PREPRINT NO. 744 (C-5)

STROKE VOLUME AND CARDIAC OUTPUT BY ECHOCARDIOGRAPHY

byBenedict Kingsley, M.Sc.Research Associate Professor of MedicineHahnemann Medical College & HospitalPhiladelphia, Pennsylvania

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STROKE VOLUME AND CARDIAC OUTPUT BY ECHOCARDIOGRAPHY

B Y

Benedict Kingsley, M. Sc.Research Associate Professor of Medicine

Hahnemann Medical College & IlospitalPhiladelphia, Pennsylvania 19102

INTRODUCTION

The advent of open-heart surgery has accentuated the need for more accurate evaluationof the ability of the heart to provide adequate blood flow. The single most important factorin evaluating the performance of the heart is the cardiac output measured in liters perminute. Cardiac output is a significant clinical entity in evaluating the performance ofp_osthetic cardiac valves, in monitoring the restoration of myocardial function in heartattach patients and in identifying the hyperkinetic (high-flow state) heart syndrome inyoungsters in innocent murmurs.

The diagnosis of low cardiac output usually rests on recognition of the effects of lowcardiac output rather than its direct measurement. Many techniques have been developed forthe estimation of cardiac output, but most of these involve venous and arterial punctures andare not applicable to bedside monitoring. (1) All cardiac output measurement techniques areindirect estimations based on various physical and mathematical assumptions. Therefore, eachtechnique contains inherent errors which must be determined and minimized.

Presently, the Fick principle and the indicator-dilution techniques utilized inconjunction with intracardiac catheterization are the accepted standards for measuring thecardiac output. The Fick principle is based on the fact that during its passage through theperipheral tissues, a certain amount of oxygen is taken up from the blood. The limitationsof this method are the difficulty of obtaining accurate samples of expired air, the need forsampling of pulmonary artery blood, and the time-consuming analysis of expired air and bloodoxygen concentrations. The indicator or dye-dilution technique involves the rapid injectionof an indicator into the circulation, with down stream analysis of its dilution concentration.This technique assumes perfect mixing of the dye in its dilution volume and neglects recircula-tion and pulsatile flow effects.

Echocardiography is a simple, non-invasive bedside technique based on reflectedultrasonic waves for measuring and monitoring cardiac output. (2)

MEASUREMENT AND APPARATUS

Prior to the clinical application of the echocardiographic technique to themeasurement of cardiac output, a simulated cardiovascular system (Figure 1) was set upin the laboratory to demonstrate the feasibility and accuracy of measuring flow by thenon-invasive technique of ultrasound. (3)

Figure 1 shows the in vitro apparatus used to measure the flow of water through aStarr-Edwards prosthetic cardiac valve. The graduated cylinder and stop watch provide ameans for measuring the actual flow. The transducer of the ultrasonoscope is positionedhead-on to the valve in order to insure that the total velocity of the ball is being recorded.

Figure 2 shows the waveform representing the motion of the ball as recorded through ananalog gate. This direct recording supplies the opening velocity of the ball, the time thevalve remains open and the pump rate. Assuming that the ball moves at the same velocity asthe fluid and that steady flow conditions exist for the entire time the valve remains open,the flow can be determined from the direct recording as described below.

Assuming steady flow conditions for each stroke of the pump, the equation

AVToRF = 1000

where A = hydraulic area in cm2

V = linear flow velocity in cm/sec.

To = time valve remains open in sec.

R = pump rate

gives the flow through the valve in liters/minute.

Table 1 compares the flow measured by ultrasound with the actual flow measured withthe stop watch and graduated cylinder. The percentage error falls within + 10% which issatisfactory for in vive applications.

TABLE I

Pump rate p/min Actual Flow l/min Flow by Ultrasound 1/min Percent Error %

32.2 0.87 0.83 -4.60

37,9 1.03 1.02 -0.9743.4 1,19 1.19 0.00

49.5 1.39 1.41 +1.4454,5 1.52 1.54 +1.32

59.6 1,68 1.66 -1.1966.0 1.90 1.72 -9.46

79.0 2,58 2.80 +8.54

99.1 3.22 2.90 -9.94

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CLINICAL APPLICATIONS

Eehocardiography when applied to man provides a non-invasive reproducible techniquefor measuring stroke volume and cardiac output. Stroke volume is computed by utilizing thebasic hydraulic equation

F AVT

where A mitral valve cross-sectional area _n cm2

V blood velocity in cm/sec.

T duration of flow across the mitral valveo (diastolic filling time) in seconds

This formula assumes that the flow of blood across the mitral valve into the left

ventricle is equal to the volume of blood ejected into the aorta for each beat, that thevelocity of opening of the mitral valve leaflets is the same as that of the blood fillingthe left ventricle and that a steady flow condition prevails for the entire time the valveremains open. The average normal mitral valve area as stated in most physiology textbooksis assumed to be 5 cm 2. (4)

A typical echocardiogram of mitral valve motion is shown in Figure 3. (5) Tileopening velocity of the anterior mitral valve leaflet as represented by slope of DE correspondsto V and is measured in cm/sec. The duration of mitral valve opening corresponds to timeelapsed from point D (beginning of mitral valve opening) to point C (mitral valve closure).

Cardiac output is computed in liters/min, from the stroke volume, F, utilizing theformula

F BEATSC. O. 1000 X MIN.

assuming a regular heart rate.

In case of arrhythmia addition of all stroke volumes in one minute will also yieldcardiac output.

In vivo, the cardiac output of 100 patients read within 15% of outputs measured atcardiac catheterization. Typical results are shown in Table II. Cardiac output was alsomeasured by dye dilution studies in 7 patients with implanted arterio-venous shunts forlmmo-dialysis. In these 7 patients, the cardiac outputs done simultaneously by ultrasoundwere within 10%.

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CARDIAC OUTPUT _AS_E_NTS

Ultrasonic Output Dye Dilution Output % Difference

(L/min) (L/min)

7.5 7.8 -3.96.7 6.5 +3.0

5.7 6.2 +8.05.7 5.4 +5.5

5.4 5.0 4-8.04.9 5.2 -5.8

4.8 4.5 -6.64.6 4.0 +15.0

4.7 4.3 +9.34.6 4.5 +2.2

4.6 4.5 + 2,24.4 4.9 -14.3

4.2 4.2 0.03.7 3.4 +9.1

3.4 3.4 0.0

3.3 3.7 -10.87,7 7.5 +2.68.7 8.5 +2.3

8.1 8.3 -2.4

7,5 7.8 -3.83.4 3.4 0.04.2 4.9 -14.3

11.2 1]..5 - 2.6

SUMbiARY

Echocardiography is a non-invasive, diagnostic technique of proven value in estimatingthe output of the heart. 1N VITRO, the validity of this ultrasonic technique has been con-firmedemployingapulsatilepump, rubbertubingandaSTARR-EDW_DSprostheticvalve. INIVO, the cardiac output read within ]5% of outputs measured at cardiac catheterization. Thisbedside technique may also be valuable in monitoring the restoration of myocardial functionin heart attack patients and in evaluating the performance of prosthetic cardiac valveseliminating the need for cardiac catheterization in many cases.

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REFERENCES

1. Berne, R.M. and Levy, M.N.: Cardiovascular Physiology.The C.V. Mosby Company, Saint Louis, 1967, pp.80-81; 170-174.

2. Bell, D.S. and Kingsley, B.: Ultrasonic Measurement of FlowThrough A Prosthetic Mitral Valve . Proceedings of the 8thInternational Conference on Medical and Biological Engineering,Chicago, Illinois. July, 1969.

3. Bell, D.S.: Evaluation of the Performance of the Starr-EdwardsMitral Valve by Ultrasound. Master's Degree Thesis, The GraduateSchool, Penn State University, March, 1969.

4. Gorlin, R. and Gorlin, S.G.: Ilydraulic Formula For CalculationOf The Area Of The Stenotic Mitral Valve, Other Cardiac Valves,and Central Circulatory Shunts. American Heart Journal, 41: 1-27,January, 1951.

5. Kingsley, B., Flint, G., Jr., Raber, G., and Segal, Bo: AnotherLook at Echocardiography (Concepts in Biomedical Engineering).Am. J. Cardiol., January, 1967.

SYSTEM FOR DETERMINING FLOW BY ULTRASOUND AND BYMEASURING FILLING TIME OFA KNOWN VOLUME

Transducer

Ball Valve __Ultra- I/ sonoscopeJ

FIGURE I

! J

I - i_i

FIGURE 2

_.---Diastolic Filling Period (To)---_rE/

I

Upstroke i

ECHO

I11O

tCLOSED INITIAL FULL CLOSED

OPENING OPENING

FIGURE 3