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Biomedical subjects

Stefan Möhlenkamp

Publications and source records attributed to Stefan Möhlenkamp.

18 recordsLinked to original sources
PubMed2006

Transient integral boundary layer method to calculate the translesional pressure drop and the fractional flow reserve in myocardial bridges.

BACKGROUND: The pressure drop-flow relations in myocardial bridges and the assessment of vascular heart disease via fractional flow reserve (FFR) have motivated many researchers the last decades. The aim of this study is to simulate several clinical conditions present in myocardial bridges to determine the flow reserve and consequently the clinical relevance of the disease. From a fluid mechanical point of view the pathophysiological situation in myocardial bridges involves fluid flow in a time dependent flow geometry, caused by contracting cardiac muscles overlying an intramural segment of the coronary artery. These flows mostly involve flow separation and secondary motions, which are difficult to calculate and analyse. METHODS: Because a three dimensional simulation of the haemodynamic conditions in myocardial bridges in a network of coronary arteries is time-consuming, we present a boundary layer model for the calculation of the pressure drop and flow separation. The approach is based on the assumption that the flow can be sufficiently well described by the interaction of an inviscid core and a viscous boundary layer. Under the assumption that the idealised flow through a constriction is given by near-equilibrium velocity profiles of the Falkner-Skan-Cooke (FSC) family, the evolution of the boundary layer is obtained by the simultaneous solution of the Falkner-Skan equation and the transient von-Kármán integral momentum equation. RESULTS: The model was used to investigate the relative importance of several physical parameters present in myocardial bridges. Results have been obtained for steady and unsteady flow through vessels with 0 - 85% diameter stenosis. We compare two clinical relevant cases of a myocardial bridge in the middle segment of the left anterior descending coronary artery (LAD). The pressure derived FFR of fixed and dynamic lesions has shown that the flow is less affected in the dynamic case, because the distal pressure partially recovers during re-opening of the vessel in diastole. We have further calculated the wall shear stress (WSS) distributions in addition to the location and length of the flow reversal zones in dependence on the severity of the disease. CONCLUSION: The described boundary layer method can be used to simulate frictional forces and wall shear stresses in the entrance region of vessels. Earlier models are supplemented by the viscous effects in a quasi three-dimensional vessel geometry with a prescribed wall motion. The results indicate that the translesional pressure drop and the mean FFR compares favourably to clinical findings in the literature. We have further shown that the mean FFR under the assumption of Hagen-Poiseuille flow is overestimated in developing flow conditions.

Animals↗
PubMed2006

Algorithms for converting random-zero to automated oscillometric blood pressure values, and vice versa.

Many surveys and cohort studies have used a random-zero sphygmomanometer blood pressure device (RZS) to measure subjects' blood pressure and to assess the value of blood pressure in predicting cardiovascular events. Recent studies used automated oscillometric blood pressure devices (AODs) that systematically measure higher blood pressure values than RZSs do, hampering comparability of values between these studies. In 2000-2003, the authors randomly used both an RZS and an AOD in an ongoing cohort study in Germany. This analysis aimed to compare blood pressure values by device and to develop an algorithm to convert estimates of blood pressure values from one device to the other. In a randomized subset of 2,365 subjects aged 45-75 years, each subject was measured three times with each device in a randomized order. The mean difference (AOD-RZS) between the devices was 3.9 mmHg for systolic blood pressure and 2.6 mmHg for diastolic blood pressure. The authors found that linear regression models including age, sex, and blood pressure level can be used to convert RZS blood pressure values to AOD blood pressure values, and vice versa. Results may help to better compare blood pressure values in epidemiologic studies that used different blood pressure devices.

Aged↗
PubMed2006

Coronary atherosclerosis and cardiovascular risk in masters male marathon runners. Rationale and design of the "marathon study".

BACKGROUND: Regular physical exercise is recommended to reduce cardiovascular mortality. And yet, atherosclerosis is the main cause of exercise-associated death in persons beyond age 35. The need for risk stratification in marathon runners is under discussion. The predictive value of modern imaging- and non-imaging-based markers of risk that can be used for risk stratification in masters endurance athletes still deserves exploration. METHODS: Male runners > 50 years who have completed at least five marathon races during the preceding 3 years and do not suffer from coronary artery disease, angina nor diabetes mellitus are studied to assess the predictive value of established and modern imaging- based and biochemical cardiovascular risk factors. Laboratory parameters including clinical chemistry, hematology and hormone measurements are determined. Lifestyle-related risk factors, psychosocial and socioeconomic variables are explored using standardized questionnaires. Coronary, carotid, femoral and aortic atherosclerosis is measured using electronbeam computed tomography and ultrasound. In addition, a resting ECG, a bicycle stress test and heart rate variability are performed. Myocardial morphology and function are assessed using echocardiography and magnetic resonance imaging. Participants are invited to compete in a marathon race to quantify the association of coronary atherosclerosis with marathon-related changes of cardiac troponin levels and the extent of marathon-induced inflammation. At the cellular level, the effect on the amount of circulating progenitor cells (EPCs) is determined by FACS analysis. Changes in laboratory parameters and hormone levels are also studied. Annual long-term follow-up including hospital records and death certificates is performed. Data are compared with those from a general unselected cohort from the Heinz Nixdorf Recall Study. CONCLUSION: This study should contribute to cardiovascular risk assessment in the growing number of masters marathon runners with a focus on assessing the predictive value of modern imaging techniques and biochemical markers for comprehensive risk stratification.

Adult↗
PubMed2006

[Aortic valve stenosis: from valvuloplasty to percutaneous heart valve].

Calcified aortic stenosis is the predominant valve disease. Patients affected are most commonly elderly people, who often show associated comorbidities like reduced left ventricular function, impaired renal function, and pulmonary hypertension. The risk of open-heart surgery is elevated. Balloon aortic valvuloplasty enables a reduction of symptoms, an increase in physical performance, and, therefore, an improved quality of life. However, a reduction in mortality cannot be reached with this method. New techniques and improved equipment induced a "revival" of balloon aortic valvuloplasty, which has been introduced almost 20 years ago. In addition, brachytherapy after balloon valvuloplasty has recently been investigated and represents an interesting approach to reduce early restenosis. The technical improvement of balloon valvuloplasty is the percutaneous heart valve, which is under present clinical investigation. The antegrade/transseptal and retrograde approaches are used, as is the transapical access to the left ventricle. Even if long-term results are not yet available and the procedures still require technical improvement, especially minimization of catheter size, percutaneous valve replacement is a new chapter in the treatment of the calcified aortic stenosis.

Age Factors↗
PubMed2006

Prevalence of peripheral arterial disease - results of the Heinz Nixdorf recall study.

BACKGROUND: This report presents population-based estimates of the prevalence of peripheral arterial disease (PAD), chronic critical limb ischemia (CLI), and Moenckeberg's medial calcinosis (MC) in Germany. PATIENTS AND METHODS: From the year 2000 to 2003, a total of 4,814 subjects aged 45-75 years were included in the study. In 30 of the subjects (0.6%), determination of the ankle brachial index (ABI) was not possible, leaving 4,735 subjects (99.4%) in the data set. PAD was considered present in all subjects with an ABI < 0.9 in one leg, and/or a history of prior treatment for PAD. CLI was considered present if the highest ankle artery pressure measured < 70 mmHg. Prevalence of MC was calculated for ABI cut-off values of 1.3 and 1.5. FINDINGS: The overall prevalence of PAD according to the ABI criteria was 6.4% among men and 5.1% among women. After accounting for history of PAD, the prevalence increased to 8.2% among men and 5.5% among women. Taking the ABI criteria and medical history into account, males had a higher prevalence of PAD, with large increases in males aged 65-69 and 70-75 years. Chronic CLI was rare in the investigated population, and was found in only five older subjects (0.1%). With the criterion of ABI > 1.3, about 13.3% of males and 6.9% of females had MC. In contrast to PAD, the prevalence of MC did not increase with age. With the criterion of ABI > 1.5, MC was present in only 1.1% and 0.5% of men and women, respectively, but only 30 (0.6%) subjects had incompressible ankle arteries with a cuff pressure > 260 mmHg. CONCLUSION: Prevalences of PAD based only on ABI generally underestimate the true prevalence of PAD in population-based studies. CLI predominantly affects older subjects. In addition, cut-off values for MC must be newly determined.

Aged↗
PubMed2005

Population-based assessment of subclinical coronary atherosclerosis using electron-beam computed tomography.

AIMS: Coronary artery calcification (CAC) is determined as a measure of the extent of coronary atherosclerosis and can be used for expanded cardiovascular risk stratification. It was our aim to establish reference CAC scores in a population-based unselected European cohort. METHODS AND RESULTS: The Heinz Nixdorf Recall study (HNR) recruited a total of 4814 participants aged 45-74 years. Cardiovascular risk factors and medications were recorded, and CAC was measured using electron-beam CT (EBCT). CAC score distribution was compared with previous studies in subjects who were self-referred, volunteered, or were physician-referred. Of the 4472 (92.9%) subjects free of clinical coronary artery disease, the CAC score was available in 4275 (95.3%) (2027 men, 2248 women). CAC scores were lower in particular in the higher age groups (> or = 60 years) in men than in the previous studies. Also, in most age groups (except the highest, 70-74 years), subjects with no cardiovascular medications had significantly lower CAC scores than subjects using cardiovascular medications. CONCLUSIONS: The current report characterises the distribution of EBCT-derived CAC scores in a European unselected population. Compared with previous reports, CAC scores were lower in our cohort, in particular in subjects not receiving cardiovascular medications. Classification of the CAC score may underestimate true risk if previously published referral cohorts are used as the reference.

Aged↗
PubMed2005

[Normal coronary angiography with myocardial bridging: a variant possibly relevant for ischemia].

Approximately 20-30% of patients with cardiac chest pain have a normal coronary angiogram. In about 5% of these patients, a myocardial bridge can be identified. The characteristic feature is systolic compression of an epicardial vessel, usually the left anterior descending artery (LAD), with the angiographic "milking effect". Using modern imaging techniques, such as intravascular ultrasound (IVUS), intracoronary Doppler ultrasound (ICD) and intracoronary pressure wires, the pathophysiological consequence of myocardial bridging could be established. While previously considered a clinically insignificant variant, ICD recordings demonstrated an increased flow velocity in the tunneled segment. Frame-by-frame IVUS analysis revealed a delayed relaxation after systolic compression, which may extend significantly into diastole. This explains both the impaired coronary flow reserve and ischemia. In IVUS, a circular or eccentric rhythmic compression of the vessel is visible, which may be partial or complete. Latest computed tomography technology can also be used to visualize myocardial bridging noninvasively. Provocation tests, such as application of nitroglycerin, orciprenaline, dobutamine or atrial stimulation, may augment systolic compression and explain symptoms and the beneficial effect of beta-blockers. In severe cases (i.e. limiting symptoms with ischemia despite medication), surgical myotomy may be performed after careful appraisal of the benefit-risk ratio. A high restenosis and complication rate associated with coronary stenting precludes a general recommendation of this interventional approach. Whether drug-eluting stents help to overcome this limitation remains to be shown. Long-term prognosis is good, although previous investigations have been performed in a limited number of cases only.

Clinical Trials as Topic↗
PubMed2004

Determinants of coronary blood flow in humans: quantification by intracoronary Doppler and ultrasound.

The direct determinants of coronary flow are lumen area and blood flow velocity; however, the precise mechanisms that control these factors are not fully understood. The aim of the present study was to assess by which mechanisms lumen area and coronary flow velocity interact with hemodynamic and morphometric factors, thereby influencing coronary flow. Intracoronary Doppler and ultrasound measurements were performed in 28 patients without coronary lumen irregularities. Flow velocity and lumen cross-sectional area were measured in the proximal segments of all three coronary arteries. Global lumen cross-sectional area and global flow were obtained by adding up the values of all three coronary arteries. Left ventricular mass was assessed by echocardiography. Stress-mass-heart rate and pressure-rate products reflecting myocardial oxygen demand were calculated. Global coronary flow increased during adenosine-induced hyperemia from 197 +/- 72 to 637 +/- 204 ml/min (P < 0.001). Global coronary flow closely correlated with the stress-mass-heart rate product (r = 0.62; P < 0.001). Looking at the two constituents of flow separately, global coronary cross-sectional area was closely related to left ventricular muscle mass (r = 0.61; P < 0.001), whereas mean coronary flow velocity at rest showed a strong linear relation with the pressure-rate product (r = 0.64; P < 0.001). There was no interaction between cross-sectional area and blood flow velocity in any of the coronary vessels. Coronary lumen size and flow velocity, the two determinants of coronary flow, are principally determined by different physiological factors. Long-term flow adaptation is achieved by an increase in coronary lumen size, whereas short-term myocardial oxygen requirements are met by changes in resting flow velocity.

Blood Flow Velocity↗
PubMed2003

[Noninvasive computed tomographic coronary angiography as a complement to coronary calcium quantification in symptomatic patients].

BACKGROUND: Invasive, selective coronary angiography remains the "gold standard" of direct visualization of epicardial coronary arteries. Technical advances in recent years and improvements in image quality in both electron beam computed tomography (EBT) and multislice spiral/helical computed tomography (MSCT) brought along an increasing interest in the potential clinical role of noninvasive computed tomographic coronary angiography (CTCA). POTENTIAL AND LIMITATIONS: Measurement of coronary calcification permits quantitative estimation of overall coronary plaque burden and thereby allows assessment of cardiovascular risk and likelihood of the presence of a significant stenosis. However, the precise site and degree of stenoses cannot be measured. Contrast-enhanced CTCA lumenography permits visualization of epicardial coronary artery stenoses with a sensitivity and specificity of about 90%. Noncalcified plaques may also be detected in individual cases, but very few data are available on this aspect of CTCA. Image artifacts due to rapid motion, especially in the distal segments of the right and circumflex coronary arteries, may preclude reliable assessment of 20-30% of these segments. Also, in-stent restenoses and distal bypass anastomoses will, in the foreseeable future, remain difficult to confidently diagnose by CTCA. Combined assessment of calcified plaque burden and CTCA may enhance diagnostic accuracy especially in patients with low or moderate calcium scores. In the presence of heavy calcifications, stenoses may be masked. INDICATIONS: Noninvasive CT-based evaluation of coronary arteries seems useful in patients with a low to intermediate pretest likelihood for significant coronary artery disease (CAD). This holds for several ACC/AHA class II indications described for invasive, selective coronary angiography and for few class I indications. Further prospective studies are required to establish the clinical value of combined assessment of coronary calcium quantification and CTCA.

Calcinosis↗
PubMed2003

Intramyocardial blood volume, perfusion and transit time in response to embolization of different sized microvessels.

OBJECTIVE: To study the role of the coronary microcirculation in response to different-sized microemboli, we measured changes in intramyocardial microvascular blood volume (Bv), perfusion (F) and transit time (TT) and also microvascular patterns of injury. METHODS: Bv, F and TT were quantitated in 24 pigs at baseline and again 2 min after repeat injections of 10- or 100-microm microspheres at rest or during intracoronary adenosine infusion. The association of Bv and TT was assessed in the microsphere pigs and in nine control pigs. Microvascular injury was studied on gross-pathologic and histologic samples. RESULTS: At rest, initial injection of 10-microm microspheres led to increases in Bv and F, but progressively decreased with additional injections. In contrast, even small numbers of 100-microm microspheres always led to decreases in Bv and F. Injection of microspheres during adenosine-induced vasodilation always resulted in decreases in peak Bv and F irrespective of their diameters, but microvascular TTs remained unaltered. In control pigs, however, TTs were inversely related to adenosine-induced changes in Bv. Histologically, 100-microm microspheres resulted in patchy distribution of microcirculatory plugging, while 10-microm microspheres induced contiguous hemorrhagic myocardial injury. CONCLUSION: Microsphere-induced changes in intramyocardial Bv and F and the associated pattern of myocardial injury are related to the size of embolized microvessels and the initial perfusion state. Microvascular functional volume reserve mechanisms appear to play a key role accompanying flow- and TT-preservation.

Adenosine↗
PubMed2003

Prognostic value of extensive coronary calcium quantities in symptomatic males--a 5-year follow-up study.

AIMS: Coronary calcium scores (CSs) have been shown to predict future events in patients presenting for first-time evaluation of CAD. Long-term outcome data on symptomatic subjects with advanced CAD are limited. In this study, we evaluated the prognostic value of very high coronary CSs in symptomatic males undergoing angiography and analyzed the impact of event definitions on identification of risk predictor's. METHODS AND RESULTS: Fifty consecutive symptomatic males with electron beam computed tomography (EBT)-based CSs >1000 were matched 1:2 by age with symptomatic males with scores between 400-1000 and 100-400. All 150 patients underwent coronary angiography. CAD risk factors were ascertained. Events were analyzed after 5 years for: (1) hard coronary events (coronary death and myocardial infarction); (2) overall hard events (adding stroke and non-coronary deaths); and (3) all events (including long-term revascularizations). During follow-up, 17 deaths, two infarctions and three strokes occurred in 21 patients; 38 patients underwent 43 revascularizations. Events occurred earlier and more frequently in patients with scores >1000. Left main disease was the only independent predictor of hard coronary events (hazard ratio, 4.5; 95% confidence interval, 1.1-17.8). Left main disease (4.3; 1.4-13.0) and CSs (1.7; 1.1-2.5) independently predicted overall hard events. Only CSs>90th percentile independently predicted all events (2.5; 1.3-4.8). CONCLUSIONS: Symptomatic males with extensive CSs carry an even higher risk for future events than other symptomatic males with advanced CAD. In these patients, EBT-based calcified plaque burden and angiographic indices of disease severity may have a complementary role in predicting future cardiovascular events.

Calcinosis↗
PubMed2003

Coronary artery calcium and its relationship to coronary artery disease.

Electron-beam computed tomography (EBCT) and the recent generation of multi-slice computed tomography scanners (MSCT) permit high-resolution imaging of the beating heart and the coronary arteries. The visualization of coronary calcium offers the opportunity to non-invasively obtain direct information on coronary anatomy and plaque burden. For clinical purposes, coronary calcium represents the presence of arteriosclerotic plaques. Coronary calcium is deposited in an actively regulated process related to lipid content of and apoptosis within coronary plaques. The amount of coronary calcium is related to the extent of coronary plaque disease, which has substantial diagnostic and prognostic implications. Visualization of coronary calcium by cardiac CT allows to non-invasively detect and localize coronary plaques and describe their distribution in the coronary tree. Approximately 50% to 70% of all plaques are calcified. Calcium cannot be used to reliably identify plaques at risk for developing complications such as rupture or erosion with ensuing thrombus formation. However, data are accumulating that indicate that calcium is an indicator of coronary arteriosclerotic disease activity. A scan negative for coronary calcium has a high negative predictive value indicating absence of stenotic coronary artery disease and an excellent short- to mid-term prognosis. Studies using serial CT scans indicate that the annual progression of coronary calcium varies between 30% to 50% in symptomatic or high-risk individuals and 0% to 20% in patients treated effectively with lipid-lowering medication. An increased rate of progression of coronary calcium seems to indicate a substantially increased risk for adverse cardiac events.

Animals↗
PubMed2003

Imaging of myocardial microvasculature using fast computed tomography and three-dimensional microscopic computed tomography.

In the past, much attention in imaging research was focused on the macroscopic morphology and patency of arteries. Only recently, research and clinical interest have shifted to the microcirculation and its impact on the long-term prognosis in patients with vascular diseases. This focus and newly developed therapeutic strategies require high-resolution imaging modalities, which do not focus exclusively on the macroscopic aspects of the arterial tree. Provided here is a comprehensive perspective of using computed tomography technology to image and quantify the function and morphology of myocardial and vascular adventitial microvessels in normal and disease states.

Blood Volume↗
PubMed2002

The latest on the calcium story.

Coronary calcium scanning is no longer restricted to specialized centers but has become widely available as a test that can easily be performed in clinical practice. There have been parallel developments in scientific evaluation of coronary calcium scanning in numerous research studies and the practical use of this test. A number of reports have clarified the relation between the unstable coronary plaque and coronary calcium. Whereas calcium is a frequent feature of plaque rupture, there is too much overlap with more stable plaques to be specific. However, data are accumulating to indicate that calcium is associated with coronary atherosclerotic disease activity. Indeed, reports presented within the past year have demonstrated that very high calcium scores, in particular those >1,000, indicate a significantly increased cardiovascular risk. The advent of cardiac spiral computed tomography (CT) has made coronary calcium scanning widely available. As this methodology is continuously evolving, a number of challenges regarding standardization of test ordering, performance, and interpretation have arisen. Currently, 4-slice spiral CT scanners are most widely used. Protocols with prospective or retrospective electrocardiographic triggering are used, which appear to yield results similar to the Agatston coronary calcium score compared with electron-beam CT, despite the technical differences. New scoring algorithms are being evaluated. With electron-beam and spiral CT we have, for the first time, a readily available test for direct diagnostic visualization of the coronary arteries in clinical practice. This represents the opportunity and the challenge of a proof of principle.

Calcinosis↗
PubMed2002

Assessment of clinically silent atherosclerotic disease and established and novel risk factors for predicting myocardial infarction and cardiac death in healthy middle-aged subjects: rationale and design of the Heinz Nixdorf RECALL Study. Risk Factors, Evaluation of Coronary Calcium and Lifestyle.

BACKGROUND: In view of consistently high cardiovascular morbidity and mortality rates, international efforts are aimed at developing tools for more precise risk prediction to allow preventive treatment targeted at high-risk individuals. Direct visualization of anatomic, preclinical atherosclerotic disease has the potential for individualized risk discrimination. Further, a variety of risk factors are actively evaluated, including markers of the activity of atherosclerotic disease, thrombogenic risk, and genetic polymorphisms. METHODS: The Heinz Nixdorf RECALL (Risk Factors, Evaluation of Coronary Calcium and Lifestyle) study is a population-based, prospective cohort study of the comparative value of modern risk stratification techniques for "hard" cardiac events. It is designed and powered to define the relative risk associated with the specific extent of coronary atherosclerosis measured by means of electron-beam computed tomography (EBCT)-derived coronary calcium quantities for myocardial infarction and cardiac death in 5 years in 4200 males and females aged 45 to 75 years in an unselected urban population from the large, heavily industrialized Ruhr area. Additionally, the predictive values of conventional cardiovascular risk factors, new candidate and socioeconomic risk factors, certain genetic polymorphisms, and direct signs of subclinical disease are examined with the ankle-brachial index, resting and stress electrocardiograms, and determination of carotid artery intima-media thickness. Prospective clinical risk-benefit and health economic analyses are an inherent part of the study. Study findings with established clinical significance are reported to the participants, but the EBCT findings are withheld until the conclusion of the study. CONCLUSIONS: The Heinz Nixdorf RECALL study will define appropriate methods for identifying high-risk subgroups in the general urban population who may derive the greatest benefit from preventive treatment.

Aged↗
PubMed2002

Quantification of myocardial microcirculatory function with X-ray CT.

A mathematical model of the intramyocardial coronary microcirculation is used to explore the validity of a fast CT imaging method for characterizing the myocardial microcirculatory functional status. The fast CT method depends on the demonstrated CT-based estimation of myocardial perfusion (F) and the intramyocardial blood volume (B(v)). The observed curvilinear myocardial blood volume-to-flow relationship, empirically fitted to B(v) = a.F + b.F(0.5), is a signature of the underlying early pathophysiologic processes thought to be involved in systemic disease processes, such as atherosclerosis, hypertension, and diabetes mellitus. The sensitivity and specificity of the CT-based estimate of this characteristic relationship is explored by altering the characteristics of the vascular diameter-to-flow relationship and the variation in the fraction of capillaries perfused at different coronary flows. The simulation results also indicate that if the vascular diameters change so that the vascular resistance corresponds to the change in flow, then the empirically observed myocardial B(v)-to-F relationship holds well.

Animals↗