Exercise radionuclide assessment of left ventricular function before and after coronary bypass surgery

Quantification of coronary artery bypass graft flow by magnetic resonance phase velocity mapping

OBJECTIVES

Determination of the true coronary artery bypass graft function requires quantification of the flow rate within the graft. The purpose of the present study was to assess the feasibility of characterizing and quantifying graft flow by magnetic resonance phase velocity mapping.

MATERIALS AND METHODS

Twenty-seven patients with 41 angiographically patent coronary artery bypass grafts underwent electrocardiographically gated magnetic resonance phase velocity mapping. Imaging was performed at 0.6 Tesla using a surface coil. Velocity maps of the bypass grafts were obtained throughout the cardiac cycle with a temporal resolution of 50 ms and a spatial resolution of 1.9 x 1.2 x 5 mm3, allowing calculation of phasic and mean graft flow.

RESULTS

Adequate flow measurements were obtained in 84% (41 out of 49) of the grafts. Coronary artery bypass graft flow was characterized by a biphasic pattern with a first peak during systole and a second peak during diastole. Average maximum systolic and diastolic velocities over the cross-section of the grafts were 14 +/- 8 cm/s and 15 +/- 9 cm/s, respectively. Mean coronary artery bypass graft cross-sectional area was 0.28 +/- 0.13 cm2. Mean volume flow was 87 +/- 59 ml/min.

CONCLUSION

Flow in coronary artery bypass grafts can be characterized and measured noninvasively by magnetic resonance phase velocity mapping.

Bedside assessment of myocardial performance in the critically ill

No measurement of myocardial performance currently available in the ICU can be regarded is ideal. Table 2 summarises the main features of the major monitoring techniques. As many of the indices of myocardial performance are interdependent, quantifying the contribution of each component to overall cardiac function is not possible currently, and the clinical utility of monitoring each individually is not therefore established. Bedside measurements of LV dimensions, volumes and ejection fraction, and the other indices of systolic and diastolic function can now be made, but the case for their routine use in influencing clinical practice remains unproven. Transoesophageal echocardiography has an important and established diagnostic role and has been used successfully for continuous monitoring during surgery, but practical considerations seriously limit its potential for routine use. Radionuclide techniques allow the measurement of many of the same parameters and have the potential for continuous use, but practical problems and the additional risk of radiation exposure may limit this application in the critical care environment. Doppler techniques are non-invasive, provide continuous data and are simple to operate, but the data provided has important limitations. Although the pulmonary artery catheter has been in use for over twenty years, questions regarding the information is provides concerning myocardial function remain and the extent to which it should influence therapeutic decisions is still controversial. However with the development of additional facilities, particularly the continuous measurement of cardiac output the pulmonary artery catheter seems likely to remain the mainstay of bedside monitoring of myocardial performance in the critically ill in the immediate future.

Nonangiographic assessment of coronary artery bypass graft patency

Coronary artery bypass graft patency can be assessed using the indirect techniques of evaluating patients' symptoms and exercise tolerance, changes in stress electrocardiogram, radioisotope regional perfusion, and myocardial wall contraction. The direct techniques assess graft patency directly by visualizing grafts using conventional computed tomography (CT), ultrafast CT, magnetic resonance imaging, digital subtraction angiography, and echocardiography. The advantages and disadvantages of each of these modalities are reviewed. At the present time, ultrafast CT and possibly magnetic resonance imaging and Doppler appear to be the only techniques besides angiography that can consistently evaluate bypass graft patency. Although they have the advantage of being minimally invasive, they cannot show graft stenosis or sequential graft patency. These techniques are best used in following patients after coronary bypass graft surgery and ruling out graft closure as the source of chest pain.

Exercise first-pass radionuclide assessment of left and right ventricular function and valvular regurgitation in symptomatic mitral valve prolapse

The changes in right ventricular (RV) and left ventricular (LV) function and in regurgitant fractions on first-pass exercise radionuclide angiography (RNA) were assessed in 29 consecutive patients with symptomatic mitral valve prolapse (MVP). The mean right ventricular ejection fraction (RVEF) was 35 +/- 8% at rest and 46 +/- 15% after exercise (p less than 0.001). The mean left ventricular ejection fraction (LVEF) was 62 +/- 11% at rest and 74 +/- 13% after exercise (p less than 0.001). Seven of 29 patients had an abnormal RV response and 6 had an abnormal LV response. Eight had abnormal wall motion after exercise. A total of 12/29 patients (41%) had one or more abnormalities. The mean left-sided regurgitant fraction before exercise was 27 +/- 17% in 21/29 patients (72%) and 31 +/- 21% after exercise (p = ns). An additional 5 patients (17%) developed left-sided regurgitation after exercise. These findings indicate that wall motion abnormalities and abnormal RVEF and LVEF responses to exercise occur in symptomatic MVP patients. In addition, 26/29 (89.6%) had left-sided regurgitation after exercise. Since the presence of a murmur did not correlate with the presence of mitral regurgitation by RNA, then symptomatic patients with MVP should have first-pass exercise RNA to assess the presence of regurgitation at rest and after exercise. Antibiotic prophylaxis is recommended in MVP patients with systolic murmurs or with regurgitation. Since patients without murmurs can have regurgitation, further study is necessary to determine the need for endocarditis prophylaxis in these patients.

Long-term follow-up by exercise radionuclide angiography of patients after valve replacement for aortic regurgitation

The long-term effects of valve replacement for chronic isolated aortic regurgitation as assessed by first-pass exercise radionuclide angiography have never been reported. We studied 20 males and 5 females before, 15 months postoperatively, and from 29 to 109 (mean 62 +/- 21) months following valve replacement with exercise radionuclide angiography. Mean peak heart rate did not change for the three studies. Peak systolic blood pressure decreased from 201 +/- 42 mmHg to 185 +/- 24 mmHg at 15 months and further declined to 177 +/- 32 mmHg by the long-term study (p less than 0.03). The mean resting left ventricular ejection fraction improved from 44 +/- 15% preoperatively to 57 +/- 18% at 15 months (p less than 0.002) with no further improvement by the long-term evaluation. The postexercise ejection fraction improved from 42 +/- 13% preoperatively to 61 +/- 21% at 15 months (p less than 0.002) also with no change by the long-term study. The duration of exercise improved from 9.7 +/- 4.6 min to 11.9 +/- 3.4 min (p less than 0.03) at 15 months with no additional improvement long term. Improvement in resting and postexercise ejection fraction and in exercise duration is maximal at 15 months. Accuracy and cost containment suggest that assessment of the maximal change in ejection fraction by exercise radionuclide angiography after aortic valve replacement in asymptomatic patients be limited to the 15-month interval.