vol. 41, no. 12, 1962 themeasurement of cerebralblood...

Journal of Clinical InvestigationVol. 41, No. 12, 1962

THE MEASUREMENTOF CEREBRALBLOODFLOWBY EXTERNALISOTOPE COUNTING*

By SHELDONH. STEINER,t KWANHSU, LEO OLINER, AND ROYH. BEHNKE

(From the Medical and Radioisotope Services, Veterans Administration Hospital, and theDepartment of Medicine, Indiana University School of Medicine,

Indianapolis, Ind.)

(Submitted for publication December 22, 1961; accepted September 1, 1962)

The Kety-Schmidt nitrous oxide method (2) ispresently the only well-accepted technique for thequantitative estimation of human cerebral per-fusion. In spite of theoretical objections (3) andpractical limitations it has provided a wealth ofotherwise unobtainable information. The pro-cedure is tedious, however, and has remained pre-dominantly a laboratory research tool.

Efforts have continued to find an accurate,more adaptable method, but with limited success.Sapirstein and Hanusek (4-6) have reportedmeasurements of regional blood flow in laboratoryanimals by use of K42, Rb86, and 4-iodo831 antipy-rine, and Sapirstein has reported observationsconcerning the applicability of these materials tomeasurement of human cerebral perfusion (7, 8).This investigation was designed to evaluate,clarify, and extend those observations by employ-ing 4-iodo'8lantipyrine and Rb86Cl in the quan-titative estimations of human cerebral blood flow.The effects of altered arterial pCO2 upon cerebralhemodynamics were evaluated by use of this re-search method.

THEORETICAL CONSIDERATIONS

Regional blood flow by indicator distribution.After a rapid intravenous injection of an indicatorsubstance, mixing occurs during passage throughthe central circulation. The initial distributionthroughout the body is directly proportional tothe regional distribution of the cardiac output.The problem inherent in this method of determin-ing organ blood flow is that of quantifying the

* Supported by U. S. Public Health Service GrantsH4080, H6308, and HTS5363 from the National HeartInstitute, and by the Indiana Heart Association. Pre-sented in part before the 34th Annual Meeting of theCentral Society for Clinical Research, Chicago, Ill., No-vember, 1961, and published in part (1).

t VA Clinical Investigator.

fraction of injected indicator received by an or-gan during this first circulation. It would be for-tunate if a substance existed that could be totallyextracted by each organ through which it passed;problems dealing with recirculation would beavoided. With such a substance, organ contentwould increase as the arterial bolus was delivered,and since venous drainage of the substance wouldbe zero, the organ content after initial localizationwould be independent of time. In that case thefractional organ uptake of the injected dose wouldbe proportional to the fractional organ distribu-tion of the cardiac output. Sources for error inthis theoretical system would be the presence ofpulmonary indicator losses and arteriovenousshunts.

It would be possible to make measurements ofregional blood flow, however, if the substance be-came redistributed sufficiently slowly after ini-tial localization. This might allow extrapolationof the organ washout rate back to maximal or-gan content at zero time of initial delivery. Sap-irstein (4) has reported that the net redistributionrate for K42 and Rb86 after initial localization ofa bolus injection for most organs of the rat anddog is slow enough not to be statistically detect-able for 3 minutes. The applicability of theseisotopes to measurement of regional blood flowprobably depends on the nature of the distribu-tion of body potassium and the rate of membraneion transport. Walker and Wilde (9) have dem-onstrated -that the injected isotope bolus of K'2 ofhigh specific activity, which is initially diluted in arelatively small blood volume, is to a large degreelost from the vascular compartment after the firstcirculation has washed through. Since the organcontent is initially zero and the arterial bolus con-centration is high, the tracer is transferred chieflyinto the organ pool, since it is considerably largerthan the vascular reservoir for potassium. Most

2221

STEINER, HSU, OLINER, AND BEHNKE

;t-, --i-.:4k!./131. Ad*T ,

. . ... ... ... ..,.. . .. . . . , .*.v,,-

iG. TH- !I' ' RT-T AiAID C.T ,N. ETIO

FIG. 1. THE CEPH-ALIC 4-IODO'31ANTIPY'IuNu COUNTRATE AFTER A RAPID CENTRAL VENOUSINJECTION.

of the radioactivity is rapidly cleared from thecirculation, and the remainder is diluted. Net re-distribution is slow, and for a considerable periodafter the first circulation of the bolus, the organfraction of the injected dose reflects the fractionalorgan distribution of the cardiac output. Thewashout curve approaches a relatively horizontalline.

The major objection to this method for meas-uring regional blood flow in animals is that the en-tire time-course of localization cannot be viewed,since the organs must be removed for countingpurposes. Early redistribution from regions withsmall exchangeable pools-e.g., brain-or fromarteriovenous shunts give falsely low flow fractionsfor these organs. Conversely, there are false in-creases in the flow fractions to organs with largeexchangeable pools, owing to the redistribution ofthe tracer out of proportion to the initial distribu-tion of the cardiac output. This objection wouldbe removed if the organ content could be con-tinuously observed during accumulation of thetracer ( 10). In that case, early redistributioncould be detected and possibly corrected for byextrapolation.

Cerebral blood flow. The principles of thismethod have been applied to the quantification ofhuman cerebral blood flow (1, 7, 8). In order tomeasure uptake by an organ of a radioisotope invivo, it is necesary to observe the organ quanti-tatively with appropriate detectors. Gammaemis-sion from the human brain has not been satisfac-torily separated from that of adjacent noncerebralcephalic structures. For the present, therefore,it has been necessary to determine cerebral bloodflow from the difference between the total cephalic

flow-brain, head, and neck-and the noncerebralcephalic flow-head and neck. 4-Iodo13'anti-pyrine and Rb86Cl have been used to measure totalcephalic blood flow and noncerebral cephalic bloodflow, respectively.

Total cephalic blood flowe. Sapirstein andHanusek (6), Hansen (11), and Reinmuth andScheinberg (12) have reported that 4-iodo13l-antipyrine readily crosses the "blood-brain bar-rier." During the first circulation after a venousinjection of 4-iodo13'antipyrine, the quantity de-livered to the head represents the cephalic frac-tional distribution of the cardiac output. There isa reasonably large, exchangeable, cephalic reser-voir for the isotope, and the washout rate afterinitial localization is relatively slow (Figure 1;the rate meter time constant is 2.0 seconds).

The arteriovenous differences for 4-iodo3'3-antipyrine were determined in two subjects inorder further to elucidate the nature of the cephalicdistribution and washout of the isotope. Sampleswere taken from the brachial artery and the in-ternal jugular vein by retrograde arm vein cathe-terization with fluoroscopic placement of the cathe-ter tip just cephalad to the junction with the sub-clavian vein at the base of the neck. At this pointthe internal jugular vein has received a majorportion of the venous drainage from the head andneck (13). The arterial and venous concentrationchanges with time are shown for one of these sub-jects during the minute and a half after central ve-lOtos (superior vena cava) injection (Figure 2).The arterial concentration describes an indicatordilution curve. The venous curve gradually in-creases during the first circulation, and it then re-mains slightly above the arterial concentration

2222

CEREBRALBLOODFLOWBY EXTERNALISOTOPE COUNTING

during the subsequent minute of observation. Thevenous blood demonstrates significant concentra-tion of label during the period of observation, andthis accounts for the early redistribution of thecephalic 4-iodo103antipyrine. A large portion ofthe tracer delivered by the arterial blood is retainedby the head, and redistribution is relatively slow.

Mathematical model. Since significant cephalicwashout of 4-iodo131antipyrine-approximately 15to 20 per cent-occurs during initial localization,it is necessary to compute the value for cephaliccontent existing before washout has occurred. Inorder to make this calculation, a model has beendevised that requires several simplifying assump-tions. The heart is represented by a single cham-ber of fixed volume whose contents are uniformlyand instantly labeled at the moment of injection.Flow of blood through the chamber is constantfor the period of observation. A portion, A,, ofthe uniformly labeled pool at zero time is destinedfor the head. The quantity of label in the heartdestined for the cephalic structures at time t isA1t. From this is derived Equation 1,

dA1/dt =-K1Al (t > 0).

The head is assumed to be a single, fixed-volumemixing chamber that accumulates indicator as itleaves the heart. Distortion imposed by passagethrough the arteries leading to the head is con-sidered negligible. Consequently, cephalic uptakecan be represented as a first-order exponentialfunction.

However, the experimental observations (Fig-ure 1) show significant washout during localiza-tion. This is assumed to follow a first-order ex-ponential function. The rate of change of cephalic

6,000

.--* Brac hi a IArtery

- 4,000 l l - -- Internal Jugular Vein

u) 2,000

20 40 60 80In ject J M E

FIG. 2. BRACHIAL ARTERIAL AND INTERNAL JUGULAR

VENOUSCONCENTRATIONSOF 4-IODO"lANTIPYRINE AFTER A

4 /AC RAPID CENTRALVENOUSINJECTION.

7.06.05.0

4.0

3.0

2.0

AL

. 1.0'_ 0.8

0.7z 0.6

0.5

0.4

_ 0.3

<

0.2

Washout T 1/2 - 140 Seconds

K, 0.00495

Extraction T 1/2 . 4.2 Seconds

K, . 0.165

r0 10 20 30 40 50 60 70 80 90 100 110 120 130 14C

Ink T M E (Seconds)

FIG. 3. SEMILOGARITHMIC PLOT OF OBSERVEDCEPHALICCONCENTRATIONDATA FOR 4-IODO 'ANTIPYRIDINE AFTER ARAPID CENTRAL VENOUS INJECTION. Extrapolation ofwashout portion of curve is represented by the line L2.The difference between the extrapolated portion of L2 andthe buildup portion of the cephalic concentration datais represented by the line L1. T. is the time of appear-ance in the head of label.

indicator, A2, can then be represented by the dif-ference between first-order exponential functions.The amount of indicator in the head at zero timeis given by A2 = 0 and at time t by AJt so that byEquation 2,

(A .,dt KA, - K.A. (t > 0).

Solution of Etquation I gives Equation 3.

Al = Ae- tt (t '0),

and so'lution of Equation 2 gives Equation 4,

A., = [K,/K1 - K.] [A0 (erKgt - e`at) (t . 0)Where K2 <<< K1, Equation 4 gives Equation 5,

limit A = (K1/K1 - K.) A~e-Kd

This equation is represented by the line L., inFigure 3.

The quantity of label delivered to the head, As,.can be determined graphically from the differencebetween two exponential functions. Equation 4subtracted from Equation 5 gives Equation 6,

Q = (K1/K, -K2) A~e-iI,where Qt is an approximation of the cardiac labelstill to be delivered to the head at time t and isrepresented in Figure 3 by the line L1.

2223

2224 ~~~~STEINER,HSU, OLINER, AND BEHNKE

When t =0, L, intercepts L, at the coordinateKA0/K1 - K2; for values of K1»>> K2, its valueis not very different from Ao. A0 may be foundby determining K1 and K, from a semilogarithmicplot of the observed data (Figure 3). The effectsof recirculation of label are assumed to be neg-ligilble.

Critique of the model. This simplified modelhas several advantages. It allows for a consistentand reproducible method of fixing the value forcephalic indicator content. This method elimi-nates the need for bilateral jugular cannulation,which would otherwise be required to make the de-termination of cephalic blood flow by use of theconventional Fick equation. In the situationwhere indicator loss from the heart is delayed orwhere cerebral blood flow is large, the values forK1 and K2 may not be so very different. Themodel then provides a means graphically to de-termine cephalic content from K1, K,, and thecoordinate K1A0/K - K2.

In Figure 3, the appearance of label in the headdoes not occur at zero time as set forth in themodel. The appearance is actually a finite num-ber of seconds after injection and has been desig-nated to. If there were no distortion or dispersionof the tracer during transit from injection site toappearance site, the time required for it to traversethe vascular system would be immaterial. SinceK1 i so much greater than K2, an error introduced

in the value of K1 by distortion or dispersionwould not be expected significantly to alter the de-termination -of AO.

The model assumes cephalic washout is a first-order exponential function. Since K1 »>> K2, thecontribution from the term (- K1/K1 - K2)

A0e-KIt of Equation 4 is transient, and the slopeof the washout K2 ought to be a function solelyof the disappearance of indicator from the headvery shortly after peak concentration has beenachieved. A semilogarthmic plot of the washout(Figure 3, line L,) is a straight line, verifying theassumption of the first-order exponential nature ofthe washout slope. Caution must be exercised,however, to insure that the time constant of thedetecting apparatus is sufficiently short to respondto early exponential cephalic washout having a dif-ferent rate constant. With time constants asshort as 0.5 second, early cephalic indicator wash-out was not observed to follow an exponentialcurve different from that of the major decay com-ponent. A 2-second time constant did not alterthe curves, but served to minimize random count-ing fluctuations, simplifying the graphical plot.

Recirculation of indicator was considered to be'negligible during the period of experimental ob-servation. The volume of distribution of 4-iodo'-antipyrine during the first minute after injectionwas found to be somewhat greater than 20 L.This volume is roughly four times greater than

- - I.

7-K

':0

2.0

10 sec. 1

t.::.Injection 20 me.*Rb86CI

- !-77T.zi

FIG. 4. THE CEPHALic RBMGLCOUNTRATE AFTER A RAPID CENTRALVENOUSINJECTION.

2224

.

-,.J-.!1 ; '. : I-I- -;- -t- ....

.I-I

.i

I


that of the vascular compartment. In addition,the initial bolus is delivered in a volume consider-ably less than the total vascular volume. There-fore, the contribution from recirculating label atany time must represent only a small portion ofthe amount initially delivered to the cephaliccompartment.

The error introduced for the value of the cepha-lic portion A,0 of the cardiac indicator by use ofthe value for K1AQ/K1 - K2 was found to be 2.5+ 0.4 per cent in 9 experiments in 5 subjects.This was considered to be sufficiently good forthe purposes of this investigation. If greater ac-curacy is required, A, could be determined fromthe coordinate K1Ao/K, - K2, and from the val-ues obtained for K1 and K2 by standard methodsfor analysis of the semilogarithmic plot of the ob-served data.

Noncerebral cephalic blood flow. The non-cerebral cephalic fraction of the cardiac output isdetermined from an injection of Rb86Cl. After arapid central venous injection of the label, thecephalic count rate describes an indicator dilutioncurve (Figure 4), and then a plateau for a con-siderable time. Since there is virtually no readilyexchangeable cerebral reservoir for cationic ma-terials, the indicator dilution pattern seen initiallyrepresents shunting of the isotope through thebrain. There is, however, a large, rapidly ex-changeable reservoir for Rb06 in non-nervous tis-sue (5, 9), and after the first circulation haswashed through, the residual cephalic plateau isgreater than 10 per cent of the injected dose. Thisprobably does not represent an appreciable numberof counts from the vascular compartment, sincethe apparent minute volume of distribution aver-ages somewhat more than 60 L, and cephalic bloodvolume appears to be relatively small. The stablecephalic plateau, therefore, represents the approxi-mate noncerebral cephalic fraction of the cardiacoutput.

Two problems exist in the interpretation of theexact nature of the cephalic Rb86 plateau after ini-tial localization of the injected dose. The first is,to what extent there exists a cerebral Rb80 reser-voir that may contribute to the total count rate.Sapirstein suggests (8) that the normal humanbrain has a very small reservoir for K42, and in-deed he suggests that the reservoir may be con-

600

M 400-

u 200

.-. Brachial Artery

........oCerebralVenous

--- M ned Venous

0 5 10 15

T M E (Mioutes

FIG. 5. CONSTANTPULMONARYARTERY INFUSION OF

RB CL.

fined to the non-nervous elements. The reservoirfor Rb06 is very small in normal subjects. It hasnot been possible to demonstrate significant cere-bral uptake of Rb86 in man by measuring the cere-bral arteriovenous difference during a 10-minuteconstant infusion (Figure 5). Mealey, Sweet, andBrownell (personal communication) report thatnormal human brain, biopsied within 15 min-utes after rapid intravenous injection of Rb86,contains 0.25 ± 0.3 per cent per kg of the injecteddose in the cerebrum in 5 subjects. In a totalof 35 samples taken from 14 patients up to 180minutes after injection, the maximal value of 0.4 +0.3 per cent per kg was seen during the third hour.The cephalic Rb86 fraction would then be over-estimated by slightly less than 0.5 per cent of theinjected dose. No correction was made for thisresidue.

The second major objection to this method fordetermining regional blood flow is the large cere-bral shunt for Rb86, i.e., the effects of arteriove-nous shunts upon the redistribution of the injecteddose out of proportion to the initial distributionof the cardiac output. It is readily seen from thecephalic Rb86 count rate (Figure 4) that the brainrepresents a substantial shunt whose Rb86 contentmust be redistributed to organs with exchangeablepools during subsequent circulations. Sheppard,Overman, Wilde, and Sangren (14) have re-ported that in the dog the major organs except forthe brain-liver, kidney, and extremity-exhibitlittle shunting for K42. Thus, the material recir-culated from the brain probably makes up thegreatest portion of the total body shunt. In thatcase, the noncerebral cephalic fraction of the car-diac output is overestimated by the fraction of Rb86redistributed to it from the Rb8i6 shunt in propor-

2225


BISMUTHTUBEWELL

PHANTOM RECORDERSTANDARD

FIG. 6. SCHEMATIC REPRESENTATION OF CEPHALIC COUNTERAND HEADSHIELD; CALIBRATING PHANTOMSHOWNIN THE LOWERINSERT.

tion to the noncerebral cephalic blood flow. It wasassumed that the only major shunt different forRb86 and 4-iodo13tantipyrine was represented bythe brain. Therefore, the noncerebral cephalicblood flow is the cephalic fraction of the injectedRb86-less the cerebral Rb86 residue, assumed tobe zero for this investigation-multipled by thecardiac output, and is overestimated by the re-distribution of the cerebral flow fraction to non-cerebral cephalic structures in proportion to thenoncerebral cephalic flow. Cerebral blood flow isthe difference between the total cephalic bloodflow-determined from the zero-time maximalcontent of the cephalic 4-iodo131antipyrine multi-plied by the cardiac output-and the noncerebralcephalic fraction, multiplied by cardiac outputand corrected for the recirculation of Rb86 fromthe brain shunt to the noncerebral cephalic struc-tures in proportion to the noncerebral cephalicblood flow. Sapirstein's report (8) that this canbe solved as a simultaneous equation is repeatedhere for continuity. Where Ft = cephalic bloodflow, Fm= noncerebral cephalic blood flow, FBr =

cerebral blood flow, C.O. = cardiac output, Fr1131

= cephalic fraction of 4-iodo131antipyrine deter-mined from zero time extrapolation of the cephaliccount rate, FrRb86 = cephalic fraction of Rb86CI,and FrrB = cerebral flow fraction of the C.O.

FJI131 C.O. = Fn,. + Fm = Ft and [i]

FrRb86 - C.O. = Ft,, + Fill" F.Br. Iii]These equations are solved as a quadratic function,

FBr =

[(Fri131 - 1) + VI(1 - FJ13)2 + 4(FE11"- FRb86) ]co

L ~~~~~~2 [iii]

EXPERIMENTALMETHODS

Protocol. Sixteen male patients 20 to 40 years of age,in the postabsorptive state, without premedication, andwith minor illnesses, served as subjects. A venous cath-eter was placed into the superior vena cava, and a brachialarterial needle was inserted. Each subject rested withhis head inside the well detector for at least 30 minutesbefore the measurements were made. Control measure-ments were made in all subjects, and replicate measure-ments in % of the group. In the remainder, arterialpCO, was altered either by voluntary hyperventilation orby breathing 5 per cent CO2 in air for 10 minutes; meas-urements were then repeated.

2226


Isotopes. Approximately 20 Ac of Rb'Cl 1 of highspecific activity in 2 ml isotonic saline was injected andthe count rate recorded for 2 min. Four to 6 Atc of4-iodo1"'antipyrinel in 2 ml isotonic saline was then in-jected and the count rate recorded.2

The purity of the 4-iodo"t'antipyrine was checked by apaper chromatographic separation in a 1: 1 ethanol:toluene solvent system. Strips were scanned for distri-bution of radioactivity. Ninety-five to 98 per cent of theactivity migrated with an Rf of 0.95 (15) during the use-ful life of the isotope.

Isotope counting equipment. The detector is a gamma-ray-sensitive, bismuth-tube well sufficiently large to con-tain the human head (Nucleonic Corporation of Amer-ica). The background count rate with shielding is 2,500cpm. The well is surrounded by 2 inches of lead, andhas a movable lead collar at the orifice that excludesvirtually all radiation emanating from below the subject'sneck, except for a small region at the base of the neck(Figure 6). The count rate of a dose of either Rb56 or4-iodo13lantipyrine from the orifice in the collar, as com-p)ared to the count rate wvithin the well, was less than 2per cent.

The position of the subject's head within the well iscritical, particularly since count rate is inversely pro-portional to the square of the distance, and the dimensionsof this counting system are large. It was necessary toplace each subject in the same position as the calibratingphantom, since motion within the well near the orificewhere the head and neck were confined altered the countrate significantly.

Count rates were recorded through a linear rate meter(Nuclear-Chicago model 1510A) onto a strip chart re-corder at 6 inches per minute (Texas Instrument Com-pany).

Calibrating standard. The standards were preparedby making duplicate injections of the isotopes into 100-mlvolumetric flasks through the venous inj ection catheter(100 per cent standard). A known fraction of the sub-stance for injection was placed in a "standard" water-filled phantom head (16), and quantification of thecephalic fraction of the injected dose was performed bycomparing the cephalic count rates with the count rate ofthe phantom (Figure 6). In this system, the count ratewas relatively independent of the isotope distributionwithin the phantom. For example, when RbV was placedin the periphery of the phantom, with the center por-tion taken up by a water-filled balloon, the count rates

vere not different. This is probably the result of severalsimultaneously operating variables such as changes in

'Obtained from Abbott Laboratories, North Chicago,Ill.

2 Dosimetry:4-Lodol'-

antipyrine, Rb8"C,referred to referred toshole body muscle

T eff. (days)q (R = 3 rem/13 weeks)q (R = 0.1 rem/week)

7.6851 /Ac

38 ,uc

15.1626 ,uc

65 sac

distance, radiation absorption and scattering in the fluidmedium, and end loss from the detector.

The phantom, however, was not satisfactory unless itcontained a neck piece, since 20 per cent of the totalcounts observed within the well were counted in a sampleplaced externally in the position of the neck. Althoughthe size of the phantom takes no account of individualvariations in head size, fluid volumes from 4 to 6 L hadno effect upon the total count rate. Uneven isotope dis-tribution within the human brain, however, may contributeto error in quantification of the cephalic flow fraction.This will require further evaluation, particularly indisease states where large areas may lack adequate per-fusion.

Cardiac output. Cardiac output was measured by use ofthe indicator dilution principle (17) by direct arterialsampling through a linear photodensitometer (GuilfordInstrument Company) with indocyanine as the indicator.Cardiac output was determined before and immediatelyafter measurement of cephalic fractions. The results wereaveraged for the final calculations.

Blood gases. Oxygen content, oxygen capacity, andcarbon dioxide content were determined on arterial bloodsamples during each procedure with duplicate checkswithin 0.2 vol per cent by the manometric technique ofVan Slyke and Neill (18). Arterial pH was measuredwith a Cambridge model R pH-meter and corrected tobody temperature with Rosenthal's factor (19). ArterialpCO. was calculated from the line graph of Van Slykeand Sendroy (20).

.4rterial pressure and vascular resistance. Mean ar-terial pressure was recorded with the reference point inthe second intercostal space at the mid-chest with aStatham p23G strain gauge coupled to a carrier amplifierand direct-writing recorder (Sanborn model 150).

Total peripheral resistance (TPR) was calculated fromthe following relationship: TPR (dyne-sec-cm-') =[Mean arterial blood pressure (mm Hg) X 1,332]/cardiac output (ml per sec). The contribution of leftatrial pressure was ignored (21). Cerebral vascular re-sistance was calculated from a similar relationship bysubstituting cerebral blood flow in milliliters per secondfor cardiac output and omitting the correction for jugu-lar venous pressure.

RESULTS

Table I shows the results for cerebral blood flowwith the isotope technique in 16 resting male pa-tients. Normal values are demonstrated for ar-terial oxygen saturation, serum CO, content (volper cent), arterial pH, and calculated pCO,. Thetotal cephalic flow fraction determined from thezero time extrapolation of the cephalic 4-iodo131-antipyrine content was 21.4 + 3.6 per cent of theiujectedl dose. The noncerebral cephalic frac-tion of the injected dose, not corrected for thecerebral Rb86 reservoir, was 10.0 + 1.7 per cent

2227

2228 STEINER, HSU, OLINER, AND BEHNKE

TABLE I

Cerebral blood flow-resting values in normal patients

Arterial blood gases Cerebral circulation

Non- Cor-Body cerebral rected Cerebral

surface Serum Cephalic cephalic cerebral Cardiac bloodSubject Age area Sat. C0g pH pC02 fraction fraction fraction output flow

M2 70 vol% mmHg %% L/min ml/minH.M. 38 1.98 93.7 63.5 7.40 46 19.6 8.9 11.6 5.15 597G.F. 38 1.98 94.9 54.9 7.37 43 23.9 12.0 13.3 6.21 825G.J. 37 1.77 96.0 61.5 7.41 44 20.0 7.0 13.9 5.59 777H.P. 41 2.15 96.0 53.2 7.40 39 16.5 8.2 9.0 6.11 550J.O. 41 1.71 99.7 53.5 7.41 38 20.1 10.0 11.1 5.97 663H.B. 40 1.84 95.8 57.2 7.40 42 22.5 11.3 12.4 4.36 541A.B. 29 1.80 97.3 57.0 7.41 41 24.3 10.8 14.9 5.71 851D.J. 36 1.80 97.5 50.1 7.41 36 14.9 8.6 6.7 6.34 425J.B. 28 1.76 97.3 55.0 7.39 41 26.6 13.5 15.3 5.99 874HI.P. 30 1.72 96.1 54.8 7.37 43 23.8 10.7 14.5 6.10 885J.K. 42 1.83 94.1 52.9 7.39 39 20.0 10.0 11.0 4.86 535S.J. 33 1.68 98.2 62.2 7.41 44 25.9 11.0 16.4 5.99 982W.S. 27 1.84 98.2 54.0 7.42 38 18.6 8.4 11.0 5.07 558R.S. 24 1.73 94.4 58.5 7.37 45 22.2 9.4 14.0 5.85 819M.1 36 1.69 94.5 54.4 7.42 38 25.8 11.2 16.2 5.79 938R.E. 44 1.96 99.7 58.0 7.40 42 17.t 8.9 8.9 5.96 530

Mean 35 1.83 96.5 56.3 7.40 40.6 21.4 10.0 12.5 5.69 709SD + 5.8 +-.13 +1.8 +4.0 +.016 +2.9 +3.6 +1.7 +2.8 +.53 +178

of the injected Rb86. The cerebral fraction of the Rb86 shunt, was 12.5 ± 2.8 per cent of the cardiaccardiac output, as the difference between the total output. Mean cardiac output for this group wascephalic fraction and the noncerebral cephalic frac- 5.69 + 0.53 L per min. The total cerebral bloodtion corrected for the redistribution of the cerebral flow, as the product of the cardiac output and the

TABLE II

Cerebral blood flow-replicate resting determinations


Non- Cor-cerebral rected

Serum Cephalic cephalic cerebral Cardiac CerebralControl Sat. C02 I)H pCO2 fraction fraction fraction output blood flow

% vol% mmHg % % % Limin ml/min

A.B. 97.3 57.0 7.41 41 24.3 10.8 14.9 5.71 851D.J. 97.5 50.1 7.41 36 14.9 8.6 6.7 6.34 425J.K. 94.1 52.9 7 39 39 20 0 10.0 11.0 4.86 535S.J. 98.2 62.2 7.41 44 25.9 11.0 16.4 5.99 982R.S. 94.4 58.5 7.37 45 22.2 9.4 14.0 5.85 819

Mean 96.3 56.1 7.40 41 21.5 9.9 12.6 5.75 722Sample SD +1.9 i4.7 ±.018 i3.7 ±4.3 ±1.0 +3.8 ±t.55 ±232

A.B. 95.3 55.1 7.41 39 25.3 11.3 15.5 5.99 928D.J. 95.0 49.4 7.42 34 15.8 8.3 7.6 6.08 462J. K. 91.6 55.5 7.40 41 19.1 10.4 9.6 4.36 419S.J. 98.0 57.9 7.42 40 25.8 10.5 16.6 5.48 910R.S. 95.7 58.6 7.37 46 21.8 7.9 14.9 5.94 885

Mean 95.1 55.3 7.40 40 21.6 9.7 12.8 5.57 721Sample SD ±2.3 ±3.6 ±.021 ±t4.3 +4.2 ±1.5 ±44.0 ±.71 +256

Coefficient of .90 .85 .86 .82 .98 .89 .96 .88 .96correlation (r)

t Test <.001 <.01 <.01 <.01 <.001 <.001 <.001 <.001 <.001


TABLE III

Cerebral blood flow-hyperventilation compared to control values



Serum Cephalic cephalic cerebral Cardiac CerebralSat. C02 pH pCO2 fraction fraction fraction output blood flow

%o Vol% mmHg %o o Limin ml/minControl

H.M. 93.7 63.5 7.40 46 19.6 8.9 11.6 5.15 597G.F. 94.9 54.9 7.37 33 23.9 12.0 13.3 6.21 825G.J. 96.0 61.5 7.41 44 20.0 7.0 13.9 5.59 777J.0. 99.7 53.5 7.41 38 20.1 10.0 11.1 5.97 663H.B. 95.8 57.2 7.40 42 22.5 11.3 12.4 4.36 541

Mean 96.1 58.1 7.40 41 21.2 9.8 12.5 5.46 681Sample SD :+2.2 ±4.3 A.016 ±5.2 ±1.9 ±2.5 ±1.2 ±0.73 ±119

HyperventilationH.M. 98.9 51.7 7.63 24 12.5 9.0 3.6 6.07 219G.F. 97.4 40.0 7.59 14 20.8 12.7 9.2 7.80 718G.J. 98.7 42.6 7.59 20 13.9 6.5 7.9 6.63 524J.0. 95.9 41.1 7.61 19 12.5 8.6 4.5 6.75 304H.B. 98.1 46.2 7.59 22 17.5 12.0 6.2 5.33 330

Mean 97.8 44.3 7.60 20 15.4 9.8 6.3 6.52 419Sample SD ±1.2 ±4.7 ± 057 ±3.8 ±3.7 ±2.6 ±2.3 ±.58 ±201

t Test NS <.005 <.005 <.005 <.005 NS <.005 <.005 <.005

cerebral fraction of the cardiac output, averaged termination in 5 of the subjects. Replicate resting709 ± 178 ml per min. These results agree with determinations were performed within 20 minutesthose reported by use of the Kety-Schmidt nitrous in the same manner as the initial determination.oxide method (2, 22). The coefficient of correlation for each parameter

Table II demonstrates the reliability of the de- measured is presented with the t test of significance

TABLE IV

Cerebral blood flow-S per cent CO2 compared with control values



Serum Cephalic cephalic cerebral Cardiac CerebralSat. C02 pH pCO2 fraction fraction fraction output blood flow

r% vol% mmHg %oS % L/min ml/minControl

J.B. 97.3 55.0 7.39 41 26.6 13.5 15.3 5.99 874H.P. 96.1 54.9 7.37 43 23.8 10.7 14.5 6.10 885WV.S. 98.2 54.0 7.42 38 18.6 8.4 11.0 5.07 558R.E. 99.7 58.0 7.40 41 17.1 8.9 8.9 5.96 530M.J. 94.5 54.4 7.42 38 25.8 11.2 16.2 5.79 938

Mean 97.2 55.3 7.40 40 22.4 10.5 13.2 5.78 757Sample SD +2.0 ±t1.6 ±.021 ±2.2 ±t4.3 ±2.0 ±3.1 ±0.41 ±196

5 per cent CO2J.B. 98.0 57.8 7.34 48 26.4 11.3 16.7 7.16 1196H.P. 96.8 57.0 7.33 49 25.2 11.1 15.6 6.02 939WV.S. 97.0 55.5 7.36 44 23.1 11.8 12.6 5.92 746R.E. 99.0 56.5 7.37 44 18.5 8.9 10.4 6.07 631M.J. 98.8 55.5 7.40 40 26.7 11.2 17.1 5.94 1016

Mean 97.9 56.5 7.36 45 24.0 10.9 14.5 6.22 906Sample SD ±1.0 ±1.0 +.087 +3.6 ±3.4 ±1.1 ±2.9 ±.53 ±223

t Test NS <.01 <.01 <.005 <.05 NS <.005 <.05 <.005

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TABLE V

Effects of voluntary hyperventilation and 5 per cent C02inhalation upon total peripheral resistance and cerebral

vascular resistance*

Total Cerebralperipheral vascularresistance p resistance p

dyne-sec-cm-5 dyne-sec-cm-5Controlt 1,291+ 66 10,452+1,691Hyperventilation 1,135 +159 <0.05 20,313 +9,048 <0.05

Control t 1,329 +239 11,051 +4,7465 per cent C02 1,300+231 NS 9,6014+3,534 <0.05

* Values are means ASD; N=5 in each group mean.t Differences between controls are not significant.

for the coefficients (23). The results in the sameindividual are reproducible in the resting state.

Tables III, IV, and V show the effects of al-tered arterial pCO2 upon cardiac output and cere-bral hemodynamics. Voluntary hyperventilationdiminished mean arterial pCO2 to 20 mmHg andincreased arterial pH to 7.60. Cardiac output sig-nificantly increased 1.06 L per minute, and the cer-ebral fraction of the cardiac output was reducedfrom a normal value of 12.5 ± 1.2 to 6.3 + 2.3 percent. The cerebral blood flow fell from 681 ± 119to 419 ± 201 ml per minute. Although total pe-ripheral resistance fell slightly, the cerebrovascularresistance approximately doubled. Inhaling 5 percent CO2 in air for 10 minutes increased arterialPCO2 5 mmHg and cardiac output 440 ml perminute. The cerebral fraction of the cardiac out-put increased from 13.2 ± 3.1 to 14.5 ± 2.9 percent. Thus the cerebral blood flow increased from757 + 196 to 906 ± 223 ml per minute. The re-sults for CO2 inhalation demonstrate an increase intotal cerebral blood flow as well as a greater cere-bral distribution of the cardiac output. Althoughtotal vascular resistance remained essentially un-changed, there was a decrease in cerebrovascularresistance. The significance of the observed differ-ences was determined with each subject serving-ashis own control (22).

DISCUSSION

The exchangeable cephalic reservoir for4-iodo13lantipyrine is sufficiently large that earlynet redistribution is slow, and the initial washoutcan be readily extrapolated toward zero time oforgan localization. The fact that 4-iodo13lantipy-rine does not equilibrate with body water, but is

concentrated in the liver (24), probably does notalter the initial arterial distribution during the firstminute after injection. Sapirstein has reported(8) that the cephalic uptake of 4-iodo'31antipyrineremains relatively constant in man after initial lo-calization, thereby simplifying the extrapolationto maximal organ content. This is presumed tobe due to the similarity of the cephalic extractionratio and the extraction ratio for the rest of thebody rather than to zero washout. In preliminarystudies in resting subjects, the values reported forthe cerebral fraction of the cardiac output couldnot be confirmed without using a shielded welland a geometrically proportioned phantom headand neck. Lack of suitable shielding (8) alsoobscured the cephalic washout, demonstrated in allstudies reported here, by failing to exclude thebody contribution to the well. The concept of sim-ilar extraction ratios for the head and body doesnot appear to be experimentally correct nor theo-retically sound. This circumstance could onlyoccur at a unique combination of flow rates. Al-terations in flow rates to any organ, or differentcombinations of flow rates in different subjectswould probably disturb any balance that mightexist coincidentally. Only in the situation whereorgans could appropriately change their extrac-tion ratios with changes in flow would equal ex-traction by head and body be likely to be meaning-ful, and there is no evidence that this phenomenonoccurs.

It would seem more reasonable to describe therelatively slow net redistribution of the cephalicindicator content in terms of the size of the ex-changeable pool available for dispersion of 4-iodo131-antipyrine. The volume of distribution is con-siderably greater than the volume of -the vascularcompartment, and the cephalic distribution pool isconsiderably larger than the available pool withinthe vascular compartment perfusing the head atany one time. Thus, the washout rate would reas-onably depend upon several factors, including therate of perfusion, the arterial concentration of in-dicator, and the relative pool sizes available withinthe exchanging regions of the organ and the vas-cular compartment perfusing those regions.

To simplify the calculation of maximal cephaliccontent, AO, of 4-iodo131antipyrine at zero time, weelected to use the graphical approximation that in-troduced a 2.5 per cent error in the calculation.

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Rlecalculation of the control values for cerebralb)lood flow in 16 subjects from the formulationof the true, maximal 4-iodo"''antipyrine contentreduced the cephalic fraction from 21.4 to 20.9 percent of the cardiac output. The mean total flowwas reduced to 667 from 709 ml per min. Thislower value agrees with the estimates made by thenitrous oxide method with extrapolation to infinitetime (25).

At present, the major experimentally unvali-dated assumption involves the empirical acceptanceof negligible loss or rapid destruction of the4-iodo131antipyrine during the initial pulmonarypassage. Since the pulmonary reservoir in gen-eral is small for most substances, only a modestloss of 4-iodo13lantipyrine probably occurs, evenif equilibration with lung tissue is complete.Sapirstein and Hanusek (6) have reported that,at least for the rat, there is a period during thelatter part of the first pulmonary 4-iodo13lantipy-rine passage that the residue represents 7 per centof the injected dose. He states, however, thatthis is rapidly washed out. Even a loss this greatwould only introduce a minimal error in the calcu-lation of total cephalic blood flow. This questionis difficult to answer experimentally in man, andhas not been effectively evaluated to date. Thesimilarity, however, of the results reported forthis study to those reported by an accepted methodof estimating human cerebral perfusion tends toconfirm the validity of the assumption.

The net redistribution of Rb86Cl is sufficientlyslow that organ uptake is approximately repre-sentative of functional regional perfusion. Thisappears to be valid for a considerably longer pe-riod than for 4-iodo131antipyrine, probably be-cause of the greater extravascular pool for Rb86.The brain is a significant exception, since there isvirtually no exchangeable cerebral Rb86 pool(< 0.5 per cent injected dose). The redistribu-tion of the cerebral Rb86 fraction causes an over-estimation of the noncerebral cephalic flow be-cause it is taken up out of proportion to the initialdistribution of the cardiac output during subse-quent recirculations. To correct for this redistri-bution, it must be assumed that the brain repre-sents the sole source of this redistribution fraction.This assumption cannot be wholly valid, sincethere are regions of nervous tissue, with blood

supply, not observed by the well. Probably neitherthe quantity of this tissue nor its rate of perfusionis sufficient for major concern. The correction forthe redistribution of Rb86 to the noncerebral cepha-lic structures represents approximately 10 per centof the total cephalic Rb86 uptake, and thereforewould only slightly alter the final calculations ifit were underestimated. The pulmonary indicatorlosses for K42 and Rb86 have been variously re-ported between 1.9 and 2.7 per cent of the in-jected dose in rats, dogs, and rabbits (6, 9, 14),and therefore represent negligible error.

The values calculated for cerebral blood flowduring rest, voluntary hyperventilation, and breath-ing 5 per cent CO2 in air agree closely with thosepreviously reported by use of the Fick principleand inert gas exchange (2, 22). Hyperventila-tion resulted in a significant decrease in totalperipheral vascular resistance. The cerebral ves-sels exhibit marked vasoconstriction, and totalcerebrovascular resistance doubled. Five per centCO, inhalation minimally increased cardiac outputand did not alter total calculated peripheral re-sistance. The cerebral resistance vessels dilated,however, as shown by a significant reduction oftotal cerebrovascular resistance.

This investigation describes a method for rapid,quantitative measurement of total cerebral bloodflow as the fraction of the cardiac output. Thetechnique requires the steady state only for theperiod of data generation. The measurementsmay be easily repeated. Equipment has been de-signed that may allow for the simultaneous deter-mination of all three parameters required to de-termine cerebral flow by this technique. A perma-nent record could then be produced every 2 min-utes and be reduced electronically to final form.

SUMMARY

The initial distribution of 4-iodo131antipyrine isused to estimate the total cephalic fraction-brain,head, and neck-of the cardiac output. Sincethere is no readily exchangeable cerebral Rb86reservoir, and since there is a large, rapidly ex-changeable, noncerebral cephalic pool, the stablecephalic content of Rb86Cl corrected for recircu-lation is used to estimate the noncerebral cephalicfraction-just head and neck-of the cardiac out-put. The difference between the total cephalic

2231


flow fraction and the noncerebral cephalic flowfraction represents the total cerebral blood flowfraction, which when multiplied by cardiac output,determines the total cerebral blood flow.

The effects of voluntary hyperventilation andbreathing 5 per cent CO2 in air upon cardiac out-put, cerebral blood flow, and cerebral vascular re-sistance were evaluated with this technique. Vol-untary hyperventilation increased cardiac output,reduced cerebral blood flow, and doubled cere-brovascular resistance. Five per cent CO2 mini-mally increased cardiac output, increased cerebralblood flow, and significantly reduced cerebrovas-cular resistance. These results agree with thosereported by the nitrous oxide method, but havethe advantage of estimating total cerebral bloodflow as a fraction of the cardiac output.

ACKNOWLEDGMENT

The authors wish to acknowledge the helpful coopera-tion of Dr. Alvin S. Hyde, Acceleration Section, Aero-space Medical Laboratory, Wright Patterson Air ForceBase, Ohio, in providing some of the equipment used inthis investigation.

REFERENCES

1. Steiner, S. H., Hsu, K., Oliner, L., and Behnke,R. H. Cerebral blood flow by external isotopecounting. Physiologist 1961, 4, 116.

2. Kety, S. S., and Schmidt, C. F. Nitrous oxide methodfor quantitative determination of cerebral bloodflow in man: theory, procedure, and normal values.J. clin. Invest. 1948, 27, 476.

3. Sapirstein, L. A., and Ogden, E. Theoretic limita-tions of the nitrous oxide method for the determina-tion of regional blood flow. Circulat. Res. 1956,4, 245.

4. Sapirstein, L. A. Fractionation of the cardiac outputof rats with isotopic potassium. Circulat. Res.1956, 4, 689.

5. Sapirstein, L. A. Regional blood flow by fractionaldistribution of indicators. Amer. J. Physiol. 1958,193, 161.

6. Sapirstein, L. A., and Hanusek, G. E. Cerebralblood flow in the rat. Amer. J. Physiol. 1958,193, 272.

7. Sapirstein, L. A. Determination of blood flow tohand with radio potassium. Fed. Proc. 1958, 17,141.

8. Sapirstein, L. A. Measurement of cephalic and cere-bral blood flow fractions of the cardiac output inman. J. clin. Invest. 1962, 41, 1429.

9. Walker, W. G., and Wilde, W. S. Kinetics of ra-diopotassium in the circulation. Amer. J. Physiol.1952, 170, 401.

10. Sapirstein, L. A., and Goodwin, R. S. Measurementof blood flow in the human hand with radioactivepotassium. J. appl. Physiol. 1958, 13, 81.

11. Hansen, D. B., quoted by S. S. Kety. Measurementof local blood flow by the exchange of an inert,diffusible substance. Meth. med. Res. 1960, 8, 228.

12. Reinmuth, 0. M., and Scheinberg, P. The use ofI.3. labeled iodoantipyrine in the determination ofrapid changes in cerebral blood flow (abstract).Neurol. 1961, 11, 271.

13. Gray's Anatomy of the Human Body. C. M. Goss,Ed. Philadelphia, Lea & Febiger, 1948.

14. Sheppard, C. W., Overman, R. R., Wilde, W. S., andSangren, W. C. Disappearance of K4' from thenon-uniformly mixed circulation pool in dogs.Circulat. Res. 1953, 1, 284.

15. Talso, P. J., Lahr, T. N., Spafford, N., Ferenzi, G.,and Jackson, H. R. 0. A comparison of the vol-ume of distribution of antipyrine, N-acetyl-4-amino-antipyrine and I"' 4-iodoantipyrine in human be-ings. J. Lab. clin. Med. 1955, 46, 619.

16. Hayes, R. L., and Brucer, M. Compartmentalizedphantoms for the standard man, adolescent andchild. Int. J. appl. Radiat. 1960, 9, 113.

17. Hamilton, W. F., Moore, J. W., Kinsman, J. M.,and Spurling, R. G. Simultaneous determinationof the pulmonary and systemic circulation timesin man and of a figure related to the cardiac output.Amer. J. Physiol. 1928, 84, 338.

18. Van Slyke, D. D., and Neill, J. M. Determination ofgases in blood and other solutions by vacuum ex-traction and manometric measurement. J. biol.Chem. 1924, 61, 523.

19. Rosenthal, T. B. The effect of temperature on pHof blood and plasma in vitro. J. biol. Chem. 1948,173, 25.

20. Van Slyke, D. D., and Sendroy, J., Jr. Line chartsfor graphic calculations by Henderson-Hasselbalchequation and for calculating plasma CO., contentfrom whole blood content. J. biol. Chem. 1928,79, 781.

21. Warren, J. V., and Gorlin, R. Calculation of vas-cular resistance. Meth. med. Res. 1958, 7, 98.

22. Kety, S. S., and Schmidt, C. F. The effects of al-tered tensions of carbon dioxide and oxygen oncerebral blood flow and cerebral oxygen consump-tion of normal young men. J. clin. Invest. 1948,27, 484.

23. Li, J. C. R. Introduction to Statistical Inference.Ann Arbor, Mich., Edwards Brothers, 1957.

24. Sapirstein, L. A. Distribution of 4-Iodoantipyrinein the rat. Fed. Proc. 1961, 20, 75.

25. Lassen, N. A., and Lane, M. H. Validity of internaljugular blood for study of cerebral blood flow andmetabolism. J. appl. Physiol. 1961, 16, 313.

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vol. 41, no. 12, 1962 themeasurement of cerebralblood...

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