Effects of hemodialysis on coronary blood flow

A Closed-Loop Modeling Framework for Cardiac-to-Coronary Coupling

The mechanisms by which cardiac mechanics effect coronary perfusion (cardiac-to-coronary coupling) remain incompletely understood. Several coronary models have been proposed to deepen our understanding of coronary hemodynamics, but possibilities for in-depth studies on cardiac-to-coronary coupling are limited as mechanical properties like myocardial stress and strain are most often neglected. To overcome this limitation, a mathematical model of coronary mechanics and hemodynamics was implemented in the previously published multi-scale CircAdapt model of the closed-loop cardiovascular system. The coronary model consisted of a relatively simple one-dimensional network of the major conduit arteries and veins as well as a lumped parameter model with three transmural layers for the microcirculation. Intramyocardial pressure was assumed to arise from transmission of ventricular cavity pressure into the myocardial wall as well as myocardial stiffness, based on global pump mechanics and local myofiber mechanics. Model-predicted waveforms of global epicardial flow velocity, as well as of intramyocardial flow and diameter were qualitatively and quantitatively compared with reported data. Versatility of the model was demonstrated in a case study of aortic valve stenosis. The reference simulation correctly described the phasic pattern of coronary flow velocity, arterial flow impediment, and intramyocardial differences in coronary flow and diameter. Predicted retrograde flow during early systole in aortic valve stenosis was in agreement with measurements obtained in patients. In conclusion, we presented a powerful multi-scale modeling framework that enables realistic simulation of coronary mechanics and hemodynamics. This modeling framework can be used as a research platform for in-depth studies of cardiac-to-coronary coupling, enabling study of the effect of abnormal myocardial tissue properties on coronary hemodynamics.

Changes in left anterior descending coronary artery flow profiles after coronary artery bypass grafting examined by means of transthoracic Doppler echocardiography

OBJECTIVE

We sought to investigate the changes of velocity profiles in the left anterior descending coronary artery after coronary artery bypass grafting using transthoracic Doppler echocardiography.

METHODS

Forty-five patients who received a bypass graft to the left anterior descending coronary artery were studied. Before coronary artery bypass grafting, Doppler velocity profiles of the distal left anterior descending coronary artery were recorded with transthoracic Doppler echocardiography. Peak systolic velocity, mean systolic velocity, peak diastolic velocity, mean diastolic velocity, total velocity time integral, systolic velocity time integral, and diastolic velocity time integral were measured. Three weeks after coronary artery bypass grafting, left anterior descending coronary artery antegrade flow in the distal portion of the anastomosis was obtained by using the same method. Coronary angiography was performed before and 3 weeks after coronary artery bypass grafting.

RESULTS

The overall success rate of measuring the left anterior descending coronary artery flow was 60.0% preoperatively and 80.0% postoperatively. In 25 patients, in whom all parameters were obtained both before and after coronary artery bypass grafting, the following increased significantly after coronary artery bypass grafting: peak systolic velocity (14.86 +/- 7.50 vs 25.07 +/- 17.02 cm/s, P =.0045), mean systolic velocity (9.86 +/- 5.42 vs 18.03 +/- 12.94 cm/s, P =.0026), peak diastolic velocity (24.26 +/- 12.54 vs 48.28 +/- 31.66 cm/s, P =.0021), mean diastolic velocity (14.94 +/- 6.65 vs 30.36 +/- 20.71 cm/s, P =.0022), diastolic velocity time integral (7.22 +/- 2.88 vs 15.55 +/- 10.39 cm, P =.0009), total velocity time integral (10.50 +/- 4.48 vs 19.27 +/- 12.63 cm, P =.0034), and diastolic-to-systolic velocity time integral ratio (3.09 +/- 1.53 vs 4.97 +/- 2.75, P =.0044). Angiography showed graft patency and no significant change in left anterior descending coronary artery stenosis in all patients.

CONCLUSIONS

Transthoracic Doppler echocardiography showed a significant increase in some parameters in left anterior descending coronary artery flow after coronary artery bypass grafting. Measurement of left anterior descending coronary artery flow by means of transthoracic Doppler echocardiography might be a noninvasive method to evaluate the effect of bypass grafting on the left anterior descending coronary artery.

Central and peripheral adaptations to physical training in patients with end-stage renal disease

Renal replacement treatment options are life-saving treatments for patients with end-stage renal disease (ESRD). However, prolonged survival in patients with ESRD is associated with various functional and morphological disorders from almost all systems. Anaemia, deconditioning, cardiac dysfunction. impairment of cardiac autonomic control and skeletal muscle weakness and fatigue, primarily because of 'uraemic' myopathy and neuropathy, are the main predisposing factors for their poor functional ability. Physical training is being recommended as a complementary therapeutic modality. There are generally 3 methods of exercise training applied in patients with ESRD: (i) the supervised outpatient programme that is held in a rehabilitation centre; (ii) a home exercise rehabilitation programme; and (iii) exercise rehabilitation programme during the first hours of the haemodialysis treatment in the renal unit. All the available training data show that the application of an exercise training programme in patients with ESRD enhances their physical fitness. This improvement is due to central and mainly peripheral adaptations. Exercise training in these patients increases aerobic capacity, causes favourable left ventricular functional adaptations, reduces blood pressure in patients with hypertension, modifies other coronary risk factors, increases the cardiac vagal activity and suppresses the incidence of cardiac arrhythmias. Moreover, exercise training has beneficial effects on muscle structural and functional abnormalities. These central and peripheral adaptations to exercise training cause an increase in their functional capacity and offer them achance of a better quality of life. Moreover, exercise training improves exercisee tolerance of renal post-transplant patients.

Cardiac effects of exercise rehabilitation in hemodialysis patients

Exercise training has well documented beneficial effects in a variety of cardiac disorders. End stage renal disease patients present many cardiovascular complications and suffer from impaired exercise capacity. No study to date has adequately examined the cardiac responses to exercise training in renal patients on hemodialysis (HD). To determine the effects of an exercise rehabilitation program on the left ventricular function at rest and during submaximal effort, 38 end-stage renal disease patients on maintenance HD were randomised into three groups. Sixteen of them (group A--mean age 46.4+/-13.9 years), without clinical features of heart failure, participated in a 6-month supervised exercise renal rehabilitation program consisting of three weekly sessions of aerobic training, 10 (group B--mean age 51.4+/-12.5 years) followed a moderate exercise program at home, and the other 12 (group C--mean age 50.2+/-7.9 years) were not trained and remained as controls. The level of anemia and the HD prescription remained constant during the study. Fifteen sex- and age-matched sedentary individuals (group D--mean age 46.9+/-6.4 years) were the healthy controls. All subjects at the start and end of the program underwent physical examination, laboratory tests, treadmill exercise testing, M-mode and 2-D echocardiograms performed at rest and at peak of supine bicycle exercise. Left ventricular volumes (EDV, ESV) and mass (LVM) were measured and ejection fraction (EF), stroke volume index (SVI) and cardiac output index (COI) were calculated by standard formulae. The maximal oxygen consumption increased by 43% (P<0.001) and the exercise time by 33% (P<0.001) after training in group A, by 17% (P<0.001) and 14% (P<0.01), respectively, in B, and both remained unchanged in group C. Training in group A was also associated with an increase in LVIDd (from 52.1+/-6.4 to 54.0+/-6.1 mm, P<0.001) and LVM (226+/-67 to 240+/-84 g, P<0.05) at rest with no change noted in groups B and C. Following a 6-month exercise training in group A an increase was also found in the resting EF by 5% (P<0.01) and SVI by 14% (P<0.001). There was no change found in groups B and C. Supine bicycle exercise after training in group A was associated with an improvement in EF by 14% compared to the pre-training change (P<0.001), SVI by 14% (P<0.001) and COI by 73% (P<0.001). These changes from rest to submaximal exercise were less pronounced in group B following training at home. The untrained patients demonstrated no changes in LV systolic function over the 6-month period. These results demonstrate that intense exercise training improves LV systolic function at rest in HD patients; both intense and moderate physical training leads to enhanced cardiac performance during supine submaximal exercise.

Potential use of transthoracic echocardiography in the assessment of coronary flow reserve

Coronary flow reserve provides a gold standard assessment of the epicardial and microvascular coronary circulation. However, measurement of coronary flow reserve is limited by the invasiveness or complexity of the methods hitherto available. We investigated whether transthoracic echocardiography could be used to assess coronary flow reserve. We imaged distal left anterior descending coronary artery diameter and flow in 14 healthy volunteers, both at rest and during intravenous infusion of adenosine (140 microg/kg per minute). Volunteers were men, with an average (+/-SD) age of 28.4 +/- 6.3 years. Complete data were acquired in 11 cases. Average distal left anterior descending coronary artery diameter was 0.213 +/- 0.03 cm. Velocity time integral rose from 8.6 +/- 2.1 cm to 27.7 +/- 5.6 cm with adenosine infusion. Heart rate rose from 64.7 +/- 9. 8 to 75.3 +/- 11.7 bpm. The Doppler angle of incidence to flow was 42.4 +/- 8.7 degrees. Resting distal left anterior descending coronary artery flow was therefore calculated as 13.4 +/- 3.2 mL/min and hyperemic flow as 51.2 +/- 16.2 mL/min, yielding a coronary flow reserve of 3.81 +/- 0.6. We conclude that coronary flow reserve can be assessed in a selected population with the use of transthoracic echocardiography and an intravenous infusion of adenosine. The simplicity of this noninvasive technique suggests that it could become a useful tool for measurement of coronary flow reserve if imaging success rates can be optimized.