intervention can help to delay these events.

How can I gauge a patient's vascular age using BP+?

BP+ algorithms calculate peripheral Augmentation Index (AI). Augmentation index is known to increase with age.

Arrythmias and Heart Rhythm

What information does BP+ give on arrhythmias?

BP+ provides a rhythm strip over 10 seconds with each pulse being clearly identifiable.

BP+ also calculates a numerical measure of short term pulse rate variability.

Two examples of arrhythmias as presented by a cardiovascular monitor are shown below:

How does BP+ calculate Pulse Rate Variability?

BP+ calculates root mean squared successive difference (RMSSD) as described here.

Exercise

How is exercise related to arterial health?

Aerobic exercise tends to make your arteries less stiff and more elastic. Immediately following exercise, arterial wave reflection is much reduced. The long term effect of exercise is commonly to increase arterial elasticity at rest.

What type of exercise has benefits?

Aerobic exercise has been shown to improve the elasticity of arteries. However, strength-based training alone does not. Flexibility is also associated with arterial stiffness.

A selection of external links:

• Large Artery Stiffness: Implications For Exercise Capacity And Cardiovascular Risk
• Flexibility exercises like Pilates and yoga could prevent, treat stiff arteries
• Cardiorespiratory Fitness, Physical Activity, and Arterial Stiffness
• Aerobic Exercise No Big Stretch For Older Adults But Helps Elasticity Of Arteries

 

What can BP+ tell me about exercise?

BP+ lets you see the effect of exercise on your arteries in a way that blood pressure alone does not. BP+ information is useful in assessing:

• Warm up - how fast the arteries expand in response to the start of exercise, and when they are ready for a full work-out.
• Immediate effects of exercise - how arteries have dilated to improve blood flow to the rest of the body.
• Recovery - how fast the arteries return to their resting state.
• Long-term benefits - measure the improvement in Augmentation Index in response to exercise.

 

What is a typical response to exercise?

Aerobic exercise will decrease augmentation index and increase pulse rate with little change in blood pressure. In the period following exercise, the augmentation index will increase towards its original value. Pulse rate will decrease towards the resting pulse rate. A typical response is shown in the chart below.

The BP+ peripheral pulse wave is shown below. Exercise causes a very significant change in pulse shape.

What happens during a warm-up?

When exercise is first started, the heart works hard to deliver blood to the arteries but the arteries have not had a chance to respond. The increase in heart-work increases blood pressure.

During the warm-up phase, the arteries begin to dilate to help deliver more blood to the rest of the body. This reduces the load on the heart and blood pressure returns to normal even though pulse rate is elevated.

Managing Hypertension

How can BP+ be useful for prescribing hypertension medications?

Measuring arterial stiffness with BP+ can help:

• Decide which borderline hypertensive patients may benefit from medication: patients that present with blood pressures such as 137/89 may benefit from treatment. An elevated arterial stiffness is evidence that treatment may be warranted.
• Choose an appropriate class of hypertension medication: Choosing whether to prescribe an ACE inhibitor, ARB, CCB, Beta blocker or diuretic can be difficult. You need to take into account many factors. However, if a patient has high arterial stiffness, ACE inhibitors and ARBs may be more appropriate as they help to relax blood vessels. Beta-Blockers may be contra indicated as they may increase arterial stiffness.
• Determine the dose of medication to prescribe: Medicines have side-effects. BP+ makes the dose-response relationship visible and allows the most cost-effective dose to be chosen.

 

How can BP+ help stratify pre-hypertensive patients?

If a patient has moderately high blood pressure, you may wish to consider their augmentation index to help determine whether he or she needs treatment. For example, moderately high blood pressure with moderate arterial stiffness may indicate nothing of particular concern. However, moderately high blood pressure with high arterial stiffness may indicate your patient is at increased risk of cardiovascular events later in life.

The following device screen shots show a range of arterial stiffness values for acceptable to moderate blood pressures.

What blood pressure medications affect arterial stiffness?

Certain kinds of blood pressure medications affect arterial stiffness directly (they are vasoactive), which in turn reduces blood pressure. Other classes of medication have little direct effect on arterial stiffness.

Vasoactive blood pressure medications include:

• Angiotensin-converting enzyme inhibitors (ACE inhibitors)
• Angiotensin II receptor antagonists, also called angiotensin II receptor blockers (ARBs)

 

Non-vasoactive blood pressure medications include:

• Beta-blockers
• Diuretics

 

How does BP+ measure the effect of vasoactive medications?

An example of the effect of an ARB on augmentation index (AI) is given below. You can see AI increases substantially when the patient stops taking his medication (AI increases prior to significant changes in BP). AI returns to normal once medication is resumed.

How do I know what is a good level of medication?

The appropriate dose of medication is dependent on a number of factors. One consideration may be to improve arterial stiffness towards a level that is normal as measured in people who are generally healthy.

References

1. M.J. Roman, R.B. Devereux, J.R. Kizer, E.T. Lee, J.M. Galloway, T. Ali, J.G. Umans, and B.V. Howard, “Central Pressure More Strongly Relates to Vascular Disease and Outcome Than Does Brachial Pressure. The Strong Heart Study,” Hypertension, May. 2007, p. HYPERTENSIONAHA.107.089078.
2. M.J. Roman, R.B. Devereux, J.R. Kizer, P.M. Okin, E.T. Lee, W. Wang, J.G. Umans, D. Calhoun, and B.V. Howard, “High Central Pulse Pressure Is Independently Associated With Adverse Cardiovascular Outcome: The Strong Heart Study,” J Am Coll Cardiol, vol. 54, Oct. 2009, pp. 1730-1734.
3. A. Avolio, “Central Aortic Blood Pressure and Cardiovascular Risk: A Paradigm Shift?,” Hypertension, vol. 51, Jun. 2008, pp. 1470-1471.
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.
6. R.A. Payne, I.B. Wilkinson, and D.J. Webb, “Arterial Stiffness and Hypertension: Emerging Concepts,” Hypertension, vol. 55, 2009, pp. 9-14.
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.
8. M. Shimizu and K. Kario - Review: Role of the augmentation index in hypertension - Therapeutic Advances in Cardiovascular Disease, vol. 2, Feb. 2008, pp. 25-35.
9. 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.
10. B.P. McGrath, Y.L. Liang, D. Kotsopoulos, and J.D. Cameron - Impact of physical and physiological factors on arterial function - Clinical and Experimental Pharmacology & Physiology, vol. 28, Dec. 2001, pp. 1104-1107.
11. I.B. Wilkinson, H. MacCallum, L. Flint, J.R. Cockcroft, D.E. Newby, and D.J. Webb - The influence of heart rate on augmentation index and central arterial pressure in humans - The Journal of Physiology, vol. 525, May. 2000, pp. 263-270.
12. 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.
13. J. Nürnberger, A. Keflioglu-Scheiber, A.M. Opazo Saez, R.R. Wenzel, T. Philipp, and R.F. Schäfers, - Augmentation index is associated with cardiovascular risk - Journal of Hypertension, vol. 20, Dec. 2002, pp. 2407-2414.
14. A. Wykretowicz, K. Adamska, P. Guzik, T. Krauze, and H. Wysocki - Indices of vascular stiffness and wave reflection in relation to body mass index or body fat in healthy subjects - Clinical and Experimental Pharmacology & Physiology, vol. 34, Oct. 2007, pp. 1005-1009.

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