Aortic elastic properties with transesophageal echocardiography with automated border detection: validation according to regional differences between proximal and distal descending thoracic aorta

Pulsed Doppler Signal Processing for Use in Mice: Applications

We have developed a high-frequency, high-resolution Doppler spectrum analyzer (DSPW) and compared its performance against an adapted clinical Medasonics spectrum analyzer (MSA) and a zero-crossing interval histogram (ZCIH) used previously by us to evaluate cardiovascular physiology in mice. The aortic velocity (means +/- SE: 92.7 +/- 2.5 versus 82.2 +/- 1.8 cm/s) and aortic acceleration (8194 +/- 319 versus 5178 +/- 191 cm/s2) determined by the DSPW were significantly higher compared to those by the MSA. Aortic ejection time was shorter (48.3 +/- 0.9 versus 64.6 +/- 1.8 ms) and the isovolumic relaxation was longer (17.6 +/- 0.6 versus 13.5 +/- 0.6 ms) when determined by the DSPW because it generates shorter temporal widths in the velocity spectra when compared to the MSA. These data indicate that the performance of the DSPW in evaluating cardiovascular physiology was better than that of the MSA. There were no significant differences between the aortic pulse wave velocity determined by using the ZCIH (391 +/- 16 cm/s) and the DSPW (394 +/- 20 cm/s). Besides monitoring cardiac function, we have used the DSPW for studying peripheral vascular physiology in normal, transgenic, and surgical models of mice. Several applications such as the detection of high stenotic jet velocities (> 4 m/s), vortex shedding frequencies (250 Hz), and subtle changes in wave shapes in peripheral vessels which could not obtained with clinical Doppler systems are now made possible with the DSPW.

Echocardiographic assessment of aortic elastic properties with automated border detection in an ICU: in vivo application of the arctangent Langewouters model

We studied whether combined pressure and transesophageal ultrasound monitoring is feasible in the intensive care unit (ICU) setting for global cardiovascular hemodynamic monitoring [systemic vascular resistance (SVR) and total arterial compliance (CPPM)] and direct estimation of local ascending and descending aortic mechanical properties, i.e., distensibility and compliance coefficients (DC and CC). Pressure-area data were fitted to the arctangent Langewouters model, with aortic cross-sectional area obtained via automated border detection. Data were measured in 19 subjects at baseline, during infusion of sodium nitroprusside (SNP), and after washout. SNP infusion lowered SVR from 1.15 ± 0.40 to 0.80 ± 0.32 mmHg·ml−1·s ( P < 0.05), whereas CPPM increased from 0.87 ± 0.46 to 1.02 ± 0.42 ml/mmHg ( P < 0.05). DC and CC increased from 0.0018 ± 0.0007 to 0.0025 ± 0.0009 l/mmHg ( P < 0.05) and from 0.0066 ± 0.0028 to 0.0083 ± 0.0026 cm2/mmHg ( P < 0.05), respectively, at the descending, but not ascending, aorta. The Langewouters model fitted the descending aorta data reasonably well. Assessment of local mechanical properties of the human ascending aorta in a clinical setting by automated border detection remains technically challenging.

Estimation of myocardial performance in CABG patients

Myocardial performance index (MPI) permits a relative easy estimation of global left ventricular (LV) systolic and diastolic function. It was shown that MPI inversely correlated strongly with the maximum derivative of LV pressure with respect to time (+dP/dtmax). The current study evaluated the change of MPI during and immediately after coronary artery bypass surgery (CABG) surgery and analyzed the relationship between MPI and hydraulic energy (exemplified by preload adjusted maximal power-PAMP) during that same period. The study was conducted in 45 patients undergoing CABG. After induction of anaesthesia, 10 min after revascularization and 2 h postoperatively, haemodynamics were assessed. Preload was characterized by LV end-diastolic area indexed for BSA (LVEDAI); afterload was estimated by arterial elastance (Ea) and indexed systemic vascular resistance (SVRI). Global myocardial performance was indicated in terms of MPI and contractility was achieved by PAMP. Myocardial performance index increased postoperatively (0.44 +/- 0.13, 0.37 +/- 0.17 and 0.50 +/- 0.16, respectively; P < 0.001). Preload adjusted maximal power did not alter significantly (1.90 +/- 1.24, 2.02 +/- 1.34 and 2.12 +/- 1.00 W cm(-2)*10(4), respectively). Left ventricular enddiastolic area indexed did not change. Arterial elastance augmented to 0.76 +/- 0.39, 0.80 +/- 0.40 and 1.01 +/- 0.43 mmHg ml(-1), respectively; P < 0.001. Systemic vascular resistance did not change. A relationship was found between 1-MPI/LVEDAI2 and PAMP (R2 = 0.50). This study shows that in the setting of CABG surgery, MPI appears to be a good measure of global LV function.

Analysis of the regional pulse wave velocity by Doppler: methodology and reproducibility

Increased arterial stiffness is observed in a number of cases. The analysis of the regional functional arterial properties is of interest to determine the role of a given risk factor on the vascular wall and in some diseases such as atherosclerosis. We analysed the pulse wave velocity (PWV) measured by the Doppler method with 2D guidance and its reproducibility in different arterial segments in 15 men with coronary artery disease. Regional Doppler PWV was defined as the distance between the extremities of a given segment divided by the transit time calculated by Doppler. Intra- and interobserver reproducibilities of the Doppler measurements were studied in all of the subjects. The variation coefficients were low, maximum at the level of ascending aorta and minimal at the level of iliac segment. This good reproducibility was validated by the Bland-Altman method. Moreover, using this Doppler technique, we found a progressive increase in PWV from the ascending aorta to the iliac segment. These data demonstrate that noninvasive Doppler analysis is a feasible and reproducible method to determine regional PWV.

Aortic leucine-to-glutamate pathway: metabolic route and regulation of contractile responses

Rat aortic endothelium is differentiated regionally for three signal pathways capable of regulating the cGMP content of the underlying smooth muscle. Formation of nitric oxide (NO) from L-arginine and of glutamate from L-leucine increase cGMP; however, formation of prostaglandin H2 (PGH2) decreases cGMP. All three have peak activity in the windkessel area just distal to the aortic arch and decrease peripherally. We report evidence that the biochemical route of the leucine-to-glutamate (Leu-->Glu) pathway is via metabolism of leucine to acetyl CoA, that the controlling reaction of the pathway is mediated by the branched chain alpha-ketoacid dehydrogenase complex (BCDC), and that glutamate formation via the Leu-->Glu pathway is a major source of aortic segment free glutamate in vitro. Interruption of the pathway by treatment of precontracted rat aortic rings in vitro with each of three classes of inhibitors (leucine analogs, competitors for the BCDC reaction, or inhibitors of L-glutamate transport) enhances contractile responses. The enhancement requires an intact endothelium and is not owing to reductions in NO formation. The results support the hypothesis that the Leu-->Glu pathway functions in the regulation of aortic contractility and compliance.

Bedside quantification of atherosclerosis severity for cardiovascular risk stratification: a prospective cohort study

OBJECTIVES

We sought to assess the ability of a new noninvasive method to quantify atherosclerosis severity and to examine its power to predict cardiovascular events.

BACKGROUND

Drug prevention of cardiovascular events is effective but costly, leading to a debate about who should receive this treatment. Patient selection is often based on surrogate markers, but quantification of atherosclerosis severity is desirable.

METHODS

Atherosclerosis severity was quantified by determination of specific aortic wall elastance in transthoracic echocardiography, applying the biomechanics of pulse wave propagation. After validating the method in 52 patients by measuring aortic plaque burden in transesophageal echo directly, another 336 patients were prospectively studied by monitoring atherosclerotic events at one year and comparing the results with conventional risk stratification.

RESULTS

Specific aortic elastance was well correlated with plaque burden (p < 0.0001) and largely independent of confounding variables. Specific aortic elastance predicted the primary end point of "atherosclerotic death, myocardial infarction or stroke" at one year (p < 0.0002). Event rate at one year in the lowest specific elastance tertile was 1.8% (CI 0.0% to 4.3%), in the middle tertile 5.4% (CI 1.1% to 9.7%) and in the highest tertile 12.7% (CI 6.3% to 19%). Secondary end points supported these findings. Stepwise multivariate analysis identified specific aortic elastance, prior atherosclerotic events and left ventricular ejection fraction as independent risk predictors. Specific elastance was of incremental value to clinically identified variables.

CONCLUSIONS

Bedside measurement of specific aortic elastance allows assessment of atherosclerosis severity. It predicts the risk for future atherosclerotic events beyond conventional risk factors, promising better targeting of pharmacologic prevention and improved cost effectiveness.

Smooth muscle relaxation and local hydraulic impedance properties of the aorta

Smooth muscle relaxation is expected to yield beneficial effects on hydraulic impedance properties of large vessels. We investigated the effects of intravenous diltiazem infusion on aortic wall stiffness and local hydraulic impedance properties. In seven anesthetized, closed-chest dogs, instantaneous cross-sectional area and pressure of the descending thoracic aorta were measured using transesophageal echocardiography combined with acoustic quantification and a micromanometer, respectively. Data were acquired during a vena caval balloon inflation, both at the control condition and with diltiazem infusion. At the operating point, diltiazem reduced blood pressure in all dogs but did not alter aortic dimensions or wall stiffness. Over the observed pressure range, aortic area-pressure relationships were linear. Whereas diltiazem affected the slope of this relationship variably (no change in 3 dogs, increase in 1 dog, decrease in 3 dogs), the zero-pressure area intercept was significantly increased in every case such that higher area was observed at any given pressure. When comparisons were made at a common level of wall stress, wall stiffness was either increased or unchanged during diltiazem infusion. In contrast, diltiazem decreased wall stiffness in every case when comparisons were made at a common level of aortic midwall radius. Aortic characteristic impedance and pulse wave velocity, components of left ventricular hydraulic load that are determined by aortic elastic and geometric properties, were affected variably. A comparison of wall stiffness at matched wall stress appears inappropriate for assessing changes in smooth muscle tone. Because of the competing effects of changes in vessel diameter and wall stiffness, smooth muscle relaxation is not necessarily accompanied by the expected beneficial changes in local aortic hydraulic impedance. These results can be reconciled by recognizing that components other than vascular smooth muscle (e.g., elastin, collagen) contribute to aortic wall stiffness.

Assessment of elastic properties of the descending thoracic aorta by transesophageal echocardiography with acoustic quantification in patients with a stroke

Previous studies have described the use of transesophageal echocardiography (TEE) with acoustic quantification (AQ) in assessing aortic elastic properties. We hypothesized that patients with a prior history of stroke (ST) may have a higher risk of atherosclerotic change in great vessels compared to nonstroke subjects (NST) and thus have decreased elastic properties. We assessed the elastic properties of the descending thoracic aorta (DTA) by TEE in ST patients and compared them with data in NST patients. Subjects included 31 with ST without any evidence of emboli originating from the heart (age 51 +/- 10 years, M:F = 20:11) and 25 age-matched NST (M:F = 8:17). Patients with significant valvular heart disease including aortic and mitral regurgitation, left ventricular dysfunction (ejection fraction < 55%), and congenital heart disease were excluded. Compliance (C), distensibility (D), and stiffness index (SI) were measured using AQ and M-mode measurement at a level of the left atrium. We scored atherosclerotic risk factors (ARF) such as a history of diabetes, hypertension, smoking, hypercholesterolemia, and the presence of atheroma of DTA. There was no evidence of atheroma of DTA in NST. There were no significant differences in heart rate and systolic and diastolic blood pressure between ST and NST patients. Fractional area change (FAC) of DTA was significantly lower in ST than in NST patients (3.2 +/- 1.6 vs 5.4 +/- 2.5%, P = 0.000). ST patients had significantly lower C (1.2 +/- 0.4 vs 1.5 +/- 0.7 x 10(-3) cm2 mmHg(-1), P = 0.039), lower D (0.8 +/- 0.3 vs 1.5 +/- 0.8 x 10(-3) mmHg(-1), P = 0.000), and higher SI (10.3 +/- 8.8 vs 5.3 +/- 2.9, P = 0.006) than NST patients. ST patients without atheroma of DTA (n = 21) also had significantly lower C (1.1 +/- 0.4 vs 1.5 +/- 0.7 x 10(-3) cm2 mmHg(-1), P = 0.038) and lower D (3.5 +/- 1.4 vs 4.8 +/- 2.4 x 10(-3) mmHg(-1), P = 0.021) than NST patients. There was a significant positive correlation between SI and the score of ARF (r = 0.51, P = 0.000). The regional elastic properties of DTA measured by TEE with AQ and M-mode method were abnormal in ST. Therefore, TEE with AQ technique may have a possible clinical application for the detection of early atherosclerotic changes such as alteration of elastic properties in morphological normal DTA.

Effects of heart rate on arterial compliance in men

1. Arterial compliance is a major determinant of left ventricular afterload. In keeping with earlier experimental data obtained in isolated arterial segments, it has recently been shown in the rat that arterial compliance decreases with an increase in heart rate (HR) induced by atrial pacing. 2. To elucidate the potential relevance of this effect in humans, we investigated nine male volunteers (age 20-30 years; mean 26 years). Systemic arterial compliance (SAC) was measured with the diastolic area method and carotid-to-femoral and femoral-to-dorsalis pedis pulse wave velocities (PWV) were measured to determine regional changes in compliance. Heart rate was first lowered with intravenous metoprolol to 56 +/- 2 b.p.m. and then increased by transoesophageal atrial pacing to 80 and 100 b.p.m. 3. Increasing HR from 56 +/- 2 to 80 b.p.m. by pacing increased mean arterial pressure (MAP) from 78 +/- 2 to 98 +/- 1 mmHg (P < 0.001) and then to 102 +/- 2 mmHg (P = NS). Systemic arterial compliance fell from 0.48 +/- 0.06 to 0.33 +/- 0.04 arbitrary compliance units (ACU; P < 0.01), carotid-to-femoral PWV increased from 6.1 +/- 0.3 to 6.8 +/- 0.4 m/s (P < 0.001) and femoral-to-dorsalis pedis PWV increased from 8.9 +/- 0.4 to 10.1 +/- 0.5 m/s (P < 0.001). Pacing at 100 b.p.m did not change MAP, but did lead to a further decrease in SAC (to 0.24 +/- 0.03 ACU; P < 0.05) and further increases in carotid-to-femoral (7.3 +/- 0.4 m/s; P = NS) and femoral-to-dorsalis pedis PWV (11.3 +/- 0.4 m/s; P < 0.001). 4. We conclude that systemic, central and peripheral compliances decrease in vivo with an increase in HR induced by atrial pacing.

Aortic elastic properties in patients with repaired coarctation of aorta

Patients with successfully repaired coarctation of the aorta have impaired distensibility. Age at surgery as well as left ventricular mass index are factors that are related to the distensibility index.