Scand. J . clin. Lab. Invest., Vol. 36, 1976.
Measurement of Cardiac Output by Impedance Cardiography in Patients with Myocardial Infarction Scand J Clin Lab Invest Downloaded from informahealthcare.com by Karolinska Institutet University Library on 06/25/14 For personal use only.
Comparative Evaluation of Impedance and Dye Dilution Methods S. GABRIEL, J.-H. ATTERHOG, L. OR6 & L.-G. EKELUND
Coronary Care Unit, Dept. of Medicine, and Dept. of Clinical Physiology, Karolinska sjukhuset, Stockholm, Sweden
Gabriel, S., Atterhog, J.-H., Oro, L. & Ekelund, L.-G. Measurement of Cardiac Output by Impedance Cardiography in Patients with Myocardial Infarction. Comparative Evaluation of Impedance and Dye Dilution Methods. Scand. J. clin. Lab. Invest. 36, 29-34, 1976. The cardiac output was measured simultaneously by the impedance cardiography and dye dilution methods in 10 patients with acute myocardial infarction 2-3 weeks after admission to the Coronary Care Unit. The impedance cardiac output was on the average 9.7% hlgher than the dye dilution cardiac output. The reproducibility of impedance cardiac output was 4.1%, compared with 5.1% for the dye dilution method. The study showed a satisfactory reliability of impedance in predicting the relative change of cardiac output in response to tilting from the supine to the 30" head-up position, to a 10" head-down position, and to the intravenous administration of propranolol. Key-words: Body tilt; cardiac output; dye dilution technique; impedance cardiograph; myocardial infarction S. Gabriel, M.D., Dept. of Clinical Physiology, Karolinska sjukhuset, 4 1 0 4 01 Stockholm 60, Sweden
In recent years impedance cardiography, a noninvasive method developed by Kubicek et al. (6,7), has been used for the measurement of stroke volume andcardiac output (I-6,9,10,13). In different clinical conditions and in normal subjects impedance cardiography has given fairly accurate measurements of the cardiac output when compared with the dye dilution and Fick methods (1, 2, 4, 6, 9, 10). There is no doubt that such a convenient non-invasive method would be suitable for patients with myocardial infarction. However, these patients may have a poor cardiac function and abnormal myocardial contractility, which may interfere with the accuracy of the method (8). The purpose of this study was to f a t h e r evaluate impedance cardiography as a method for the measurement of cardiac output in patients with clinically stable myocardial infarction and to evaluate
the validity of the method when applied in conditions producing cardiocirculatory changes. MATERIAL AND METHODS Ten patients, 9 men and 1 woman, mean age 54 years, range 43-70 years, with acute myocardial infarction were examined 2-3 weeks after admission to the Coronary Care Unit (CCU). All patients were informed about the purpose of the study, and consent was obtained from each patient. They had sinus rhythm and were free from valvular disease or clinical signs of heart failure. Simultaneous dye dilution curves and impedance cardiograms were recorded at rest in the following situations: a) in supine position after 15 min, b) after 5 min of tilting 30" with the head upwards (30"4), c) after 5 min of tilting 10"with
30
S. Gabriel, J.-H. Atterhog, L. Oro & L.-G. Ekelund
phonocordioqmm
Scand J Clin Lab Invest Downloaded from informahealthcare.com by Karolinska Institutet University Library on 06/25/14 For personal use only.
OSCILLATOR
Fig. 1. The position of the electrodes and the basic system of impedance cardiograph.
VOLTAGE RCK UP DETECTION
the head downwards (lOoJ), d) in the supine position 3 min after intravenous injection of 0.1 mg/kg body weight propranolol (Inderalm). Impedance cardiogriiphy
The method for measuring the cardiac output (CO) has been described previously (6, 7). Four disposable self-adhering aluminized mylar strip electrodes manufactured by the 3M Co., Minnesota, were attached circumferentially to the patient, as seen in Fig. 1. The inner electrodes (2 and 3) were placed around the base of the neck and at the level of the upper border of the manubrium sterni and the xiphisternal joint, respectively. The outer two electrodes (1 and 4) were placed at least 3 cm from the inner electrodes. The electrodes were attached to an IFM Minnesota Impedance Cardiograph, Model 304 A, which supplies a constant sinusoidal alternating current of 4 mA at a frequency of 100 kHz, which is transmitted between electrodes 1 and 4. A voltage gradient is generated between electrodes 2 and 3 proportional to the thoracic impedance (Zo); the latter is read directly on the digital display. A crystal microphone placed at the apes was used to record a phonocardiogram. Four output signals were recorded on a multichannel graphic recorder (Mingograph, Siemens-Elema) : 1) electrocardiogram (ECG), 2) the impedance change during the cardiac cycle ((AZ), 3) the first derivative thoracic impedance (dZ/dt), 4) phonocardiogram. The stroke volume was calculated according to the formula given by Kubicek et al. (6, 7), d V = p ( L / Z O ) ~ . T * ~rnin, Z/~ where ~ dV = stroke volume (ml), p = resistivity of blood (taken as 135 ohm-cm), L = shortest distance (cm) between the
inner electrodes (mean of the front and back measurements), Zo = the mean thoracic impedance in ohms, T = the ventricular ejection time in seconds, and dZ/dt min = the maximal deflection from zero line for dZ/dt, ohms/sec, occurring during the cardiac cycle. In each subject the average of 5 beats was used to calculate the stroke volume. The cardiac output was calculated by multiplying the stroke volume by the heart rate; the latter was obtained from the ECG recording. Dye dilution cardiuc output
A 60-cm Teflon catheter was inserted in an antecubital vein in the right arm by the Seldinger technique. A similar catheter was inserted by the same technique in the left brachial artery. In one patient both catheters were inserted in the right arm. The arterial catheter was connected to a Beckman cardiodensitometer. For each measurement 1 ml of a solution containing 5 mg of Indocyanine Green dye (Cardio Green) was injected intravenously from a calibrated syringe. Arterial blood was withdrawn at a rate of 30 ml/ min and afterwards was reinfused through the venous catheter. The cardiac output was calculated according to the on-line calibration equation as developed by Scheel et al. (11) and Vollm & Rolett (12). Procedure
The patients were examined without premedication. They were kept on the current medication except for any p-adrenergic blocking agent, which was not given on the day of examination. The tape electrodes were attached to the patients as
Cardiac Output by Impedance Cardiography
31
Scand J Clin Lab Invest Downloaded from informahealthcare.com by Karolinska Institutet University Library on 06/25/14 For personal use only.
Table I. Comparison between impedance and dye dilution, stroke volume, and cardiac output in different body positions, in ten patients with myocardial infarction*
Position
Difference Difference A-B in yo Impedance method Dye dilution method A-B of B A
Number Heart of rate, measure- beats/ ments min
Supine Tilting 3Oof Tilting 10"4 Supine after propranolol All measurements
Zo, ohms
SV, ml
sv, ml
co, I/min
s v , co, ml I/min
sv, co, %
%
71312 25.5k2.4 70519 5.0751.50 64f16 45851.13 6.0 0.49 9.4 10.7 82510 26.4f2.4 54511 4.4150.82 48&15 3.8950.99 6.0 0.52 12.0 13.4 73514 25.7k2.0 71522 5.0851.63 70&20 4.995150 1.0 0.09 1.4 1.8
21
61510 25.8k1.8 73522 4.4351.26 6 3 j 1 5 3.8550.92 10.0 0.58 15.9 15.1
86
72+13
25.852.2 67520 4.75j1.36 62f18
4.3351.23
5.0 0.42
8.1
9.7
S.D. SV=stroke volume; CO=cardiac output; Zo=mean thoracic impedance.
described above, after which the arterial and venous catheters were inserted. In every patient two and sometimes three simultaneous dye dilution curves and impedance cardiograms were obtained in each of the four positions mentioned above. In changing from position (b) to position (c) and from the latter to position (d), the patients were kept in the supine position for 5 min. RESULTS The absolute values of cardiac output estimated by impedance cardiography were in general higher Impedance Cardiac Output, i/min
1-
5-
supine tilting 301 tilting 10.1 A after Prooranolol 0
3-
all measurements r.0.853 p..O.OOl 11.86
1
1
3
co,
I/min
22 21 22
* Mean valuesf1
1
B
7 9 Dye Dilution Cardiac Outout. I/min 5
. ,.
Fig. 2. Relationship between absolute values of impedance (y) and dye dilution (x) cardiac output in 10 patients with myocardial infarction. y=0.689+ 0.937 x, S.E.M. 0.715.
than the CO measured by the dye dilution technique (Table I). The difference was largest after intravenous propranolol in supine position, averaging 0.58 I/min or 15.1%, and lowest in the tilting position 10.4, in which it averaged 0.09 I/min or 1.8%. In the supine position the mean impedance CO was 0.49 l/min or 10.7%; in the tilting 30"t position it was 0.52 I/min or 13.4%, higher than the dye dilution CO. For all measurements the absolute impedance CO was 0.42 I/min, or 9.7% higher than the absolute dye dilution CO. The difference was, however, not significant (P>0.05). There was a significant correlation between the absolute values of CO estimated by the impedance method and those measured by the dye dilution method (Fig. 2). The correlation was highly significant in all four positions studied (P0.05.In the tilting loo$ position and after propranolol Zo was practically unchanged. DISCUSSION The present study showed a close relationship between values for cardiac output obtained with impedance cardiography and the dye dilution technique, the mean impedance cardiac output
Table 111. Reproducibility of impedance and dye dilution cardiac output* Method Cardiac output, l/min
Impedance Dyedilution
2
d
4.693 4.317
-0.031 -0.048
S.D.4 S.E.M.d
c%
t
P
n
0.271 0.313
4.1 5.1
-0.688 -0.909
NS
72 72
0.045 0.052
NS
* %=mean value; z=mean of differences between the first and second measurement; S.D.d=standard deviation of the differences; S.E.M.d=standard error of the differences; C=coefficient of variation for the differences; t=t test of the differences; P=level of significance; n=number of measurements.
Cardiac Output by Impedance Cardiography
33
Table IV. Relative changes of impedance and dye dilution cardiac output in response to cardiocirculatory changes. The cardiac output is expressed as mean value in per cent of mean supine value
Scand J Clin Lab Invest Downloaded from informahealthcare.com by Karolinska Institutet University Library on 06/25/14 For personal use only.
Position Tilting 30" t Tilting 10" 4 Supine after propranolol
Number of measurements 19 20 18
being 9.7% higher than the mean dye dilution cardiac output. The difference varied by 1.8%15.1% depending on the body position. This is in agreement with earlier studies in different clinical conditions and in normal subjects (1, 6, 9). The difference could be explained by the individual variation of the blood resistivity. In the present study the blood resistivity was assumed to be constant in all positions studied. Lababidi et al. (9) reported 5.570 higher impedance cardiac output in children with congenital heart disease without shunts or valvular disease. Baker et al. (1) compared cardiac output values obtained by dye dilution and impedance cardiography in ten narmal men, using the blood resistivity values as determined from the subject hematocrit. The impedance cardiac output calculated from the published data was 8% higher than the dye dilution cardiac output. The regression equation, given by these authors, describing this relationship was similar to that in the present study. In an earlier study Harley & Greenfield (2), using a less developed impedance technique, found in 13 normal subjects that the ratio of impedance cardiac output to dye dilution cardiac output was 1.34 during rest and 1.23 following isoproterenol. However, they reported poor correlation between the two estimates. The reproducibility of impedance cardiac output was somewhat better than that of the dye dilution cardiac output. Similar results were reported in the early studies of the method by Kubicek et al. (6) and later by Lababidi et al. (9). For the clinician it is sometimes more important to assess the relative changes in cardiac output than to measure its absolute value. The present study included estimations in different body positions and after injection of propranolol, conditions known to cause cardiovascular changes and to influence the cardiac output.
-
3 Scand. J. clin. Lab. Invest.
Impedance cardiac output Dye dilution cardiac output
% supine 86.2 102.0 91.8
% change
% supine
- 13.8
84.3 108.5 89.1
+ 2.0 -
8.2
% change
- 15.7 $- 8.5
- 10.9
Good agreement was obtained between the two methods in reflecting the relative changes in cardiac output, especially on tilting 30"T and in supine position after propranolol. In the tilting 10.4 position, however, the change of dye dilution cardiac output was somewhat larger (8.5%,) than that of impedance cardiac output (2%). As mentioned above, the impedance cardiac output increased 6% in three patients when they were tilted 10.4, and the heart rate increased 6 beats/ min, while their dye dilution cardiac output decreased 2%. In one patient the impedance cardiac output increased 9% when the heart rate increased 19 beats/min, while the dye dilution cardiac output decreased 1 yo. These findings support the observations made by Baker et al. (1) that impedance values reflect more closely the expected change in cardiac output than do the dilution values. The relative changes of impedance cardiac output in the present study are generally in agreement with earlier studies in normal subjects by Kubicek et al. (6) and Judy et al. (4). Our results, however, differ from those of Zambrano & Spodick (13), who, in patients with stablecoronary disease, reported hardly any change in the impedance cardiac output or stroke volume in response to immediate tilt compared to normal subjects. This was explained by the increased tolerance to orthostatic stress in patients with cardiovascular disease. In conclusion, the present study shows a good validity of the impedance method in the measurement of cardiac output and its relative changes. The reproducibility of the impedance cardiac output was relatively high in spite of the presence in some patients of distortions in the impedance cardiogram, a factor which may introduce an error in the calculation of cardiac output (8)
34
S . Gabriel, J.-H. Atterhog, L. Oro & L.-G. Ekelund
REFERENCES
Scand J Clin Lab Invest Downloaded from informahealthcare.com by Karolinska Institutet University Library on 06/25/14 For personal use only.
1 . Baker, L. E., Judy, W. V., Geddes, L. E., Langley, F. M. & Hill, D. W. The measurement of cardiac output by means of electrical impedance. Cardiovasc. Res. Cent. Bull. 9, 135, 1971. 2. Harley, A. & Greenfield, J. C., Jr. Determination of cardiac output in man by means of impedance plethysmography. Aerospace Med. 39, 248, 1968. 3. Hill, D. W. & Lowe, H. J. The use of the electricalimpedance technique for the monitoring of cardiac output and limb bloodflow during anaesthesia. Med. biol. Eng. 534, Sept. 1973. 4. Judy, W. V., Langley, F. M., McCowen, K. D., Stinnett, D. M., Baker, L. E. & Johnson, P. C. Comparative evaluation of the thoracic impedance and isotope dilution methods for measuring cardiac output. Aerospace Med. 40, 532, 1969. 5. Kottke, F. J., Kubicek, W. G. & Olson, M. E. Evaluation of cardiac competence during rehabilitation following myocardial infarction. Israel J. med. Sci. 9, 568, 1973. 6. Kubicek, W. G., Karnegis, J. N., Patterson, R. P., Witsoe, D. A. & Mattson, R. H. Development and evaluation of an impedance cardiac output system. Aerospace Med. 37, 1208, 1966. 7 . Kubicek, W. G., Patterson, R. P. & Witsoe, D. A. Impedance cardiography as a noninvasive method to monitor cardiac function and other parameters
Aeceived 5 March 1975 Accepted 13 November 1975
of the cardiovascular system. International conference on bioelectrical impedance. Ann. N.Y. Acad. Sci. 724, 1970. 8. Kubicek, W. G., Kottke, F. J., Ramos, M. U., Patterson, R. P., Witsoe, D. A., LaBree, J. W., Remole, W., Layman, T. E., Schoening, H. & Smith, D. The Minnesota impedance cardiograph - theory and applications. Biomed. Eng. J. 9, 410, 1974. 9. Lababidi, Z., Ehmke, D. A., Durnin, R. P., Leaverton, P. E. & Lauer, R. M. Evaluation of impedance cardiac output in children. Pediatrics 47, 870, 1971. 10. Pomerantz, M., Delgado, F. & Eiseman, B. Unsuspected depressed cardiac output following blunt thoracic or abdominal trauma. Surgery 70, 865, 1971. 11. Scheel, K. W., Watson, D. G. & Lehan, P. H. An improved on-line calibrator for dye-dilution curves. J. appl. Physiol. 26, 667, 1969. 12. Vollm, K. R. & Rolett, E. L. Calibration of dyedilution curves by a dynamic method. J. appl. Physiol. 26, 147, 1969. 13. Zambrano, S. S. & Spodick, D. H. Comparative responses to orthostatic stress in normal and abnormal subjects. Evaluation by impedance cardiography. Chest 65, 4, 1974.
Measurement of Cardiac Output by Impedance Cardiography in Patients with Myocardial Infarction Scand J Clin Lab Invest Downloaded from informahealthcare.com by Karolinska Institutet University Library on 06/25/14 For personal use only.
Comparative Evaluation of Impedance and Dye Dilution Methods S. GABRIEL, J.-H. ATTERHOG, L. OR6 & L.-G. EKELUND
Coronary Care Unit, Dept. of Medicine, and Dept. of Clinical Physiology, Karolinska sjukhuset, Stockholm, Sweden
Gabriel, S., Atterhog, J.-H., Oro, L. & Ekelund, L.-G. Measurement of Cardiac Output by Impedance Cardiography in Patients with Myocardial Infarction. Comparative Evaluation of Impedance and Dye Dilution Methods. Scand. J. clin. Lab. Invest. 36, 29-34, 1976. The cardiac output was measured simultaneously by the impedance cardiography and dye dilution methods in 10 patients with acute myocardial infarction 2-3 weeks after admission to the Coronary Care Unit. The impedance cardiac output was on the average 9.7% hlgher than the dye dilution cardiac output. The reproducibility of impedance cardiac output was 4.1%, compared with 5.1% for the dye dilution method. The study showed a satisfactory reliability of impedance in predicting the relative change of cardiac output in response to tilting from the supine to the 30" head-up position, to a 10" head-down position, and to the intravenous administration of propranolol. Key-words: Body tilt; cardiac output; dye dilution technique; impedance cardiograph; myocardial infarction S. Gabriel, M.D., Dept. of Clinical Physiology, Karolinska sjukhuset, 4 1 0 4 01 Stockholm 60, Sweden
In recent years impedance cardiography, a noninvasive method developed by Kubicek et al. (6,7), has been used for the measurement of stroke volume andcardiac output (I-6,9,10,13). In different clinical conditions and in normal subjects impedance cardiography has given fairly accurate measurements of the cardiac output when compared with the dye dilution and Fick methods (1, 2, 4, 6, 9, 10). There is no doubt that such a convenient non-invasive method would be suitable for patients with myocardial infarction. However, these patients may have a poor cardiac function and abnormal myocardial contractility, which may interfere with the accuracy of the method (8). The purpose of this study was to f a t h e r evaluate impedance cardiography as a method for the measurement of cardiac output in patients with clinically stable myocardial infarction and to evaluate
the validity of the method when applied in conditions producing cardiocirculatory changes. MATERIAL AND METHODS Ten patients, 9 men and 1 woman, mean age 54 years, range 43-70 years, with acute myocardial infarction were examined 2-3 weeks after admission to the Coronary Care Unit (CCU). All patients were informed about the purpose of the study, and consent was obtained from each patient. They had sinus rhythm and were free from valvular disease or clinical signs of heart failure. Simultaneous dye dilution curves and impedance cardiograms were recorded at rest in the following situations: a) in supine position after 15 min, b) after 5 min of tilting 30" with the head upwards (30"4), c) after 5 min of tilting 10"with
30
S. Gabriel, J.-H. Atterhog, L. Oro & L.-G. Ekelund
phonocordioqmm
Scand J Clin Lab Invest Downloaded from informahealthcare.com by Karolinska Institutet University Library on 06/25/14 For personal use only.
OSCILLATOR
Fig. 1. The position of the electrodes and the basic system of impedance cardiograph.
VOLTAGE RCK UP DETECTION
the head downwards (lOoJ), d) in the supine position 3 min after intravenous injection of 0.1 mg/kg body weight propranolol (Inderalm). Impedance cardiogriiphy
The method for measuring the cardiac output (CO) has been described previously (6, 7). Four disposable self-adhering aluminized mylar strip electrodes manufactured by the 3M Co., Minnesota, were attached circumferentially to the patient, as seen in Fig. 1. The inner electrodes (2 and 3) were placed around the base of the neck and at the level of the upper border of the manubrium sterni and the xiphisternal joint, respectively. The outer two electrodes (1 and 4) were placed at least 3 cm from the inner electrodes. The electrodes were attached to an IFM Minnesota Impedance Cardiograph, Model 304 A, which supplies a constant sinusoidal alternating current of 4 mA at a frequency of 100 kHz, which is transmitted between electrodes 1 and 4. A voltage gradient is generated between electrodes 2 and 3 proportional to the thoracic impedance (Zo); the latter is read directly on the digital display. A crystal microphone placed at the apes was used to record a phonocardiogram. Four output signals were recorded on a multichannel graphic recorder (Mingograph, Siemens-Elema) : 1) electrocardiogram (ECG), 2) the impedance change during the cardiac cycle ((AZ), 3) the first derivative thoracic impedance (dZ/dt), 4) phonocardiogram. The stroke volume was calculated according to the formula given by Kubicek et al. (6, 7), d V = p ( L / Z O ) ~ . T * ~rnin, Z/~ where ~ dV = stroke volume (ml), p = resistivity of blood (taken as 135 ohm-cm), L = shortest distance (cm) between the
inner electrodes (mean of the front and back measurements), Zo = the mean thoracic impedance in ohms, T = the ventricular ejection time in seconds, and dZ/dt min = the maximal deflection from zero line for dZ/dt, ohms/sec, occurring during the cardiac cycle. In each subject the average of 5 beats was used to calculate the stroke volume. The cardiac output was calculated by multiplying the stroke volume by the heart rate; the latter was obtained from the ECG recording. Dye dilution cardiuc output
A 60-cm Teflon catheter was inserted in an antecubital vein in the right arm by the Seldinger technique. A similar catheter was inserted by the same technique in the left brachial artery. In one patient both catheters were inserted in the right arm. The arterial catheter was connected to a Beckman cardiodensitometer. For each measurement 1 ml of a solution containing 5 mg of Indocyanine Green dye (Cardio Green) was injected intravenously from a calibrated syringe. Arterial blood was withdrawn at a rate of 30 ml/ min and afterwards was reinfused through the venous catheter. The cardiac output was calculated according to the on-line calibration equation as developed by Scheel et al. (11) and Vollm & Rolett (12). Procedure
The patients were examined without premedication. They were kept on the current medication except for any p-adrenergic blocking agent, which was not given on the day of examination. The tape electrodes were attached to the patients as
Cardiac Output by Impedance Cardiography
31
Scand J Clin Lab Invest Downloaded from informahealthcare.com by Karolinska Institutet University Library on 06/25/14 For personal use only.
Table I. Comparison between impedance and dye dilution, stroke volume, and cardiac output in different body positions, in ten patients with myocardial infarction*
Position
Difference Difference A-B in yo Impedance method Dye dilution method A-B of B A
Number Heart of rate, measure- beats/ ments min
Supine Tilting 3Oof Tilting 10"4 Supine after propranolol All measurements
Zo, ohms
SV, ml
sv, ml
co, I/min
s v , co, ml I/min
sv, co, %
%
71312 25.5k2.4 70519 5.0751.50 64f16 45851.13 6.0 0.49 9.4 10.7 82510 26.4f2.4 54511 4.4150.82 48&15 3.8950.99 6.0 0.52 12.0 13.4 73514 25.7k2.0 71522 5.0851.63 70&20 4.995150 1.0 0.09 1.4 1.8
21
61510 25.8k1.8 73522 4.4351.26 6 3 j 1 5 3.8550.92 10.0 0.58 15.9 15.1
86
72+13
25.852.2 67520 4.75j1.36 62f18
4.3351.23
5.0 0.42
8.1
9.7
S.D. SV=stroke volume; CO=cardiac output; Zo=mean thoracic impedance.
described above, after which the arterial and venous catheters were inserted. In every patient two and sometimes three simultaneous dye dilution curves and impedance cardiograms were obtained in each of the four positions mentioned above. In changing from position (b) to position (c) and from the latter to position (d), the patients were kept in the supine position for 5 min. RESULTS The absolute values of cardiac output estimated by impedance cardiography were in general higher Impedance Cardiac Output, i/min
1-
5-
supine tilting 301 tilting 10.1 A after Prooranolol 0
3-
all measurements r.0.853 p..O.OOl 11.86
1
1
3
co,
I/min
22 21 22
* Mean valuesf1
1
B
7 9 Dye Dilution Cardiac Outout. I/min 5
. ,.
Fig. 2. Relationship between absolute values of impedance (y) and dye dilution (x) cardiac output in 10 patients with myocardial infarction. y=0.689+ 0.937 x, S.E.M. 0.715.
than the CO measured by the dye dilution technique (Table I). The difference was largest after intravenous propranolol in supine position, averaging 0.58 I/min or 15.1%, and lowest in the tilting position 10.4, in which it averaged 0.09 I/min or 1.8%. In the supine position the mean impedance CO was 0.49 l/min or 10.7%; in the tilting 30"t position it was 0.52 I/min or 13.4%, higher than the dye dilution CO. For all measurements the absolute impedance CO was 0.42 I/min, or 9.7% higher than the absolute dye dilution CO. The difference was, however, not significant (P>0.05). There was a significant correlation between the absolute values of CO estimated by the impedance method and those measured by the dye dilution method (Fig. 2). The correlation was highly significant in all four positions studied (P0.05.In the tilting loo$ position and after propranolol Zo was practically unchanged. DISCUSSION The present study showed a close relationship between values for cardiac output obtained with impedance cardiography and the dye dilution technique, the mean impedance cardiac output
Table 111. Reproducibility of impedance and dye dilution cardiac output* Method Cardiac output, l/min
Impedance Dyedilution
2
d
4.693 4.317
-0.031 -0.048
S.D.4 S.E.M.d
c%
t
P
n
0.271 0.313
4.1 5.1
-0.688 -0.909
NS
72 72
0.045 0.052
NS
* %=mean value; z=mean of differences between the first and second measurement; S.D.d=standard deviation of the differences; S.E.M.d=standard error of the differences; C=coefficient of variation for the differences; t=t test of the differences; P=level of significance; n=number of measurements.
Cardiac Output by Impedance Cardiography
33
Table IV. Relative changes of impedance and dye dilution cardiac output in response to cardiocirculatory changes. The cardiac output is expressed as mean value in per cent of mean supine value
Scand J Clin Lab Invest Downloaded from informahealthcare.com by Karolinska Institutet University Library on 06/25/14 For personal use only.
Position Tilting 30" t Tilting 10" 4 Supine after propranolol
Number of measurements 19 20 18
being 9.7% higher than the mean dye dilution cardiac output. The difference varied by 1.8%15.1% depending on the body position. This is in agreement with earlier studies in different clinical conditions and in normal subjects (1, 6, 9). The difference could be explained by the individual variation of the blood resistivity. In the present study the blood resistivity was assumed to be constant in all positions studied. Lababidi et al. (9) reported 5.570 higher impedance cardiac output in children with congenital heart disease without shunts or valvular disease. Baker et al. (1) compared cardiac output values obtained by dye dilution and impedance cardiography in ten narmal men, using the blood resistivity values as determined from the subject hematocrit. The impedance cardiac output calculated from the published data was 8% higher than the dye dilution cardiac output. The regression equation, given by these authors, describing this relationship was similar to that in the present study. In an earlier study Harley & Greenfield (2), using a less developed impedance technique, found in 13 normal subjects that the ratio of impedance cardiac output to dye dilution cardiac output was 1.34 during rest and 1.23 following isoproterenol. However, they reported poor correlation between the two estimates. The reproducibility of impedance cardiac output was somewhat better than that of the dye dilution cardiac output. Similar results were reported in the early studies of the method by Kubicek et al. (6) and later by Lababidi et al. (9). For the clinician it is sometimes more important to assess the relative changes in cardiac output than to measure its absolute value. The present study included estimations in different body positions and after injection of propranolol, conditions known to cause cardiovascular changes and to influence the cardiac output.
-
3 Scand. J. clin. Lab. Invest.
Impedance cardiac output Dye dilution cardiac output
% supine 86.2 102.0 91.8
% change
% supine
- 13.8
84.3 108.5 89.1
+ 2.0 -
8.2
% change
- 15.7 $- 8.5
- 10.9
Good agreement was obtained between the two methods in reflecting the relative changes in cardiac output, especially on tilting 30"T and in supine position after propranolol. In the tilting 10.4 position, however, the change of dye dilution cardiac output was somewhat larger (8.5%,) than that of impedance cardiac output (2%). As mentioned above, the impedance cardiac output increased 6% in three patients when they were tilted 10.4, and the heart rate increased 6 beats/ min, while their dye dilution cardiac output decreased 2%. In one patient the impedance cardiac output increased 9% when the heart rate increased 19 beats/min, while the dye dilution cardiac output decreased 1 yo. These findings support the observations made by Baker et al. (1) that impedance values reflect more closely the expected change in cardiac output than do the dilution values. The relative changes of impedance cardiac output in the present study are generally in agreement with earlier studies in normal subjects by Kubicek et al. (6) and Judy et al. (4). Our results, however, differ from those of Zambrano & Spodick (13), who, in patients with stablecoronary disease, reported hardly any change in the impedance cardiac output or stroke volume in response to immediate tilt compared to normal subjects. This was explained by the increased tolerance to orthostatic stress in patients with cardiovascular disease. In conclusion, the present study shows a good validity of the impedance method in the measurement of cardiac output and its relative changes. The reproducibility of the impedance cardiac output was relatively high in spite of the presence in some patients of distortions in the impedance cardiogram, a factor which may introduce an error in the calculation of cardiac output (8)
34
S . Gabriel, J.-H. Atterhog, L. Oro & L.-G. Ekelund
REFERENCES
Scand J Clin Lab Invest Downloaded from informahealthcare.com by Karolinska Institutet University Library on 06/25/14 For personal use only.
1 . Baker, L. E., Judy, W. V., Geddes, L. E., Langley, F. M. & Hill, D. W. The measurement of cardiac output by means of electrical impedance. Cardiovasc. Res. Cent. Bull. 9, 135, 1971. 2. Harley, A. & Greenfield, J. C., Jr. Determination of cardiac output in man by means of impedance plethysmography. Aerospace Med. 39, 248, 1968. 3. Hill, D. W. & Lowe, H. J. The use of the electricalimpedance technique for the monitoring of cardiac output and limb bloodflow during anaesthesia. Med. biol. Eng. 534, Sept. 1973. 4. Judy, W. V., Langley, F. M., McCowen, K. D., Stinnett, D. M., Baker, L. E. & Johnson, P. C. Comparative evaluation of the thoracic impedance and isotope dilution methods for measuring cardiac output. Aerospace Med. 40, 532, 1969. 5. Kottke, F. J., Kubicek, W. G. & Olson, M. E. Evaluation of cardiac competence during rehabilitation following myocardial infarction. Israel J. med. Sci. 9, 568, 1973. 6. Kubicek, W. G., Karnegis, J. N., Patterson, R. P., Witsoe, D. A. & Mattson, R. H. Development and evaluation of an impedance cardiac output system. Aerospace Med. 37, 1208, 1966. 7 . Kubicek, W. G., Patterson, R. P. & Witsoe, D. A. Impedance cardiography as a noninvasive method to monitor cardiac function and other parameters
Aeceived 5 March 1975 Accepted 13 November 1975
of the cardiovascular system. International conference on bioelectrical impedance. Ann. N.Y. Acad. Sci. 724, 1970. 8. Kubicek, W. G., Kottke, F. J., Ramos, M. U., Patterson, R. P., Witsoe, D. A., LaBree, J. W., Remole, W., Layman, T. E., Schoening, H. & Smith, D. The Minnesota impedance cardiograph - theory and applications. Biomed. Eng. J. 9, 410, 1974. 9. Lababidi, Z., Ehmke, D. A., Durnin, R. P., Leaverton, P. E. & Lauer, R. M. Evaluation of impedance cardiac output in children. Pediatrics 47, 870, 1971. 10. Pomerantz, M., Delgado, F. & Eiseman, B. Unsuspected depressed cardiac output following blunt thoracic or abdominal trauma. Surgery 70, 865, 1971. 11. Scheel, K. W., Watson, D. G. & Lehan, P. H. An improved on-line calibrator for dye-dilution curves. J. appl. Physiol. 26, 667, 1969. 12. Vollm, K. R. & Rolett, E. L. Calibration of dyedilution curves by a dynamic method. J. appl. Physiol. 26, 147, 1969. 13. Zambrano, S. S. & Spodick, D. H. Comparative responses to orthostatic stress in normal and abnormal subjects. Evaluation by impedance cardiography. Chest 65, 4, 1974.
