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Effect of Postural Stress on Left Ventricular Performance Using the Continuous-Wave Doppler Technique
Effect of Postural Stress on Left Ventricular Performance Using the Continuous-Wave Doppler Technique* Ricardo A. Brown, Ph.D.; Kathleen A. McCormick, Ph.D.; Peter V. \&itkevicius, M.D.; and jerome L. Fleg, M.D. To evaluate the effect of postural shifts on cootinuous-wave
Doppler indices of left ventricular perlOrmance in normal man, we recorded Doppler sipals suprasteroally in 69 healthy voluoteers, ranging in age from !0 to 86 yean, in the supine position and I min after assumption of sitting and standing postures. AD indices clec:reased progressively with increasing orthostasis: peak acceleration (PIA): 15.6:t4.5 mlsl to 14.8:t4.0 mlsl to 13.6:t4.6 mlsl; peak velocity (PKV): 0.64:t0.18 m/s to 0.58:t0.17 m/s to 0.56:t0.17 mls; stroke distaoce (SD): 11.4:t3.7 em to 9.8:t3.4 em to 8.0:t1.8 em; SD x heart rate (VIB): 717:t171 em to 655:tl68 em to 57!:t117 em, from supine to sitting to standing, respectively (p
he study of the effects of postural changes on T cardiac performance in normal subjects is important in that it provides a model for assessment of performance under various pathophysiologic conditions influenced by posture. In the past two decades, the effects of posture on cardiac performance have been investigated extensively, yet the results remain controversial. Some authors have shown that cardiac output decreased on changing from recumbency to a sitting position, a.s whereas others reported no change at all. 8 •7 Some of the differences in the results may be related to differences in age, sex, health, medication status of the subjects studied, and the diversity of measuring techniques used. However, consistent among these investigations was a decrease in stroke volume with increasing orthostasis. During this same period, several investigators, using either Doppler echocardiography and/or electromagnetic flow probes, confirmed that ascending aortic blood flow velocity and its derivatives were sensitive indices of global left ventricular performance.s- 13 Given the technical difficulty associated with monitoring postural changes in cardiac performance using two-dimensional (2D) echocardiography and radionuclide imaging, the continuous-wave Doppler technique might provide a convenient .noninvasive means *From the Laboratory of Behavioral Sciences (Drs. Brown and McCormick), Laboratory ol CardiovascuJar Science (Dn. Vaitkevicius and Fleg), NatioDallnstitute on Aging, NatioDal Institutes ol Health, Gerontolog)' Research Center, -Baltimore, Md, and the Department of Physiology (Dr. Brown), Wayne State University, Detroit, Mich. Manuscript received November 8; revision~ February 9:1. Reprint 1'8qf168U: Dr. Brown, Gerontology BaeGich Center; NflflontJl lrutitute on Agjng, 4940 &.tern Aomae, llolflmor8 21224 738
heart rate increased modestly from 6!.4 :t 10.0 bpm supine, to 66.9:tl1.4 bpm sitting, to 71.3:t9.9 bpm stMding (p<.OOI~ Similar postural changes in Doppler variables were seen in all three age groups (10 to 44 years; 45 to 64 years; and 65 to 86 years~ Thus, orthostasis in normal subjects is accompanied by a reduction in all cootinuouswave Doppler indices of left ventricular performance, regardless of age. (Chat 1991; 100:138-43)
or
ANOVA• analysis variaace; BLSA. • Baltimwe I.oacitudiaal Study Agio&; KA•peak acceJentioa; PKV•peak modal
or
velocity; SD•Strob clisiaace or systolic 8ow velocity inlep"ll); VJH Veloeity integral-Mart rate product
=
for monitoring the cardiac response to postural shifts. To date and to our knowledge, there has been only one investigation using this technique to study the effect of posture on left ventricular performance. 14 These investigators showed an orthostatic decline in peak acceleration and peak velocity from supine to standing. An age-related decline among these parameters was also shown. However, left ventricular performance in the sitting position was not studied nor was the Dopple:Mlerived estimate of cardiac output calculated. The present investigation was undertaken to do the following: (1) study the effects on left ventricular performance of postural shifts from supine-to-sitting and sitting-to-standing, using the continuous-wave Doppler methodology; (2) examine the influence of age on these responses to orthostasis; and (3) determine the mechanisms used to adjust left ventricular performance under increasing degrees of orthostasis. METHODS
SelscHon
of Subject 1bpulation
The 69 subjects of this study (41 men, 28 women) were volunteer participants fro~ the Baltimore Longitudinal Study on Aging (BLSA) and ranged in age from 20 to 86 years. The demographic characteristics of the BLSA participants have been described in detail elsewhere.,. This population is predominantly white, welleducated, uppel'-middle class with above-average income and access to good health care. The subjects receive 21/a days of extensive physical, physiologic, and psychologic euminaHnns biennially at the Gerontology Besearch Center in Baltimore, Md. All subjects undergo a detailed physical examination on each visit. Subjects were included in the study provided they had no clinical evidence of cardiac, pulmonary. or any other signi6cant disease, had a resting blood pressure below 16«W5 mm Hg, received no medication that could a8"ect cardiopulmonary function, and demonstrated a nonischemic ECG response to maximal treadmill exercise.
C~nuow-
Wtwe Doppler Recording of Aortic Flow lelocity
Ascending aortic blood Bow velocity was evaluated transcutaneously from the suprasternal notch with the use of a small handheld nonimaging transducer connected to the input stage Qf a continuous-wave Doppler Bowmeter (Exerdop, Quinton Instruments Co). The transducer, which functions as both a transmitter (at 3 MHz) and receiver, is positioned so that the intercept angle is assumed to be parallel to the direction of Bow. The device is equipped with an LED readout that is proportional to the degree to which Doppler signals are parallel to the axis of aortic blood Bow. Unidirectional signals are bandpass filtered with 3-dB cutoft" frequencies of 480 Hz and U.S kHz, prior to entry into a constant amplitude velocity circuit that tracks instantaneous peak modal velocity (PKV) over a 11111ge of 0.2 to 2.5 m/s. Peak acceleration (PKA) and the systolic Bow velocity integral (also termed stroke distance, or SD) are internally derived through differentiation and integration, respectively. Peak acceleration is detected over a range o£2 to99m/sls. Validation oftheaccuracyofPICV and PKA recorded, using the continuous-wave Doppler with values obtained using electromagnetic Bowmetry, has resulted in correlations of0.95 and 0.96, respectively.•• Stroke distance is a one-dimensional approximation of stroke volume that does not take into account the crosssectional area of the aorta. 11 TherefOre, the product of SD and heart rate (the velocity integral heart rate product, or VIH) yields a Dopplel"derived one-dimensional estimate of cardiac output. Excellent correlations between VIH and invasively obtained cardiac output at rest and over a wide range of activity have been reported ...... All subjects were studied in the postabsorptive state between 9 AM and noon. After informed consent was obtained, baseline continuous-wave Doppler measurements of left venbicular performance were obtained in the supine position after 15 minutes of rest. Doppler measurements were then repeated in the sitting and standing positions, approximately 2 min after the assumption of each posture. The algorithm (Exerdop) excludes beats with a signalto-noise ratio below 10 dB and those occurring outside the limits of an empirically derived stack average of the previous eight data points. A minimum of ten acceptable beats ful611ing these criteria was required in each position for inclusion in the study...... \\\:! recorded the percentage of acceptable beats for each posture. The left brachial arterial blood pressure was determined by auscultation using a mercury sphygmomanometer. The diastolic blood pressure was indicated by Korotkoft" phase five. Heart rate and rhythm were continuously monitored using electrocardiographic leads aVF, V,, andv•.
DGta AriGly&i& Statistical difFerences for Doppler variables in the three positions were compared by a one-way analysis of variance (ANOVA) for repeated measures. Linear regression analysis was used to determine correlation between variables. To find out ifsignificant linear and/or quadratic trends were present, an orthogonal polynomial model was employed. To determine whether age affected the Doppler measurements, we divided the sample into three age groups: group 1, 20 to 44 years (17 men, 11 women); group 2, 45 to 64 years (14 men, ten women); and group 3, 65 to 86 years (ten men, seven women). The postural responses of these groups were compared using repeated measures ANOVA. A level of p<0.05 was considered statistically significant. Data are reported as the mean± the SD unless otherwise stated.
REsuLTS Population Characteristics
The mean age of the 69 subjects was 50.2 ± 18.3 years (range, 20 to 86 years); body mass index averaged 25.3±5.0 kg/m2 , and baseline seated blood pressure
was 130.7±20.9178.1±9.3 mm Hg. There were no gender differences with regard to age, body mass index, or resting blood pressure. Ninety percent of the participants were nonsmokers.
Postural Effects on Doppler \hriables and Heart Rate
For the entire group, the average percentage of acceptable beats was 73 ± 23, 69 ± 22, and 70 ± 23 in the supine, sitting, and standing positions, respectively. There was no significant difference with regard to the mean values of acceptable beats among these three postures. The postural effects on Dopplel'derived variables and heart rate are shown in Thble I. Peak acceleration (PKA), peak velocity (PKV), SD, and the velocity integral heart rate product (VIH) decreased by linear trend analysis with increasing orthostasis. By contrast, heart rate increased significantly and linearly on the assumption of an upright posture. Blood pressure was 130.7 ± 20.9178.1 ± 9.4 mm Hg when sitting, and did not change significantly on standing (124.8±19.6176.4±9.6 mm Hg; p=NS for both systolic and diastolic pressures). &-cent Change in Doppler \hriables and Heart Rate Supine-to-Sitting: The percentage of change in
Doppler variables and heart rate from the supine-tositting position for the group is shown in Figure lA. After the assumption of the sitting position, heart rate increased significantly (7.4 percent), but SD declined substantially ( -11.4 percent). The decrements in PKA (-7.5 percent) and PKV (-6.9 percent) were also significant, but the decrease in VIH (- 5.5 percent) was not. Thus, the decrease in SD that occurred with this postural shift was essentially offset by the increase in heart rate. Sitting-to-Standing: Figure IB shows the percentage of change in Doppler variables and heart rate from the sitting-to-standing position. As was seen in Figure lA, the change in SD (- 15.7 percent) is greater than that for any other variables during this postural shift. The increase in heart rate (8.1 percent) was not sufficient to offset the large decline in SD, Table 1-Steodr/-State Doppkr-~Jerived Vcaritlbla and Hetn1 Bt&te ira Supine, Sitting, and Standing Poatioru
Supine Mean SD Sitting Mean SD Standing Mean SD p(ANOVA) p (Linear trend)
HR,
VIH,
bpm
em/min
PICA, m/sls
PKv, m/s
15.6 4.5
0.64 0.18
11.4
62.4
3.7
10.0
717 272
14.0 4.0
0.58 0.17
9.8 3.4
66.9 12.1
655 268
13.6 4.6 0.001 0.001
0.56 0.17 0.001 0.001
8.0 2.8 0.001 0.001
71.3 9.9 0.001 0.001
572 217 0.001 0.001
SD, em
CHEST I 100 I 3 I SEPTEMBER. 1991
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Correlation betweeen Postural Changes in VIH, Heart Rate, and SD
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VIH
l. Effects of postural shifts on the change in Doppler
variables and heart rate. Bars represent the average percent change ± S EM fi>r each variable. Top lbnel = supine-to-sitting postural shift; hotto m panel= sitting-to-standing postural shift; HR =heart rate; SD =stroke distance; PKA =peak acceleration; PKV =peak velocity; VIH =velocity integral-heart rate product; one asterisk= p
resulting in a significant fall in VIH (- 8.5 percent). Small nonsignificant decreases in PKA ( -1.6 percent) and PKV (- 2.6 percent) were seen. Age Effect: The effect of age on Doppler-derived variables and heart rate is presented in Figure 2. There was a significant age-related attenuation ofPKA (Fig 2A), PKV (Fig 2B), SO (Fig 2C), and VIH (Fig 20) in the supine and sitting positions. In the standing position, although there was an age-related decline in PKV, there were no significant differences in PKA, SD, or VIH across the three age groups. Similarly, although not shown, there was no age effect on heart rate in any posture. The effect of age on the postural changes in Dopplerderived variables and heart rate was also analyzed. There was a significant age-related attenuation of the 740
postural decline in PKA (supine-to-sitting, p<0.05) and SD (sitting-to-standing, p<0.009). However, age had no effect on the change in PKV, SD, heart rate, or VIH in response to postural shifts. The change in the Doppler-derived estimate of cardiac output, VIH, was also expressed as a continuous function of age by linear regression analysis. The correlations of the change in VIH with age were r= -0.12 from supineto-sitting position and r = -0.21 from sitting-to-standing position (both NS).
Regression analyses were performed to determine whether it was the change in heart rate or in SD that was more important in mediating the postural change in VIH. The postural change in VIH as a function of the postural change in SD, from supine-to-sitting position, is shown in Figure 3A; the corresponding sitting-to-standing relationship is shown in Figure 3C. Strong positive relationships between the change in VIH and the change in SD were seen in response to both postural shifts. By contrast, there was no correlation between the change in VIH and the change in heart rate in response to the supine-to-sitting postural shift (Fig 3B), and only a weak correlation between these variables when shifting from sitting to standing (Fig 30). The influence of age on the relationships between the postural change in VIH, heart rate, and SD is summarized in Table 2. In assumption of the sitting position (Table 2A), there was no significant correlation between the change in VIH and the change in heart rate across the three age groups. However, a strong positive correlation was seen between the change in VIH and that for SD for all three age groups, as seen in the overall sample. The magnitude of the correlation coefficients indicates that 81 to 91 percent of the postural change in VIH can be accounted for by the change in SD . The slope of this relationship was similar across all age groups. Thus, for a given change in SO, the change in VIH was essentially the same for the elderly as it was for the younger or middle-aged groups. By contrast, during the sitting-to-standing postural shift (Table 2B), the change in VIH is more tightly correlated with the change in SD with advancing age while postural heart rate changes have a similar modest effect on VIH changes across age groups. DISCUSSION
The results of this study show that in normal man the average values for all continuous-wave Doppler indices of left ventricular performance, including PKA, PKV, SD, and the Doppler-derived estimate of cardiac output (VIH) decrease with increasing levels of orthostasis. The decrease in VIH seen with increasing degrees of orthostasis occurred despite an increase Effect of Postural Stress on LV Performance (Brown et el)
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in heart rate. Therefore, the mechanism for the decrease in VIH was through a significant decrease in SD, the Doppler counterpart of stroke volume. Despite age-related declines in supine values for all Doppler indices, the directional changes in these indices with orthostasis and the strong dependence of VIH changes on changes in SD were similar across age. Several prior investigations have shown upright cardiac output to decline below supine values or remain the same as supine values. 1"7 In these studies, stroke volume was consistently diminished with orthostasis. Although beat-by-beat indices of left ventricular performance were not reported in early studies, more recent reports suggest that ejection fraction was minimally affected by posture. 6 •7 An early report by Granath et al,1 using right-heart catheterization to evaluate left ventricular performance, showed that older men had reduced cardiac outputs and stroke volumes at rest compared with younger men in the recumbent position, but had similar values in the sitting position. 51 Using radionuclide cardiac imaging, Rodeheffer et al7 found a reduction in stroke volume, from 101 ml supine to 93 ml sitting, but no significant postural change in cardiac output in 64 men, ages 25 to 86 years; similar &ndings were noted regardless of age. More recently, Lazarus et al, 14 using the continuous-wave Doppler, noted an age-related decline in PKA and P.KV in the supine but not in the standing
FIGURE 2. Age trends in Doppler variables as a function of position. Closed circles= young (20 to 44 years); open triangles= middle aged (45 to 64 years); X= old (65 to 86 years); Panel A= peak acceleration (PICA); panel B =peak velocity (PICV); panel C =stroke distance (SD); panel D =velocity integral-heart rate product (VIH). In the supine and sitting positions, a significant age group ell"ect is noted for all four Doppler variables (p<0.01 by analysis of variance); however, only PKV was related to age In the standing position.
position; values for SD and VIH were not reported. In the present study, the Doppler-derived estimate of cardiac output, VIH, decreased with age in both the supine and sitting postures, but not in the standing position. Although this age-related decline in SD and VIH is in apparent contrast to both echocardiographic Table 2-l.inear Begnruion ofPo.turol Claanga in Doppler-l>erived \briGbla tmd lleGr1 &te in Thrw Age Groupa•
Age, yr
A. Supine-sitting VIHvsHR
20-44 45-64
65-86 VIH vsSD B. Sitting-standing VIHvsHR
VIH vs SD
20-44 45-64 65-86 20-44 45-64 65-86 20-44 45-64
65-86
Slope Coefficient of Linear Regression 4.83 0.95 3.74 63.4 56.9 64.0 4.01 3.79 7.13 55.0 59.3 67.5
rp 0.29 0.07 0.27
0.14 0.76 0.29
0.90 0.94 0.95
0.001 0.001 0.001
0.49 0.43 0.43
0.01 0.05 0.08
0.63 0.81 0.94
0.001 0.001 0.001
*HR =heart rate; SD =stroke distance; VIH =stroke distance x heart rate; age groups: 20 to 44 years, n = 28; 45 to 64 years, n=24; and 65 to 86 years, n= 17. CHEST I 100 I 3 I SEPTEMBER, 1991
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and radionuclide estimates of resting stroke volume and cardiac output in other samples from within the BLSA, 7.J0·11 it is important to emphasize that the DoppleNlerived variables are one-dimensional estimates of stroke volume and cardiac output. To convert these Doppler variables to actual volumes, they must be multiplied by the aortic cross-sectional area, which is known to increase with advancing age.• Further· more, anatomic factors might result in a greater angle between the Doppler beam and the direction of ascending aortic blood flow in older subjects. It is therefore possible that actual stroke volume and cardiac output may not be age related in the present population. Experimental evidence suggests that peak acceleration (PKA) of blood from the left ventricle is associated with both the maximum force generated by the heart in early systole and the maximal initial velocity of shortening of the left ventricular muscle. 11 Thus, it is thought that the PKA of blood from the ascending aorta is an index of cardiac contractility and a manifestation of left ventricular performance. In our study. the decrease in PKA that occurs with increasing degrees of orthostasis suggests a posture-mediated decrease in contractility. This hypothesis is supported by the parallel decreases in PKY, SD, and VIH with
t-DI
, •. 186 P•NS
I
• I
SITIING- STANDING
• •
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(,) ..J
F'JcuRE 3. Correlation of postural changes in VIH vs postural changes in SD and heart rate for all subjects. Panels A and B =supine-to-sitting postural changes; panels C and D =sitting-to-standing postural changes; VIH = ~locity integral-heart rate product; SD =stroke distance; and HR =heart rate.
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rl'~ c • •I
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these postural shifts. However, it has been suggested recently that DoppleNlerived indices of left ventricular performance, particularly PKA and PICv, are preload and afterload sensitiveJI-M and thus are not pure indices of contractility. Investigations have demonstrated that PKA, PICv, and the systolic velocity integral (SD) are directly related to preload and inversely related to afterload. D-ll3 Although blood pressure, a crude measure of afterload, was not significantly influenced by orthostasis in our sample, sizable orthostatic reductions in left ventricular preload could have caused parallel decreases in Doppler indices without invoking changes in contractility. per se. Furthermore, Harrison et al• have shown that PKA, PICv, and the flow velocity integral are attenuated in proportion to pacing-induced graded increments in heart rate. This observation is also consistent with our findings although the orthostasic increases in heart rate were modest. Certainly. all of these factors should be considered when using Doppler methodology to evaluate left ventricular performance. In this study, although heart rate accelerated in a graded manner with increasing degrees of orthostasis, the postural change (supine-to-sitting and sitting-tostanding) in VIH was shown to be correlated primarily with the postural change in SD rather than that of
heart rate in all age groups. Specifically, as one changes from supine to sitting posture, the change in VIH that occurs is primarily due to the change in SD, regardless of age. When one changes from sitting to a standing position, the change in VIH is influenced by both SD and heart rate in an age-dependent manner: with advancing age, there is greater reliance on SD. Similar findings regarding cardiac output changes from supine to sitting were noted by Rodeheffer et al7 using radionuclide blood pool imaging in another subset of the BLSA population. A possible limitation of this study involves our dependence on a digital printout with a display of the raw velocity tracings. Although the instrumentation (Exerdop) does not allow one to identify artifact with certainty, careful operator attention to maximize the auditory signal intensity and the sophisticated moving average algorithm used to exclude outlying data points probably eliminate the majority of artifactual signals. In conclusion, the present study shows that orthostatic stress causes graded reductions in all commonly used continuous-wave Doppler indices of left ventricular performance, regardless of age. Although the average supine values ofall Doppler variables decrease with age, the older subject adjusts to postural changes much like his younger counterpart; ie, postural changes in VIH are primarily mediated by changes in SD rather than heart rate. Thus, continuous-wave Doppler provides a convenient noninvasive tool to evaluate postural changes in left ventricular performance. The normative data from this and other studies 14 should be applicable to the evaluation of patients with orthostasic symptoms, as well as the results of pharmacologic interventions. ACKNOWLEDGMENTS: The authors gratefully acknowledge Dr. Bernard "I;_~_ngel for assistance with statistical analysis, as well as Jeanettea Wright, Monteray Jackson, and Carolyn Eames for their technical assistance, and Mary Lou Wright for secretarial support. REFERENCES 1 Wang Y, Marshall RJ, Shepard JT. The eiJect of changes in posture and of graded exercise on stroke volume in man. J Clin Invest 1960; 39:1051-61 2 Granath A, Jonsson B, Strandell T. Studies on the central circulation at rest and during exercise in supine and sitting position. Acta Med Scand 1961; 169:125-26 3 Reeves JT, Grover RF, Filley GF, Blount SG. Circulatory changes in man during mild supine exercise. J Appl Physiol1961; 16:27982 4 Reeves JT, Grover RF, Filley GF, Blount SG. Cardiac output in normal resting man. J Appl Physiol1961; 16:27~78 5 Bevegard S, Holmgren A, Jonsson B. Circulatory studies in well trained athletes at rest and during heavy exercise, with special reference to stroke volume and the influence of body position. Acta Physiol Scand 1963; 57:2«h50 6 Poliner LR, Dehmer GJ, Lewis SE, Parker Rw, Blomquist G, Willerson JT. Left-ventricular performance in normal subjects: a comparison of the response to exercise in the upright and supine posture. Circulation 1980; 62:528-34 7 RodeheiJer RJ, Gerstenblith G, Beard E, Fleg JL, Becker LC, Weisfeldt ML, et al. Postural changes in cardiac volumes in men
in relation to adult age. Exp Gerontol1986; 21:367-78 8 Stein PD, Sabbah HN, Albert DE, Snyder JE. Continuous wave Doppler for the noninvasive evaluation of aortic blood velocity and rate of change in velocity: evaluation in dogs. Med Instrum 1987; 21:177-82 9 Bennjltt ED, Else W, Miller GAH, Sutton GC, Miller HC, Noble MIM. Maximum acceleration of blood from the left ventricle in patients with ischemic heart disease. Clin Sci 1974;
46:49-59 10 Sabbah UN, Khaja F, Brymer JF, McFarland TM, Albert DE, Snyder JE, et al. Noninvasive evaluation of left ventricular performance based on peak aortic blood acceleration measured with a continuous-wave velocity meter. Circulation 1986; 74:323-29 11 Rushmer RF. Initial ventricular impluse: a potential key to cardiac evaluation. Circulation 1964; 29:268-83 12 Noble MIM, Trenchard D, Guz A. Left ventricular ejection in conscious dogs: measurement and significance of maximum acceleration of blood from the left ventricle. Circ Res 1966; 19:139-47 13 Wallmeyer K, Wann SL, Sagar KB, Kalbfleisch J, Klopfenstein HS. The influence of preload and heart rate on Doppler echocardiographic indexes of left ventricular performance: comparison with invasive indexes in an experimental preparation. Circulation 1986; 74:181-86 14 Lazarus M, Dang TY, Gardin JM, Allfie A, Henry WL. Evaluation of age, gender, heart rate, and blood pressure changes and exercise conditioning on Doppler measured aortic blood flow velocity during upright treadmill testing. Am J Cardiol1988; 62:439-43 15 Shock NW, Greulich RC, Andres R, Arenberg D, Costa PT, Lakatta EG, et al. Normal Human Aging: The Baltimore Longitudinal Study of Aging. NIH publication No. 84-2450 16 Stein PD, Sabbah HN, Albert DE, Snyder JE. Blood velocity and acceleration: comparison of continuous-wave Doppler with electromagnetic flowmetry. Fed Proc 1985; 44:1565 (abstract) 17 Huntsman LL, Stewart DK, Barnes SR, Franklin SB, Colocousis JS, Hessel EA. Noninvasive Doppler determination of cardiac output in man: clinical validation. Circulation 1983; 67:593-601 18 NishimuraRA, Callahan MJ, SchaiJHV, Ilstrup DM, Miller FA, 'Illjik AJ. Noninvasive measurement of cardiac output by continuous-wave Doppler echocardiography: initial experience and review of the literature. Mayo Clio Proc 1984; 59:484-89 19 Lau CP, Camm AJ. Hole ofleft ventricular function and Dopplerderived variables in predicting hemodynamic benefits of rateresponsive pacing. Am J Cardiol1988; 62:9096-911 20 Gerstenblith G, Frederiksen J, Yin FCP, Lakatta EG, Weisfeldt ML. Echocardiographic assessment of a normal aging population. Circulation 1977; 56:273-78 21 RodeheiJer RJ, Gerstenblith G, Beard E, Fleg JL, Becker LC, Weisfeldt ML, et al. Exercise cardiac output is maintained with advancing age in healthy human subjects: cardiac dilatation and increased stroke volume compensate for a diminished heart rate. Circulation 1984; 69:203-13 22 Bedotto JB, Eichhorn EJ, Grayburn PA. EIJects of left-ventricular preload and afterload on ascending aortic blood velocity and acceleration in coronary artery disease. Am J Cardiol1989; 64:~
23 Harrision MR, Clifton DG, Berk MR, DeMaria AN. EIJect of blood pressure and afterload on Doppler-echocardiographic measurements of left ventricular systolic function in normal subjects. Am J Cardiol1989; 64:905-08 24 Gardin JM. Doppler measurements of aortic blood flow velocity and acceleration: load-independent indexes of left-ventricular performance? Am J Cardiol1989; 64:935-36 25 Harrison MR, Clifton GD, Sublett KL, DeMaria AN. EIJect of heart rate on Doppler indexes of systolic function in humans. J Am Coli Cardiol1989; 14:929-35 CHEST I 100 I 3 I SEPTEMBER, 1991
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Doppler indices of left ventricular perlOrmance in normal man, we recorded Doppler sipals suprasteroally in 69 healthy voluoteers, ranging in age from !0 to 86 yean, in the supine position and I min after assumption of sitting and standing postures. AD indices clec:reased progressively with increasing orthostasis: peak acceleration (PIA): 15.6:t4.5 mlsl to 14.8:t4.0 mlsl to 13.6:t4.6 mlsl; peak velocity (PKV): 0.64:t0.18 m/s to 0.58:t0.17 m/s to 0.56:t0.17 mls; stroke distaoce (SD): 11.4:t3.7 em to 9.8:t3.4 em to 8.0:t1.8 em; SD x heart rate (VIB): 717:t171 em to 655:tl68 em to 57!:t117 em, from supine to sitting to standing, respectively (p
he study of the effects of postural changes on T cardiac performance in normal subjects is important in that it provides a model for assessment of performance under various pathophysiologic conditions influenced by posture. In the past two decades, the effects of posture on cardiac performance have been investigated extensively, yet the results remain controversial. Some authors have shown that cardiac output decreased on changing from recumbency to a sitting position, a.s whereas others reported no change at all. 8 •7 Some of the differences in the results may be related to differences in age, sex, health, medication status of the subjects studied, and the diversity of measuring techniques used. However, consistent among these investigations was a decrease in stroke volume with increasing orthostasis. During this same period, several investigators, using either Doppler echocardiography and/or electromagnetic flow probes, confirmed that ascending aortic blood flow velocity and its derivatives were sensitive indices of global left ventricular performance.s- 13 Given the technical difficulty associated with monitoring postural changes in cardiac performance using two-dimensional (2D) echocardiography and radionuclide imaging, the continuous-wave Doppler technique might provide a convenient .noninvasive means *From the Laboratory of Behavioral Sciences (Drs. Brown and McCormick), Laboratory ol CardiovascuJar Science (Dn. Vaitkevicius and Fleg), NatioDallnstitute on Aging, NatioDal Institutes ol Health, Gerontolog)' Research Center, -Baltimore, Md, and the Department of Physiology (Dr. Brown), Wayne State University, Detroit, Mich. Manuscript received November 8; revision~ February 9:1. Reprint 1'8qf168U: Dr. Brown, Gerontology BaeGich Center; NflflontJl lrutitute on Agjng, 4940 &.tern Aomae, llolflmor8 21224 738
heart rate increased modestly from 6!.4 :t 10.0 bpm supine, to 66.9:tl1.4 bpm sitting, to 71.3:t9.9 bpm stMding (p<.OOI~ Similar postural changes in Doppler variables were seen in all three age groups (10 to 44 years; 45 to 64 years; and 65 to 86 years~ Thus, orthostasis in normal subjects is accompanied by a reduction in all cootinuouswave Doppler indices of left ventricular performance, regardless of age. (Chat 1991; 100:138-43)
or
ANOVA• analysis variaace; BLSA. • Baltimwe I.oacitudiaal Study Agio&; KA•peak acceJentioa; PKV•peak modal
or
velocity; SD•Strob clisiaace or systolic 8ow velocity inlep"ll); VJH Veloeity integral-Mart rate product
=
for monitoring the cardiac response to postural shifts. To date and to our knowledge, there has been only one investigation using this technique to study the effect of posture on left ventricular performance. 14 These investigators showed an orthostatic decline in peak acceleration and peak velocity from supine to standing. An age-related decline among these parameters was also shown. However, left ventricular performance in the sitting position was not studied nor was the Dopple:Mlerived estimate of cardiac output calculated. The present investigation was undertaken to do the following: (1) study the effects on left ventricular performance of postural shifts from supine-to-sitting and sitting-to-standing, using the continuous-wave Doppler methodology; (2) examine the influence of age on these responses to orthostasis; and (3) determine the mechanisms used to adjust left ventricular performance under increasing degrees of orthostasis. METHODS
SelscHon
of Subject 1bpulation
The 69 subjects of this study (41 men, 28 women) were volunteer participants fro~ the Baltimore Longitudinal Study on Aging (BLSA) and ranged in age from 20 to 86 years. The demographic characteristics of the BLSA participants have been described in detail elsewhere.,. This population is predominantly white, welleducated, uppel'-middle class with above-average income and access to good health care. The subjects receive 21/a days of extensive physical, physiologic, and psychologic euminaHnns biennially at the Gerontology Besearch Center in Baltimore, Md. All subjects undergo a detailed physical examination on each visit. Subjects were included in the study provided they had no clinical evidence of cardiac, pulmonary. or any other signi6cant disease, had a resting blood pressure below 16«W5 mm Hg, received no medication that could a8"ect cardiopulmonary function, and demonstrated a nonischemic ECG response to maximal treadmill exercise.
C~nuow-
Wtwe Doppler Recording of Aortic Flow lelocity
Ascending aortic blood Bow velocity was evaluated transcutaneously from the suprasternal notch with the use of a small handheld nonimaging transducer connected to the input stage Qf a continuous-wave Doppler Bowmeter (Exerdop, Quinton Instruments Co). The transducer, which functions as both a transmitter (at 3 MHz) and receiver, is positioned so that the intercept angle is assumed to be parallel to the direction of Bow. The device is equipped with an LED readout that is proportional to the degree to which Doppler signals are parallel to the axis of aortic blood Bow. Unidirectional signals are bandpass filtered with 3-dB cutoft" frequencies of 480 Hz and U.S kHz, prior to entry into a constant amplitude velocity circuit that tracks instantaneous peak modal velocity (PKV) over a 11111ge of 0.2 to 2.5 m/s. Peak acceleration (PKA) and the systolic Bow velocity integral (also termed stroke distance, or SD) are internally derived through differentiation and integration, respectively. Peak acceleration is detected over a range o£2 to99m/sls. Validation oftheaccuracyofPICV and PKA recorded, using the continuous-wave Doppler with values obtained using electromagnetic Bowmetry, has resulted in correlations of0.95 and 0.96, respectively.•• Stroke distance is a one-dimensional approximation of stroke volume that does not take into account the crosssectional area of the aorta. 11 TherefOre, the product of SD and heart rate (the velocity integral heart rate product, or VIH) yields a Dopplel"derived one-dimensional estimate of cardiac output. Excellent correlations between VIH and invasively obtained cardiac output at rest and over a wide range of activity have been reported ...... All subjects were studied in the postabsorptive state between 9 AM and noon. After informed consent was obtained, baseline continuous-wave Doppler measurements of left venbicular performance were obtained in the supine position after 15 minutes of rest. Doppler measurements were then repeated in the sitting and standing positions, approximately 2 min after the assumption of each posture. The algorithm (Exerdop) excludes beats with a signalto-noise ratio below 10 dB and those occurring outside the limits of an empirically derived stack average of the previous eight data points. A minimum of ten acceptable beats ful611ing these criteria was required in each position for inclusion in the study...... \\\:! recorded the percentage of acceptable beats for each posture. The left brachial arterial blood pressure was determined by auscultation using a mercury sphygmomanometer. The diastolic blood pressure was indicated by Korotkoft" phase five. Heart rate and rhythm were continuously monitored using electrocardiographic leads aVF, V,, andv•.
DGta AriGly&i& Statistical difFerences for Doppler variables in the three positions were compared by a one-way analysis of variance (ANOVA) for repeated measures. Linear regression analysis was used to determine correlation between variables. To find out ifsignificant linear and/or quadratic trends were present, an orthogonal polynomial model was employed. To determine whether age affected the Doppler measurements, we divided the sample into three age groups: group 1, 20 to 44 years (17 men, 11 women); group 2, 45 to 64 years (14 men, ten women); and group 3, 65 to 86 years (ten men, seven women). The postural responses of these groups were compared using repeated measures ANOVA. A level of p<0.05 was considered statistically significant. Data are reported as the mean± the SD unless otherwise stated.
REsuLTS Population Characteristics
The mean age of the 69 subjects was 50.2 ± 18.3 years (range, 20 to 86 years); body mass index averaged 25.3±5.0 kg/m2 , and baseline seated blood pressure
was 130.7±20.9178.1±9.3 mm Hg. There were no gender differences with regard to age, body mass index, or resting blood pressure. Ninety percent of the participants were nonsmokers.
Postural Effects on Doppler \hriables and Heart Rate
For the entire group, the average percentage of acceptable beats was 73 ± 23, 69 ± 22, and 70 ± 23 in the supine, sitting, and standing positions, respectively. There was no significant difference with regard to the mean values of acceptable beats among these three postures. The postural effects on Dopplel'derived variables and heart rate are shown in Thble I. Peak acceleration (PKA), peak velocity (PKV), SD, and the velocity integral heart rate product (VIH) decreased by linear trend analysis with increasing orthostasis. By contrast, heart rate increased significantly and linearly on the assumption of an upright posture. Blood pressure was 130.7 ± 20.9178.1 ± 9.4 mm Hg when sitting, and did not change significantly on standing (124.8±19.6176.4±9.6 mm Hg; p=NS for both systolic and diastolic pressures). &-cent Change in Doppler \hriables and Heart Rate Supine-to-Sitting: The percentage of change in
Doppler variables and heart rate from the supine-tositting position for the group is shown in Figure lA. After the assumption of the sitting position, heart rate increased significantly (7.4 percent), but SD declined substantially ( -11.4 percent). The decrements in PKA (-7.5 percent) and PKV (-6.9 percent) were also significant, but the decrease in VIH (- 5.5 percent) was not. Thus, the decrease in SD that occurred with this postural shift was essentially offset by the increase in heart rate. Sitting-to-Standing: Figure IB shows the percentage of change in Doppler variables and heart rate from the sitting-to-standing position. As was seen in Figure lA, the change in SD (- 15.7 percent) is greater than that for any other variables during this postural shift. The increase in heart rate (8.1 percent) was not sufficient to offset the large decline in SD, Table 1-Steodr/-State Doppkr-~Jerived Vcaritlbla and Hetn1 Bt&te ira Supine, Sitting, and Standing Poatioru
Supine Mean SD Sitting Mean SD Standing Mean SD p(ANOVA) p (Linear trend)
HR,
VIH,
bpm
em/min
PICA, m/sls
PKv, m/s
15.6 4.5
0.64 0.18
11.4
62.4
3.7
10.0
717 272
14.0 4.0
0.58 0.17
9.8 3.4
66.9 12.1
655 268
13.6 4.6 0.001 0.001
0.56 0.17 0.001 0.001
8.0 2.8 0.001 0.001
71.3 9.9 0.001 0.001
572 217 0.001 0.001
SD, em
CHEST I 100 I 3 I SEPTEMBER. 1991
731
10
A
SUPINE - SilTING
5 w
0
z<(
-5
C)
J:
(.) ~ 0
-10
Correlation betweeen Postural Changes in VIH, Heart Rate, and SD
-15
••
-20 10
•••
8
SilTING- STANDING
5 w
C)
0
J:
-5
z<(
(.) ~ 0
-10 -15
•••
-20 HR FIG URE
so
PKA
PKV
VIH
l. Effects of postural shifts on the change in Doppler
variables and heart rate. Bars represent the average percent change ± S EM fi>r each variable. Top lbnel = supine-to-sitting postural shift; hotto m panel= sitting-to-standing postural shift; HR =heart rate; SD =stroke distance; PKA =peak acceleration; PKV =peak velocity; VIH =velocity integral-heart rate product; one asterisk= p
resulting in a significant fall in VIH (- 8.5 percent). Small nonsignificant decreases in PKA ( -1.6 percent) and PKV (- 2.6 percent) were seen. Age Effect: The effect of age on Doppler-derived variables and heart rate is presented in Figure 2. There was a significant age-related attenuation ofPKA (Fig 2A), PKV (Fig 2B), SO (Fig 2C), and VIH (Fig 20) in the supine and sitting positions. In the standing position, although there was an age-related decline in PKV, there were no significant differences in PKA, SD, or VIH across the three age groups. Similarly, although not shown, there was no age effect on heart rate in any posture. The effect of age on the postural changes in Dopplerderived variables and heart rate was also analyzed. There was a significant age-related attenuation of the 740
postural decline in PKA (supine-to-sitting, p<0.05) and SD (sitting-to-standing, p<0.009). However, age had no effect on the change in PKV, SD, heart rate, or VIH in response to postural shifts. The change in the Doppler-derived estimate of cardiac output, VIH, was also expressed as a continuous function of age by linear regression analysis. The correlations of the change in VIH with age were r= -0.12 from supineto-sitting position and r = -0.21 from sitting-to-standing position (both NS).
Regression analyses were performed to determine whether it was the change in heart rate or in SD that was more important in mediating the postural change in VIH. The postural change in VIH as a function of the postural change in SD, from supine-to-sitting position, is shown in Figure 3A; the corresponding sitting-to-standing relationship is shown in Figure 3C. Strong positive relationships between the change in VIH and the change in SD were seen in response to both postural shifts. By contrast, there was no correlation between the change in VIH and the change in heart rate in response to the supine-to-sitting postural shift (Fig 3B), and only a weak correlation between these variables when shifting from sitting to standing (Fig 30). The influence of age on the relationships between the postural change in VIH, heart rate, and SD is summarized in Table 2. In assumption of the sitting position (Table 2A), there was no significant correlation between the change in VIH and the change in heart rate across the three age groups. However, a strong positive correlation was seen between the change in VIH and that for SD for all three age groups, as seen in the overall sample. The magnitude of the correlation coefficients indicates that 81 to 91 percent of the postural change in VIH can be accounted for by the change in SD . The slope of this relationship was similar across all age groups. Thus, for a given change in SO, the change in VIH was essentially the same for the elderly as it was for the younger or middle-aged groups. By contrast, during the sitting-to-standing postural shift (Table 2B), the change in VIH is more tightly correlated with the change in SD with advancing age while postural heart rate changes have a similar modest effect on VIH changes across age groups. DISCUSSION
The results of this study show that in normal man the average values for all continuous-wave Doppler indices of left ventricular performance, including PKA, PKV, SD, and the Doppler-derived estimate of cardiac output (VIH) decrease with increasing levels of orthostasis. The decrease in VIH seen with increasing degrees of orthostasis occurred despite an increase Effect of Postural Stress on LV Performance (Brown et el)
20
A
18
s~ ~
a..
• YOUNG ll. MIDDLE x OLD
B 0.9 0.8
16
~
14 12 10 15
I----I--i c
0
C/)
I
0.6
a..
~
0.5 0.4
D 900
13
I
0.7
11
700
"2
~
-
(j
9
:I:
500
7 5
L..-_.....___...1,__-l.,__....J
SUPINE
SmiNG STANDING
>
L..--~-~-~-~300
SUPINE
SITTING STANDING
in heart rate. Therefore, the mechanism for the decrease in VIH was through a significant decrease in SD, the Doppler counterpart of stroke volume. Despite age-related declines in supine values for all Doppler indices, the directional changes in these indices with orthostasis and the strong dependence of VIH changes on changes in SD were similar across age. Several prior investigations have shown upright cardiac output to decline below supine values or remain the same as supine values. 1"7 In these studies, stroke volume was consistently diminished with orthostasis. Although beat-by-beat indices of left ventricular performance were not reported in early studies, more recent reports suggest that ejection fraction was minimally affected by posture. 6 •7 An early report by Granath et al,1 using right-heart catheterization to evaluate left ventricular performance, showed that older men had reduced cardiac outputs and stroke volumes at rest compared with younger men in the recumbent position, but had similar values in the sitting position. 51 Using radionuclide cardiac imaging, Rodeheffer et al7 found a reduction in stroke volume, from 101 ml supine to 93 ml sitting, but no significant postural change in cardiac output in 64 men, ages 25 to 86 years; similar &ndings were noted regardless of age. More recently, Lazarus et al, 14 using the continuous-wave Doppler, noted an age-related decline in PKA and P.KV in the supine but not in the standing
FIGURE 2. Age trends in Doppler variables as a function of position. Closed circles= young (20 to 44 years); open triangles= middle aged (45 to 64 years); X= old (65 to 86 years); Panel A= peak acceleration (PICA); panel B =peak velocity (PICV); panel C =stroke distance (SD); panel D =velocity integral-heart rate product (VIH). In the supine and sitting positions, a significant age group ell"ect is noted for all four Doppler variables (p<0.01 by analysis of variance); however, only PKV was related to age In the standing position.
position; values for SD and VIH were not reported. In the present study, the Doppler-derived estimate of cardiac output, VIH, decreased with age in both the supine and sitting postures, but not in the standing position. Although this age-related decline in SD and VIH is in apparent contrast to both echocardiographic Table 2-l.inear Begnruion ofPo.turol Claanga in Doppler-l>erived \briGbla tmd lleGr1 &te in Thrw Age Groupa•
Age, yr
A. Supine-sitting VIHvsHR
20-44 45-64
65-86 VIH vsSD B. Sitting-standing VIHvsHR
VIH vs SD
20-44 45-64 65-86 20-44 45-64 65-86 20-44 45-64
65-86
Slope Coefficient of Linear Regression 4.83 0.95 3.74 63.4 56.9 64.0 4.01 3.79 7.13 55.0 59.3 67.5
rp 0.29 0.07 0.27
0.14 0.76 0.29
0.90 0.94 0.95
0.001 0.001 0.001
0.49 0.43 0.43
0.01 0.05 0.08
0.63 0.81 0.94
0.001 0.001 0.001
*HR =heart rate; SD =stroke distance; VIH =stroke distance x heart rate; age groups: 20 to 44 years, n = 28; 45 to 64 years, n=24; and 65 to 86 years, n= 17. CHEST I 100 I 3 I SEPTEMBER, 1991
741
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SUPINE- SITIING
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• • •• ~·:: •
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> ~
"i
.
100
w
-
0
X
~
~
I
-100
-~ • • ••••
r•.914 P< .001
• 300 200
w
~
100
X
0
~
-100
"i
•
c
SITIING- STANDING
~
-200
r
742
, •. 768 P< .001
•
•
-300 -4 -3 -2 -1
and radionuclide estimates of resting stroke volume and cardiac output in other samples from within the BLSA, 7.J0·11 it is important to emphasize that the DoppleNlerived variables are one-dimensional estimates of stroke volume and cardiac output. To convert these Doppler variables to actual volumes, they must be multiplied by the aortic cross-sectional area, which is known to increase with advancing age.• Further· more, anatomic factors might result in a greater angle between the Doppler beam and the direction of ascending aortic blood flow in older subjects. It is therefore possible that actual stroke volume and cardiac output may not be age related in the present population. Experimental evidence suggests that peak acceleration (PKA) of blood from the left ventricle is associated with both the maximum force generated by the heart in early systole and the maximal initial velocity of shortening of the left ventricular muscle. 11 Thus, it is thought that the PKA of blood from the ascending aorta is an index of cardiac contractility and a manifestation of left ventricular performance. In our study. the decrease in PKA that occurs with increasing degrees of orthostasis suggests a posture-mediated decrease in contractility. This hypothesis is supported by the parallel decreases in PKY, SD, and VIH with
t-DI
, •. 186 P•NS
I
• I
SITIING- STANDING
• •
•
(,) ..J
F'JcuRE 3. Correlation of postural changes in VIH vs postural changes in SD and heart rate for all subjects. Panels A and B =supine-to-sitting postural changes; panels C and D =sitting-to-standing postural changes; VIH = ~locity integral-heart rate product; SD =stroke distance; and HR =heart rate.
·:·
-300
X
>
rl'~ c • •I
-#
(,)
-
• • II.. •
.At~. ..•
•• •
I
0
1
2
3
CHANGE IN SO (em)
4
-
r• .417 P< .001
I
5 6-40 -30 -20 -10
0
10
20
CHANGE IN HEART RATE (bpm)
these postural shifts. However, it has been suggested recently that DoppleNlerived indices of left ventricular performance, particularly PKA and PICv, are preload and afterload sensitiveJI-M and thus are not pure indices of contractility. Investigations have demonstrated that PKA, PICv, and the systolic velocity integral (SD) are directly related to preload and inversely related to afterload. D-ll3 Although blood pressure, a crude measure of afterload, was not significantly influenced by orthostasis in our sample, sizable orthostatic reductions in left ventricular preload could have caused parallel decreases in Doppler indices without invoking changes in contractility. per se. Furthermore, Harrison et al• have shown that PKA, PICv, and the flow velocity integral are attenuated in proportion to pacing-induced graded increments in heart rate. This observation is also consistent with our findings although the orthostasic increases in heart rate were modest. Certainly. all of these factors should be considered when using Doppler methodology to evaluate left ventricular performance. In this study, although heart rate accelerated in a graded manner with increasing degrees of orthostasis, the postural change (supine-to-sitting and sitting-tostanding) in VIH was shown to be correlated primarily with the postural change in SD rather than that of
heart rate in all age groups. Specifically, as one changes from supine to sitting posture, the change in VIH that occurs is primarily due to the change in SD, regardless of age. When one changes from sitting to a standing position, the change in VIH is influenced by both SD and heart rate in an age-dependent manner: with advancing age, there is greater reliance on SD. Similar findings regarding cardiac output changes from supine to sitting were noted by Rodeheffer et al7 using radionuclide blood pool imaging in another subset of the BLSA population. A possible limitation of this study involves our dependence on a digital printout with a display of the raw velocity tracings. Although the instrumentation (Exerdop) does not allow one to identify artifact with certainty, careful operator attention to maximize the auditory signal intensity and the sophisticated moving average algorithm used to exclude outlying data points probably eliminate the majority of artifactual signals. In conclusion, the present study shows that orthostatic stress causes graded reductions in all commonly used continuous-wave Doppler indices of left ventricular performance, regardless of age. Although the average supine values ofall Doppler variables decrease with age, the older subject adjusts to postural changes much like his younger counterpart; ie, postural changes in VIH are primarily mediated by changes in SD rather than heart rate. Thus, continuous-wave Doppler provides a convenient noninvasive tool to evaluate postural changes in left ventricular performance. The normative data from this and other studies 14 should be applicable to the evaluation of patients with orthostasic symptoms, as well as the results of pharmacologic interventions. ACKNOWLEDGMENTS: The authors gratefully acknowledge Dr. Bernard "I;_~_ngel for assistance with statistical analysis, as well as Jeanettea Wright, Monteray Jackson, and Carolyn Eames for their technical assistance, and Mary Lou Wright for secretarial support. REFERENCES 1 Wang Y, Marshall RJ, Shepard JT. The eiJect of changes in posture and of graded exercise on stroke volume in man. J Clin Invest 1960; 39:1051-61 2 Granath A, Jonsson B, Strandell T. Studies on the central circulation at rest and during exercise in supine and sitting position. Acta Med Scand 1961; 169:125-26 3 Reeves JT, Grover RF, Filley GF, Blount SG. Circulatory changes in man during mild supine exercise. J Appl Physiol1961; 16:27982 4 Reeves JT, Grover RF, Filley GF, Blount SG. Cardiac output in normal resting man. J Appl Physiol1961; 16:27~78 5 Bevegard S, Holmgren A, Jonsson B. Circulatory studies in well trained athletes at rest and during heavy exercise, with special reference to stroke volume and the influence of body position. Acta Physiol Scand 1963; 57:2«h50 6 Poliner LR, Dehmer GJ, Lewis SE, Parker Rw, Blomquist G, Willerson JT. Left-ventricular performance in normal subjects: a comparison of the response to exercise in the upright and supine posture. Circulation 1980; 62:528-34 7 RodeheiJer RJ, Gerstenblith G, Beard E, Fleg JL, Becker LC, Weisfeldt ML, et al. Postural changes in cardiac volumes in men
in relation to adult age. Exp Gerontol1986; 21:367-78 8 Stein PD, Sabbah HN, Albert DE, Snyder JE. Continuous wave Doppler for the noninvasive evaluation of aortic blood velocity and rate of change in velocity: evaluation in dogs. Med Instrum 1987; 21:177-82 9 Bennjltt ED, Else W, Miller GAH, Sutton GC, Miller HC, Noble MIM. Maximum acceleration of blood from the left ventricle in patients with ischemic heart disease. Clin Sci 1974;
46:49-59 10 Sabbah UN, Khaja F, Brymer JF, McFarland TM, Albert DE, Snyder JE, et al. Noninvasive evaluation of left ventricular performance based on peak aortic blood acceleration measured with a continuous-wave velocity meter. Circulation 1986; 74:323-29 11 Rushmer RF. Initial ventricular impluse: a potential key to cardiac evaluation. Circulation 1964; 29:268-83 12 Noble MIM, Trenchard D, Guz A. Left ventricular ejection in conscious dogs: measurement and significance of maximum acceleration of blood from the left ventricle. Circ Res 1966; 19:139-47 13 Wallmeyer K, Wann SL, Sagar KB, Kalbfleisch J, Klopfenstein HS. The influence of preload and heart rate on Doppler echocardiographic indexes of left ventricular performance: comparison with invasive indexes in an experimental preparation. Circulation 1986; 74:181-86 14 Lazarus M, Dang TY, Gardin JM, Allfie A, Henry WL. Evaluation of age, gender, heart rate, and blood pressure changes and exercise conditioning on Doppler measured aortic blood flow velocity during upright treadmill testing. Am J Cardiol1988; 62:439-43 15 Shock NW, Greulich RC, Andres R, Arenberg D, Costa PT, Lakatta EG, et al. Normal Human Aging: The Baltimore Longitudinal Study of Aging. NIH publication No. 84-2450 16 Stein PD, Sabbah HN, Albert DE, Snyder JE. Blood velocity and acceleration: comparison of continuous-wave Doppler with electromagnetic flowmetry. Fed Proc 1985; 44:1565 (abstract) 17 Huntsman LL, Stewart DK, Barnes SR, Franklin SB, Colocousis JS, Hessel EA. Noninvasive Doppler determination of cardiac output in man: clinical validation. Circulation 1983; 67:593-601 18 NishimuraRA, Callahan MJ, SchaiJHV, Ilstrup DM, Miller FA, 'Illjik AJ. Noninvasive measurement of cardiac output by continuous-wave Doppler echocardiography: initial experience and review of the literature. Mayo Clio Proc 1984; 59:484-89 19 Lau CP, Camm AJ. Hole ofleft ventricular function and Dopplerderived variables in predicting hemodynamic benefits of rateresponsive pacing. Am J Cardiol1988; 62:9096-911 20 Gerstenblith G, Frederiksen J, Yin FCP, Lakatta EG, Weisfeldt ML. Echocardiographic assessment of a normal aging population. Circulation 1977; 56:273-78 21 RodeheiJer RJ, Gerstenblith G, Beard E, Fleg JL, Becker LC, Weisfeldt ML, et al. Exercise cardiac output is maintained with advancing age in healthy human subjects: cardiac dilatation and increased stroke volume compensate for a diminished heart rate. Circulation 1984; 69:203-13 22 Bedotto JB, Eichhorn EJ, Grayburn PA. EIJects of left-ventricular preload and afterload on ascending aortic blood velocity and acceleration in coronary artery disease. Am J Cardiol1989; 64:~
23 Harrision MR, Clifton DG, Berk MR, DeMaria AN. EIJect of blood pressure and afterload on Doppler-echocardiographic measurements of left ventricular systolic function in normal subjects. Am J Cardiol1989; 64:905-08 24 Gardin JM. Doppler measurements of aortic blood flow velocity and acceleration: load-independent indexes of left-ventricular performance? Am J Cardiol1989; 64:935-36 25 Harrison MR, Clifton GD, Sublett KL, DeMaria AN. EIJect of heart rate on Doppler indexes of systolic function in humans. J Am Coli Cardiol1989; 14:929-35 CHEST I 100 I 3 I SEPTEMBER, 1991
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