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1 Section of Cardiovascular Sciences, Department of Medicine, Baylor College of Medicine, Houston 77030; 2 Indus Instruments, Houston, Texas 77058; and 3 Department of Pharmacology, Berlex Biosciences, Richmond, California 94804
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ABSTRACT |
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 TOP
 ABSTRACT
 INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Apolipoprotein E-knockout (ApoE-KO) mice develop advanced
atherosclerotic lesions by 1 yr of age and have been well characterized pathologically and morphologically, but little is known regarding their
cardiovascular physiology and hemodynamics. We used noninvasive Doppler
ultrasound to measure aortic and mitral blood velocity and aortic
pulse-wave velocity in 13-mo-old ApoE-KO and wild-type (WT) mice
anesthetized with isoflurane. In other mice from the same colony, we
measured systolic blood pressure, body weight, heart weight,
cholesterol, and hematocrit. Heart rate and blood pressure were
comparable (P = not significant) between ApoE-KO and WT
mice, but significant decreases (P < 0.001) were found in body weight (
22%) and hematocrit (11%), and significant
increases were found in heart weight (+23%), aortic velocity (+60%),
mitral velocity (+81%) (all P < 0.001), and
pulse-wave velocity (+13%, P < 0.05). We also found
inflections in the aortic arch velocity signal consistent with enhanced
peripheral wave reflection. Thus ApoE-KO mice have phenotypic
alterations in indexes of peripheral vascular resistance and compliance
and significantly elevated cardiac outflow velocities and heart
weight-to-body weight ratios.
atherosclerosis; cardiac output; hypertrophy; ultrasonics
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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THE POLYPEPTIDE
APOLIPOPROTEIN E (ApoE) is important in the hepatic clearance of
circulating cholesterol. When ApoE is dysfunctional or absent, severe
hyperlipidemia occurs in humans (14) and in animal models
(25). In ApoE-knockout (ApoE-KO) mice (24), atherosclerosis develops and progresses spontaneously
(27), with lesions covering over 20% of the proximal
aortic wall at 4 mo and 50% at 13 mo (35). Because the
lesions progress with age and to some degree resemble human
atherosclerotic lesions (as shown in Fig.
1, adapted from Tse et al. in Ref.
34), ApoE-KO mice are considered a potentially important model
of human atherosclerosis (17, 22). Numerous studies have
been done to characterize the morphology, pathology, and histology of
arterial lesions in ApoE-KO mice (23, 26, 31), and several
studies have documented neurologic deficits (15, 21) and
reduced aerobic capacity (19). However, alterations to
other aspects of cardiac and vascular physiology and function have not
been well characterized in this model.
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A common consequence of atherosclerosis is an increase in the stiffness of the arterial wall, resulting in decreased vascular elasticity and compliance (1). Vascular stiffness can be evaluated by measuring pulse-wave velocity (PWV), which is known to increase with increasing arterial stiffness (5). In a previous study using invasive pressure waveforms, we found that aortic PWV was significantly increased in 13-mo-old ApoE-KO versus wild-type (WT) mice despite the finding of unchanged heart rate and blood pressure (35). We hypothesize that the cardiovascular systems of these mice may undergo significant remodeling and adaptation in response to elevated cholesterol and the presence of severe atherosclerotic lesions. Thus the present study was designed to determine 1) whether increases in PWV could be detected and measured noninvasively in intact ApoE-KO mice, 2) whether blood pressure and heart rate were normal in intact mice in the absence of anesthesia, and 3) whether the cardiovascular adaptations produced measurable changes in aortic blood flow velocity in ApoE-KO mice. As the study progressed and we found significant elevations in aortic velocity, we added measurements of mitral velocity, heart weight, and hematocrit. We chose to limit this study to 13-mo-old ApoE-KO mice and age-matched controls because at that age established lesions are present, the distribution and morphology of the lesions are well documented, and the increase in PWV reaches statistical significance (35).
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METHODS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Animals. A total of 28 13-mo-old male ApoE-KO and 25 age-matched male WT C57BL/6J mice, selected from a colony kept at Berlex Biosciences, were studied intact using noninvasive methods (10, 11, 32) or were euthanized for determination of heart weight or hematocrit. Mice were housed in the animal facility at Berlex Biosciences and at the Fondren-Brown vivarium of Baylor College of Medicine. Both facilities are American Association for Accreditation of Laboratory Animal Care approved, and experimental protocols were approved by the respective animal care and use committees. Animals were kept in rooms at controlled temperature (24°C) and lighting (14:10 h light-dark cycle) with free access to food and water. The diets of both ApoE-KO and WT mice consisted of normal chow. In general, 10 mice were studied in each group, except in some terminal experiments where projections of sample size from available studies allowed us to use a smaller number of animals.
Blood pressure in conscious mice. In one group consisting of 10 ApoE-KO and 10 WT mice, systolic blood pressures and heart rates were measured in the conscious, unanesthetized state using a tail-cuff system (Kent Scientific, Litchfield, CT). Mice were trained to stay quietly in a restrainer placed on a warm pad for a period of at least 30 min for 1-4 days before the study. On the day of the study, the trained mouse was kept quietly in a temperature-controlled restrainer for 15 min. Blood pressure was then measured repeatedly by the tail-cuff method and recorded on a data acquisition system (PowerLab 16/s, AD Instruments, Castle Hill, NSW Australia). From these data, we derived systolic blood pressure and heart rate from the average of five measurements in the unanesthetized state.
Measurements under anesthesia. In the same group of animals, noninvasive Doppler measurements were made under anesthesia. Anesthesia was induced by placing mice in the closed chamber of an anesthesia machine ventilated with 1.5% isoflurane for 3-5 min (IMPAC 6, VetEquip, Pleasanton, CA). After they were inducted, the mice were taped supine to electrocardiogram (ECG) electrodes incorporated into a temperature-controlled printed circuit board. The board includes ECG electrodes (4) under each limb, an array of 50 surface-mounted, heat-generating resistors positioned under the body of the mouse, and a temperature sensor used to control board temperature. The temperature of each mouse was monitored with a rectal probe (Physitemp, Clifton, NJ), and body temperature was maintained at 35 ± 2°C throughout the study by manually adjusting the temperature of the board between 35 and 40°C. The ECG electrodes were connected to a high-fidelity ECG amplifier with a 0.1- to 2-kHz bandwidth set to record lead II. Anesthesia was maintained during measurements by placing a coaxial tubing set from the anesthesia machine loosely over the face of the mouse. The ECG board, amplifier, and temperature controller, along with the Doppler transducers and signal processing, were developed by the authors specifically for use with mice.
Cardiac Doppler measurements.
Transvalvular mitral and aortic blood flow velocities were measured
with a 2-mm-diameter 10-MHz pulsed Doppler probe with a focal distance
of 6 mm. The probe was placed just below the sternum using minimal
pressure and angled toward the ventricular inflow and outflow tracks,
respectively, as shown in Fig. 2
(32). At each site, the sample volume depth and probe
position were adjusted to record the maximum velocity with waveform,
direction, and timing consistent with mitral or aortic velocity. We
have verified in previous studies (11, 32) that consistent
and reproducible signals are obtainable from these sites in mice
without image guidance.The typical depth setting was 4-7 mm for
transmitral recordings and 6-9 mm for transaortic recordings
(32).
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Aortic PWV.
For aortic PWV measurements, a 20-MHz Doppler probe with a focal
distance of 4 mm was placed just right of the sternum and angled to
record velocity in the aortic arch moving away from the probe at a
depth of 2-4 mm. A mark was made on the chest at the aortic arch
measurement site, and a second mark was made 40 mm distal on the
abdomen. A measurement was then taken at the second mark from the
abdominal aorta with the probe angled toward the heart at a depth of
2-3 mm, as shown in Fig. 3. Aortic
pulse-wave velocity was calculated by dividing the separation distance
(40 mm) by the difference in arrival times of the velocity pulse timed with respect to the ECG (11). In two animals in each
group, signals were also obtained from the left and right carotid
arteries in the neck and from seven to nine sites along the aorta in
1-mm intervals progressing from the aortic outflow track to the upper descending aorta using the 20-MHz probe, as shown in Fig.
4.
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Doppler data acquisition system. The Doppler probes were connected to a modular pulsed Doppler system designed by the authors originally for use with implantable probes in animals (9), and the Doppler audio output was connected to a data acquisition and signal processing system integrated into a personal computer. Briefly, the quadrature audio Doppler signals from the 10- or 20-MHz pulsed Doppler modules and the amplified ECG signal were sampled and digitized at 62.5 or 125 kHz corresponding to the sampling rate of the pulsed Doppler. A complex fast Fourier transform was calculated and displayed in real-time on the computer monitor along with the ECG for use by the operator in optimizing and adjusting the probe position. When the desired signals were obtained, a button was pressed on a remote keypad to save the last 2 s of unprocessed signals to a data file.
Signal processing.
For analysis, the digitized quadrature Doppler and ECG signals were
processed off line. The 2-s data file from each location on each mouse,
consisting of 10-20 cardiac cycles, was played back and displayed
using an adjustable 64- to 1,024-point fast Fourier transform. A
semiautomated program was used to outline the upper edge of the Doppler
spectrum. We typically used a 256-point (2-4 ms) spectral window
that was advanced through the data set at 0.1-ms intervals, resulting
in a temporal resolution of 0.1 ms and a frequency resolution of
250-500 Hz depending on the sampling rate. The analyzer
can be adjusted to optimize frequency resolution for blood velocity
measurements or temporal resolution for PWV and timing measurements.
Examples of Doppler signals are shown in Figs.
2-5.
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f) using the Doppler equation [V =
c · f/2fo · cos(
)], where
c is the speed of sound (1, 500 m/s) and fo is
the ultrasonic frequency (10 or 20 MHz) (9, 10). With
the probe positions used in this study for cardiac and aortic arch
measurements, the Doppler angle () was nearly zero ( ± 15°) such
that cos() = 0.97-1.0. For the 20-MHz Doppler probe used
for arterial measurements, the equation becomes V = 3.75 · f and for the 10-MHz Doppler probe used in
transvalvular cardiac measurements, the equation becomes
V = 7.5 · f, where V is measured
in centimeters per second and f is measured in kilohertz for both
equations. We did not attempt to quantify the magnitude of
velocity from the abdominal aorta where a 45° angle was used.
From the spectral envelopes (as illustrated in Fig. 5), the following
features were extracted from the average of at least 10 cardiac cycles:
1) heart rate; 2) pulse arrival time, defined as
the time from the peak of the R wave of the ECG to the
upstroke of velocity; 3) peak velocity;
4) stroke distance, defined as the area under the aortic
velocity curve; 5) mean velocity, defined as velocity
averaged over the cardiac period (equivalent to stroke distance
multiplied by heart rate); and 6) peak acceleration, determined from the maximum derivative of the velocity signal. From the
aortic signals (Fig. 2), we determined heart rate, peak and mean
ejection velocity, stroke distance, ejection time, and peak
acceleration. From mitral signals (Fig. 2), we determined peak and mean
filling velocity. From the difference in pulse arrival times in the
aortic arch and in the abdominal aorta (Fig. 3), we determined aortic
PWV. From the aortic arch velocity signals (Fig. 5), we determined
early (A1) and late (A2)
peak accelerations; total acceleration time, defined as the time from
upstroke to peak velocity; and acceleration time 1, defined as the time
from the upstroke of velocity to the first inflection in velocity (or minimum in acceleration), as shown in Fig. 5.
Histology and blood sampling. For histologic and post mortem examination, ApoE-KO and WT mice were killed by inhalation of CO2. The heart was then removed, blotted dry on tissue paper, and weighed. Heart weight, body weight, and heart weight-to-body weight ratios were determined from 10 ApoE-KO mice and 7 WT mice. In eight additional mice from each group, blood was drawn by heart puncture for determinations of hematocrit measured using the microcapillary centrifugation method (International Equipment, Needham, MA).
Statistical analysis.
Results are presented as means ± SE for the number of animals
(n) in each group. Differences were determined by Student's t-test, and statistical significance was defined by a
P value of <0.05. Because the body weights were
significantly lower in the ApoE-KO mice, the raw parameters summarized
in Table 1 were normalized to body weight
using the scaling relationships of Dawson (7) before
statistical comparisons were made. According to Dawson, heart rate and
acceleration scale to the 1/4 power of body weight
(BW
1/4), periods and times scale to the +1/4 power
of body weight (BW1/4), and velocities and pressures scale
to the zero power of body weight (BW0).
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RESULTS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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The data from the several groups of 13-mo-old male mice are
summarized in Table 1. Body weight was 22% lower in ApoE-KO versus WT
mice (34.5 ± 0.9 vs. 44.5 ± 1.1 g), but heart weight
was 23% higher (186 ± 7.1 vs. 151 ± 2.5 mg), resulting in
an increase of 59% in the heart weight-to-body weight ratio
(P < 0.001). Hematocrit was slightly but significantly
lower (11%, P < 0.001) in ApoE-KO versus WT mice at
41.7 ± 1.1 versus 46.6 ± 0.4%. Aortic PWV was increased in
ApoE-KO versus WT mice at 428 ± 14.5 cm/s in ApoE-KO mice and
379 ± 10.1 cm/s in WT mice (P < 0.05).
Systolic blood pressure measured in the awake, unanesthetized state was similar in both groups at 140 ± 7.6 mmHg in ApoE-KO mice and 136 ± 7.4 mmHg in WT mice [P = not significant (NS)]. Heart rate was 712 ± 10 beats/min in ApoE-KO mice and 678 ± 12.5 beats/min in WT mice measured in the conscious state (P < 0.05) and 488 ± 10.3 beats/min in ApoE-KO mice and 439 ± 10.4 beats/min in WT mice measured under anesthesia (P < 0.05). However, when normalized to body weight using the BW1/4 relationship (7), the differences in heart rate lose statistical significance, as shown in Table 1.
Transaortic and transmitral blood velocities were significantly elevated in ApoE-KO versus WT mice. Peak aortic velocity in ApoE-KO versus WT mice was 133.4 ± 7.8 cm/s versus 89.2 ± 5.8 cm/s (P < 0.01), whereas mean aortic velocity was 35.9 ± 2.7 versus 22.0 ± 1.6 cm/s, respectively (P < 0.001). Peak mitral velocities were 92 ± 7.2 cm/s in ApoE-KO mice and 47.2 ± 5.3 cm/s in WT mice (P < 0.001). Mean mitral velocities were 20.6 ± 1.7 and 11.4 ± 1.3 cm/s, respectively (P < 0.001). Because of the relatively high (close to normal) heart rates, the early and late filling waves were merged (32), and the early-to-late filling wave ratio was not analyzed in any of these mice.
Peak aortic acceleration in ApoE-KO versus WT mice was 118 ± 7.8 versus 100 ± 12 m/s2. When normalized to body weight and because of the high variance, the difference was not statistically significant (P = NS). Acceleration in the aortic arch occurred in two phases, which were designated as A1 and A2, as shown in Fig. 5. The ratio of A2 to A1, which is used here as an index of wave shape, was significantly higher in the ApoE-KO mice (2.1 ± 0.4) compared with the WT mice (0.46 ± 0.14) with a P value <0.001.
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DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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The ApoE-KO mouse has been proposed as a model of atherosclerosis in the human (22). Whereas we confirmed the development of vascular stiffening, there is also clear evidence of a hyperdynamic state with elevated blood velocity under anesthesia. Furthermore, there are unusual waveforms in blood flow, suggesting sites of wave reflections close to the heart. Both could contribute to the marked myocardial hypertrophy also seen in these animals.
Pulse-wave velocity. With the use of noninvasive methods in intact mice, we verified data taken invasively, which showed a significant increase in aortic PWV in ApoE-KO versus WT mice (35). However, the absolute magnitudes were higher when measured in intact versus surgically operated mice. PWV measured invasively with vascular tonometry was 274 cm/s in WT mice and 377 cm/s in ApoE-KO mice (35), whereas the corresponding values measured noninvasively in the present study were 379 and 428 cm/s. Because we (11) have previously shown that Doppler and tonometric methods produce the same result when measured simultaneously, the most likely cause for the difference is the higher blood pressures observed in the noninvasive studies versus the invasive studies (systolic blood pressure = 136 vs. 90 mmHg in WT mice and 140 vs. 92 mmHg in the ApoE-KO mice).
Scaling to body weight.
It has been predicted using scaling models and verified in numerous
studies that velocity in a given artery is relatively unchanged in
mammals ranging in size from mice to elephants (7). In a
previous study, we (10) reported a mean aortic velocity of
23 ± 4 cm/s in a group of 31 normal control mice weighing
24.4 ± 1.5 g. Although the body weights were considerably
lower than the 44.5 ± 1.1 g in the older WT mice studied
here, the mean aortic velocities were not different from those reported
here (22.0 ± 1.6 cm/s). Thus aortic velocity can be used as an
index of cardiac output, which automatically accounts for variations in
the size and weight of the animal provided that aortic diameter is
"normal." A similar argument holds for PWV and blood pressure,
which are also predicted to be independent of body weight
(7). The measured difference in heart rate between WT and
ApoE-KO mice, when normalized to body weight using the
BW1/4 relationship, loses its statistical significance.
Thus neither blood pressure nor heart rate is different in ApoE-KO
versus WT mice. Similarly, the differences in ejection time and aortic
acceleration are not statistically significant when normalized to body
weight (or to heart rate). A summary of the ratio of values for
selected parameters in ApoE-KO to WT mice is shown in Fig.
6.
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Transvalvular blood velocities. Peak and mean velocities were significantly increased in both mitral and aortic positions in ApoE-KO versus WT mice. The observed increases could be caused by narrowing of the valve orifices or because cardiac output is elevated in ApoE-KO mice. However, the elevations were essentially identical for the mean velocities at the mitral position and aortic position, and the likelihood of simultaneous development of stenosis to the same extent in both the high-pressure aortic valve and low-pressure mitral system seems remote. When velocity was measured at sites separated by 1-mm intervals along the aorta from the aortic root to the upper descending aorta in ApoE-KO and WT mice (Fig. 4), no significant variations in velocity were observed in either group. In ApoE-KO mice (3, 29), velocity was elevated at all sites along the aorta as well as at the mitral valve. Furthermore, other groups have found that the aortic lumen is maintained at nearly normal or slightly increased dimensions in the ApoE-KO mouse. The only stenotic lesions were observed at the carotid bifurcations (29). Therefore, we conclude that the most plausible reason for the elevated flow velocities in the ApoE-KO mouse is elevated cardiac output under anesthesia. Because blood pressure is similar in each group, this would suggest a significant reduction in peripheral vascular resistance in ApoE-KO mice.
It has been demonstrated by others (19) that ApoE-KO mice have reduced aerobic capacity, which is consistent with a decreased cardiac reserve and increased resting cardiac output. It is unclear whether elevations in resting cardiac output are a necessary part of other models of atherosclerosis. Cholesterol-fed rabbits (2), which have high serum cholesterol levels and develop atherosclerotic lesions similar to ApoE-KO mice, have elevated hind limb blood flow (8). The 50% increase in hind-limb blood flow is similar to the increases in mean aortic velocity (60%) observed in the present study and suggests an increased demand for peripheral blood flow or altered vasoregulation. Other studies (12) show that hypercholesterolemic rabbits are in a frank hypermetabolic state associated with increased oxygen consumption and presumably increased cardiac output. However, measurement of cardiac output with microspheres in rabbits fed similar diets revealed no increment in resting cardiac output (13). Furthermore, in one of the few human studies of young persons with hyperlipoproteinemia, there was no increment in cardiac output at rest or with exercise (28). There are several mechanisms to explain the increase in flow velocities, including peripheral vasodilation due to overproduction of nitric oxide in the vessel walls (2, 4, 8), compensation for impaired oxygen delivery by cholesterol-modified red cells (16, 30, 33) or reduced hematocrit (6), or increased peripheral demand associated with the subacute inflammation integral to atherosclerosis (22). Reductions in hematocrit have been observed in atherosclerotic patients (6) and also in cholesterol-fed rabbits (Y.-X. Wang, unpublished data, and P. D. Henry, personal communication) and are thought to be a compensatory response to reduce blood viscosity and thereby improve peripheral perfusion and optimize oxygen delivery. Nevertheless, the modified hemodynamic state seen in the ApoE-KO mice may contribute significantly to the accelerated atherosclerotic process because shear stress is dependent on both blood velocity and viscosity (18). Whether this observation diminishes the relevance of the ApoE-KO mouse to the human condition awaits further study.Altered aortic arch velocity waveform. One of the significant findings in this study is the alteration in the magnitude and shape of the aortic arch velocity signal, as shown in Figs. 4 and 5. With the use of the objective criteria illustrated in Fig. 5, we found measurable and significant increases in the ratio of A2 to A1 in ApoE-KO versus WT mice. To our knowledge, this type of biphasic or concave acceleration has not been reported in any systemic artery of any animal. When we studied velocity signals systematically from numerous (7-9) sites along the proximal aorta, the only location where the biphasic or concave waveforms were observed was at the aortic arch past the origin of the innominate artery. The waveforms in the aortic root, the descending aorta, and the carotid artery were indistinguishable (except in magnitude) between ApoE-KO and WT mice, as shown in Fig. 4. The most logical explanation is an alteration in vascular impedance that changes the magnitude and timing of peripheral wave reflections.
Wave reflections normally occur from major branch points and the periphery and return to the central aorta late in the cardiac cycle after the aortic valve has closed (18, 20). Vascular lesions produce additional discontinuities in impedance that can alter the magnitude and timing of reflections. A previous report (29) showed that 9-mo-old ApoE-KO mice had stenotic lesions in the external carotid artery near the bifurcation (but nowhere else) in 9 of 12 animals. The augmentation from this source of reflection should appear on the pressure wave as an enhancement or an anachrotic notch. Such features have been observed in older patients with vascular disease (18). The effect of reflections on the velocity wave depends on the direction of travel. In the carotid artery, the reflected wave travels opposite to the direction of flow and suppresses velocity, whereas in the aortic arch, the reflected wave from the carotid after entering the aorta travels in the direction of flow and enhances velocity. Indeed, carotid flow waveforms from most species, including WT mice (Fig. 4), peak much earlier in systole than aortic flow waveforms because of early peripheral reflections. On closer analysis, 7 of 10 WT mice also showed evidence of wave reflections in the aortic arch and upper descending aorta similar to what O'Rourke showed in dogs (20), as illustrated in Figs. 4 and 5, but the magnitude was such that A2 (if present) was always lower than the initial peak. If the carotid bifurcation (about 1.5-2.0 cm from the aortic arch) was the source of the reflection, the return time would be 7-10 ms at a PWV of 400 cm/s. This agrees well with the average time to the inflection (Fig. 5) of 8.6 ± 0.4 ms in ApoE-KO mice and 8.8 ± 0.6 ms in WT mice. Thus the presence of an early and strong inflection in the aortic arch velocity waveform could be indicative of carotid stenosis and/or increased aortic stiffness. It is intriguing to speculate whether this kind of velocity wave ever occurs in humans with advanced atherosclerosis. In this study, we have documented detailed alterations in velocity waveforms that have not been shown before in mice, humans, or other animals. Some of this could be attributed to the improved Doppler signal processing, which can resolve higher velocities with higher bandwidth and fidelity compared with Doppler systems adapted from clinical devices (10, 32). The current system was designed specifically for applications in mice using 10 and 20 MHz ultrasound frequencies and can resolve velocities up to 400 cm/s (Doppler shifts up to 125 kHz) with a frequency response from zero to 300 Hz after smoothing of the spectral envelope (velocity curves in Fig. 5). Temporal resolution using the spectral display alone can be as high as 0.1 ms, as shown in Fig. 3. The clinical systems that we and others (see Ref. 10) have used in the past are limited in peak velocity to ~24 kHz of Doppler shift (100 cm/s at 20 MHz), with an upper frequency limit in the envelope waveform of 50-60 Hz and a temporal resolution at the highest sweep speed of 2-4 ms.Potential errors. Although the Doppler studies are done without image guidance, we have been able to obtain consistent signals from the aortic and mitral valves from the apical position in every mouse by maximizing the velocity signals and by noting the position and depth at which valve clicks appear on the spectral display (32). However, a significant source of error in the calculation of PWV is in estimating the distance between the velocity measurement sites in the curved aorta from straight-line distance measurements on the surface, as shown in Fig. 3. We attempted to account for the effects of sample volume depth (2-4 mm) and probe angle, but the uncertainty could be as high as ± 2 mm and a similar error could arise from the curvature. Thus the uncertainty in distance could be as high as ±4 mm or 10%. The same errors would apply to each group of mice and would not be expected to alter the ratio, but the uncertainty could bias the absolute value of PWV and add to the variance. Pulse arrival time can be measured from the ECG to within 0.2 ms at each site, and the variance between beats in a given animal is ±0.5 ms. Because a minimum of 10 beats is averaged at each site and the arrival times are subtracted, the errors tend to cancel, and no bias is expected from this source.
In summary, the phenotype of the ApoE-deficient mouse is considerably more complex than previously appreciated. Although these mice have normal heart rate and blood pressure, they have elevated PWV, elevated aortic and mitral flow velocity, alterations in aortic acceleration suggestive of increased wave reflections, decreased hematocrit, and increased heart weight-to-body weight ratios. Taken together the observed changes suggest that ApoE-KO mice have increased cardiac output and stroke volume and decreased peripheral vascular resistance and compliance and cardiac hypertrophy. These new findings could have important implications in the use of the ApoE-KO mouse as a model to study human atherosclerosis.| |
ACKNOWLEDGEMENTS |
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This work was supported in part by the National Institutes of Health Grants R37-HL-22512 (to C. J. Hartley), R44-HL-52364 (to S. Madala), R03-AG-15568 (to G. E. Taffet), and P01-HL-42550 (to M. L. Entman) and by Berlex Biosciences.
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FOOTNOTES |
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Address for reprint requests and other correspondence: C. J. Hartley, Dept. of Medicine (CVS), Baylor College of Medicine, One Baylor Plaza, Houston, TX 77030 (E-mail: chartley{at}bcm.tmc.edu).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 4 February 2000; accepted in final form 23 May 2000.
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
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