Central Blood Pressure Estimation

See article vol. 30: 1132-1141

Katsuda’s study resolves the issue regarding the estimation of central blood pressure and proposes a novel estimation technique concerning the central blood pressure¹⁾.

It is well-known that the central blood pressure is superior to peripheral blood pressure in approximating the prognosis of cardiovascular disorders^{2, 3)}. One of the monumental works to verify the superiority of central blood pressure in prognostic predictability is the ASCOT-CAFE study⁴⁾. In this research, calcium antagonist (amlodipine) and angiotensin-converting enzyme inhibitor (perindopril) medication for hypertensive patients reduce central blood pressure more than beta-blocker (atenolol) and thiazide diuretic medication, although the brachial blood pressure lowering effect was same between two drugs. Furthermore, the change in the central hemodynamics was considerably related to total cardiovascular event improvement. According to these works, the significance of central blood pressure estimation has been revealed in the examination of cardiovascular risk assessment. Still, in reality, central blood pressure is not prevalent enough.

It is not generally known that the aortic systolic blood pressure (SBP) is lower than brachial SBP with the same measurement procedure⁵⁾. In other words, the upstream blood pressure is lower than the downstream blood pressure. The arterial blood pressure waveform contains two pressure waves, a forward pressure wave, and a reflected pressure wave. The forward pressure wave is produced by the heart and the reflected pressure wave is generated by pressure wave reflection which originates from around the aortic bifurcation. Thus, the arterial blood pressure waveform relies on location. The peak SBP in the brachial artery is higher than that in the ascending aorta, because the time lag between the arrival of the forward pressure wave and the reflected pressure wave in the brachial artery is smaller than that in the ascending aorta. This phenomenon is known as pulse pressure amplification. Nonetheless, the brachial cuff SBP is lower than the invasively measured ascending aortic SBP. The difference is ascribed to the measurement procedure. The blood pressure measured by cuff is lower than that in the invasive method⁶⁾. Such a challenge may inhibit the popularization of central blood pressure evaluation in clinical practice.

The participation of the reflected pressure wave in ascending aortic pressure waveform is assessed as an augmentation index⁷⁾. The augmentation index is computed by dividing the augmentation pressure by the pulse pressure. The value is negative when the inflection point occurs after peak systolic pressure in the pressure waveform. There exist three categories of aortic pressure waveforms⁸⁾. When the augmentation index is over 0.12, it is a type A pressure waveform. When the augmentation index is between 0.12 and 0, and when it is negative, the waveform is classified as type B and type C, respectively (Fig.1).

Fig.1. Aortic pressure waveforms

(a) type A pressure waveform. (b) type C pressure waveform. Arrows indicate the inflection point.

Recently designed devices that estimate central blood pressure primarily adopt two techniques for the estimation of central blood pressure, the generalized transfer function (GTF) method, and the SBP2-method⁹⁾. GTF-method is based on using the transfer function which is made from the generalization of pressure transduction properties between the ascending aorta and peripheral arteries. SBP2-method is based on the information that the radial SBP2 (the second systolic peak or shoulder of radial pressure waveform, pSBP₂ in Katsuda’s paper) is closely correlated to central SBP. The SBP2 reflects the reflected pressure wave and the first systolic peak or shoulder of the radial pressure waveform (SBP1, pSBP in Katsuda’s paper) reflects the forward pressure wave (Fig.2). These devices mainly aim to estimate the central blood pressure of the aged population, and their aortic blood pressure waveform are type A. Thus, it looks like a matter of course that the SBP2, which strongly associates with the reflected pressure wave, is closely correlated to central SBP. The degree of impact of SBP1 and SBP2 in the estimation of central blood pressure would depend on the central blood pressure waveform. The pressure waveform in aged persons is type A, but greater parts of adolescents to young people demonstrate type C aortic pressure waveform¹⁰⁾. We previously stated that the correlation between SBP1 and aortic SBP was stronger than the correlation between SBP2 and aortic SBP. Hence, the SBP2-method would be beneficial in adult patients whose aortic blood pressure waveform is type A; nevertheless, the accuracy will be low when it applies to the person whose aortic blood pressure waveform is type C.

Fig.2. Radial pressure waveform

SBP1, the first systolic peak or shoulder of radial pressure waveform; SBP2, the second peak or shoulder of radial pressure waveform.

Katsuda’s paper shows the difference in pressure waveform between the ascending aorta and right brachial artery in normal and Kurosawa and Kusanagi-hypercholesterolemic rabbits by a catheter-tip micromanometer¹⁾. The paper shows the data not only baseline but also in response to intravenous administration of angiotensin II and sodium nitroprusside. In type A pressure waveform, the correlation between SBP1 and central blood pressure is higher than the correlation between SBP2 and central SBP in normal rabbits. It is the opposite result in Kurosawa and Kusanagi-hypercholesterolemic rabbits. In the type B pressure waveform, the correlation between SBP1 and central SBP is higher than the correlation between SBP2 and central SBP. Particularly, the correlation between SBP2 and central SBP in type B pressure waveform is low compared to other correlations, although it is statistically considerable. They also examined the correlation between an average of SBP1 and SBP2 (pSBPm in Katsuda’s paper) and central SBP. The correlation of average is midway between SBP1 and SBP2 in the type A pressure waveform. In type B pressure waveform, the correlation to central SBP on average is higher than that in SBP1 and SBP2. These outcomes indicated that, in the estimation of central SBP, it would be better to consider not only SBP2 but also SBP1. Of course, identifying the optimal allocation of two pressures is challenging because it depends on the central pressure waveform. It may be necessary to use properly depending on the target. It is conceivable that adopting the average of two blood pressures is one of the good options in estimating central SBP.

Conflicts of Interest

None.

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