Sublingual nitroglycerin delays arterial wave reflections despite increased aortic "stiffness" in patients with hypertension: a Doppler echocardiography study.

Venodilatation with consequent reduction in left ventricular filling and end-diastolic wall stress is an important mechanism for the beneficial effects of nitroglycerin in ischemic heart disease and in left ventricular failure. The effects of sublingual nitroglycerin on arterial pulsatile hemodynamics are less well defined. Doppler echocardiography and the calibrated subclavian artery pulse tracing were used to assess hemodynamics in subjects with sustained arterial hypertension (n = 25) before and 5 to 10 minutes after sublingual deposition of 0.5 mg glyceryl trinitrate. Aortic characteristic impedance was calculated by averaging the modulus of the input impedance (ratio of pressure to flow) at high frequencies and by calculating the ratio of pressure and flow increments during upstroke. The pressure wave was split into forward and backward components, and the reflection coefficient (the ratio of backward to forward pressures) was calculated. Parameters of the arterial bed were estimated by using 2- and 3-element Windkessel models. Nitroglycerin delayed the return of arterial wave reflections by 17% (P =.02) and increased aortic characteristic impedance by 20% (P =. 01), but it did not influence total arterial compliance. Mean arterial pressure decreased 7% (P =.0001), but pulse pressure did not change. Stroke volume and the acceleration time of aortic root flow decreased by 13% (P =.0001) and 8% (P =.01), respectively. Cardiac output decreased 7% (P =.01), despite an increase in heart rate of 10% (P =.0001). Peripheral resistance tended to decrease (4%, P =.06). Thus, in subjects with sustained hypertension, sublingual nitroglycerin dilates peripheral, predominantly muscular arteries with a subsequent delayed return of reflected pressure waves. Reflex activation of the sympathetic nervous system with consequent increased acceleration of left ventricular ejection seems to counteract the effect of reduced mean arterial pressure (distending pressure) with respect to the "stiffness" of the aorta.

Controlled auxotonic twitch in papillary muscle: a new computer-based control approach.

Based on new advancements in digital technology, we developed a PC- and DSP-based measurement and control system for isolated papillary muscle experiments. High flexibility was obtained through a three level control. Length or force was controlled real-time with a sample frequency of 5000 Hz. Muscle length and up to three segment lengths were measured simultaneously and each of these lengths could be chosen as feedback variable. Individual algorithms were implemented for different twitch types. Batches of twitches were organized in experiment protocols. The system included a new twitch type, namely a controlled auxotonic twitch. In this twitch, the muscle acted against a virtual ideal spring, giving a proportional change in developed force and shortening. The value of the virtual spring constant could be set on-line or defined in the experiment protocol. An increasing virtual spring constant represented a smooth transition from isotonic to isometric conditions.

Influence of body size and left ventricular ejection dynamics on total arterial compliance determined using Doppler echocardiography and subclavian artery pulse tracings in healthy humans.

The aim was to investigate determinants of total arterial compliance in healthy humans. Estimates of aortic root pressures and flow were obtained non-invasively with the calibrated subclavian artery pulse tracing and Doppler echocardiography in 37 males (27-76 years) and 45 females (20-77 years). Total arterial compliance, estimated using a three-element vascular model, correlated positively with body height (r = 0.45, p < 0.01) and acceleration time of aortic root flow (r = 0.32, p < 0.01) and inversely with age (r = -0.34, p < 0.05), heart rate (r = -0.33, p < 0.01), and mean arterial pressure (r = -0.51, p < 0.01). Multivariate analysis indicated that height and heart rate contributed most to the prediction of total arterial compliance. The inclusion of mean arterial pressure within the model significantly reduced the contribution of age, but not that of body height and heart rate. After adjustment for height and heart rate, total arterial compliance did not differ significantly between gender. Thus, total arterial compliance, as assessed in this study, seems to reflect both arterial capacity and viscoelastic properties of the arterial wall. Differences in body size, heart rate and mean arterial pressure should be considered when comparing total arterial compliance in different groups.

Evaluation of reference systems for quantitative wall motion analysis from three-dimensional endocardial surface reconstruction: an echocardiographic study in subjects with and without myocardial infarction.

Six relevant computer-implemented reference systems for three-dimensional quantitative assessment of left ventricular wall motion abnormalities were compared with visual wall motion analysis of two-dimensional images. Endocardial borders were traced in three apical echocardiographic views at end-diastole and end-systole in 10 patients with myocardial infarction and 5 healthy subjects, and three-dimensional reconstruction of endocardial surfaces was performed. End-diastolic and end-systolic surfaces were aligned in a common axis system depending on the reference system, and systolic wall motion was assessed at 1,024 points on the endocardial surface. The localization of abnormal wall motion was displayed in bull's-eye maps, and the area was determined as a percentage of total endocardial area. For each reference system, the segmental concordance between three-dimensional computerized and visual assessment was determined. The best agreement between computerized and visual analysis was obtained with a reference system based on wall motion towards the major ventricular axis, whereas the poorest result was obtained using the center of left ventricular cavity as reference. Correlation between the estimated area of wall motion abnormality and visually determined wall motion score index was best using the aligned center of mitral valve plane as reference (r = .92).

Systemic arterial compliance early and late after a first acute myocardial infarction.

Systemic arterial properties and left ventricular function were assessed by Doppler echocardiography and a calibrated subclavian artery pulse tracing early (5 weeks) and late (27 months) after a first acute myocardial infarction in 19 patients aged 44-77 years and in healthy subjects matched for age, gender, and arterial blood pressure. Total arterial compliance (3-element windkessel model) was reduced by 26% (p < 0.001) from early to late assessment, and left-ventricular end-diastolic and end-systolic volumes were increased by 11 and 18%, respectively (p < 0.05). Peripheral resistance and characteristic impedance were not significantly changed. The healthy matched subjects had arterial compliance similar to patients at early assessment, but tended to be higher at late. Thus, 2.3 years after a first acute myocardial infarction with moderate left ventricular dilatation, arterial distensibility was significantly decreased whereas peripheral resistance was not changed.

Ultrasound-guided neurosurgery: a feasibility study in the 3-30 MHz frequency range.

This study, which includes seven patients, illustrates some potential values of the interactive use of ultrasound technology prior to, during and after brain tumour resection. Ultrasound B-scan and colour flow imaging were applied during open surgery using a cardiac scanner in the 3.25-7.5 MHz frequency range and an intravascular scanner with catheters at 10, 20 and 30 MHz. The tumour and vital blood vessels were localized prior to resection using low frequency imaging from the brain surface. High frequency, high resolution close-up imaging was applied during and after resection in order to identify remaining tumour tissue, as well as to detect blood vessels in the vicinity of the resection wall. The study also demonstrates that the tumour and surgical tools such as, for example, bipolar diathermy, acoustic aspirator or biopsy forceps,can be visualized simultaneously. This simplifies the localization of remaining tumour tissue.

Three-dimensional echocardiography for quantitative left ventricular wall motion analysis: a method for reconstruction of endocardial surface and evaluation of regional dysfunction.

A method for quantitative LV wall motion analysis based on 3-D reconstruction of the LV endocardial surface is presented. The reconstruction is based on a minimum of three transthoracic apical 2-D cineloops of the LV, digitally transferred from the ultrasound scanner to a computer. Images are obtained by rotating the transducer around the LV long axis. Endocardial borders are traced with an automatic edge detection algorithm with manual correction. These borders are used with a specially designed computer algorithm for reconstruction of LV cavity 3-D shape, and LV volumes, ejection fraction, and endocardial surface area can be determined. The end-diastolic and end-systolic endocardial surfaces are compared for analysis of regional wall motion. A threshold value is selected to discriminate between normal and abnormal wall motion. Regional wall motion abnormalities are displayed in a bull's eye plot, and the corresponding endocardial surface area is expressed in percent of the total endocardial area. Phase analysis is performed from reconstruction of the endocardial surface throughout the cardiac cycle, and displays regions with abnormal wall motion as being out of phase with LV volume variation. Thus, LV 3-D reconstruction performed by this method can be used for quantitative analysis of wall motion in several clinical situations, and due to the simplicity of processing the data, can be useful outside the research laboratory.

Model-based estimation of vascular parameters: evaluation of robustness and suitability of models.

Left ventricular performance depends not only on myocardial state, but also on the properties of the systemic arterial tree. These properties can be assessed from recordings of aortic root pressure and flow by the use of appropriate vascular models. Noninvasive estimates of aortic root pressure and flow can be obtained by the combined use of calibrated external subclavian artery pulse tracing and Doppler echocardiography. With recent advances in computer technology, estimation of model parameters are thus accessible in the clinical setting. We discuss the suitability of different parametric vascular models together with methods for adapting these models to the measured aortic root pressure. We compared the results obtained with simple vascular models (three-component modified Windkessel models) with those of five-component models. The simpler models gave less accurate approximation of the measured pressure waveform, but for a representative set of aortic root pressure and flow data, the simpler models provided adequate estimates of the peripheral arterial resistance, the total arterial compliance, and the proximal aortic area compliance. Furthermore, the simpler models are robust for measurement noise with simple estimation algorithms. Distal arterial pressure and flow waveforms are more oscillatory, and for these the five-component model has more robust estimation schemes with more accurate estimated parameters. Hence, we conclude that for clinical noninvasive assessment of aortic vascular properties, the simpler three-component models provide adequate information. For assessment of the peripheral arteries with large oscillations in the flow, the three-component models can give more than 10% error in the compliance estimate and more complex models can be appropriate.

Regurgitant volume in aortic regurgitation from a parameter estimation procedure.

The regurgitant volume and regurgitant orifice area as well as total peripheral resistance and arterial compliance were estimated in a cardiovascular hydromechanical simulator and in 10 patients with aortic regurgitation. A parameter estimation procedure based on a simple model of the cardiovascular system, Doppler measurements of the regurgitant jet, aortic systolic flow, and systolic and diastolic blood pressures was used. In the cardiovascular simulator the estimated regurgitant orifice area was compared with the size of a hole in the disk of a mechanical aortic valve. In the patients the regurgitant fraction was compared with semiquantitative grading from echocardiography routinely performed in our laboratory. In the hydromechanical simulator, the estimated regurgitant orifice area of 26.5 +/- 3.5 (SD) mm2 (n = 9) was not different from the true value of 24 mm2. In the patients there was a fair relationship between the estimated regurgitant fraction and the semiquantitative grading. The estimated regurgitant orifice areas varied between 1.6 and 31.2 mm2. The estimated mean values of total peripheral resistance and arterial compliance were 1.67 +/- 0.55 mmHg.s.ml-1 and 1.30 +/- 0.42 ml/mmHg, respectively.

Estimation of the pressure gradient across the fetal ductus venosus based on Doppler velocimetry.

In the fetus, the umbilical vein is directly linked to the inferior vena cava by the narrow ductus venosus. Thus, the ductus venosus blood velocity probably reflects the pressure gradient between the umbilical vein and the central venous system. In a longitudinal study that included 29 normal fetuses, pulsed Doppler velocimetry was carried out in the umbilical vein and the ductus venosus during the last half of the pregnancy. By applying the Bernoulli equation, we estimated the pressure gradient across the ductus venosus to vary between 0-3 mm Hg during the heart cycle; it remained within those ranges during gestational weeks 18-40. During fetal inspiratory movement, pressure gradients up to 22 mm Hg were estimated. The estimated ductus venosus pressure gradient seems to be within ranges compatible with known umbilical venous pressures, and may provide a new opportunity to understand central venous hemodynamics and respiratory force in the fetus once methodological limitations are controlled.

Noninvasive computerized assessment of left ventricular performance and systemic hemodynamics by study of aortic root pressure and flow estimates in healthy men, and men with acute and healed myocardial infarction.

Aortic root pressure and flow data can be used to assess left ventricular (LV) performance and properties of the systemic arterial tree. The calibrated subclavian arterial pulse trace was combined with echocardiographic imaging and Doppler velocity recordings to obtain noninvasive estimates of aortic root pressure and flow in 8 healthy subjects (group A), 12 patients with recent myocardial infarction (group B), and 8 with healed myocardial infarction and a dilated left ventricle (group C). The pressure and flow data were transferred to a computer and processed in specially designed software, including a new procedure for estimation of 3-element windkessel model parameters. There were no significant group differences for either aortic root pressure estimates or heart rate. In groups B and C, stroke and cardiac indexes were lower and total peripheral resistance higher than in group A. There were no group differences in the model estimates of total arterial compliance, whereas the characteristic impedance was greater in group C than in A, indicating a less compliant aorta in C. Both LV total and steady power were less in groups B and C than in A, whereas no group difference was found for percent oscillatory power. The reproducibility for recording was good for the aortic root pressure estimates, and lower for the derived parameters (stroke and cardiac indexes, windkessel model parameters and LV power), whereas that for interpretation was generally good. This method provides a unique noninvasive access to important parameters of LV function and the systemic circulation.

Intravascular ultrasonography. Normal and pathologic findings in the great veins.

The intravascular ultrasonographic findings in normal and diseased veins after transfemoral catheterization of 25 patients are presented. The iliac veins, the inferior and superior vena cava, the renal veins, the right atrium, both brachiocephalic veins, and the right internal jugular vein were studied. In 4 cases valves or valve-like structures were observed. Anatomic variants such as spurs and webs were seen in 3 and mural thrombi or postthrombotic wall changes in 4 patients. In one case a sphincter-like ostium venae cavae was observed. In 2 patients thin filaments within the right atrium, most likely a Chiari net, were seen. Two patients had an abnormal, stratified "artery-like" vessel wall structure. This new imaging modality has several potential applicabilities in the veins and may contribute new information about anatomy and function of the venous system. It provides cross-sectional in vivo visualization and the demonstration of motility of small intraluminal structures which cannot be revealed by traditional diagnostic methods. Because of the wide range of anatomic variation in the venous system, knowledge of its normal intravascular ultrasonographic appearance is a prerequisite for further clinical investigations.

Intravascular ultrasonographic assessment of thrombus formation on central venous catheters.

In vitro experiments were performed in order to investigate the appearance of different types of central venous catheters at intravascular ultrasonography. The experiments were repeated with artificially produced thrombi which were made adherent to the catheter wall. All thrombi larger than 1 mm could be identified. In a clinical study including 12 patients who had a central venous catheter, transfemoral intravascular ultrasonography was performed. The catheters had been in place for an average period of 54 days (range 1-360 days). In 3 patients a catheter thrombus, mural thrombus, or occlusive vein thrombosis was found. In 2 of these patients the catheter was occluded, in the 3rd patient it was malpositioned into the contralateral brachiocephalic vein. There were no complications following the ultrasonographic procedures. Mean catheterization time was 7.5 min (range 3-20 min). The advantages of this new method compared with conventional phlebographic studies and its impact on further clinical investigations are discussed.

Intravascular ultrasonographic appearance of angiographically normal arteries related to age and the occurrence of vascular disease.

In 22 individuals (mean age 52 years) the ultrasonographic images of arteries defined as normal by arteriography were studied and related to the age and medical history of the patients. The series was divided into 2 groups: patients with clinical manifestation of atherosclerosis and patients without a history of arterial disease. The study included 6 young patients (mean age 14 years) referred for angiographic documentation of abolished intracranial circulation. A typical 3-layered appearance of the arterial wall was found in young healthy individuals as well as in adult and elderly subjects. There was no difference in the ultrasonographic appearance of muscular and elastic arteries. In patients with extensive obstructive atherosclerosis affecting other parts of the arterial tree, a segment of the iliac artery can have the same appearance as seen in young healthy individuals. There are indications that severe hypertension can result in a thickening of the middle low-echogenic layer of the arterial wall. In patients with chronic renal insufficiency, small calcifications in the middle layer were a typical finding.

Flow estimation using ultrasound imaging (color M-mode) and computer postprocessing.

We have developed a method to calculate flow noninvasively in blood vessels using color Motion-mode (M-mode) and computer postprocessing. The velocity of each point in the cross-sectional area of the vessel was found from the color M-mode recording by correcting for angle both distances and velocities and by assuming a symmetrical circular velocity field. Volume flow was then found by integrating the velocity field at 5-ms intervals through the cardiac cycle. In a cardiovascular hydromechanical model, a correlation of 0.99 and p value of less than 0.001 were found between estimated and measured flow in the model (n = 8). In 20 healthy individuals, we made 31 investigations in the common carotid (CCA), internal carotid (ICA), and external carotid (ECA) artery, comparing flow in the CCA with the added flow in the ICA and ECA. The values (CCA versus ICA + ECA) correlated with r = 0.91 and p less than 0.01. Repeated investigations (n = 8) in one individual gave flow estimates of 495 +/- 50 ml/min in the CCA, 304 +/- 45 ml/min in the ICA, and 165 +/- 37 ml/min in the ECA (means +/- SD). This article shows that this system can make accurate estimation of blood flow to the brain noninvasively.

The velocity distribution in the aortic anulus in normal subjects: a quantitative analysis of two-dimensional Doppler flow maps.

The velocity distribution in the aortic anulus is commonly assumed to be uniform. A skewed velocity profile may have consequences for the accuracy of volume flow estimates by the Doppler echocardiographic technique. To assess this issue, the velocity distribution in the aortic anulus in 12 normal subjects was studied by computer analysis of digital velocity data from two-dimensional Doppler ultrasound flow maps. The velocity profiles in the aortic anulus were found to be flat but slightly skewed, with the highest velocities toward the septum. There was little interindividual variation. Our findings imply that the centerline velocity is the best estimate for the spatial mean velocity at the aortic anulus in normal subjects. The importance of this finding in patients is unknown. In normal subjects, the results suggest that stroke volume might be overestimated by approximately 15% by Doppler echocardiography if the cross-sectional velocity profile is not accounted for.

Intra-arterial ultrasound: a potential tool for the vascular surgeon. Clinical review.

Intra-arterial ultrasound is a new real-time imaging technique using a miniaturized transducer in a catheter to image diseased arteries. The small size and close proximity to the structures being imaged allow the use of frequencies from 20-40 MHz, resulting in high resolution images. In addition to the stenotic lumen, it is possible to examine the composition and eccentricity of the plaque, as well as structures outside of the vessel. Such information can be used for planning and evaluating vascular interventions. The addition of Doppler measurements allows the evaluation of function as well as anatomy. Research applications include studies of lesion progression, measurement of arterial elasticity, quantitative blood flow velocities and study of intra-arterial flow patterns. Although this technique is in its infancy, it is highly probable that it will prove to be a useful aid to the vascular surgeon.

Intravascular ultrasound guidance for catheter-based coronary interventions.

Intravascular ultrasound is a new method for visualizing the details of vascular pathology, providing high resolution images of plaque and thrombus. This review summarizes the potential applications of ultrasound imaging in the guidance of balloon angioplasty, atherectomy, laser ablation and stenting. Ultrasound differs from angiography and angioscopy in its ability to penetrate below the surface of the vessel lumen, demonstrating specific aspects about the distribution and composition of plaque. Because the different layers of the arterial wall have different acoustic properties, ultrasound catheters are able to define the layers of normal wall in comparison with plaque. Particularly in combination with therapeutic techniques designed to remove or ablate plaque, ultrasound may prove useful in maximizing the amount of plaque treated and minimizing trauma to normal vessel wall components. Combined imaging/therapeutic devices are in the pilot phase of development and show promise for enhancing the safety and efficacy of the catheter devices.

Sensitivity and speed of colour Doppler flow mapping compared with continuous wave Doppler for the detection of ventricular septal defects.

Twenty nine patients (aged from three months to 37 years) with confirmed or suspected ventricular septal defects were studied separately by three examiners who used colour flow mapping and imaging, or continuous wave Doppler and imaging, or a combined reference examination. Colour flow mapping identified 19 of the 25 patients with a ventricular septal defect, continuous wave Doppler echocardiography identified 18, and the combined reference examination identified 24. Two of four patients without ventricular septal defect had a false positive result with colour flow mapping and none had a false positive result with continuous wave Doppler examination. During the reference examination continuous wave Doppler identified 24 patients with ventricular septal defects and colour flow mapping identified 23. In two patients a second ventricular septal defect was found by colour flow mapping, and confirmed by continuous wave Doppler. There was no significant difference in time to diagnosis between the two techniques. Colour flow mapping aids identification of multiple ventricular septal defects but is not faster and has lower specificity than continuous wave Doppler. A combination of the two techniques gave the highest sensitivity and specificity.

Tissue characterization with intra-arterial ultrasound: special promise and problems.

Although we are able to identify many tissue types based on the screen image in intravascular ultrasound, there is additional information in the ultrasound signal which could be of assistance in characterization and identification of tissue. Intravascular ultrasound has several special characteristics which affect tissue characterization. These include the high transducer frequency, small transducers, short and relatively uniform path to the tissue, and limited tissue types to identify. These characteristics influence the results obtained by absolute backscatter, local statistics, frequency dependent backscatter, and angle dependency of backscatter. These effects are both positive and negative, and in many cases can be observed in clinical imaging. Another area of tissue characterization which can be performed with ultrasound is measurement of arterial wall elasticity. This can be of importance in the evaluation of mechanisms of dilatation, and the potential for complications.