doppler measurement of vortex vein blood flow in … doppler measurement of vortex vein blood flow...

Doppler measurement of vortex vein bloodflow in animals

William A. Schlegel and Carteret Lawrence

The blood flow teas measured continuously in the intact vortex vein of the anesthetized rabbitusing an ultrasonic Doppler -flowmeter toith a frequency response of approximately 4 Hz.Blood flow correlated well ivith perfusion pressure (arterial pressure minus intraocular pres-sure). The intraocidar pressure was changed stepwise by perfusion from a saline reservoir,Tonometry teas performed tuith the eye connected to the reservoir and with the valve to thereservoir closed. Blood fioto decreased with decreasing perfusion pressure in all cases.

Key words: ocular blood flow tests, vortex veins, intraocular pressure change,Schi0tz tonometry, ocular blood circulation, rabbits.

Studies on the ocular pressure-volumerelationship, homeostatic pressure mecha-nisms, tonometry, tonography, suction cuppressure decay curves, and retinal metabol-ism require an understanding of thechanges in ocular blood flow and volume.

Monnick,1 in 1870, suggested that theocular blood volume decreased duringtonometry. Since that time, many investi-gators have studied the problem of ocularblood volume and flow and made someattempts to quantitate these effects. Evenrecent studies2"7 have provided little or noinformation on rapid changes in bloodflow.

This series of experiments was an at-tempt to record the changes in outflow ofblood in an intact vortex vein associatedwith changes in intraocular pressure. Anultrasonic Doppler flowmeter was used to

From the Bishop Eye Research Center, PacificNorthwest Research Foundation, Seattle, Wash.

This investigation was supported by the NationalInstitutes of Health Grant No. NB 05098 andby the Bishop Foundation.

obtain flow recordings with approximately4 Hz. frequency response.

MethodsWhite New Zealand rabbits were anesthetized

topically with proparacaine hydrochloride and in-travenously with ethylcarbamate or sodium pento-barbital. The right femoral artery, the medianear artery, and the nose vein were cannulated.The vein cannula was directed towards the eye.The conjunctiva of the right eye was incisedsuperiorly and medially at the limbus, and thsupper lid and superior orbital margin were re-moved in most cases. The superior medial vortexvein was exposed and the ultrasonic transducerwas either placed over the point of emergence ofthe vortex vein, glued on with Eastman 910, orsutured in place. In some animals, the superiorrectus and superior oblique muscles were sec-tioned. The rabbit was heparinized with 2,000 to4,000 U.S.P. units intravenously. We cannulatedthe anterior chamber with a 21 gauge needleconnected to a supply reservoir of heparinizedsaline and used a separate 23 gauge needle formeasurement of the intraocular pressure. If thesurgical preparation on the right side was notperfect, the left side was used instead (two rab-bits).

The intraocular pressure, saline reservoir supplypressure, nose vein pressure, median ear arterypressure, and femoral artery pressure were re-corded continuously with 267BC Sanborn-Hewlett

201

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202 Schlegel and Lawrence Investigative OphthalmologyApril 1969

10 MHZ

OSCILLATOR

10 MHZCRYSTALDRIVER

AUDIO

AMP

10 MHZ

TRF RECEIVER

IFREO

METER-

1PAPER

RECORDER

IANALOG

TAPERECORDER

Fig. 1. Block diagram of Doppler flowmeter anddata system.

Packard transducers and 350-1,100 carrier pre-amplifiers. The flowmeter signal was obtainedfrom a 10 mHz. Parks Electronic TranscutaneousDoppler Flowmeter* with the use of a speciallyconstructed flow transducer applied to the supe-rior nasal vortex vein (see Fig. 1). The outputfrom this flowmeter was recorded through a modi-fied Hewlett Packard frequency meter amplifier,350-2,800. These six signals were recorded simul-taneously on the 350-7,700 Sanborn direct-writingrecording system and on a seven-track 3,955BHewlett Packard magnetic tape system. One ofthe following procedures was followed:

I. Stepwise changes in intraocular pressure(ten rabbits). The saline reservoir connected tothe anterior chamber was set at 20 mm. Hg, heldfor two to four minutes, and then raised in 10mm. Hg steps, each held for the same length oftime. When 90 mm. Hg was reached, the intra-ocular pressure was lowered in similar steps tozero mm. Hg. Then the intraocular pressure wasraised stepwise to 90 mm. Hg and returned tozero again, one to three times. In three rabbits,the pressure steps were not sequential but wererandomized (Fig. 2).

The voltages representing vortex vein flow, in-traocular pressure, and median ear arterial pres-sure from the magnetic tape system were passedthrough identical filters to remove arterial pulsa-tions. The intraocular signal was subtracted elec-tronically from the arterial signal, producing avoltage proportional to perfusion pressure,2 whichwe applied to the X-axis of the X-Y recorder.The flow voltage was applied to the Y-axis (Fig.3).

"Parks Electronic Labs., 419 S.W. 1st, Beaverton, Ore.97005.

2. Tonometry. In two of the preceding rabbits,tonometry was performed after the stepwisechanges in intraocular pressure, either with theeye at 20 mm. Hg not open to saline reservoir or

~ with the eye open to the saline reservoir at 20mm. Hg.

3. Calibration. In a different group of five rab-bits, before the application of the ultrasonictransducer, the vortex vein was cannulated ap-proximately 10 mm. from the eye with drawn-outPE-10 tubing. The cannula tip rested approxi-mately 5 mm. from the point of emergence ofthe vortex vein. This tubing was attached to ahorizontal 1 ml. pipette 5 cm. above the eye. Thetime required for a flow of 100 /iL was deter-mined for intraocular pressures of 20 and 40mm. Hg with simultaneous recording of the Dop-pler flow signal.

Results

1. Stepwise changes in intraocular pres-sure. Stepwise changes in intraocular pres-sure produced rapid changes in vortex veinflow; changes in flow were also producedby spontaneous changes in arterial pres-sure (Fig. 2). A plot of derived perfusionpressure and flow is shown in Fig. 3. Theheavy traces represent retracing of theflow-perfusion pressure curve for eachfixed intraocular pressure.

2. Tonometry. The changes in pressureand flow produced by Schi0tz indentationtonometry with open and closed manom-etry are shown in Fig. 4. The 25 cyclesper minute wave shown to some degreein all traces represents the respiratory cy-cle.

3. Calibration. Our preliminary attemptsat calibration resulted in considerablescatter, but the average flow was 325 /xhper minute for the single vein, which cor-responds to 1.3 ml. per minute for all vor-tex veins. The measured frequency shiftwas used to calculate the average velocity,approximately 50 mm. per second.

Discussion

The shift in frequency which occurswhen ultrasound is scattered or reflectedfrom moving, formed elements in the blood-stream is an example of the "Doppler" ef-fect. The change in frequency which oc-curs is proportional to the velocity of the


Volume 8Number 2

Doppler measurement of blood flow 203

NOSE VEIN PRESSURE

TIME

RELATIVE

INTRAOCULAR

FLOW

PRESS

1.0 —

0 —

80 —

40 —

0 —

mmHg

200FEMORAL ART. PRESS. | 0 0

0

10 min.

Fig. 2. Simultaneous record of nose vein pressure, vortex vein flow, intraocular pressure, andarterial pressure demonstrating typical response to changes in intraocular pressure and arterialpressure. Note the transient spontaneous decreases in femoral arterial blood pressure andtheir association with simultaneous decreases in relative blood flow (except when alreadyzero).

formed elements in the blood, The signalwhich is returned is a spectrum of fre-quencies because the blood flow in a vesselconsists of lamellae moving with differentvelocities, faster near the axis than thewall. The method for obtaining a voltageproportional to the mean velocity of themoving elements in the blood was givenby Franklin and co-workers.8

The Doppler output voltage is propor-tional to average velocity. Actual volumeflow will depend on the cross-sectional areaof the vessel being measured and the ve-locity profile across the vessel, In theseexperiments, the vortex vein diameterswere not measured, but no gross changeswere observed. In an attempt to determinewhether the placement of the transducerwas altering the cross-sectional area of thevortex vein, the transducer was fixed tothe sclera in three different ways: laid onthe surface, glued in place, and fixed withsutures. No differences were observed inthe results obtained with these three meth-ods of transducer application. The nosevein was used as an estimate of the dis-

tending pressure in the vortex vein. Thechanges recorded in the nose vein did notexceed 2 mm. Hg during a run, and thedistension of the vortex vein probably didnot change significantly. The finding thatthe flow signal was related to perfusionpressure with an extremely consistent curvein all our experiments, except at intraocu-lar pressures lower than 20 mm. Hg, is anindication that our flow signal may rep-resent volume flow.

When the intraocular pressure was re-duced below the normal pressure in therabbit, there was great variability in flowsignal. This was probably due to two fac-tors: (1) When the eye pressure was re-duced below its normal value, the venouspressure probably should have been sub-stituted for the intraocular pressure in theperfusion pressure equation: Perfusionpressure equals arterial pressure minus in-traocular pressure, as defined in Fig. 3.(2) At this pressure level, the sclera is nolonger under normal tension and there areprobably changes in the exit resistance ofthe vein.


204 Schlegel and Lawrence In ije OphthalmologyApril. 1969

ANALOG

TAPE

RECORDER

(R ART.-R I.O.P)- P. PERF

LO. !• t I T AT * 60 40 20 0

RELFLOW

Fig. 3. The signals were taken from the tape system through the analogue manifold to anX-Y recorder. Blood flow was plotted continuously against perhision pressure and the excel-lent correlation shown above was consistently obtained.

NOSE VEIN PRESSURE

RELATIVE FLOW

INTRAOCULAR PRESSURE

FEMORAL ART. PRESSURE

RESERVOIR

CONNECTED

8 —

4—v-u^0 —

1.0 —

0 —

80 —

40

0 —

200 —

100 —i

0

RESERVOIR NOT

CONNECTED

-v—"~~^- mmHg

mmHg

nwnHg

5.5gmt i lOgmt

TONOMETER WEIGHT ON EYE

PAPER SPEED 50 mm/minl/mir

Fig. 4. The effect of repeated applications of a Schi0tz tonometer on intraocular blood flowin the rabbit eye during open and closed manometiy.


Volume 8Number 2

Doppler measurement of blood floio 205

Although the scatter was large in ourrecords of intraocular pressure below 20mm. Hg, the changes in flow appeared tobecome smaller for a given change inpressure.

Bill2 indicated that ocular blood flowapproaches zero at an intraocular pressurelower than the average femoral arterialpressure. Our results indicate that zero flowoccurs close to or below the measured dia-stole pressure of the median ear artery.

In this preliminary series of experiments(Fig. 4), we did not achieve completelyopen manometry and there were somechanges in intraocular pressure upon appli-cation of the Schi0tz tonometer. However,the results would appear to indicate thatthe ocular blood flow is primarily de-pendent on the intraocular pressure duringtonometry. As the tonometer is removed,vortex vein flow decreases transiently,which probably indicates a filling of intra-ocular vascular channels. These results areconsistent with Ytteborg's9 work. A transi-ent increase in blood outflow was observedalmost invariably each time the intraocularpressure was elevated by the reservoir andwas seen about one time in three whenthe tonometer was applied to the eye.

REFERENCES1. Monnick, A. J. W.: Ein neuer Tonometer und

sein Gebrauch, Von Graefes Arch. Ophth. 16:49, 1870.

2. Bill, A.: Intraocular pressure and blood flowthrough the uvea, Arch. Ophth. 67: 336,1962.

3. Bill, A.: The drainage of blood from the uveaand the elimination of aqueous humour inrabbits, Exper. Eye Res. 1: 200, 1962.

4. Pilkerton, R., Bulle, P. H., and O'Rourke, J.:Uveal blood flow determined by the nitrousoxide method, INVEST. OPHTH. 3: 227, 1964.

5. Friedman, E., Kopald, H. H., and Smith, T.R.: Retinal and choroidal blood flow deter-mined with krypton-85 anesthetized animals,INVEST. OPHTH. 3: 539, 1964.

6. Friedman, E., and Smith, T. R.: Estimationof retinal blood flow in animals, INVEST.OPHTH. 4: 1122, 1965.

7. Trokel, S.: Quantitative studies of choroidalblood flow by reflective densitometry, INVEST,OPHTH. 4: 1129, 1965.

8. Franklin, D. L., Schlegel, W. A., and Rush-mer, R. F.: Blood flow measured by Dopplerfrequency shift of back-scattered ultrasound,Science 134: 564, 1961.

9. Ytteborg, J.: The role of intraocular bloodvolume in rigidity measurements on humaneyes, Acta ophth. 38: 410, 1960.


doppler measurement of vortex vein blood flow in … doppler measurement of vortex vein blood flow...

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