Reconstructing Obesity: The Meaning of Measures and the Measure of Meanings

Weight and muscle gain

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Also, the four tracings take longer to record than the two measurements needed with the modified Quinones method. Other methods that use area measurements of the LV cavity have also been described. The main limitation of both the 1D and 2D methods described above is that they are less accurate in patients with regional variation in systolic function, as the measurements can be obtained from a region of the LV cavity where the function is discordant from the overall ventricular function see Figure 4.

Both methods may also have reduced accuracy if the imaging planes used for the measurement are incorrect i. With the modified Simpson method, the ventricle can be foreshortened on apical images, so that the true apex will not be imaged see Figure 5. This can lead to errors if apical function is discordant, as can happen with an apical infarct.

Obese patients, patients with chronic obstructive pulmonary disease and patients with limited space between the ribs will often have poor image quality see Figure 6A. The use of echocardiography contrast has been shown to improve LVEF determination in patients with poor acoustic windows and reduce inter-observer variability see Figure 6B. Several reconstruction techniques have been developed to acquire 3D data of the heart from which LV volumes can be calculated.

Unlike other echocardiographic methods, 3D methods make minimal assumptions of LV cavity shape. They have been shown to be less variable and more accurate than other echocardiographic methods, when compared with MRI as a reference standard.

Introduction

However, the acoustic window needs to be of sufficient quality to allow the delineation of the entire LV cavity endocardial border. Because the image data are usually acquired over several heart beats, an ectopic beat or breathing during the imaging time will lead to artefacts which can alter the endocardial border see Figure 7 and different segments of the left ventricle will appear to contract at different times.

Also, LVEF is usually calculated using commercially available software that is semi-automated, requiring the user to manually assign certain points e.

If the points are improperly assigned, the LVEF may be inaccurate. The location of the endocardial border may also be incorrectly interpreted, either by the operator or the software due to poor definition, and a papillary muscle or trabecula may be interpreted as being the endocardial surface. The area of the tracing for each image slice is multiplied by the slice interval slice thickness plus image gap to determine a volume for that slice. The volumes of the slices are summed to determine an LV volume.

This method requires few assumptions of LV shape because the entire LV cavity is traced. Because of the high contrast resolution and high signal:noise ratio of MRI, the endocardial border is usually well defined. MRI is contraindicated in patients with implantable cardioverter defibrillators, most pacemakers and several other types of implanted devices. Cardiac MRI requires multiple breath holds, and image quality may be poor in patients who cannot hold their breath.

If the level of inspiration is different during the acquisition of different levels, segments of the LV may not be imaged while other segments may be imaged twice. This may lead to variability in calculated volumes and LVEF. Because data are acquired over several cardiac cycles with ECG gating, image quality will be degraded in patients with cardiac arrhythmias or ectopic beats leading to decreased accuracy.

In a study by Karamitsos et al. The automated methods usually rely on the differentiation of the LV cavity from the endocardium based on Hounsfield unit measurements.

As long as the contrast bolus timing is appropriate, there will be high contrast and spacial resolution resulting in a well defined endocardial border. Disadvantages of CT are the exposure of the patient to ionising radiation and the need for iodinated contrast material. Iodinated contrast material should be not be used in patients with iodinated contrast allergies — unless they have been pre-medicated to avoid any allergic reaction — and should be used judiciously in patients with poor renal function.

Also, variability in the selection of the ventricular basal segment will cause variability in LVEF calculation when using the Simpson method. Breathing during image acquisition can also lead to artefacts, which can reduce accuracy. Because of the need for intravenous IV contrast to delineate the endocardium, a problem that is unique to CT is the need for proper coordination of the timing of contrast injection and scanning.

LVEF can be calculated by several methods using different nuclear cardiac imaging techniques. Most commonly, planar images of the left ventricle are acquired for analysis, although SPECT images can also be acquired.

The Meaning of Measures and the Measure of Meanings

If planar imaging is used, a left anterior oblique projection with best separation of the left and right ventricle is acquired for LVEF calculation. MUGA scan measures changes in radioactivity in the left ventricle between end-diastole and end-systole, rather than truly measuring LV volumes. Assignment of LV ROI can be automated, semi-automated or manual, automated and semi-automated edge detection being most commonly used.

Image acquisition is gated with an ECG and radioactive counts are acquired over multiple cardiac cycles. Each cardiac cycle is divided into a predetermined number of intervals usually 16 or 32 , corresponding to the number of frames images per cardiac cycle. The frame with the highest counts is considered end-diastole and the frame with the lowest counts is considered end-systole. LVEF equals net counts in the end-diastolic frame minus net counts in end-systolic frame divided by net counts in end-diastole.

Net counts are calculated by subtracting counts from a background ROI placed next to the left ventricle from measured LV counts. It can be performed in patients whose body habitus might prevent or limit the reliable use of other techniques.

Obesity: An increasing problem for orthopedists

There are no absolute contraindications to this technique. This will reduce the accuracy of LVEF calculations in patients with arrhythmias. In addition, poor labelling of the RBCs can occur, which can lead to poor count rates within the blood pool and increased background counts. If background counts are overestimated e. On the other hand, if background counts are underestimated e. Differences in software edge detection algorithms have also been shown to lead to variation in the calculated LVEF.

Gated myocardial perfusion SPECT is performed by injecting a patient with a radiolabelled myocardial perfusion agent such as technetium 99m radiolabelled sestamibi or tetrofosmin. Ammonia, rubidium or fluorodeoxyglucose can be used as imaging agents. The LV functional assessment with LVEF calculation is usually done in conjunction with a myocardial perfusion study, 16—18 allowing function and perfusion to be evaluated with one test.

After injection, the radiopharmaceutical is taken up by the myocardium. After allowing a sufficient amount of time for the radiotracer to be taken up by the tissues and cleared from the blood pool, ECG-gated images are acquired. LVEF is determined quantitatively by analysing a reconstructed three-dimensional data set using software with automated edge detection.

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Because a three-dimensional data set is used, few geometric assumptions need to be made about the LV cavity shape. The software automatically determines the border between the count high LV myocardium and the count poor LV cavity. For this reason, LVEF calculation using gated myocardial perfusion is not recommended for patients with arrhythmias such as atrial fibrillation, frequent premature ectopic beats or heart block.

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If the perfusion defect is so severe that the border cannot be accurately determined, the LVEF may be inaccurate see Figure This occurs because there is blurring of the LV cavity border due to the relative poor resolution of the gamma camera compared with LV wall thickness, and because of an increase in myocardial count density with contraction see Figure As with other automated or semi-automated methods of calculating LVEF, the calculated LVEF can vary between different processing systems, even when identical raw data are being analysed.

With other techniques, LVEF is calculated with the patient truly at rest. Some people with severe ischaemia have been shown to have a true decrease in LVEF due to prolonged stunning with post-stress imaging. Some of the variation in LVEF calculations may be intrinsic to the way in which the data are processed within an individual institution e. Because of the variation that can occur between modalities and institutions, if serial measurements of the LVEF are needed, the same modality should be used to limit intermodality variation.

Garth is a sociocultural and medical anthropologist specializing in the anthropology of food. Her work addresses issues of inequality and structural violence , with regional interests in Latin American, the Caribbean, and the United States. She currently has active research projects in Cuba and Los Angeles. In Cuba, she has conducted research on household food acquisition practices and the changing Cuban food system. In Los Angeles, she has been researching the food justice movement and the organizations that work toward increasing healthy food access in low-income areas.

Both projects address issues of race and gender based inequality. Garth is currently working on a book project on household food consumption practices in Santiago de Cuba. She is also developing new research on obesity and related chronic diseases in Havana and Santiago de Cuba.

For more information on Dr. NAPA Bulletin Nancy J.

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