noninvasive ultrasonic estimation of brachial artery blood flow

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  • NONINVASIVE ULTRASONIC ESTIMATION OF BRACHIAL ARTERY BLOOD FLOW

    by Paul A. Thomas, Jr., MD

    ABSTRACT Ultrasonic instrumentation based on the Doppler phenomenon fulfflls the requirement that a blood flowmeter should sense a variable which is a function of the volume of blood moved through a blood vessel in situ. The blood flow velocity waveform recorded nonin- vasively from the brachial artery in the antecubital fossa was used as basis for quantita- tively estimating a stroke flow index. The method used in these experiments was repro- ducible in 31 volunteer subjects studied; the first and second brachial artery stroke flow indices determined on different days had a correlation coefficient of 0.909. The cal- culated minute flow index ranged from 32 t o 95 ml in these subjects. A clinical applica- tion was explored by making serial measurements before and after operation in 64 pati- ents submitting to 68 open heart operations. A significant reduction in the brachial artery blood flow velocity was recorded postoperatively in 21 of these patients. The bra- chial artery stroke flow index has potential value as an objective measure of cardiovas- cular instability.

    Indexing Words Ultrasonic Blood Flowmeter Doppler Velocimeter Brachial Artery Blood Flow

    Brachial Artery Stroke Flow

    An ideal blood flowmeter should be capable of sensing a variable which is a predictable func- tion of the volume of blood moved per unit of time through the blood vessel in situ. Ultrasonic instrumentation based on the Doppler principle fulfills this requirement. In 1961 Franklin et a1 (1) described a method of measuring blood flow by determining the sound wave frequency change from incident to reflected sound waves based on the assumption that a linear relation- ship exists between the observed frequency change and the velocity of moving elements in the stream of blood. Subsequently, Vatner et a1 (2) experimentally compared and calibrated an ultrasonic blood flowmeter with an electro- magnetic blood flowmeter and demonstrated a linear response to the arterial flow ranges studied using both instruments. The flow probes for both instruments were chronically implanted around the arteries of their experimental animals. They concluded that accurate measurement of

    From the Division of Research, The Lankenau Hospital,

    Received December 17 , 1975; revision accepted August 25.

    For reprints contact: VA West Side Hospital. 8 2 0 South

    Philadelphia, Pennsylvania 191 51 .

    1976.

    Damen Avenue, Chicago, Illinois 6 0 6 1 2 .

    regional blood flow was possible with an ultra- sonic flowmeter. Benchimol et a1 ( 3 ) simultane- ously recorded identical ultrasonic brachial artery flow velocity waveforms from both implanted and transcutaneously placed probes in a patient studied for aortic valve stenosis. Their study confirmed the potential for nonin- vasive estimation of blood flow. Ram and Bhimani ( 4 ) calibrated an ultrasonic flow probe in uitro and made transcutaneous clinical mea- surements over paired vessels of the extremities. They arrived at quantitative estimates of blood flow by evaluating the flow velocity waveforms and calculating a ratio for paired vessels. They concluded that the velocity signal of sound frequency shift was directly proportional to the volume of blood flow, provided the size of the vessel remained constant. Flax et al (5) described design and testing specifications for Doppler ultrasonic blood flowmeters to assure reliability. Reagan et a1 (6) calculated the mean velocity of femoral artery blood flow from the recorded ultrasonogram and estimated the artery cross-sectional lumen area by ultrasonic echo ranging to determine volume flow. Al- though their calculated femoral artery volume

    7 2 JOURNAL O F CLINICAL ULTRASOUND

  • FIGURE 1. The recorded peak deflection i s measured f rom the f l ow velocity tracing and the peak velocity IS calculated as a rat io o f the standard signal deflection (Equation 1) The average area beneath the prlmary waveform deflect ion IS measured planimetrically and the area beneath the callbratlon signal, pro- jected in t ime for the period o f the primary waveform deflection (shaded area), i s determined F rom the rat io o f these areas, a brachial artery stroke f l ow index i s calculated (Equation 3)

    flows were comparable to those determined by other investigators using indicator dye dilu- tion methods, they were cognizant of the probe placement difficulties inherent in the transcutan- eous method. We modified their method for clinical application. In this report we describe a noninvasive method of recording the brachial artery flow velocity waveform and the deriva- tion of a quantitative estimate of the stroke flow volume. Application of the method is illustrated by measurements made serially in cardiac surgical patients.

    MATERIAL AND METHODS

    Instrumentation: Brachial artery flow velocity measurements were made using a nondirectional, multipurpose, continuous wave 5 MHz Doppler instrument.* This instrument is sensitive to changes in blood flow velocity but not the direc- tion of blood flow. Velocity frequency shifts produced by vessel wall motion are reported to be less than 100 Hz ( 5 ) . Since these fre- quencies are below the range of linear response for the instrument used, they were filtered out electronically and presumably did not contri- bute to the shifted frequency signals recorded as blood flow velocity. The frequency shift from incident to reflected sound was detected by the transducer housing transmitting and receiving. piezoelectric crystals. The frequency shift signal * Smith Kline Instruments

    was passed through a zero-crossing circuit t o a calibrated recorder.* this ultrasonic instrument has characteristics consistent with the specifica- tions described by Flax et a1 (5 ) which overcome objections expressed regarding reliability and ac- curacy of ultrasonic blood flowmeters. The flow velocity waveform recorded from the nondirec- tional instrument was qualitatively compared with the waveform recorded from a directional ultrasonic flowmeter.**

    Calculations: The recorded flow velocity waveform produced by the passage of a bolus of blood through the incident sound beam directed into the brachial artery was used t o estimate a mean peak velocity and a stroke flow index (Fig. 1). A calibration signal of 100 mv representing a piezoelectric crystal excitation of 1000 cycles/ sec equivalent to 15.4 cm/sec velocity was re- corded with each brachial artery flow velocity determination. For convenience the recorder was adjusted t o provide a needle deflection of 20 mm from the baseline in respolise to the calibration signal. The mean peak velocity was computed from the ratio:

    15.4cm/sec x MPRmm 1. MPV = 20mm

    where MPV = mean peak velocity 15.4cmlsec = velocity of calibration signal

    MPR = amplitude of mean peak recorded

    20mm = amplitude of recorded calibration signal

    signal

    The mean peak velocity recorded was estimated by inspection of sequential flow velocity wave- forms displayed on the brachial artery flow velocity tracing. The formula derived for the cal- culation of a quantitative brachial artery stroke flow index was based on the equation relating the Doppler shift in sound wave frequency to the velocity of a moving object:

    2. v = where V =

    A f =

    ft = a = c =

    A f . C 2 f t c o s a

    velocity of the object frequency shift, transmitted t o reflected sound frequency, transmitted sound angle, acoustical axis with blood stream velocity of sound in tissue

    However, the moving object, a bolus of blood, accelerates and decelerates with each propagated

    *Hewlett Packard Viso-Cardiette Model 500 using direct current input mode.

    Delalande Electronique. **Directional Ultrasonic Flowmeter, continuous wave 4MHz.

    VOLUME 5. NUMBER 2 73

  • pulse wave in an artery; therefore, it was neces- sary to determine the mean velocity. This was done by measuring the areas under three to five consecutive velocity waveforms using a planim- eter and taking the average area to estimate the mean frequency shift. Since both the velocity of sound in tissue, 1.54 x lo5 cm/sec, and the fre- quency of the incident sound for the instrument used, 5 x lo6 cycleslsec, are constant, the mean velocity is proportional to the average area be- neath the recorded waveforms as a function of the calibration signal (Fig. 1). The brachial artery stroke flow index (SQ) was calculated as the product of the mean blood flow velocity of each bolus of blood passing through the incident sound beam and the vessel cross-sectional lumen area (A):

    3. SQ = VA average area beneath

    - recorded waveforms X 15.4cm/sec X A calibration signal area for time cosa average waveform

    The angle between the incident sound beam and the brachial artery was not determined precisely in these experiments. For noninvasive clinical measurements, the absolute anatomical course of the brachial artery in the antecubital fossa of a particular subject was not known. The ultrasound transducer was placed over the artery and was adjusted to the point at which the loud- est, highest pitched frequency shift was audible over the loudspeaker of the instrument. Trial- and-error positioning of the transducer probed directing the incident sound beam into the brachial artery consistently resulted in an angle of less than 45" with the skin surface. In so doing, the value of cos a approached 1.

    The brachial artery cross-sectional lumen area was determined by direct measurement in 95 patients who required arteriotomy and retrograde catheterization during the period of this investigation. This group included some, but not all, subjects in the brachial artery flow veloc

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