- Email: [email protected]
Transcutaneous determination of aortic blood-flow velocities in man
Transcutaneous determination vehities in man
of aortic blood-flow
Lee L. Huntsman, Ph.D. Emmeram Gams, M.D. Curtis C. Johnson, Ph.D. Eugene Fairbanks, M.S. Seattle, Wash.
While physiologic studies have indicated that infrrsrglation about aortic blood-flow dynamics may be of considerable usefulness in assessment of hmrt performance, the lack of a widely useful tacshnique has impaired clinical evaluation of this pclrtffntia1.1~2At the present time, such informaw is available by angiographic methods whereby left ventricular dimensions are determined, &amber volume estimated, and outflow rate ed as the rate of change of volume.3” Altermethods which provide direct measurement daortic flow events by means of catheter-tip flow or velocity sensors are under active developnBant.6s6 However, both of these techniques require arterial catheterization, and this repre&&nts a major restriction on their usefulness. T&us, there remains an important need for nonillllvasive methods of aortic flow or velocity deteration which are applicable to situations re the more expensive, troublesome, and rous invasive techniques are not suitable. situations include, for example: continuous toring of the postoperative or other high* patient; screening of large numbers of pea; and measurements in the presence of cardiovascular perturbations such as exercise. As noted by other investigators, there is an intr@ing possibility of determining aortic blood&w velocities transcutaneously by ultrasonic hrn the Center for Bioengineering, University of Washington, Seatth. ‘&it work wan supported by United States Public Health Service Omnts HL 07293 and GM16436, and Deutache Forschungagemiaschaft. Rsmived for publication June 14, 1974. hprint requests: Dr. Lee L. Huntsman, Center for Bioengineering, &&ere.ity of Washington, Seattle, Wash. 98195.
h&v, 1975, Vol. 89, No. 5, pp. 605-612
Doppler techniques. Potentially, an ultrasonic beam originating from a transducer in the suprasternal notch could be directed toward either the ascending aorta or the aortic arch providing quantitative blood velocity estimates for each of these locations. While the assessment of volume blood flow would require the determination of additional parameters, notably aortic diameter, just the determination of velocities could be of significance clinically. One instrument which allows such noninvasive measurements, the pulsed ultrasonic Doppler flowmeter, has undergone extensive development.‘Recent clinical trials with this device have clearly demonstrated that flow velocity information can be reliably obtained transcutaneously and used to advantage in cardiac diagnosis.8 While offering considerable measurement capability and versatility, the pulsed Doppler instrument is a complex and expensive device which requires skill and training for proper utilization. On the other hand, it has been suggested that the anatomy and physiology of blood flow in the thorax may be uniquely suited to a simplified approach using continuous wave ultrasound.g-12 Thus, it may be possible to develop a device expressly for the purpose of measuring aortic bloodflow velocities transcutaneously which is leas versatile than the pulsed instrument but much less expensive and easier to use. The present research has been undertaken to evaluate the feasibility of such an approach. ApprtXW2h
The ultrasonic Doppler technique for determining blood-flow velocities has been extensively evaluated.13r14 Sound waves emanating continu-
American
Heart Journal
606
Huntsman
et al.
Fii. 1. In principle, ultrasonic energy radiated from a transducer placed in the supraetemal notch can be directed toward the back of the aortic arch so as to intersect the aortic axis with a shallow angle.
ously from a transmitting piexo-electric crystal are partially reflected by the moving erythrocytes resulting in a Doppler frequency shift, fD, which is determined by the equation: fD =
2 f,V cos. 8 C
where fo = frequency of transmitted wave; C = velocity of sound in tissue (approximately 1,570 M. per second); 13= angle between the transducer axis and the blood flow velocity vector; and V = velocity of the erythrocytes which are responsible for scattering part of the ultrasonic energy back to the receiving crystal. The reflected wave is detected by a receiving crystal and the resulting signal processed electronically to obtain fo The difficulty with applying this simple approach to blood-velocity measurement in the thorax, as illustrated in Fig. 1, is that there are potentially many different portions of vasculature illuminated by the ultrasonic beam, Therefore, the Doppler signal will be a composite of information about erythrocytes moving with many different velocities at a variety of angles. There are, however, two premises regarding illumination of the aorta from the suprasternal notch which suggest that it may be possible to uniquely
606
identify and quantitate blood velocities in the aortic arch. The first premise is that a sound beam originating from the suprasternal notch can be directed so as to be nearly parallel to the central axis of the aorta somewhere near the top of the aortic arch. Thus, the angle between the sound beam and the velocity vector of the blood in this section of the aorta will approach zero and the cosine of this angle will approach unity. The second premise is that the velocities in the aortic arch are sufficiently high that the V cos Q product for sound waves reflected from this region is larger than for any other portion of the thoracic vasculature -at least during systole. Thus, the blood flowing in the arch will be responsible for the highest Doppler shift frequencies. There are some important implications of these premises, if they can be shown to be true. First, since the ultrasonic Doppler technique yields quantitative information about the velocity of the blood in the direction of the beam, the presence of a shallow angle between the velocity and the sound beam means that the Doppler data from this region of the aorta can be interpreted quantitatively without any unknown scale factors. Second, that portion of the vasculature which has the highest apparent velocity in the direction of the beam will be responsible for the highest Doppler shift frequencies in the return signal. Therefore, if the maximum Doppler shift frequencies can be separated from the rest of the Doppler signal, they will yield a quantitative measure of the blood velocity in a specific section of the aorta. There is the additional possibility that useful velocity signals may be available from the ascending aorta. However, anatomic variations in the orientation of the aortic root indicate that a near tangential approach of the sound beam to the flow velocity may not be assumed. In fact, the angle of incidence may actually change during a cardiac cycle. If, on the other hand, the aortic velocities are high enough and the angle of incidence shallow enough that the maximum Doppler shift frequencies are associated with the information of interest, then the data will be of considerable usefulness though not quantifiable in absolute terms. Thus, for a subject experiencing cardiovascular changes, alterations of ascending aortic velocity characteristics could be
May, 1975, Vol. 89, No. 8
Transcutaneous
_____ +-A Nl~,l
I/--
RECEIVER
DEMOWLAlW
determination
----------
of aortic blood flow
DOPPLER SIGNAL 1
FREQUENCY
-TRANSLATION 0
-
FREOUENCY
-
ANALYZER
ENVELOPE DETECT!ON
------O
ENVELOPE o”TPUT
I>
Fig. 2. Block diagram of the principal components of the continuous-wave measure aortic blood flow velocities. See text for further explanation.
eswRsed as a fraction of their control values. In addition to the great potential usefulness of taneous measurement of aortic blood flow ies, it should be noted that there are some ially important limitations of the techFor example, the technique inherently res velocity, not volume flow, measures peak velocity which has an unknown relationship to average velocity because the velocity profile is unknown, and, for the arch, measures v&city in a section of the aorta which carries an unknown fraction of cardiac output. Furthermore, the need to identify the maximum frequency components of the Doppler signal on a real-time basis poses some difficult instrumentation problems, the solution of which may impose restrictions on the technique. The severity of such limitations can be judged, however, only after the validity of the general approach has been demonstrated. Methods
All of the results reported in this study have been obtained with a continuous-wave ultrasonic Doppler instrument developed expressly for transcutaneous aortic blood flow velocity measurements. The organization and principle components of the device are shown schematically in Fig. 2. An oscillator generates a continuous wave at a frequency of 2.5 MHz. This signal is amplified and used to drive the transmitting piezo-electric crystal which radiates sound waves into the tissue at maximum power densities of less than 25 milliwatts per square centimeter. The much weaker reflected wave is amplified and demodulated to yield the Doppler signal. At the 2.5 MHz sound wave frequency, assuming an angle of incidence close to zero, each 1 KHz of Doppler signal frequency corresponds to approximately 30 cm. per second velocity of the erythrocytes. Therefore, for the velocities found in the cardiovascular system, the Doppler frequencies
Anrerican Heart Journal
ultrasonic Doppler instrument
used to
are well within the audio range and can be listened to directly. The Doppler signal is then conditioned, translated upward in frequency, and processed by a frequency analyzer circuit. The maximum excursions of this circuit are indicated by an envelope detector yielding a time trace of the maximum Doppler shift frequencies. Although ultrasonic transducers can be fabricated in a wide variety of configurat ing on the particular application, the presented here were obtained with a tra consisting of two piezo-electric crystals, semicircular disk, mounted edge-totwo-degree angle between them. Ea backed by a thin layer of air in ord the efficiency of forward The instrumentation t continuous wave ultrasonic up to the point and established our device.‘* Fo our approach is new, differing that of others. It will be described more completely elsewhere. Because the processing technique is new, however, the Doppler signal which results from demodulation has been taken a8 the standard data, not the envelope signal. Thus, for all experiments, the Doppler signal was recorded on magnetic tape for subsequent proceasing by a spectrum analyzer (Kay Sonograph) if the output from the frequency analysis circuitry proved to be open to question. For each different class of experiment, such off-line spectrum analysis was performed and the results compared to the tracings obtained from the instrument to verify that the maximum frequencies were adequately indicated. Results The geometric beam originating
relationship between a sound in the suprasternal notch and
607
Huntsman
et al.
(Antem-
Posterior 1
VIEW I (Loteml)
VIEW 4
VIEW 5
\
Geometric construction technique by which information contained in the antero-posterior and lateral x-ray films was manipulated to obtain 1, the distance from transducer to aortic axis intersection, and 0, the angle of intersection. The transducer, placed in the suprasternal notch, appeared in the x-ray films, thereby defining the position of one end of the line. Fig. 3.
Table I. Data for seven individuals
for whom the minimum angle (0) intersection of sound beam and aortic axis was determined by x-ray techniques. 1 is the distance from the suprasternal notch to the point of minimum angle intersection Patient
0
1.
6.5” 8.0” 0.0”
2. 3. 4. 5. 6. I.
Mean value Standard deviation
11.0” 4.0” 9.5” 5.0” 6.3” 3.7”
1 fem.) 7.5 7.1 8.9 8.7 8.4 6.6 9.8 8.2
1.1
the aortic arch has been investigated in seven subjects with apparently normal anatomy and cardiovascular function. With the transducer probe in place, simultaneous radiographs of the anterior-posterior view and the lateral view were taken. As shown in Fig. 3, these orthographic views allow construction of a triangle identifying the plane of the aortic arch. The transducer which also appeared in the x-ray film, provided one end-point on a line between the suprasternal notch and the point where the ultrasonic beam is
608
Fig. 4. Maximum frequency envelopes taken from the spectrum analyzer recorda for several examples of blood velocity in the arch measured by pulsed Doppler (solid lines) and continuous-wave Doppler (dotted lines).
most nearly tangent with the aortic axis. For simplicity, the second point was defined as that point in the lateral view film where a line through the suprasternal notch was tangent with the aortic center line. Although not identical with the point which minimizes the angle be tween the ultrasonic beam and the aortic ax&, this lateral view tangent point does provide a close approximation and the true minimum angle will be no greater than the angle measured by this technique. The data for all seven individuals is summarized in Table I. It is noteworthy that even with the tendency of the technique to overestimate the minimum angle, the angles as&! quite small and the assumption that cosine 8 equals unity introduces an error of less than 2 per cent. To evaluate the assumption that the greatest Doppler shifts are due to blood flowing in that portion of the aortic arch where the sound beam makes a nearly tangent approach to the velocity vector, a comparison of continuous wave and pulsed ultrasonic Doppler techniques was carried out. The pulsed Doppler instrument uses a narrow, well-defined beam and range-gating to define a small sample volume from which Doppler information is obtained.‘To carry out the comparison, the pulsed Doppler beam was first directed toward the aortic arch and beam direction and range were varied to the location where the maximum apparent velocities in the aortic arch were obtained. Doppler signals from this region of the arch were then recorded. Next, the continuous (CW) device with its broader beam and lack of range-gating was applied to the same individual with approximately the same beam direction and recordings made of the Doppler signals obtained. In both cases, the Doppler signals were processed with a Kay Sonograph spectrum analyzer and tracings were made of the maxMay, 1975, Vol. 89, No. 6
Trunscutuneous
determination
ofaortic
blood fEoLL
line envelope detector (solid line).
m Doppler shift frequencies as a function of ample tracings from one of the individuals ich this comparison was carried out are in Fig. 4. The close similarity between the pulsed and the CW data was consistently found and the differences appear to be within the range of normally occurring short-term variations and experimental error. This result indicates that the mjrrimum frequencies in the CW Doppler spectrum may be validly attributed to that portion of the aortic arch where the ultrasonic beam is nearly tangent to the blood-velocity vector. An initial evaluation of transcutaneous aortic ty measurements in humans was carried attempting arch and ascending aorta ments in 20 apparently normal subjects. were obtained with the subjects resting n the supine position. Acceptable records ained from the arch for all but two of the and from the ascending aorta for all but ect. The spectrum analyzer and envelope reeMs for a single cardiac cycle in one subject are shown superimposed in Fig. 5. The ability of the instrument to provide an indication of the cy envelope is clearly demonstrated. In the velocity obtained from the envelope &&&or was in the range of 90 to 100 per cent of tin value obtained from off-line analysis, and any mrepancy was consistent for a given subject. ‘pfpical tracings obtained for the ascending aorta the arch from one subject are illustrated in 6. The presence of a lower peak velocity in t&e ascending aorta than in the arch was obd in most subjects (never was the arch > suggesting that the angle between the sound beam and the velocity is often significantly rent from zero for the ascending aorta case. Preliminary comparison of the Doppler velocity signal with volume flow rate was carried out by
American
Heart Journal
Fig. 6. Sample blood flow velocity traces obtained from the ascending aorta, A, and the aortic arch, B, of one individual.
making transcutaneous velocity determinations on two baboons which had electromagnetic flow sensors chronically implanted on the root of the aorta. Several weeks after surgery, the animals were lightly sedated with ketamine, 5 mg. per kilogram intramuscularly, and confined in a sitting position with a special chair which allowed the measurements to be made by hand-holding the ultrasonic transducer in the suprasternal notch. Apparently because lung or some other tissue which greatly attenuates sound waves intervened, useful velocity signals were not available from the arch. Adequate signals were obtained from the ascending aorta, however, and these were recorded, along with the signal from the electromagnetic flowmeter, for later comparison. Because the angle of the sound beam relative to the flow velocity is not known and
609
Huntsman et al.
Fig. 7. Time course of the response of the ascending aorta volume blood flow (EMF) and velocity (CW Doppler) in the baboon to norepinephrine infusion. The JSMF signal was uncalibrated since only relative changes were of interest.
Table II. Ratio
of changes of Doppler velocity characteristics to corresponding electromagnetic flow meter changes following inotropic interventions. Each is expressed as per cent of control
7 Isoproterenol (4 mg./min., intravenously) n=12 Metho-Hexital (5 mg./Kg. intravenously) n= 3
1.04f0.03
0.84f0.10
1.37 zkO.06
1.01
1.00
1.08
and flow caused by these drugs, expressed as a percentage of control valves, are summarized in Table II. The time course of one animal’s response to isoproterenol is illustrated in Fig. 7. The deviations from proportionality for changea of acceleration and stroke volume occur primarily because of the lack of a clearly defined envelope during the low velocity periods, though it may also be true that the velocity pro& changes, becoming more blunt, under conditioag of increased inotropism. Minor modifications of the instrumentation subsequent to these experiments have indicated that the correlation of isoproterenol-induced changes of flow and velocity may be substantially better than that indicated by these preliminary results. Disamsion
probably not zero (the baboon has a very short ascending aorta), quantitative comparisons of volume flow-rate and maximum instantaneous velocity were not possible. Rather, the time course of the flow and velocity traces were compared qualitatively and the response of each signal to positive and negative inotropic interventions was evaluated. These interventions consisted of isoproterenol infusions (4 mg. per minute intravenously) administered twelve times in two animals and methohexitol given three times in the two animals. The changes of velocity 610
The results of these experiments indicate that it is possible to measure aortic blood flow velocities and accelerations in man by transcutaneous ultrasonic Doppler methods. The basic premises underlying the technique have been validated by the x-ray data demonstrating a shallow angle of incidence of sound beam and aortic axis and by the continuous wave-pulsed Doppler comparison demonstrating that aortic flow causes the high& Doppler shift frequencies. While it is important to remember that ultrasonic access to the aortic arch is required in addition to suitable geometry, the successful use of the technique in 18 of a0
May, 1975, Vol. 89, No. 5
Transcutaneous
normal subjects suggests that this may not be a major difficulty. These findings indicate that it should be possible to develop an instrument to quantitatively measure aortic blood flow velocities in man which is acceptable for clinical use. Furthermore, it appears that such an instrument may well be relatively simple, inexpensive, and easy to use. The prototype device used in this study meets most of these objectives. However, even greater ease of use could be obtained with a more diverging sound beam which would allow detection of arch velocities without precise aiming of the transducer. The feasibility of this modification has been demonstrated by the trial use of a beam with a 28” divergence. There are, however, two difficulties which have hindered this change. First, the diverging beams, with congruent transmitting and receiving patterns from the two crystals, are very difficult to fabricate in a compact form. Second, the diverging beam places stringent requirements on the electronics of the instrument. Since the sound energy is transmitted over a wide angle and received from a wide angle, the energy received from the region of interest is small. A very good signal-to-noise ratio is therefore required in order to extract acceptable velocity information. While it is important to be clear about the fact that the transcutaneous ultrasonic technique gives information about blood velocity, not volume flow rate, the clinical usefulness of this method is not likely to rest on the establishment of a known quantitative relationship of the measured velocity to volume flow rate. For many potential uses of this technique, the magnitude of such flow characteristics as peak flow, acceleration, and stroke volume is of less importance than information about their rate and direction of change from some control state. If the Doppler velocity data can be shown to vary in the same way as corresponding flow variables, determination of relative changes will be possible without auxiliary measurements. The data comparing maximum velocity and volume flow in the ascending aorta of the baboon suggest that it may be possible to demonstrate such a correlation. Based on these highly encouraging preliminary results, a systematic evaluation of the technique is being undertaken. First, the transcutaneous accessibility of aortic velocity information together with the convenience and reliability of
American Heart Journal
determination
of aortic blood flow
the method will be determined for wide variations of subject anatomy and disease state. Second, the degree to which Doppler velocity characteristics correlate with corresponding flow characteristics will be evaluated by performing comparisons with measurements made by standard techniques such as angiography, cardiac output determinations, and possibly catheter-tip flow meters. Finally, the clinical usefulness of the information provided by the technique will be weighed against the effort and expense involved in making the measurement. If this proves successful, the availability of a convenient method for obtaining aortic velocity information transcutaneously could be of profound importance in several areas of clinical cardiology. Perhaps the most immediate application would be to patient monitoring where the detection of short-term changes is of primary importance. The potential low cost of the instrument and its suitability for use by paramedical staff should make it particularly attractive for this type of application. For diagnostic purposes, the method may have particular usefulness under such circumstances as stress testing where invasive techniques are generally unsuitable. Ultimately, there may be potential for achieving an indication of the heart’s mechanical activity which is as simple to obtain and widely useful as the electrocardiogram is for the heart’s electrical activity. Summary
A transcutaneous ultrasonic Doppler technique for measurement of aortic blood-flow velocities has been developed and compared to more established techniques in order to evaluate its potential usefulness. It is possible by this method to quantitate blood velocity in both the ascending aorta and the aortic arch with ease and reliability. Ultrasonic access to the aorta from the suprasternal notch proved adequate in more than 90 per cent of the normal subjects examined. If further clinical trials prove as encouraging, this technique may be of significant value for patient monitoring and cardiac diagnosis. REFERENCES
1. Ruahmer, R. F.: Initial ventricular impulse: a potential key to cardiac evaluation, Circulation 26~268, 1964. 2. Noble, M. I. M., Trewehard, D., and Guz, A.: Left ventricular ejection in conscious dogs: I. Measurement and
611
Huntsman
et al.
significance of the maximum acceleration of blood from the left ventricle, Circ. Res. 19:139, 1966. 3. Dodge, H. T., Sandler, H., Ballew, D. W., and Lord, J. D.: The use of biplane angiocardlography for the measurement of left ventricular volume in man, AM. HEART J. 60:762, 1960. 4. Rackley, C. E.: Measurement of ventricular volume in man, Circulation 44z975, 1971. 5. Peterson, K. L., Uther, J. B., Shabetai, R., and Brauward, E.: Assessment of left ventricular performance in man. Instantaneous tension-velocity-length relations obtained with the aid of an electromagnetic velocity catheter in ascending aorta, Circulation 47:924, 1973. 6. Benchimol, A., Desser, K. B., and Gartlan, J. L., Jr.: Left ventricular blood-flow velocity in man studied with the Doppler ultrasonic flowmeter, AM. HEART J. 85:3,1973. 7. Baker, D. W.: Pulsed ultrasonic Doppler blood-flow sensing, IEEE Trans. Sonics Ultrasonics, SU- 17, No. 3, July, 1970.
612
8. Johnson, S. L., Baker, D. W., Lute, R. A., and Dodge, H. T.: Doppler echocardiography, Circulation 4Q:810, 1973. 9. Light, H., and Gross, G.: Cardiovascular data by transcutaneous aortovelography, in: Blood Flow Measurement, Roberts, C., editor. Baltimore, 1972, Williams and Wilkins, Company. 10. Light, L. H.: Noninjurious ultrasonic technique for observing flow in the human aorta, Nature 22431119, 1969. 11. Light, L. H.: Transcutaneous observation of blood velocity in the ascending aorta in man, Biol. Cardiol. 26:214, 1969. 12. Mackay, R. S.: Noninvasive cardiac output measurement, Microvasc. Res. 4~438, 1972. 13. Franklin, D. L., Schlegl, W., and Rushmer, R. F.: Blood flow measured by Doppler frequency shift of back-scattered ultrasound, Science 134~664, 1961. 14. Roberts, C., Editor: Blood Flow Measurement, Baltimore, 1972, The Williams & Wilkins Company.
May, 1975, Vol. 89, No. 6
of aortic blood-flow
Lee L. Huntsman, Ph.D. Emmeram Gams, M.D. Curtis C. Johnson, Ph.D. Eugene Fairbanks, M.S. Seattle, Wash.
While physiologic studies have indicated that infrrsrglation about aortic blood-flow dynamics may be of considerable usefulness in assessment of hmrt performance, the lack of a widely useful tacshnique has impaired clinical evaluation of this pclrtffntia1.1~2At the present time, such informaw is available by angiographic methods whereby left ventricular dimensions are determined, &amber volume estimated, and outflow rate ed as the rate of change of volume.3” Altermethods which provide direct measurement daortic flow events by means of catheter-tip flow or velocity sensors are under active developnBant.6s6 However, both of these techniques require arterial catheterization, and this repre&&nts a major restriction on their usefulness. T&us, there remains an important need for nonillllvasive methods of aortic flow or velocity deteration which are applicable to situations re the more expensive, troublesome, and rous invasive techniques are not suitable. situations include, for example: continuous toring of the postoperative or other high* patient; screening of large numbers of pea; and measurements in the presence of cardiovascular perturbations such as exercise. As noted by other investigators, there is an intr@ing possibility of determining aortic blood&w velocities transcutaneously by ultrasonic hrn the Center for Bioengineering, University of Washington, Seatth. ‘&it work wan supported by United States Public Health Service Omnts HL 07293 and GM16436, and Deutache Forschungagemiaschaft. Rsmived for publication June 14, 1974. hprint requests: Dr. Lee L. Huntsman, Center for Bioengineering, &&ere.ity of Washington, Seattle, Wash. 98195.
h&v, 1975, Vol. 89, No. 5, pp. 605-612
Doppler techniques. Potentially, an ultrasonic beam originating from a transducer in the suprasternal notch could be directed toward either the ascending aorta or the aortic arch providing quantitative blood velocity estimates for each of these locations. While the assessment of volume blood flow would require the determination of additional parameters, notably aortic diameter, just the determination of velocities could be of significance clinically. One instrument which allows such noninvasive measurements, the pulsed ultrasonic Doppler flowmeter, has undergone extensive development.‘Recent clinical trials with this device have clearly demonstrated that flow velocity information can be reliably obtained transcutaneously and used to advantage in cardiac diagnosis.8 While offering considerable measurement capability and versatility, the pulsed Doppler instrument is a complex and expensive device which requires skill and training for proper utilization. On the other hand, it has been suggested that the anatomy and physiology of blood flow in the thorax may be uniquely suited to a simplified approach using continuous wave ultrasound.g-12 Thus, it may be possible to develop a device expressly for the purpose of measuring aortic bloodflow velocities transcutaneously which is leas versatile than the pulsed instrument but much less expensive and easier to use. The present research has been undertaken to evaluate the feasibility of such an approach. ApprtXW2h
The ultrasonic Doppler technique for determining blood-flow velocities has been extensively evaluated.13r14 Sound waves emanating continu-
American
Heart Journal
606
Huntsman
et al.
Fii. 1. In principle, ultrasonic energy radiated from a transducer placed in the supraetemal notch can be directed toward the back of the aortic arch so as to intersect the aortic axis with a shallow angle.
ously from a transmitting piexo-electric crystal are partially reflected by the moving erythrocytes resulting in a Doppler frequency shift, fD, which is determined by the equation: fD =
2 f,V cos. 8 C
where fo = frequency of transmitted wave; C = velocity of sound in tissue (approximately 1,570 M. per second); 13= angle between the transducer axis and the blood flow velocity vector; and V = velocity of the erythrocytes which are responsible for scattering part of the ultrasonic energy back to the receiving crystal. The reflected wave is detected by a receiving crystal and the resulting signal processed electronically to obtain fo The difficulty with applying this simple approach to blood-velocity measurement in the thorax, as illustrated in Fig. 1, is that there are potentially many different portions of vasculature illuminated by the ultrasonic beam, Therefore, the Doppler signal will be a composite of information about erythrocytes moving with many different velocities at a variety of angles. There are, however, two premises regarding illumination of the aorta from the suprasternal notch which suggest that it may be possible to uniquely
606
identify and quantitate blood velocities in the aortic arch. The first premise is that a sound beam originating from the suprasternal notch can be directed so as to be nearly parallel to the central axis of the aorta somewhere near the top of the aortic arch. Thus, the angle between the sound beam and the velocity vector of the blood in this section of the aorta will approach zero and the cosine of this angle will approach unity. The second premise is that the velocities in the aortic arch are sufficiently high that the V cos Q product for sound waves reflected from this region is larger than for any other portion of the thoracic vasculature -at least during systole. Thus, the blood flowing in the arch will be responsible for the highest Doppler shift frequencies. There are some important implications of these premises, if they can be shown to be true. First, since the ultrasonic Doppler technique yields quantitative information about the velocity of the blood in the direction of the beam, the presence of a shallow angle between the velocity and the sound beam means that the Doppler data from this region of the aorta can be interpreted quantitatively without any unknown scale factors. Second, that portion of the vasculature which has the highest apparent velocity in the direction of the beam will be responsible for the highest Doppler shift frequencies in the return signal. Therefore, if the maximum Doppler shift frequencies can be separated from the rest of the Doppler signal, they will yield a quantitative measure of the blood velocity in a specific section of the aorta. There is the additional possibility that useful velocity signals may be available from the ascending aorta. However, anatomic variations in the orientation of the aortic root indicate that a near tangential approach of the sound beam to the flow velocity may not be assumed. In fact, the angle of incidence may actually change during a cardiac cycle. If, on the other hand, the aortic velocities are high enough and the angle of incidence shallow enough that the maximum Doppler shift frequencies are associated with the information of interest, then the data will be of considerable usefulness though not quantifiable in absolute terms. Thus, for a subject experiencing cardiovascular changes, alterations of ascending aortic velocity characteristics could be
May, 1975, Vol. 89, No. 8
Transcutaneous
_____ +-A Nl~,l
I/--
RECEIVER
DEMOWLAlW
determination
----------
of aortic blood flow
DOPPLER SIGNAL 1
FREQUENCY
-TRANSLATION 0
-
FREOUENCY
-
ANALYZER
ENVELOPE DETECT!ON
------O
ENVELOPE o”TPUT
I>
Fig. 2. Block diagram of the principal components of the continuous-wave measure aortic blood flow velocities. See text for further explanation.
eswRsed as a fraction of their control values. In addition to the great potential usefulness of taneous measurement of aortic blood flow ies, it should be noted that there are some ially important limitations of the techFor example, the technique inherently res velocity, not volume flow, measures peak velocity which has an unknown relationship to average velocity because the velocity profile is unknown, and, for the arch, measures v&city in a section of the aorta which carries an unknown fraction of cardiac output. Furthermore, the need to identify the maximum frequency components of the Doppler signal on a real-time basis poses some difficult instrumentation problems, the solution of which may impose restrictions on the technique. The severity of such limitations can be judged, however, only after the validity of the general approach has been demonstrated. Methods
All of the results reported in this study have been obtained with a continuous-wave ultrasonic Doppler instrument developed expressly for transcutaneous aortic blood flow velocity measurements. The organization and principle components of the device are shown schematically in Fig. 2. An oscillator generates a continuous wave at a frequency of 2.5 MHz. This signal is amplified and used to drive the transmitting piezo-electric crystal which radiates sound waves into the tissue at maximum power densities of less than 25 milliwatts per square centimeter. The much weaker reflected wave is amplified and demodulated to yield the Doppler signal. At the 2.5 MHz sound wave frequency, assuming an angle of incidence close to zero, each 1 KHz of Doppler signal frequency corresponds to approximately 30 cm. per second velocity of the erythrocytes. Therefore, for the velocities found in the cardiovascular system, the Doppler frequencies
Anrerican Heart Journal
ultrasonic Doppler instrument
used to
are well within the audio range and can be listened to directly. The Doppler signal is then conditioned, translated upward in frequency, and processed by a frequency analyzer circuit. The maximum excursions of this circuit are indicated by an envelope detector yielding a time trace of the maximum Doppler shift frequencies. Although ultrasonic transducers can be fabricated in a wide variety of configurat ing on the particular application, the presented here were obtained with a tra consisting of two piezo-electric crystals, semicircular disk, mounted edge-totwo-degree angle between them. Ea backed by a thin layer of air in ord the efficiency of forward The instrumentation t continuous wave ultrasonic up to the point and established our device.‘* Fo our approach is new, differing that of others. It will be described more completely elsewhere. Because the processing technique is new, however, the Doppler signal which results from demodulation has been taken a8 the standard data, not the envelope signal. Thus, for all experiments, the Doppler signal was recorded on magnetic tape for subsequent proceasing by a spectrum analyzer (Kay Sonograph) if the output from the frequency analysis circuitry proved to be open to question. For each different class of experiment, such off-line spectrum analysis was performed and the results compared to the tracings obtained from the instrument to verify that the maximum frequencies were adequately indicated. Results The geometric beam originating
relationship between a sound in the suprasternal notch and
607
Huntsman
et al.
(Antem-
Posterior 1
VIEW I (Loteml)
VIEW 4
VIEW 5
\
Geometric construction technique by which information contained in the antero-posterior and lateral x-ray films was manipulated to obtain 1, the distance from transducer to aortic axis intersection, and 0, the angle of intersection. The transducer, placed in the suprasternal notch, appeared in the x-ray films, thereby defining the position of one end of the line. Fig. 3.
Table I. Data for seven individuals
for whom the minimum angle (0) intersection of sound beam and aortic axis was determined by x-ray techniques. 1 is the distance from the suprasternal notch to the point of minimum angle intersection Patient
0
1.
6.5” 8.0” 0.0”
2. 3. 4. 5. 6. I.
Mean value Standard deviation
11.0” 4.0” 9.5” 5.0” 6.3” 3.7”
1 fem.) 7.5 7.1 8.9 8.7 8.4 6.6 9.8 8.2
1.1
the aortic arch has been investigated in seven subjects with apparently normal anatomy and cardiovascular function. With the transducer probe in place, simultaneous radiographs of the anterior-posterior view and the lateral view were taken. As shown in Fig. 3, these orthographic views allow construction of a triangle identifying the plane of the aortic arch. The transducer which also appeared in the x-ray film, provided one end-point on a line between the suprasternal notch and the point where the ultrasonic beam is
608
Fig. 4. Maximum frequency envelopes taken from the spectrum analyzer recorda for several examples of blood velocity in the arch measured by pulsed Doppler (solid lines) and continuous-wave Doppler (dotted lines).
most nearly tangent with the aortic axis. For simplicity, the second point was defined as that point in the lateral view film where a line through the suprasternal notch was tangent with the aortic center line. Although not identical with the point which minimizes the angle be tween the ultrasonic beam and the aortic ax&, this lateral view tangent point does provide a close approximation and the true minimum angle will be no greater than the angle measured by this technique. The data for all seven individuals is summarized in Table I. It is noteworthy that even with the tendency of the technique to overestimate the minimum angle, the angles as&! quite small and the assumption that cosine 8 equals unity introduces an error of less than 2 per cent. To evaluate the assumption that the greatest Doppler shifts are due to blood flowing in that portion of the aortic arch where the sound beam makes a nearly tangent approach to the velocity vector, a comparison of continuous wave and pulsed ultrasonic Doppler techniques was carried out. The pulsed Doppler instrument uses a narrow, well-defined beam and range-gating to define a small sample volume from which Doppler information is obtained.‘To carry out the comparison, the pulsed Doppler beam was first directed toward the aortic arch and beam direction and range were varied to the location where the maximum apparent velocities in the aortic arch were obtained. Doppler signals from this region of the arch were then recorded. Next, the continuous (CW) device with its broader beam and lack of range-gating was applied to the same individual with approximately the same beam direction and recordings made of the Doppler signals obtained. In both cases, the Doppler signals were processed with a Kay Sonograph spectrum analyzer and tracings were made of the maxMay, 1975, Vol. 89, No. 6
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ofaortic
blood fEoLL
line envelope detector (solid line).
m Doppler shift frequencies as a function of ample tracings from one of the individuals ich this comparison was carried out are in Fig. 4. The close similarity between the pulsed and the CW data was consistently found and the differences appear to be within the range of normally occurring short-term variations and experimental error. This result indicates that the mjrrimum frequencies in the CW Doppler spectrum may be validly attributed to that portion of the aortic arch where the ultrasonic beam is nearly tangent to the blood-velocity vector. An initial evaluation of transcutaneous aortic ty measurements in humans was carried attempting arch and ascending aorta ments in 20 apparently normal subjects. were obtained with the subjects resting n the supine position. Acceptable records ained from the arch for all but two of the and from the ascending aorta for all but ect. The spectrum analyzer and envelope reeMs for a single cardiac cycle in one subject are shown superimposed in Fig. 5. The ability of the instrument to provide an indication of the cy envelope is clearly demonstrated. In the velocity obtained from the envelope &&&or was in the range of 90 to 100 per cent of tin value obtained from off-line analysis, and any mrepancy was consistent for a given subject. ‘pfpical tracings obtained for the ascending aorta the arch from one subject are illustrated in 6. The presence of a lower peak velocity in t&e ascending aorta than in the arch was obd in most subjects (never was the arch > suggesting that the angle between the sound beam and the velocity is often significantly rent from zero for the ascending aorta case. Preliminary comparison of the Doppler velocity signal with volume flow rate was carried out by
American
Heart Journal
Fig. 6. Sample blood flow velocity traces obtained from the ascending aorta, A, and the aortic arch, B, of one individual.
making transcutaneous velocity determinations on two baboons which had electromagnetic flow sensors chronically implanted on the root of the aorta. Several weeks after surgery, the animals were lightly sedated with ketamine, 5 mg. per kilogram intramuscularly, and confined in a sitting position with a special chair which allowed the measurements to be made by hand-holding the ultrasonic transducer in the suprasternal notch. Apparently because lung or some other tissue which greatly attenuates sound waves intervened, useful velocity signals were not available from the arch. Adequate signals were obtained from the ascending aorta, however, and these were recorded, along with the signal from the electromagnetic flowmeter, for later comparison. Because the angle of the sound beam relative to the flow velocity is not known and
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Fig. 7. Time course of the response of the ascending aorta volume blood flow (EMF) and velocity (CW Doppler) in the baboon to norepinephrine infusion. The JSMF signal was uncalibrated since only relative changes were of interest.
Table II. Ratio
of changes of Doppler velocity characteristics to corresponding electromagnetic flow meter changes following inotropic interventions. Each is expressed as per cent of control
7 Isoproterenol (4 mg./min., intravenously) n=12 Metho-Hexital (5 mg./Kg. intravenously) n= 3
1.04f0.03
0.84f0.10
1.37 zkO.06
1.01
1.00
1.08
and flow caused by these drugs, expressed as a percentage of control valves, are summarized in Table II. The time course of one animal’s response to isoproterenol is illustrated in Fig. 7. The deviations from proportionality for changea of acceleration and stroke volume occur primarily because of the lack of a clearly defined envelope during the low velocity periods, though it may also be true that the velocity pro& changes, becoming more blunt, under conditioag of increased inotropism. Minor modifications of the instrumentation subsequent to these experiments have indicated that the correlation of isoproterenol-induced changes of flow and velocity may be substantially better than that indicated by these preliminary results. Disamsion
probably not zero (the baboon has a very short ascending aorta), quantitative comparisons of volume flow-rate and maximum instantaneous velocity were not possible. Rather, the time course of the flow and velocity traces were compared qualitatively and the response of each signal to positive and negative inotropic interventions was evaluated. These interventions consisted of isoproterenol infusions (4 mg. per minute intravenously) administered twelve times in two animals and methohexitol given three times in the two animals. The changes of velocity 610
The results of these experiments indicate that it is possible to measure aortic blood flow velocities and accelerations in man by transcutaneous ultrasonic Doppler methods. The basic premises underlying the technique have been validated by the x-ray data demonstrating a shallow angle of incidence of sound beam and aortic axis and by the continuous wave-pulsed Doppler comparison demonstrating that aortic flow causes the high& Doppler shift frequencies. While it is important to remember that ultrasonic access to the aortic arch is required in addition to suitable geometry, the successful use of the technique in 18 of a0
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Transcutaneous
normal subjects suggests that this may not be a major difficulty. These findings indicate that it should be possible to develop an instrument to quantitatively measure aortic blood flow velocities in man which is acceptable for clinical use. Furthermore, it appears that such an instrument may well be relatively simple, inexpensive, and easy to use. The prototype device used in this study meets most of these objectives. However, even greater ease of use could be obtained with a more diverging sound beam which would allow detection of arch velocities without precise aiming of the transducer. The feasibility of this modification has been demonstrated by the trial use of a beam with a 28” divergence. There are, however, two difficulties which have hindered this change. First, the diverging beams, with congruent transmitting and receiving patterns from the two crystals, are very difficult to fabricate in a compact form. Second, the diverging beam places stringent requirements on the electronics of the instrument. Since the sound energy is transmitted over a wide angle and received from a wide angle, the energy received from the region of interest is small. A very good signal-to-noise ratio is therefore required in order to extract acceptable velocity information. While it is important to be clear about the fact that the transcutaneous ultrasonic technique gives information about blood velocity, not volume flow rate, the clinical usefulness of this method is not likely to rest on the establishment of a known quantitative relationship of the measured velocity to volume flow rate. For many potential uses of this technique, the magnitude of such flow characteristics as peak flow, acceleration, and stroke volume is of less importance than information about their rate and direction of change from some control state. If the Doppler velocity data can be shown to vary in the same way as corresponding flow variables, determination of relative changes will be possible without auxiliary measurements. The data comparing maximum velocity and volume flow in the ascending aorta of the baboon suggest that it may be possible to demonstrate such a correlation. Based on these highly encouraging preliminary results, a systematic evaluation of the technique is being undertaken. First, the transcutaneous accessibility of aortic velocity information together with the convenience and reliability of
American Heart Journal
determination
of aortic blood flow
the method will be determined for wide variations of subject anatomy and disease state. Second, the degree to which Doppler velocity characteristics correlate with corresponding flow characteristics will be evaluated by performing comparisons with measurements made by standard techniques such as angiography, cardiac output determinations, and possibly catheter-tip flow meters. Finally, the clinical usefulness of the information provided by the technique will be weighed against the effort and expense involved in making the measurement. If this proves successful, the availability of a convenient method for obtaining aortic velocity information transcutaneously could be of profound importance in several areas of clinical cardiology. Perhaps the most immediate application would be to patient monitoring where the detection of short-term changes is of primary importance. The potential low cost of the instrument and its suitability for use by paramedical staff should make it particularly attractive for this type of application. For diagnostic purposes, the method may have particular usefulness under such circumstances as stress testing where invasive techniques are generally unsuitable. Ultimately, there may be potential for achieving an indication of the heart’s mechanical activity which is as simple to obtain and widely useful as the electrocardiogram is for the heart’s electrical activity. Summary
A transcutaneous ultrasonic Doppler technique for measurement of aortic blood-flow velocities has been developed and compared to more established techniques in order to evaluate its potential usefulness. It is possible by this method to quantitate blood velocity in both the ascending aorta and the aortic arch with ease and reliability. Ultrasonic access to the aorta from the suprasternal notch proved adequate in more than 90 per cent of the normal subjects examined. If further clinical trials prove as encouraging, this technique may be of significant value for patient monitoring and cardiac diagnosis. REFERENCES
1. Ruahmer, R. F.: Initial ventricular impulse: a potential key to cardiac evaluation, Circulation 26~268, 1964. 2. Noble, M. I. M., Trewehard, D., and Guz, A.: Left ventricular ejection in conscious dogs: I. Measurement and
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significance of the maximum acceleration of blood from the left ventricle, Circ. Res. 19:139, 1966. 3. Dodge, H. T., Sandler, H., Ballew, D. W., and Lord, J. D.: The use of biplane angiocardlography for the measurement of left ventricular volume in man, AM. HEART J. 60:762, 1960. 4. Rackley, C. E.: Measurement of ventricular volume in man, Circulation 44z975, 1971. 5. Peterson, K. L., Uther, J. B., Shabetai, R., and Brauward, E.: Assessment of left ventricular performance in man. Instantaneous tension-velocity-length relations obtained with the aid of an electromagnetic velocity catheter in ascending aorta, Circulation 47:924, 1973. 6. Benchimol, A., Desser, K. B., and Gartlan, J. L., Jr.: Left ventricular blood-flow velocity in man studied with the Doppler ultrasonic flowmeter, AM. HEART J. 85:3,1973. 7. Baker, D. W.: Pulsed ultrasonic Doppler blood-flow sensing, IEEE Trans. Sonics Ultrasonics, SU- 17, No. 3, July, 1970.
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8. Johnson, S. L., Baker, D. W., Lute, R. A., and Dodge, H. T.: Doppler echocardiography, Circulation 4Q:810, 1973. 9. Light, H., and Gross, G.: Cardiovascular data by transcutaneous aortovelography, in: Blood Flow Measurement, Roberts, C., editor. Baltimore, 1972, Williams and Wilkins, Company. 10. Light, L. H.: Noninjurious ultrasonic technique for observing flow in the human aorta, Nature 22431119, 1969. 11. Light, L. H.: Transcutaneous observation of blood velocity in the ascending aorta in man, Biol. Cardiol. 26:214, 1969. 12. Mackay, R. S.: Noninvasive cardiac output measurement, Microvasc. Res. 4~438, 1972. 13. Franklin, D. L., Schlegl, W., and Rushmer, R. F.: Blood flow measured by Doppler frequency shift of back-scattered ultrasound, Science 134~664, 1961. 14. Roberts, C., Editor: Blood Flow Measurement, Baltimore, 1972, The Williams & Wilkins Company.
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