November
Beppu et al.
American
33. Kimura M, Naito H, Ohta M, et al. Measurement of four chambers’ volumes and ventricular masses by cardiac CT examination (in Japanese). J Cardiogr 1983;13:605-15. 34. Refsum H, Junemann M, Lipton MJ, Skioldebrand C, Carlsson E, Tyberg JV. Ventricular diastolic pressure-volume relations and the pericardium: effects of changes in blood volume and pericardial effusion in dogs. Circulation 1981;64:997-1004.
Heart
1990 Journal
35. Smiseth OA, Refsum H, Junemann M, et al. Ventricular diastolic pressure-volume shifts during acute ischemic left ventricular failure in dogs. J Am Co11 Cardiol 1984;3:966-77. 36. Wood FC, Wolferth CC. The tolerance of certain cardiac patients for various recumbent positions (trepopnea). Am J Med Sci 1937;193:354-78.
Noninvasive assessment of cardiac output children using impedance cardiography
in
This study evaluated the effect of intracardiac shunting on the accuracy of thoracic bioimpedance-derived cardiac output determinations. Twenty-six patients, ranging in age from 3 months to 17 years, underwent cardiac catheterization during which simultaneous Fick and impedance measurements of cardiac output were obtained. The subjects were divided into three groups: 10 children with no intracardiac shunts, nine children with predominant left-to-right intracardiac shunts, and seven children with predominant right-to-left intracardiac shunts. Positive correlations between impedance and Fick-derived cardiac output determinatlons were obtained in the non-shunt group (I = 0.84), with lower correlations in the left-to-right shunt group (I = 0.70). In the right-to-left shunt group, the impedance derived cardiac output correlated with Fick pulmonary flow (r = 0.82), but the variability was unacceptably large. Although further study is needed, impedance cardiography appears to have validity as a methodology in pediatric critical care and cardiovascular health research. (AM HEART J 1990;120:1166.)
David S. Braden, MD, Linda Leatherbury, William B. Strong, MD. Augusta, Ga.
MD, Frank A. Treiber,
The validity of impedance cardiography is an important issue because of its potential usefulness as a noninvasive monitoring methodology in pediatric critical care medicine. Impedance cardiography can be used in the intensive care unit to monitor changes in hemodynamic parameters (e.g., cardiac output, systemic vascular resistance) as well as to gauge responses in these parameters to pharmacologic therapy.iv4 This would be helpful in both the postoperative cardiac patient as well as in the child with noncardiac heart failure or shock. Impedance cardiography also has potential usefulness in clinical research assessing the pathophysiology of essential hypertension and other cardiovascular diseases such
From the Section of Cardiology, Medical College of Georgia, Supported by National Institutes of Health Grants HL41781 and HL35073 and by research grant awards from the Research Institute and the Department of Pediatrics at the Medical College of Georgia. Received for publication Dec. 26, 1989; accepted May 28, 1990. Reprint requests: David S. Braden, MD, Section of Cardiology, Medical College 4/l/23479
1166
of Georgia.
1120
15th
St., Augusta,
GA 30912.3710.
PhD, and
as the evaluation of cardiovascular reactivity to psychologic and physical stressors.5, s Impedance cardiography (thoracic bioimpedance) utilizes beat-to-beat analysis of the electrical impedance of the thoracic cavity as a means to measure stroke volume. By multiplying stroke volume (SV) by the heart rate (HR), the thoracic bioimpedance methodology permits the calculation of cardiac output (CO). Impedance cardiography is easily performed and carries no known risk or element of discomfort. The impedance cardiography technique has been validated in animal studies, primarily in the rat and canine populations. 7-11In these studies, impedance cardiography was compared with either thermodilution or electromagnetic flow probe studies and was found to be accurate in showing relative and directional changes in stroke volume. Validation studies in humans have typically involved adults and results have shown that impedance cardiography provides accurate stroke volume and cardiac output values when compared with dye dilution techniques as well as angiographic techniques.‘2-16 A few validation studies have been conducted in children.‘7-20 To
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Table
Cardiac output
by impedance
cardiography
1167
I. Characterization of patient subgroups
Non-shunt (n = 10) L-R shunt (n = 9) R-L shunt (n = 7)
Age (4
Weight (kg)
Height (cm)
BSA Cm’)
L (cm)
Hgb kmldU
67.6 i 19.8 (3.5-204.5) 57.8 ?L 16.7 (3.0-131.5) 62.7 k 23.9 (15.0-157.5)
23.8 ? 6.4 (6.4-68.2) 16.8 f 4.0 (4.2-39.2) 16.3 + 3.5 (8.4-31.5)
107.8 * 10.7 (63-177.8) 98.4 * 11.7 (57-149) 102.0 + 11.6 (7.6-149.5)
0.83 t 0.15 (0.34-1.77) 0.67 k 0.12 (0.26-1.27) 0.69 k 0.10 (0.46-1.14)
20.0 f 1.4 (17.0-30.0) 19.1 * 1.1 (16.0-25.0) 18.3 f 1.6 (15.0-25.0)
11.7 + 0.3 (10.5-12.9) 11.6 k 0.4 (10.0-13.9) 15.9 + 1.1 (10.4-19.8)
All data present as x + SD with range in parentheses. L-R, Left-to-right; R-L, right-to-left: SaOz, arterial saturation
in percent:
BSA, body surface area; L, thoracic
date, the validity of impedance cardiography has not been’ assessed in children with right-to-left intracardiac shunts. This study investigated the accuracy of impedance cardiography in children without intracardiac shunts, in those with predominant leftto-right intracardiac shunts, and in those with predominant right-to-left intracardiac shunts. METHODS Subjects.
All children underwent a complete hemodynamic cardiac catheterization toconfirm their diagnosis,to evaluate the extent of a congenital heart defect, or to determine the successof corrective cardiovascular surgery. Approval for the study wasobtained from the Human Assurance Committee at the Medical College of Georgia. Twenty-six subjects participated and were divided into three groupsbasedon the presenceor absenceof intracardiac shunting, and the predominant direction of the intracardiac shunt. Table I summarizesthe subjects’basal data. The first group consistedof 10 children with no intracardiac shunts. This group was composedof children with right- or left-sided obstructive lesionssuch as aortic or pulmonary stenosis,aswell aschildren with successfully repaired intracardiac defects. The secondgroup consisted of nine children with acyanotic left-to-right intracardiac defects. Seven children with predominant right-to-left shunts made up the third group. Catheterization. The children were sedated approximately 30 minutes prior to their procedure using standard agents. Supplemental sedation was occasionally required with intravenous ketamine (0.5 to 1 mg/kg). For mostcases, the right femoral vein wasusedfor right heart catheterization with percutaneousentry after infiltration of the right inguinal region with 1% lidocaine. Antegrade catheterization was performed utilizing an end-hole wedgecatheter and/or a Berman angiographic catheter (4F to 7F) (Arrow International Inc., Reading, Pa.). Left heart catheterization wasperformed after percutaneousentry into the right or left femoral artery utilizing a 4F to 6F Cook pigtail catheter (Cook Inc., Bloomington, Ind.). In two non-shunt patients, only right-sided catheterization was performed, with the arterial saturations obtained by pulse oximetry. Cardiac output assessment. Systemic and pulmonary blood flows were calculated by the direct Fick method. Di-
SaOz 95.2 2 0.4 (93.0-97.0) 95.3 * 0.3 (94.0-97.0) 84.1 ? 2.4 (75.0-92.0)
length; Hgb, hemoglobin.
rection and magnitude of shunting were alsocalculated using this method. The superior vena caval or right atria1 mid-lateral wall sample was used as the mixed venous sample in children with intracardiac shunts. For those children without intracardiac shunts, the pulmonary arterial samplewasusedas the mixed venous sample.Oxygen saturation wasmeasuredwith a Unistat Oximeter (American Optical, Buffalo, N.Y.) utilizing 0.2 to 0.3 ml aliquots of blood. Oxygen content wascalculated by multiplying the oxygen saturation times the hemoglobin in grams per deciliter ~1.36. For 19 of the patients, oxygen consumption was measured using a Waters instruments continuous flow-through head box (Waters Instruments, Inc., Rochester, Minn.). In seven children, oxygen consumption was assumedutilizing guidelines from the Boston Children’s Hospital with the aid of the child’s age,weight, and heart rate.21Comparison of these children’s oxygen consumption in milliliters per kilogram did not differ significantly from those that were measured (measured values = 6.9 + 2.1 ml/kg; assumedvalues = 7.6 -t 3.5 ml/ kg; p = 0.60). Cardiac output was measuredby impedancecardiography with the child as calm and sedateas possible,either during the time of oxygen consumption measurementand/ or during the attainment of simultaneous arterial and venous saturations. The BoMed NCCOM3 Model 6 thoracic bioimpedancemonitor (BoMed Medical Instruments, Ltd., Irvine, Calif.) wasusedto measurethe electrical bioimpedanceof the thoracic segment.Eight spot electrodes were positioned on both lateral aspectsof the neck and the chest,asdemonstratedin Fig. 1.The NCCOM3 determines stroke volume (SV) by using changesin electrical conductivity of the thoracic segmentthat are due to changesin volume and velocity of blood flow. The thoracic area is measuredwith the two setsof electrodes (one current-injecting, one sensing)positioned at the xiphoid notch in the midaxillary line and at the angle of the jaw on eachside. A high-frequency low-voltage alternating current is injected at 2.5 mA at 70 kHz. The NCCOM3 monitor calculatesthe stroke volume for every heart beat according to the following equation: Stroke volume (SV) = VEPT X VET X EVI/TFI. The VEPT is the physical volume of electrically participating thoracic tissuein milliliters and it is determined by dividing the cube of the patient’s thoracic length by the
1168
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FRONTAL VIEW Fig.
Table
II. Determination
1. Illustration
of impedance
of L value*
Age (mos)
qO of Height
0 2 4 6 8 10 12 14 16 18 20 22 24 and up
29 28 27 26 25 24 23 22 21 20 19 18 17
*From BoMed Medical Instruments with permission.
NCCOMB
for L
User’s Manual.
November 1990 Heart Journal
POSTERIORVIEW cardiography
lead placements.
Statistical analyses. Means and standard deviations, correlations, regression analyses, and Student dependent t tests were computed using the SPSS/PC+ statistical package (SPSS, Inc., Chicago, Ill.). RESULTS
Reproduced
conversion factor 4.25. The thoracic length or the L value is determined by multiplying a constant times the child’s height. The constant is dependent upon the age of the child, as illustrated in Table II. The VET represents the ventricular ejection time in seconds. The EVI is the ejection velocity index in ohms per second, and TFI is the thoracic fluid index in ohms. These last three values, as well as the CO, HR, and SV, are displayed on the face of the NCCOMS monitor. All data were obtained with the instrument in the research slow mode, which automatically deletes artifacts and provides an average of SV, HR, and CO values across successive lo-second intervals. The number of beats actually screened varied dependent upon the HR of the child. In an effort to decrease variability, 8 to 10 successive lo-second intervals were assessed, with mean values determined and used in all statistical analyses.
The means, standard deviations, and ranges of the systemic flow (Qs) (i.e., the cardiac output indexed by body surface area in meters squared); the similarly indexed pulmonary flow (Qp); the ratio of the pulmonary flow to the systemic flow (&p/&s); and the cardiac index as derived by impedance cardiography (Qz) are presented for each group in Table III. These various parameters are presented for each patient by group in order of increasing body surface area in Table IV. Non-shunt. In the non-shunt group, impedance-derived cardiac output correlated well with Fick-derived systemic flow (r = 0.84; p < 0.005). Accuracy varied, with overestimations as high as 29.2% and underestimations as high as 22.9 % . Patient No. 1 was status post repair of truncus arteriosus with moderate truncal insufficiency. Patient J had chronic rheumatic fever with ventricular dysfunction and significant mitral insufficiency. When these two patients were removed from the analysis, correlations improved only slightly (r = 0.86; p < 0.005) and variability of estimations decreased to a range of + 17.0 % to -18.2%. Left-to-right shunt. Correlations between impedance-derived cardiac flow and Fick-derived cardiac flow were lower in the left-to-right shunt group
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Cardiac output
by impedance
cardiography
1169
III. Summary of Fick and impedance blood flows (5 + SD; range)
Table
Qz Non-shunt (n = 10) L-R shunt (n = 9) R-L shunt (n = 7)
Qs
4.44 57 0.34
QP
4.59 + 0.35 (2.41-6.35) 4.60 _? 0.26 (3.60-6.06) 4.38 * 0.44 (2.30-5.86)
(2.70-5.99) 5.32 + 0.51
(3.99-8.65) 4.11 + 0.99 (1.88-9.50)
All flows in liters/minute/square meter. Qs, Systemic flow; Qp, pulmonary flow; Qz, cardiac index by impedance
cardiography;
IV. Summary of patients, descriptive and hemodynamic
Table
QPIQs
4.59 zk 0.35 (2.41-6.35) 8.51 2 2.41 (4.30-24.25) 4.23 k 0.94 (1.99-9.33)
other abbreviations
1.00 2.06 + 0.41 (1.15-4.12) 0.94 +- 0.15 (0.60-1.72)
as in Table I.
data by group
(mo)
Sex
Race
Weight (kg)
BSA (m2)
Qs
Qz
Qp
f% diff)
(% diff)
(mllminlm2)
35 13 14 56 73 30 78 66 138 204.5
M M M M M M M M F M
W B B B B B W B W B
6.4 8.5 9.6 13.3 13.6 16.1 19.6 33.7 48.9 68.2
0.34 0.43 0.46 0.60 0.62 0.67 0.79 1.15 1.46 1.77
4.4 5.1 4.7 5.0 5.5 3.3 6.4 4.4 4.8 2.4
4.8 4.4 5.5 4.5 5.6 2.7 6.0 4.1 3.7 3.1
4.4 5.1 4.7 5.0 5.5 3.3 6.4 4.4 4.8 2.4
+ 9.1 -13.7 +17.0 -10.0 + 1.8 -18.2 - 6.3 - 6.8 -22.9 t29.2
t 9.1 -13.7 t17.0 -10.0 t 1.8 -18.2 - 6.3 - 6.8 -22.9 +29.2
206 209 175* 155 165* 155 165 135 192 131
K L M N 0
15 3 5.5 16 65
E R S
92.5 73 119 131.5
M F F M M F M M M
W W W B B W B B W
4.2 4.7 6.4 9.3 15.2 26.5 24.0 27.5 39.2
0.26 0.28 0.34 0.46 0.66 0.81 0.90 1.02 1.27
6.1 4.3 4.6 4.1 5.7 4.5 3.6 4.1 4.6
8.7 6.8 5.7 4.4 5.2 4.3 4.0 4.3 4.6
24.3 5.5 12.3 5.9 7.7 5.2 4.3 16.7 5.9
+42.6 t58.1 t23.9 + 7.3 - 8.8 - 4.4 tll.l t 4.9 0.0
-64.2 t23.6 -53.7 -23.7 -32.5 -17.3 - 7.0 -74.3 -22.0
194* 204* 148 156 170* 135* 124 150* 164
21 15 15.5 31 49 157.5 150
F M M M F M M
W B W W W B W
9.2 11.5 8.4 13.0 12.7 27.7 31.5
0.46 0.51 0.55 0.58 0.58 1.00 1.14
4.3 5.9 3.9 2.3 4.2 5.4 4.7
4.2 3.9 2.6 1.9 2.0 9.5 4.7
2.6 4.7 3.1 2.0 4.9 9.3 3.0
- 2.3 -33.9 -33.3 -17.4 -52.4 +75.9 0.0
t61.5 -17.0 -16.1 - 5.0 -59.2 t 2.2 t56.7
147 171 160 147 141 168 140
Age Patient
Qz-Qs
Qz-QP
voz
Non-shunt A B C D E F G H I J
L-R shunt
R-L shunt T U V w X Y Z
Abbreviations as in Tables I and III. *Assumed VOp.
(r = 0.70; p < 0.05). In this group, impedance
cardiography tended to overestimate systemic flow by an average of 14.96%) with a range of +58.1% to -8.8%. The greatest disparities were seen in Patients K, L, and M, all of whom had body surface areas less than 0.35 m2. Repeat correlations excluding these three patients improved the correlation between impedance-derived cardiac flow and Fick-derived cardiac flow (r = 0.95; p < 0.005). In the left-to-right shunt group, impedance-derived cardiac flow was not sig-
nificantly correlated p = 0.11).
with pulmonary
flow (r = 0.57;
Right-to-left shunt. In the right-to-left shunt group, the correlation between impedance-derived cardiac flow and Fick-derived cardiac flow was not significant (r = 0.63; p = 0.13). There was a wide range of variation observed in this group with underestimations by as much as 52.4% and overestimations of cardiac flow by as much as 75.9 % . Although impedance-derived cardiac flow in this group correlated better with
1170
Braden
et al
American
November 1990 Heart Journal
NON-SHUNT
O““”i,o--
08
16
24
32
40
NCCOM-3
LEFT-TO-RIGHT
48
64
56
(Umin/m*
72
80
88
)
SHUNT
RIGHT-TO-LEFT
SHUNT
C
r = 0.63
/, 00
08
16
24
32
40
NCCOM-3
48
(Umin/m
56
64
72
80
88
NCCOM-3
2,
(Umlnim’
)
2. Prediction of Fick-derived systemic blood flow cQ.s)by impedance cardiography (NCCOMS). A, Non-shunt group; 6, left-to-right shunt group; and C, right-to-left shunt group. The second lines in (6) and (C) represent the relationship between Fick-derived pulmonary blood flow (Qp) and impedance-derived flow (NCCOM3) in these groups. Fig.
pulmonary
flow than with systemic
flow (r = 0.82;
p < 0.05), the range of differences was unacceptably
large (-59.2% to +61.5%). Regression equations for these three subgroups are presented in Fig. 2. In the non-shunt group, the mean difference between Fick impedance-derived cardiac output was less than 5% ; r was 0.84, slope was 0.86, and intercept was 0.78. The mean difference was -16% in the left-to-right shunt group; r was 0.70, slope was 0.35, and intercept was 2.68. In the rightto-left shunt group, the mean difference between the two methods was 6 % ; r was 0.63, slope was 0.28, and intercept was 3.24. DISCUSSION
Only a few validation studies of impedance cardiography have been conducted in children, and most
have employed other impedance instrumentation (e.g., Minnesota Impedance Cardiograph, Minneapolis, Minn.). ‘T-z0 Lababidi et alli showed that-impedance cardiography (Minnesota Impedance Cardiograph Model 202) accurately predicted CO in children without intracardiac shunts or valvular insufficiency. In children with left-to-right shunts, impedance cardiography correlated better with Fickderived pulmonary blood flow than it did with the systemic blood flow. l7 Studies in premature neonates have also shown that impedance cardiography-derived CO determinations correlated better with the pulmonary flow than with the systemic blood flow. In fact, the magnitude of the cardiac-related deflection in the impedance signal has been found to be useful in the prediction of the magnitude of ductal shunting in these neonates.l*, lg More recently, Miles et
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alS2’ compared impedance cardiography (Minnesota Impedance Cardiograph Model 1304B) derived cardiac output determinations with Fick cardiac output determinations in the catheterization laboratory utilizing measured oxygen consumption values. They observed moderate correlations between the impedance cardiography and Fick cardiac output determinations in both children with no intracardiac shunts (1. = 0.69) and in children with left-to-right intracardiac shunts (r range = 0.69 to 0.89). To date, no study has assessed the accuracy of thoracic bioimpedance in determining CO in children with right-to-left intracardiac shunts. This study evaluated the accuracy of thoracic bioimpedance in three separate groups: children with no cardiac shunts, children with left-to-right intracardiac shunts, and children with predominant right-to-left intracardiac shunts. In this study, cardiac output determinations derived from the BoMed (NCCOM3 Model 6) bioimpedance monitor were positively correlated with Fick-derived systemic flow in patients without intracardiac shunts. Our findings (r = 0.84) were comparable with those of other pediatric studies that used the Minnesota Cardiograph.‘7e,20 As has been noted by Lababidi et al., l7 the accuracy of impedance cardiography is diminished when patients with valvular insufficiency are included in the analyses. In this study, the range of variability improved after the removal of the two patients with left-sided valvular regurgitant lesions, although there was minimal increase in the correlation. This study reconfirmed previous evidence that impedance cardiography is accurate in non-shunt lesions without evidence of significant valvular insufficiency.17 This is not surprising, since total volume output is being compared with Fick forward CO in both regurgitant and shunt lesions. Correlations between impedance-derived cardiac flow and Fick-derived cardiac flow were lower in the left-to-right shunt group. However, these correlations improved with the exclusion of patients of smaller body dimensions. As previously noted by Introna et a1.,22 problems may exist in the determination of thoracic length in smaller children when using impedance cardiography. Therefore further work is needed to assess the accuracy of alternate methods of determining thoracic length in very small pediatric patients. Variability in accuracy of CO assessment in children may also be secondary to volume changes in the thoracic cavity as well as HR variability. In the right-to-left shunt group, impedance-derived cardiac flow failed to correlate significantly with Fick-derived cardiac flow. Although a signifi-
Cardiac output by impedance cardiography
I I71
cant correlation was observed between Fick-derived pulmonary flow and impedance-derived flow, the extreme range of variation in this group suggested that impedance cardiography is unacceptable as a noninvasive measure of either systemic or pulmonary flow in patients with right-to-left shunts. In summary, this study confirmed the accuracy of the BoMed (NCCOM3 Model 6) thoracic bioimpedante instrument in the determination of CO in children with no intracardiac defects, including those with valvular insufficiency. Therefore thoracic bioimpedance or impedance cardiography is useful in the noninvasive monitoring and follow-up of postoperative cardiac surgery patients, as well as in the management of noncardiac intensive care patients.23 Its major value is in determining changes in CO, rather than in determining absolute u&es of CO. Impedance cardiography may also prove useful in cardiovascular reactivity research in which individuals are exposed to physical (e.g., forehead cold stimulation, postural change, handgrip) and/or psychological (e.g., challenging video games, mental arithmetic) stressors. 24Typically, blood pressure and HR responses are assessed, with CO and systemic vascular resistance rarely evaluated. Cardiovascular reactivity has been associated as a possible mechanism in the development of cardiovascular diseases.5j 6 Impedance cardiography may therefore prove useful in the assessment of the pathophysiology of coronary heart disease and hypertension. The authors thank Mrs. Brigid Pursley and Mrs. Pam Hawkins for secretarial assistance, Thomas Rhodes for statistical support, and Bob Stephenson, Chris Williams, Mary Montgomery, and Terry Fulmer for assistance in data collection.
REFERENCES
1. Bernstein DP. Continuous noninvasive real-time monitoring of stroke volume and cardiac output by thoracic electrical bioimpedance. Crit Care Med 1986;14:898. 2. Appel PL, Krom HB, Mackabee J, Fleming AW, Shoemaker WC. Comparison of measurements of cardiac output by bioimpedance. Crit Care Med 1986;14:933. 3. McKinley DF, Pollack MM. A comparison of thoracic bioimpedance to thermodilution cardiac output in critically ill children [Abstract]. Crit Care Med 1987;15:358. 4. Donovan KD, Dobb GJ, Woods PD, Hockings BE. Comparison of transthoracic electrical impedance and thermodilution methods for measuring cardiac output. Crit Care Med 1986; 14:1038. 5. Anderson NB. Racial differences in stress-induced cardiovascular reactivity and hypertension: current status and substantive issues. Psycho1 %ll 1989;105:89. 6. Krantz DS, Manuck SB. Acute psychophysiologic reactivity and risk of cardiovascular disease: a review and methodologic critique. Psycho1 Bull 1984;96:435. I. Gotshall RW. Breav-Pilcher JC. Boelcskevv BD. Cardiac out,put in adult and neonatal rats utilizing impedance cardiography. Am J Physiol 1987;253:H1298. 8. Gotshall RW, Miles DS. Noninvasive assessment of cardiac I
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Evaluation of impedance cardiac output in children. Pediatrics 1971;7:870. Costeloe K, Stocks d, Godfrey S, Mohapatra SN, Hill DW. Cardiac output in the neonatal period using impedance cardiography. Pediatr Res 1977;11:1171. Cotton RB, Lindstrom DP, Olsson T, Riha M, Graham TP, Selstram U, Catterton WZ. Impedance cardiographic assessment of symptomatic patent ductus arteriosus. J Pediatr 1980;96:711. Miles DS, Gotshall RW, Golden JC, Tuuri DT, Beckman RH. Dillon T. Accuracy of electrical impedance cardiography for measuring cardiac output in children with congenital heart defects. Am J Cardiol 1988:61:612. Lock JE, Keane JF, Fellows KE. Hemodynamic evaluation of congenital heart disease. In: Lock JE, et al, eds. Diagnostic and interventional catheterization in congenital heart disease. Boston: Martinus Nijhoff Publishers, 1987:51. Introna RPS, Pruett JK, Crumrine RC, Cuadrado AR. Use ot transthoracic bioimpedance to deterine cardiac output in pediatric patients. Crit Care Med 1988;16:1101. Gastfriend RJ, Van De Water JM, Leonard ML, Macko P, Lynch PR. Impedance cardiography: current status and clinical applications. Am Surg 1986;52:636. Treiber FA, Musante L, Braden DS, Arensman F, Strong WB, Levy M, Leverett S. Racial differences in hemodynamic responses to the cold face stimulus in children and adults. Psychosom Med 1990;52:286.
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33. Kimura M, Naito H, Ohta M, et al. Measurement of four chambers’ volumes and ventricular masses by cardiac CT examination (in Japanese). J Cardiogr 1983;13:605-15. 34. Refsum H, Junemann M, Lipton MJ, Skioldebrand C, Carlsson E, Tyberg JV. Ventricular diastolic pressure-volume relations and the pericardium: effects of changes in blood volume and pericardial effusion in dogs. Circulation 1981;64:997-1004.
Heart
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35. Smiseth OA, Refsum H, Junemann M, et al. Ventricular diastolic pressure-volume shifts during acute ischemic left ventricular failure in dogs. J Am Co11 Cardiol 1984;3:966-77. 36. Wood FC, Wolferth CC. The tolerance of certain cardiac patients for various recumbent positions (trepopnea). Am J Med Sci 1937;193:354-78.
Noninvasive assessment of cardiac output children using impedance cardiography
in
This study evaluated the effect of intracardiac shunting on the accuracy of thoracic bioimpedance-derived cardiac output determinations. Twenty-six patients, ranging in age from 3 months to 17 years, underwent cardiac catheterization during which simultaneous Fick and impedance measurements of cardiac output were obtained. The subjects were divided into three groups: 10 children with no intracardiac shunts, nine children with predominant left-to-right intracardiac shunts, and seven children with predominant right-to-left intracardiac shunts. Positive correlations between impedance and Fick-derived cardiac output determinatlons were obtained in the non-shunt group (I = 0.84), with lower correlations in the left-to-right shunt group (I = 0.70). In the right-to-left shunt group, the impedance derived cardiac output correlated with Fick pulmonary flow (r = 0.82), but the variability was unacceptably large. Although further study is needed, impedance cardiography appears to have validity as a methodology in pediatric critical care and cardiovascular health research. (AM HEART J 1990;120:1166.)
David S. Braden, MD, Linda Leatherbury, William B. Strong, MD. Augusta, Ga.
MD, Frank A. Treiber,
The validity of impedance cardiography is an important issue because of its potential usefulness as a noninvasive monitoring methodology in pediatric critical care medicine. Impedance cardiography can be used in the intensive care unit to monitor changes in hemodynamic parameters (e.g., cardiac output, systemic vascular resistance) as well as to gauge responses in these parameters to pharmacologic therapy.iv4 This would be helpful in both the postoperative cardiac patient as well as in the child with noncardiac heart failure or shock. Impedance cardiography also has potential usefulness in clinical research assessing the pathophysiology of essential hypertension and other cardiovascular diseases such
From the Section of Cardiology, Medical College of Georgia, Supported by National Institutes of Health Grants HL41781 and HL35073 and by research grant awards from the Research Institute and the Department of Pediatrics at the Medical College of Georgia. Received for publication Dec. 26, 1989; accepted May 28, 1990. Reprint requests: David S. Braden, MD, Section of Cardiology, Medical College 4/l/23479
1166
of Georgia.
1120
15th
St., Augusta,
GA 30912.3710.
PhD, and
as the evaluation of cardiovascular reactivity to psychologic and physical stressors.5, s Impedance cardiography (thoracic bioimpedance) utilizes beat-to-beat analysis of the electrical impedance of the thoracic cavity as a means to measure stroke volume. By multiplying stroke volume (SV) by the heart rate (HR), the thoracic bioimpedance methodology permits the calculation of cardiac output (CO). Impedance cardiography is easily performed and carries no known risk or element of discomfort. The impedance cardiography technique has been validated in animal studies, primarily in the rat and canine populations. 7-11In these studies, impedance cardiography was compared with either thermodilution or electromagnetic flow probe studies and was found to be accurate in showing relative and directional changes in stroke volume. Validation studies in humans have typically involved adults and results have shown that impedance cardiography provides accurate stroke volume and cardiac output values when compared with dye dilution techniques as well as angiographic techniques.‘2-16 A few validation studies have been conducted in children.‘7-20 To
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Table
Cardiac output
by impedance
cardiography
1167
I. Characterization of patient subgroups
Non-shunt (n = 10) L-R shunt (n = 9) R-L shunt (n = 7)
Age (4
Weight (kg)
Height (cm)
BSA Cm’)
L (cm)
Hgb kmldU
67.6 i 19.8 (3.5-204.5) 57.8 ?L 16.7 (3.0-131.5) 62.7 k 23.9 (15.0-157.5)
23.8 ? 6.4 (6.4-68.2) 16.8 f 4.0 (4.2-39.2) 16.3 + 3.5 (8.4-31.5)
107.8 * 10.7 (63-177.8) 98.4 * 11.7 (57-149) 102.0 + 11.6 (7.6-149.5)
0.83 t 0.15 (0.34-1.77) 0.67 k 0.12 (0.26-1.27) 0.69 k 0.10 (0.46-1.14)
20.0 f 1.4 (17.0-30.0) 19.1 * 1.1 (16.0-25.0) 18.3 f 1.6 (15.0-25.0)
11.7 + 0.3 (10.5-12.9) 11.6 k 0.4 (10.0-13.9) 15.9 + 1.1 (10.4-19.8)
All data present as x + SD with range in parentheses. L-R, Left-to-right; R-L, right-to-left: SaOz, arterial saturation
in percent:
BSA, body surface area; L, thoracic
date, the validity of impedance cardiography has not been’ assessed in children with right-to-left intracardiac shunts. This study investigated the accuracy of impedance cardiography in children without intracardiac shunts, in those with predominant leftto-right intracardiac shunts, and in those with predominant right-to-left intracardiac shunts. METHODS Subjects.
All children underwent a complete hemodynamic cardiac catheterization toconfirm their diagnosis,to evaluate the extent of a congenital heart defect, or to determine the successof corrective cardiovascular surgery. Approval for the study wasobtained from the Human Assurance Committee at the Medical College of Georgia. Twenty-six subjects participated and were divided into three groupsbasedon the presenceor absenceof intracardiac shunting, and the predominant direction of the intracardiac shunt. Table I summarizesthe subjects’basal data. The first group consistedof 10 children with no intracardiac shunts. This group was composedof children with right- or left-sided obstructive lesionssuch as aortic or pulmonary stenosis,aswell aschildren with successfully repaired intracardiac defects. The secondgroup consisted of nine children with acyanotic left-to-right intracardiac defects. Seven children with predominant right-to-left shunts made up the third group. Catheterization. The children were sedated approximately 30 minutes prior to their procedure using standard agents. Supplemental sedation was occasionally required with intravenous ketamine (0.5 to 1 mg/kg). For mostcases, the right femoral vein wasusedfor right heart catheterization with percutaneousentry after infiltration of the right inguinal region with 1% lidocaine. Antegrade catheterization was performed utilizing an end-hole wedgecatheter and/or a Berman angiographic catheter (4F to 7F) (Arrow International Inc., Reading, Pa.). Left heart catheterization wasperformed after percutaneousentry into the right or left femoral artery utilizing a 4F to 6F Cook pigtail catheter (Cook Inc., Bloomington, Ind.). In two non-shunt patients, only right-sided catheterization was performed, with the arterial saturations obtained by pulse oximetry. Cardiac output assessment. Systemic and pulmonary blood flows were calculated by the direct Fick method. Di-
SaOz 95.2 2 0.4 (93.0-97.0) 95.3 * 0.3 (94.0-97.0) 84.1 ? 2.4 (75.0-92.0)
length; Hgb, hemoglobin.
rection and magnitude of shunting were alsocalculated using this method. The superior vena caval or right atria1 mid-lateral wall sample was used as the mixed venous sample in children with intracardiac shunts. For those children without intracardiac shunts, the pulmonary arterial samplewasusedas the mixed venous sample.Oxygen saturation wasmeasuredwith a Unistat Oximeter (American Optical, Buffalo, N.Y.) utilizing 0.2 to 0.3 ml aliquots of blood. Oxygen content wascalculated by multiplying the oxygen saturation times the hemoglobin in grams per deciliter ~1.36. For 19 of the patients, oxygen consumption was measured using a Waters instruments continuous flow-through head box (Waters Instruments, Inc., Rochester, Minn.). In seven children, oxygen consumption was assumedutilizing guidelines from the Boston Children’s Hospital with the aid of the child’s age,weight, and heart rate.21Comparison of these children’s oxygen consumption in milliliters per kilogram did not differ significantly from those that were measured (measured values = 6.9 + 2.1 ml/kg; assumedvalues = 7.6 -t 3.5 ml/ kg; p = 0.60). Cardiac output was measuredby impedancecardiography with the child as calm and sedateas possible,either during the time of oxygen consumption measurementand/ or during the attainment of simultaneous arterial and venous saturations. The BoMed NCCOM3 Model 6 thoracic bioimpedancemonitor (BoMed Medical Instruments, Ltd., Irvine, Calif.) wasusedto measurethe electrical bioimpedanceof the thoracic segment.Eight spot electrodes were positioned on both lateral aspectsof the neck and the chest,asdemonstratedin Fig. 1.The NCCOM3 determines stroke volume (SV) by using changesin electrical conductivity of the thoracic segmentthat are due to changesin volume and velocity of blood flow. The thoracic area is measuredwith the two setsof electrodes (one current-injecting, one sensing)positioned at the xiphoid notch in the midaxillary line and at the angle of the jaw on eachside. A high-frequency low-voltage alternating current is injected at 2.5 mA at 70 kHz. The NCCOM3 monitor calculatesthe stroke volume for every heart beat according to the following equation: Stroke volume (SV) = VEPT X VET X EVI/TFI. The VEPT is the physical volume of electrically participating thoracic tissuein milliliters and it is determined by dividing the cube of the patient’s thoracic length by the
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FRONTAL VIEW Fig.
Table
II. Determination
1. Illustration
of impedance
of L value*
Age (mos)
qO of Height
0 2 4 6 8 10 12 14 16 18 20 22 24 and up
29 28 27 26 25 24 23 22 21 20 19 18 17
*From BoMed Medical Instruments with permission.
NCCOMB
for L
User’s Manual.
November 1990 Heart Journal
POSTERIORVIEW cardiography
lead placements.
Statistical analyses. Means and standard deviations, correlations, regression analyses, and Student dependent t tests were computed using the SPSS/PC+ statistical package (SPSS, Inc., Chicago, Ill.). RESULTS
Reproduced
conversion factor 4.25. The thoracic length or the L value is determined by multiplying a constant times the child’s height. The constant is dependent upon the age of the child, as illustrated in Table II. The VET represents the ventricular ejection time in seconds. The EVI is the ejection velocity index in ohms per second, and TFI is the thoracic fluid index in ohms. These last three values, as well as the CO, HR, and SV, are displayed on the face of the NCCOMS monitor. All data were obtained with the instrument in the research slow mode, which automatically deletes artifacts and provides an average of SV, HR, and CO values across successive lo-second intervals. The number of beats actually screened varied dependent upon the HR of the child. In an effort to decrease variability, 8 to 10 successive lo-second intervals were assessed, with mean values determined and used in all statistical analyses.
The means, standard deviations, and ranges of the systemic flow (Qs) (i.e., the cardiac output indexed by body surface area in meters squared); the similarly indexed pulmonary flow (Qp); the ratio of the pulmonary flow to the systemic flow (&p/&s); and the cardiac index as derived by impedance cardiography (Qz) are presented for each group in Table III. These various parameters are presented for each patient by group in order of increasing body surface area in Table IV. Non-shunt. In the non-shunt group, impedance-derived cardiac output correlated well with Fick-derived systemic flow (r = 0.84; p < 0.005). Accuracy varied, with overestimations as high as 29.2% and underestimations as high as 22.9 % . Patient No. 1 was status post repair of truncus arteriosus with moderate truncal insufficiency. Patient J had chronic rheumatic fever with ventricular dysfunction and significant mitral insufficiency. When these two patients were removed from the analysis, correlations improved only slightly (r = 0.86; p < 0.005) and variability of estimations decreased to a range of + 17.0 % to -18.2%. Left-to-right shunt. Correlations between impedance-derived cardiac flow and Fick-derived cardiac flow were lower in the left-to-right shunt group
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Cardiac output
by impedance
cardiography
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III. Summary of Fick and impedance blood flows (5 + SD; range)
Table
Qz Non-shunt (n = 10) L-R shunt (n = 9) R-L shunt (n = 7)
Qs
4.44 57 0.34
QP
4.59 + 0.35 (2.41-6.35) 4.60 _? 0.26 (3.60-6.06) 4.38 * 0.44 (2.30-5.86)
(2.70-5.99) 5.32 + 0.51
(3.99-8.65) 4.11 + 0.99 (1.88-9.50)
All flows in liters/minute/square meter. Qs, Systemic flow; Qp, pulmonary flow; Qz, cardiac index by impedance
cardiography;
IV. Summary of patients, descriptive and hemodynamic
Table
QPIQs
4.59 zk 0.35 (2.41-6.35) 8.51 2 2.41 (4.30-24.25) 4.23 k 0.94 (1.99-9.33)
other abbreviations
1.00 2.06 + 0.41 (1.15-4.12) 0.94 +- 0.15 (0.60-1.72)
as in Table I.
data by group
(mo)
Sex
Race
Weight (kg)
BSA (m2)
Qs
Qz
Qp
f% diff)
(% diff)
(mllminlm2)
35 13 14 56 73 30 78 66 138 204.5
M M M M M M M M F M
W B B B B B W B W B
6.4 8.5 9.6 13.3 13.6 16.1 19.6 33.7 48.9 68.2
0.34 0.43 0.46 0.60 0.62 0.67 0.79 1.15 1.46 1.77
4.4 5.1 4.7 5.0 5.5 3.3 6.4 4.4 4.8 2.4
4.8 4.4 5.5 4.5 5.6 2.7 6.0 4.1 3.7 3.1
4.4 5.1 4.7 5.0 5.5 3.3 6.4 4.4 4.8 2.4
+ 9.1 -13.7 +17.0 -10.0 + 1.8 -18.2 - 6.3 - 6.8 -22.9 t29.2
t 9.1 -13.7 t17.0 -10.0 t 1.8 -18.2 - 6.3 - 6.8 -22.9 +29.2
206 209 175* 155 165* 155 165 135 192 131
K L M N 0
15 3 5.5 16 65
E R S
92.5 73 119 131.5
M F F M M F M M M
W W W B B W B B W
4.2 4.7 6.4 9.3 15.2 26.5 24.0 27.5 39.2
0.26 0.28 0.34 0.46 0.66 0.81 0.90 1.02 1.27
6.1 4.3 4.6 4.1 5.7 4.5 3.6 4.1 4.6
8.7 6.8 5.7 4.4 5.2 4.3 4.0 4.3 4.6
24.3 5.5 12.3 5.9 7.7 5.2 4.3 16.7 5.9
+42.6 t58.1 t23.9 + 7.3 - 8.8 - 4.4 tll.l t 4.9 0.0
-64.2 t23.6 -53.7 -23.7 -32.5 -17.3 - 7.0 -74.3 -22.0
194* 204* 148 156 170* 135* 124 150* 164
21 15 15.5 31 49 157.5 150
F M M M F M M
W B W W W B W
9.2 11.5 8.4 13.0 12.7 27.7 31.5
0.46 0.51 0.55 0.58 0.58 1.00 1.14
4.3 5.9 3.9 2.3 4.2 5.4 4.7
4.2 3.9 2.6 1.9 2.0 9.5 4.7
2.6 4.7 3.1 2.0 4.9 9.3 3.0
- 2.3 -33.9 -33.3 -17.4 -52.4 +75.9 0.0
t61.5 -17.0 -16.1 - 5.0 -59.2 t 2.2 t56.7
147 171 160 147 141 168 140
Age Patient
Qz-Qs
Qz-QP
voz
Non-shunt A B C D E F G H I J
L-R shunt
R-L shunt T U V w X Y Z
Abbreviations as in Tables I and III. *Assumed VOp.
(r = 0.70; p < 0.05). In this group, impedance
cardiography tended to overestimate systemic flow by an average of 14.96%) with a range of +58.1% to -8.8%. The greatest disparities were seen in Patients K, L, and M, all of whom had body surface areas less than 0.35 m2. Repeat correlations excluding these three patients improved the correlation between impedance-derived cardiac flow and Fick-derived cardiac flow (r = 0.95; p < 0.005). In the left-to-right shunt group, impedance-derived cardiac flow was not sig-
nificantly correlated p = 0.11).
with pulmonary
flow (r = 0.57;
Right-to-left shunt. In the right-to-left shunt group, the correlation between impedance-derived cardiac flow and Fick-derived cardiac flow was not significant (r = 0.63; p = 0.13). There was a wide range of variation observed in this group with underestimations by as much as 52.4% and overestimations of cardiac flow by as much as 75.9 % . Although impedance-derived cardiac flow in this group correlated better with
1170
Braden
et al
American
November 1990 Heart Journal
NON-SHUNT
O““”i,o--
08
16
24
32
40
NCCOM-3
LEFT-TO-RIGHT
48
64
56
(Umin/m*
72
80
88
)
SHUNT
RIGHT-TO-LEFT
SHUNT
C
r = 0.63
/, 00
08
16
24
32
40
NCCOM-3
48
(Umin/m
56
64
72
80
88
NCCOM-3
2,
(Umlnim’
)
2. Prediction of Fick-derived systemic blood flow cQ.s)by impedance cardiography (NCCOMS). A, Non-shunt group; 6, left-to-right shunt group; and C, right-to-left shunt group. The second lines in (6) and (C) represent the relationship between Fick-derived pulmonary blood flow (Qp) and impedance-derived flow (NCCOM3) in these groups. Fig.
pulmonary
flow than with systemic
flow (r = 0.82;
p < 0.05), the range of differences was unacceptably
large (-59.2% to +61.5%). Regression equations for these three subgroups are presented in Fig. 2. In the non-shunt group, the mean difference between Fick impedance-derived cardiac output was less than 5% ; r was 0.84, slope was 0.86, and intercept was 0.78. The mean difference was -16% in the left-to-right shunt group; r was 0.70, slope was 0.35, and intercept was 2.68. In the rightto-left shunt group, the mean difference between the two methods was 6 % ; r was 0.63, slope was 0.28, and intercept was 3.24. DISCUSSION
Only a few validation studies of impedance cardiography have been conducted in children, and most
have employed other impedance instrumentation (e.g., Minnesota Impedance Cardiograph, Minneapolis, Minn.). ‘T-z0 Lababidi et alli showed that-impedance cardiography (Minnesota Impedance Cardiograph Model 202) accurately predicted CO in children without intracardiac shunts or valvular insufficiency. In children with left-to-right shunts, impedance cardiography correlated better with Fickderived pulmonary blood flow than it did with the systemic blood flow. l7 Studies in premature neonates have also shown that impedance cardiography-derived CO determinations correlated better with the pulmonary flow than with the systemic blood flow. In fact, the magnitude of the cardiac-related deflection in the impedance signal has been found to be useful in the prediction of the magnitude of ductal shunting in these neonates.l*, lg More recently, Miles et
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alS2’ compared impedance cardiography (Minnesota Impedance Cardiograph Model 1304B) derived cardiac output determinations with Fick cardiac output determinations in the catheterization laboratory utilizing measured oxygen consumption values. They observed moderate correlations between the impedance cardiography and Fick cardiac output determinations in both children with no intracardiac shunts (1. = 0.69) and in children with left-to-right intracardiac shunts (r range = 0.69 to 0.89). To date, no study has assessed the accuracy of thoracic bioimpedance in determining CO in children with right-to-left intracardiac shunts. This study evaluated the accuracy of thoracic bioimpedance in three separate groups: children with no cardiac shunts, children with left-to-right intracardiac shunts, and children with predominant right-to-left intracardiac shunts. In this study, cardiac output determinations derived from the BoMed (NCCOM3 Model 6) bioimpedance monitor were positively correlated with Fick-derived systemic flow in patients without intracardiac shunts. Our findings (r = 0.84) were comparable with those of other pediatric studies that used the Minnesota Cardiograph.‘7e,20 As has been noted by Lababidi et al., l7 the accuracy of impedance cardiography is diminished when patients with valvular insufficiency are included in the analyses. In this study, the range of variability improved after the removal of the two patients with left-sided valvular regurgitant lesions, although there was minimal increase in the correlation. This study reconfirmed previous evidence that impedance cardiography is accurate in non-shunt lesions without evidence of significant valvular insufficiency.17 This is not surprising, since total volume output is being compared with Fick forward CO in both regurgitant and shunt lesions. Correlations between impedance-derived cardiac flow and Fick-derived cardiac flow were lower in the left-to-right shunt group. However, these correlations improved with the exclusion of patients of smaller body dimensions. As previously noted by Introna et a1.,22 problems may exist in the determination of thoracic length in smaller children when using impedance cardiography. Therefore further work is needed to assess the accuracy of alternate methods of determining thoracic length in very small pediatric patients. Variability in accuracy of CO assessment in children may also be secondary to volume changes in the thoracic cavity as well as HR variability. In the right-to-left shunt group, impedance-derived cardiac flow failed to correlate significantly with Fick-derived cardiac flow. Although a signifi-
Cardiac output by impedance cardiography
I I71
cant correlation was observed between Fick-derived pulmonary flow and impedance-derived flow, the extreme range of variation in this group suggested that impedance cardiography is unacceptable as a noninvasive measure of either systemic or pulmonary flow in patients with right-to-left shunts. In summary, this study confirmed the accuracy of the BoMed (NCCOM3 Model 6) thoracic bioimpedante instrument in the determination of CO in children with no intracardiac defects, including those with valvular insufficiency. Therefore thoracic bioimpedance or impedance cardiography is useful in the noninvasive monitoring and follow-up of postoperative cardiac surgery patients, as well as in the management of noncardiac intensive care patients.23 Its major value is in determining changes in CO, rather than in determining absolute u&es of CO. Impedance cardiography may also prove useful in cardiovascular reactivity research in which individuals are exposed to physical (e.g., forehead cold stimulation, postural change, handgrip) and/or psychological (e.g., challenging video games, mental arithmetic) stressors. 24Typically, blood pressure and HR responses are assessed, with CO and systemic vascular resistance rarely evaluated. Cardiovascular reactivity has been associated as a possible mechanism in the development of cardiovascular diseases.5j 6 Impedance cardiography may therefore prove useful in the assessment of the pathophysiology of coronary heart disease and hypertension. The authors thank Mrs. Brigid Pursley and Mrs. Pam Hawkins for secretarial assistance, Thomas Rhodes for statistical support, and Bob Stephenson, Chris Williams, Mary Montgomery, and Terry Fulmer for assistance in data collection.
REFERENCES
1. Bernstein DP. Continuous noninvasive real-time monitoring of stroke volume and cardiac output by thoracic electrical bioimpedance. Crit Care Med 1986;14:898. 2. Appel PL, Krom HB, Mackabee J, Fleming AW, Shoemaker WC. Comparison of measurements of cardiac output by bioimpedance. Crit Care Med 1986;14:933. 3. McKinley DF, Pollack MM. A comparison of thoracic bioimpedance to thermodilution cardiac output in critically ill children [Abstract]. Crit Care Med 1987;15:358. 4. Donovan KD, Dobb GJ, Woods PD, Hockings BE. Comparison of transthoracic electrical impedance and thermodilution methods for measuring cardiac output. Crit Care Med 1986; 14:1038. 5. Anderson NB. Racial differences in stress-induced cardiovascular reactivity and hypertension: current status and substantive issues. Psycho1 %ll 1989;105:89. 6. Krantz DS, Manuck SB. Acute psychophysiologic reactivity and risk of cardiovascular disease: a review and methodologic critique. Psycho1 Bull 1984;96:435. I. Gotshall RW. Breav-Pilcher JC. Boelcskevv BD. Cardiac out,put in adult and neonatal rats utilizing impedance cardiography. Am J Physiol 1987;253:H1298. 8. Gotshall RW, Miles DS. Noninvasive assessment of cardiac I
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