3.3 Results
3.3.4 Sloshing
2.9.1.2 PROGNOSTIC IMPLICATION OF LVH
The Framingham Heart study has unequivocally demonstrated the prognostic value of echocardiographically detected LVH.106 Subjects with LVH are older, more obese, have higher blood pressures and are more likely to have pre-existing coronary disease. LVH predicts an increased risk of cardiovascular morbidity and death, even after adjustment for other major risk factors (age, blood pressure, obesity, pulse pressure, diabetes, cholesterol profile, hypertension treatment, cigarette smoking). A limitation of these data is that the Framingham Heart Study is composed predominantly of white adults. The prevalence of echocardiographic LVH is reported to be higher in black adults and LVH is associated with a doubling of mortality in both black and white cohorts.106
2.10 MEASURES OF BODY FAT
2.10.1.2 WAIST / HIP RATIO
Central obesity is often assumed by the easily measureable waist circumference and the waist-to-hip ratio. Since waist circumference alone is more strongly correlated with visceral fat than the waist-hip ratio, guidelines tend to focus on waist circumference to estimate disease risk.107 These widely used cut-off points (102cm for men, 88cm for women) were originally based on a replacement of the classification of BMI. It has been suggested that waist circumference is more sensitive to changes in energy balance such as decreased physical activity than BMI.107 Hip circumference is more strongly associated with leg fat mass, but also leg lean mass (particularly in men).107
2.10.1.3 SKINFOLD MEASUREMENTS
Early studies on fat distribution often used skinfold measurements to characterize subcutaneous fat distribution such as ratios of skinfolds like the triceps / subscapular skinfold ratio, ratio of the sum of trunk folds to the sum of skinfolds on extremities. The most promising and simple indicator of central fat distribution using skinfolds seems to be the subscapular skinfold.107 A prospective study showed that the subscapular skinfold predicted coronary heart disease in men independently of BMI and other cardiovascular risk factors.108 A few other studies however have shown that correlations of waist circumference with cardiometabolic risk factors were either similar or stronger compared with those of the subscapular skinfold.
2.10.2 BIOELECTRIC IMPEDANCE
The bioelectric impedance technique is based on the principle that electrical conductivity of fat is far less than that of fat free mass. It is a reliable and valid approach for the estimation of human body composition.109,110 It is safe, noninvasive, provides rapid measurements, requires little operator skill and subject cooperation and the equipment is portable. This method has been validated, found to be accurate, precise and free of
bias.111,112
The OMRON BF510 body composition monitor measures body fat percentage using the bioelectrical impedance method.75 Visceral fat is also measured using the same device. It is a hand held impedance analyzer that uses in addition to arm and foot impedance, weight, height, age and sex by passing an extremely weak current of 50kHz and less than 500µA through the body to derive the body fat percentage. The Omron body composition monitor has been validated in a Pan European study in collaboration with research centers in the United States.113,114 The study suggests that the monitor provides a reliable tool for the assessment of body fat percentage in epidemiological studies.113 Normal reference values are as shown in tables 4 and 5 in the following chapter.
2.10.3 DUAL ENERGY X-RAY ABSORPTIOMETRY
Dual energy x-ray absorptiometry (DXA) is used primarily to measure bone mineral density. It can also be used to measure total body composition and fat content. Two
distinct low energy x-ray beams penetrate the bone and soft tissue. The system uses a body scanner. The application of DXA in obese individuals is subject to practical limitations. Many obese patients are simply too wide for the scan field and because of this DXA manufacturers do not recommend scanning individuals whose body weight exceeds 100kg115
2.10.4 UNDER WATER WEIGHING
Under water weighing (hydrostatic weight) is a method of determining body composition using Archimedes’ principle of displacement. It is based upon the assertion that the density of fat mass and fat-free mass are constant, lean tissue such as bone and muscle are more dense than water and fat tissue is less dense. Therefore a person with more body fat will weigh less under water and be more buoyant. Someone with more muscle will weigh more under water.
2.10.5 AIR DISPLACEMENT PLETHYSMOGRAPHY
Air displacement plethysmography is also used for measuring human body composition.
It utilizes the inverse relationship between pressure and volume (Boyle’s law) to measure body volume directly.
2.10.6 COMPUTED TOMOGRAPHY
Computed tomography is an imaging technique capable of directly measuring abdominal fat with a very high degree of accuracy and precision.116 CT tissue area and volume measurements have been shown to be highly reproducible. An average error of + 0.6%
for whole body adipose tissue volume has been reported.117 Rossner et al118 also found good agreement between CT and cadaveric adipose tissue areas and ratios. It is non-invasive but it’s potential is limited by high equipment cost, high technical skill requirements and exposure to ionizing radiation. It can be used to quantify intra-abdominal fat using a single intra-abdominal L4/L5 image, as it correlates highly with visceral adipose tissue volume.119
2.10.7 MAGNETIC RESONANCE IMAGING
Magnetic resonance imaging is essentially the same as CT in the estimation of body composition at the tissue level. It is a non x-ray based method in which a magnetic field excites water and fat molecules in the body producing a measurable signal. It is very safe as it uses no ionizing radiation. It’s use is however limited by the high cost of equipment and analysis.