Pulse Wave Analysis (PWA) consists in the recording of brachial pressure waves, conversion into central pressure wave forms and subsequent detailed evaluation. In addition to systolic and diastolic central pressure, the analysis of the central wave yields valuable diagnostic such as augmentation pressure, augmentation index, diastolic function, and even estimation of cardiac output.
Considering the broad clinical use of brachial pressure for far more than 100 years, the clinical need for central pressure determination is not obvious. In health, brachial and central pressures are closely related; nevertheless, the central wave may be of scientific interest for a host of reasons. As soon as Arterial Stiffness is involved, brachial and central pressures can be considerably different, therefore knowledge of central pressures will become essential in the treatment and follow-up of hypertension and other vascular diseases. Saying it bluntly, in cardio-vascular disease, brachial pressure may be an indicator, but only central pressure will give a proper answer.

The groundbreaking work of O’Rourke described the relation between brachial and central pressure waves [24] as early as 1970. By introducing a global transfer function, he determined the complex but fixed relation between brachial and central waves. The transfer function holds throughout adulthood, not only in health but also in cardio-vascular and heart disease, as O’Rourke was able to show. Visually it is impossible to derive a central from a brachial pressure wave. But with the advent of real-time Fast Fourier Transforms central pressure is readily available in modern vascular diagnostic devices.

As you can see from the figure, the beauty of the central wave lies in the fact that the superimposed incident and reflected wave can be differentiated and analyzed. A multitude of diagnostic information can be derived:

Abrev	Parameter	Dimension
AoBP	Central aortic syst/diast pressure	mmHg
AoPP	Central aortic pulse pressure	mmHg
Aug Press	Augmentation Pressure	mmHg
Aug Index	Augmentation Index	%
MAP	Mean arterial pressure	mmHg
SEVR	Subendocardial Viability Ratio Buckberg Index
ESP	Central endsystolic pressure	mmHg
HR	Heart rate	1/min
CO	Cardiac Output	L/min
SV	Stroke Volume	ml
TPR	Total peripheral resistance Systemic vascular resistance	PU
Beats	Number of recorded beats for evaluation

Table with parameters derived from the central pressure wave

Wave reflection is a complex phenomenon. The incident pulse pressure wave drives blood flow directed towards the periphery and vascular bed. At bifurcations and the vascular bed though, the pressure wave is redirected against the heart while blood flow maintains its direction. The concept of pressure wave reflection is well explained in [25], with many analogies that decipher the underlying physiology, supported by many clinical examples.

Clinical Relevance of Central Pressure

When considering the clinical importance of PWA testing, two aspects should be considered. First and foremost, PWA allows non-invasive assessment of central aortic pressure, undisputed the closest link to left ventricular load. Secondly, in PWA testing Augmentation Pressure or Augmentation Index (AIx), the ratio of Augmentation pressure to central pulse pressure, is evaluated, in order to assess the reflecting properties of the arterial tree. In every day practice, brachial pressure is still the standard of care and the commonly used gauge for cardiac load. The international societies, such as the European Society of Cardiology, have acknowledged the value of central pressure evaluation in treating hypertension [21]. Besides maintaining (brachial) blood pressure in tight boundaries, the societies also recognized the urgent need for more relevant descriptors, such as central pressure and AIx. A very thorough outline of PWA testing and interpretation is given in [17]. Although the testing modality described in the latter paper consists in operator dependent applanation tonometry it can be readily replaced by less cumbersome plethysmographic oscillometry as executed by Vicorder®.

While AIx and its clinical value have been extensively described in the scientific literature over the past years [27], no reliable normative data have been published. From the few studies that are available in this regard, it becomes evident that AIx may be a good bio-marker in youth and middle-aged subjects as AIx rises steadily in the first two decades of age, but less suitable for an elderly population as AIx remains fairly constant over the higher decades. The age dependent increase in Aix is nonlinear and different in men and women.

The reflecting properties of the arterial tree may be explained by AIx, but AIx does not necessarily explain arterial stiffness. There is common ground between AIx and PWV, but crucial differences exist [28]. In disease, at least in nephrotic patients, PWV is definitely a much more powerful parameter, allowing better recognition and differentiation of arterial stiffness than Aix [25]. PWV has a linear age dependence that is equal in both genders. In general, PWV is more valuable in any population beyond the mid ages, has a better reproducibility and a lower variation than Aix. In this chapter, we referred to Augmentation evaluated from brachial/central pressure waves. To our knowledge and the common opinion in scientific literature Aix derived from finger tip measurements has little in common with the above.