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DOI:10.1016/S0003-4975(10)61943-11985;39:401-402Ann Thorac Surg

Gordon Wright and Anthony FurnessWhat Is Pulsatile Flow?

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Thoracic Surgeons. Print ISSN: 0003-4975; eISSN: 1552-6259.

and the Southern Thoracic Surgical Association. Copyright 1985 by The Society ofis the official journal of The Society of Thoracic SurgeonsThe Annals of Thoracic Surgery

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EDITORIAL

What Is Pulsatile Flow?Gordon Wright, Ph.D., and Anthony Furness, Ph.D.

The most surprising feature of the literature and the pro-longed discussions on the subject of pulsatile and non-pulsatile blood flow is that pulsatile flow has not beendefined. It is anomalous that scientific investigationsand discussions can proceed with no clear demarcationof the factors being compared. The most discerning au-thors pay only token acknowledgment to this problemby referring to the pressure pulse amplitude measuredat some peripheral site by uncharacterized manometrictechniques, and to the pulse repetition frequency (pulserate). However, no evidence that pressure pulse am-plitude or pulse rate has any physiological significancehas been presented.

The benefits of pulsatile flow are now well known andare prominent in the cardiopulmonary bypass literature,as reviewed by one of us [l] nd Mavroudis [2]. In oppo-sition, there is a growing population of authors report-ing that they have been unable to detect any notabledifferences between pulsatile and nonpulsatile flow [3].

One reason for this dichotomy (and there are others)may be that different investigators have used differenttypes of pulsatile flow. As suggested by Philbin [3],some types of pulsatile flow are more effective than non-pulsatile flow in preserving tissue function while othersare not. The one type of pulsatile flow we can beconfident about is the type produced in the aorta by thenormal left ventricle, and it has been suggested that thismight form the basis of a working definition [4]. n theother hand, it is clear that some benefits can be obtainedusing pulsatile pumps that produce very bizarre arterialpressure and flow waveforms compared with normal

The urgent requirement for a descriptive techniquethat is capable of distinguishing among different typesof pulsatile flow is apparent. In common usage, the flowof blood from the heart to the aorta is described as atime-history of events-an ejection phase and a recov-ery phase of ventricular action. We may extend this de-scription by adding factors such as ejection phase dura-tion, peak flow, and pressure pulse amplitude, but thetime domain description cannot provide a comprehen-sive definition of pulsatile flow because the morphologyof the waveform is not included.A more complete description can be obtained by theuse of frequency domain techniques by which the com-plex pressure or flow waveform is transformed to amean level plus a series of sine waves of differing am-plitude, frequency, and phase (harmonics) [6, 71. Thepower of left ventricular ejection is then derived as theproduct of left ventricular mean pressure and flow plus

(Fig 1) [51.

From the Department of Biological Sciences, University of Keele, Keele,StaffordshireST5 5BG, England.

the products of pressure and flow at each harmoniclevel. This gives us a clue to one of the principal advan-tages of pulsatile flow: for equal mean blood flow rates,pulsatile flow contains more energy (power x time),because nonpulsatile flow is completely represented bythe mean value, whereas it is necessary to add a series ofsine waves of different amplitude, frequency, and phaseto describe pulsatile flow. It is important to recognizethat this is not just a mathematical trick. It has real phys-iological significance and may be the most fundamentalbenefit of pulsatile flow.

The dynamic properties of the vascular system aresimilarly obtained as the quotient of mean pressure andmean flow (peripheral vascular resistance) plus the quo-tients of the corresponding harmonics. Phase differ-ences between pressure and flow for corresponding har-monics are caused by compliance and inertance in thevascular system. The important consideration here isthat unlike a purely resistive system, a system with com-pliant and inertive components is capable of both tem-porary storage and lateral transmission of energy.

The transient storage of energy in the walls of themajor arteries has the effect of off-loading the heart, orpump, from the high-resistance peripheral vascularbeds. The effect of this is that pulsatile flow meets alower impedance than nonpulsatile flow, because thecardiovascular system is predominantly compliant. AsFigure 2 shows, aortic input impedance is dependent onthe frequency content (in terms of sine waves) of thecomplex waveform.

Thus, pulsatile flow produced by the normal left ven-tricle gives us two important and fundamental advan-tages over nonpulsatile flow. First, at equal mean bloodflow rates, pulsatile flow contains more energy. Some ofthis energy is transmitted downstream where it per-forms work, such as tissue perfusion and the promotionof metabolic exchange, and some energy is directed ra-dially where it encourages the movement of tissue fluidsand the formation and flow of lymph. Second, pulsatileflow is more easily transmitted to the peripheral vascularbeds because the major arteries are compliant. Thesetwo factors may be responsible for the plethora of physi-ological and pathological advantages that have been re-ported in controlled pump trials. Failures to detect ad-vantages in some studies may be due to the fact that thefrequency content (in terms of sine waves) of the pumpoutput may not have been optimal for the patients onwhom it was used. It should be noted that the importantpoint is what occurs in the aorta, so that our concept of"the pump" has to include the aortic line and cannula.Acompliant arterial line or a small-bore cannula can con-siderably modify the pulse. It should also be noted thatcurrently available pulsatile pumps can be operated in a

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402 The Annals of Thoracic Surgery Voi 39 No 5 May 1985

Pressure

3001

-100 I *B

Fig 1 . Pressure and flo w wavefo rms generated in a model of the hu-man vascular system by ( A )a conventional roller pum p (Sarns), (B)a modijied roller pump (Stiickert), and (0 positive-displacementpulsatile p um p (P oly sta n). The modified roller pump can be operatedin nonpulsatile or pulsa tile mode wi th a continuous range of adju st-ment between these extremes.

"01.OO t i

1s ,

, -0 -.--+-..0O F 1 2 3 4 5 6 7 8 9

2 1

Radians- 2 1( Phase1

Fig 2 . Impedance spectrum in pig aorta. The impedance modulus isv e y high for non pulsatileflow (zero frequency). Phase refers to thedelay of flow behind pressure. Posit ive values of phase indicate iner-tance, and negative values indicate compliance.

variety of modes (see Fig 1).Some modes of operationmay be better matched to the aortic input impedancethan others.

The importance of accurate measurements should bestressed. Measuring pulsatile pump output from thepump meter display is totally inadequate; even when anelectromagnetic flowmeter is employed, certain fre-

C

quency-dependent corrections are necessary. Also, thesite of measurement is of great importance since thepressure and flow pulses vary considerably between theaorta and the peripheral arteries [6]. Finally, it will beapparent from this discussion of impedance that mea-surements of peripheral vascular resistance are of littleconsequence, because this value is derived from thequotient of mean pressure and mean flow, and does notinclude the considerable effects of compliance and iner-tance in pulsatile flow.

This brief and oversimplified discussion has not an-swered the original question, but it at tempts to point theway in a field of unnecessary confusion. The basic strat-egy in pulsatile flow studies is quite unreproachable.The type of pulsatile flow we need is not the type pro-duced by a pump with that mode labeled on it, but thetype that is ideally matched to the cardiovascular sys-tem. The only type of pulsatile flow we can be sureabout at present is that generated by the normal leftventricle. Comparative studies in which the pump out-put fails to approach this ideal may have demonstrableclinical benefits, but in our opinion, it is difficult to de-termine the relevance of these studies to the discussionsconcerning the relative merits of pulsatile and nonpul-satile flow.References1. Wright G: Brain damage in dogs resulting from pulsatile and

non-pulsatile blood flows in extracorporeal circulation. Uni-versity of Keele Ph.D. thesis, 19712. Mavroudis C: To pulse or not to pulse (collective review).

Ann Thorac Surg 25:259, 19783. Philbin DM. Should we pulse? J Thorac Cardiovasc Surg

M805, 19824. Sanderson JM , Morton PG, Tolloczko TS, et al: The Morton-

Keele pump: a hydraulically activated pulsatile pump for usein extracorporeal circulation. Med Biol Eng, March 1973, p182

5. TaylorKM, Bain WH, DavidsonKG, et al: Comparative clini-cal study of pulsatile and non-pulsatile perfusion in 350 con-secutive patients. Thorax 37:324, 1982

6. McDonald DA:Blood Flow in Arteries. Second edition. Lon-don, Arnold, 1974

7. Mdnor WR: Hemodynamics. Baltimore, Williams& Wilkins,1982

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DOI:10.1016/S0003-4975(10)61943-11985;39:401-402Ann Thorac Surg

Gordon Wright and Anthony FurnessWhat Is Pulsatile Flow?

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