The Nuclear Stethoscope: A Simple Device for Generation of

Left Ventricular Volume Curves

HENRY N. WAGNER, Jr.

ROBERT WAKE

EDWARD NICKOLOFF

T. K. NATARAJAN

Baltimore, Maryland

Initial evaluation has begun of a system for displaying left ventricular time-activity curves, relating the intraventricular content of radioactivity with the cardiac cycle as determined by the patient’s electrocardiogram. Major problems include proper positioning of the detector, correction for background radioactivity outside the ventricle and calibration of the device to permit conversion of measurement of radioactivity to measurement of ventricular volumes.

One of the most important advances in the diagnosis of cardiovascular disease is the use of radioactive tracers to measure regional myocardial perfusion and the motion of the ventricular walls in coronary and other types of heart disease. Valuable as these studies are proving to be, they suffer from one major drawback: the expense of the equipment required. To perform such a study with the scintillation camera and its associated data processing requires more than $100,000 worth of equipment, and the study ordinarily takes up to 1 hour to perform.

To make certain of these studies more readily available-for example, for monitoring a group of patients in a coronary care unit-we have de- signed, constructed and carried out initial clinical evaluation of a simple device called a “nuclear stethoscope,” that we hope will bring some of the potential uses of cardiovascular nuclear medicine to small medical centers and hospitals and possibly some day even to the physician’s office (Fig. 1). The device is compact and inexpensive because it dispenses with spatial resolution. In other words, it does not produce images of the heart but portrays a time-dependent image of the rate of filling and emptying of the left ventricle as a whole.

Method

The patient is given an injection of technetium-99m albumin, which becomes distributed rapidly throughout the heart and blood vessels. The amount of tracer within the heart is measured by a simple crystal scintillation detector, small enough to be held by hand against the patient’s chest as the heart fills and empties. The detector must be positioned so that radioactivity within the ven- tricle is being detected although activity from other sources surrounding the ventricles must be taken into account. This is referred to as background activity. The crystal detector is attached by way of intermediate electronic equipment to a recording device that displays the activity being viewed by the detector in relation to the patient’s electrocardiogram. The device is designed to divide the interval between successive R waves into 48,96 or 192 intervals or “time slots.” For each of these intervals, the heart’s content of radioactive tracer is measured and depicted on a television screen in the form of a developing left ventricular time-activity curve.

From the Divisions of Nuclear Medicine and Ra- diation Health, The Johns Hopkins Medical Insti- tutions, Baltimore, Md.

Address for reprints: Henry N. Wagner, Jr., MD, Divisions of Nuclear Medicine and Radiation Health, 615 N. Wolfe St., Baltimore, Md. 21205.

The image of the left ventricular time-activity curve is produced by displaying simultaneously on a television screen the activity in 48 to 192 memory channels beginning at the time of the patient’s electrocardiographic R wave. The parallel lines of the television raster correspond to memory channels. The count rate stored in a particular memory channel causes the video signal for the corre- sponding raster line to be maintained at a binary level for a corresponding time interval. This produces a bar graph in which the length of each bar indicates the

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NUCLEAR STETHOSCOPE-WAGNER ET AL

FIGURE 1. Illustration of the principle of the nuclear stethoscope. The electrocardiographic leads provide the pulses for dividing ventricular radioactivity into the proper time slots after the QRS complex.

LEFT VENTRICULAR VOLUME CURVE

FIGURE 2. Photogr .aph of the PI rototype.

FIGURE 3. Left ventricular time-activity curve on the left, background activity on the right. The vertical axis is ac- tivity; the horizontal axis is time after the QRS com- plex.

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amount of radioactivity being seen by the radiation detector during the corresponding time of the cardiac cycle. As each memory channel accumulates radioactivity count rates during several hundred cardiac cycles, each bar lengthens to indicate the cumulative increase in activity during each time slot until an interpretable left ventricular time-activity curve is ob- tained.

The prototype device: In the first version of the device, a crystal scintillation detector, 2.54 cm in diameter, was combined with parallel hole lead collimators 2.54 cm in length and 0.95 to 1.27 cm in diameter (Fig. 2). The detector was placed over the chest wall at a point medial to the point of maximal cardiac impulse with the axis of the detector in a manner similar to the left anterior oblique angiographic view.

The radioactive tracer in the blood produces output pulses that are counted and stored over brief repeating intervals of time in a multichannel memory. The first channel of memory is gated by detection of the QRS complex in the patient’s electrocardiogram, the recording of which is an intrinsic part of the device. Thereafter, the channels of memory are suc- cessively addressed so that each successive radioactivity count rate over repeating time intervals is stored in corresponding memory channels. With each successive heartbeat, the process is repeated and new counts are added to the counts alread in storage in a particular channel.

While the count rates are being recorded, the memory channels are read out to a television type display in which each memory channel is correlated with a horizontal line sweep. At the beginning of each line, the video input signal is switched from a low level to a high level where it remains for a period of time that corresponds to the accumulated total in the memory channels, thus writing solely on the television screen whose length is determined by the progressively accumulated radioactivity counts.

Since successive lines of the television display correspond to the different intervals of the cardiac cycle, the physician can observe the progressive build-up of an image over a period of about 1 minute that reflects the rising and falling of the amount of the tracer within the field of view of the detector (Fig. 3). If the detector is positioned properly, the curve will correspond to the left ventricular time-activity curve. With proper calibration to permit conversion of the amount of ra-

FIGURE 5. Gated scintillation camera images are compared with a left ventricular volume curve obtained in the same patient. Images in lefl pan- els, systolic frames; images in center panels, diastolic frames. Upper right panel shows volume curve measured by coupling the output of a scin- tillation camera to the nuclear stethoscope. Lower right panel shows background activity associated with the volume curve.

dioactivity to the amount of blood in cubic centimeters, the left ventricular volume curve can be obtained.

The device is also designed to record the time course of the tracer as it passes through the heart after the initial intrave- nous injection. On the basis of these results with the prototype, the design has been modified and a new device is under con- struction.

The left ventricular time-activity cuwe: In Figure 3, the vertical axis displays accumulated counts in each time interval after the QRS complex corresponding to the zero intercept on the horizontal axis. Each bar corresponds to a time interval after the QRS complex. In this case full scale on the horizontal axis is approximately 1 second. The video screen displays the updated contents of all the time intervals at once. The X axis

I EJECTtON FRACTION

%

l

a l / l

N.S. l b l e0 l

50- l 0

0 l O

25- /

0: e

b 0

0*

U I

25 1

so 75 Id0 o/o

G.C.

FIGURE 4. Relation between ejection fraction measured with the nu- clear stethoscope (N.S.) (vertical axis) and with the scintillation camera (G.C.) method of measuring ventricular dimensions in 44 patients. The open and closed circles represent data measured by two different in- vestigators.

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represents time and the Y axis represents radioactivity. The full extent of the X axis corresponds to an average conglom- erate cardiac cycle; the displayed curve represents the varia- tion in radioactivity during repetitive cardiac cycles. The image builds up, one cardiac cycle after the other, until the physician has obtained the required information. The right panel of Figure 3 shows background activity obtained by po- sitioning the detector over the left lung away from the heart.

In use, the portable instrument is brought to the patient’s bedside, and the patient is given an injection of aamTc-labeled human serum albumin in a dose of 1 to 20 mCi. About 10 minutes after injection, the physician or technologist places the detector over the patient’s pre~ordium and, observing the count ratemeter while positioning the detector in the esti- mated left anterior oblique angiographic posit&n, locates the point of maxima1 radioactivity. Then, the image is observed for about 30 seconds with minor changes in the position of the detector until the maximal stroke volume is observed, that is, the point when the difference between maximal activity cor- responding to end-diastole and minimal activity corre- sponding to end-systole is greatest. Care is taken to avoid the left atrium.

After the left ventricular curve has been obtained, the de- tector is moved to the left to obtain the background activity (Fig. 3). The detector is moved to the left until the rise and fall of radioactivity in relation to the electrocardiogram is no longer observed. The period of observation of background and of left ventricular activity must be identical so that the former can be subtracted from the latter.

Results

The normal left ventricular time-activity curve: In normal persons at rest, the left ventricular time- activity curve is usually similar to that in Figure 3. Im- mediately after the QRS complex, the preejection pe- riod is evident; the ventricle has begun contracting, but the aortic valve has not opened and the left ventricle has not yet begun to empty. When the intraventricular pressure increases sufficiently to open the aortic valve against the pressure within the aorta, the activity level recorded by the detector falls as blood flows out of the heart and levels off after the aortic valve closes. It begins rising again as the heart refills. When filling is com- pleted, there is another flat region in the curve, the isovolumetrie diastolic period, or diastasis, before contraption of the ventricular muscle starts another cycle. In the normal resting person, the intraventricular volume is level for about one third of the cardiac cycle. With exercise, the isovolumetric diastolic period di- minishes or disappears completely.

The maximal activity corresponds to end-diastolic volume, whereas the low point of activity corresponds to end-systolic volume; after background activity has been subtracted, subtracting end-systolic from end- diastolic volume gives stroke volume which, divided by the diastolic volume, gives ejection fraction. Stroke volume averaged over 30 seconds to 1 minute gives cardiac output.

Comparison with other methods: Currently we are comparing the results obtained with the nuclear stethoscope with those obtained in the same patients with nuclear imaging studies, echocardiography and, in some cases, contrast angiography. The correlation of the ejection fraction measured in 44 patients with the scintillation camera and the nuclear stethoscope is shown in Figure 4. The camera method consisted of measuring the ventricular dimensions from camera images and the use of the Dodge, Sandler formulas for calculation of end-systolic and end-diastolic volumes.

These studies should give a clearer idea of what the still experimental nuclear stethoscope can and cannot do. We intend to see how closely specific abnormalities in the ventricular volume curves correlate with anatomic or pathologic abnormalities revealed by other meth- ods.

Discussion

Clinical implication: It is too early to evaluate the diagnostic potential of the nuclear stethoscope in rela- tion to the more elaborate cardiovascular nuclear medicine procedures where diagnostic value seems al- ready well established. It is likely that the device may find its greatest use in association with the scintillation camera, which can be used as a detector to achieve more precise localization of the left ventricle (Fig. 5). Par- ticularly in patient monitoring, one or more nuclear stethoscopes may be able to provide the cardiologist with rapid feedback on the effect of a given regimen, such as digitalis or counterpulsation. It may also help in quantifying the effects of exercise in the recovery period after myocardial infarction, and possibly, in adjusting the dose of medications for maximal benefit to the ventricular function of the individual patient.

Future directions: Problems still requiring inves- tigation are the size and configuration of the collimator, the rules that should be followed to ensure correct po- sitioning of the detector, the correction required for background activity, and the accuracy of the device in measuring preejection period, left ventricular ejection time, the rates of emptying and filling of the ventricle, end-systolic and end-diastolic volumes and ejection fraction. At present, it seems that it is most accurate in measurement of time intervals, moderately accurate in measurement of ejection fraction and least accurate in measuring absolute volumes because of problems with calibration of activity to yield volumes and problems of correcting for background.

1.

Reference

Wagner HN Jr, Natarajan TK, Strauss HW, et al: The nuclear stethoscope: a bedside device for continuous monitoring of ven- tricular function (abstr). Circulation 52: Suppl II: N-70, 1975

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