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changing equipment during longitudinal studies.141 Development of software is underway to permit an automated assessment of the myocardial vector traces obtained using color tissue Doppler, and this may improve performance of the technique.142 Global Cardiac Function Tei Index First described in 1997 from measurements made in adults,143 the Tei index has been used in fetal echocardiography to describe changes in myocardial performance with gestational age, and in conditions of altered loading such as TTTS.144–146 This index is a measure of global cardiac function and uses pulsed wave Doppler of the mitral inflow and aortic outflow waveforms Although it is technically easy to record, its reproducibility has been questioned, and various modifications (Fig 6.15), by using the valvar ejection clicks,144 have been developed As for long axis measurements, different ultrasound machines produce different values, and results are not comparable from machine to machine Other authors have used myocardial tissue velocities to produce a Doppler tissue Tei index.147 The waveforms produced are more difficult to interpret than with pulsed Doppler, and caution should be exercised in their interpretation FIG 6.15 Doppler recording of simultaneous inflow into the left ventricle and flow through the aortic valve The Tei index is calculated as shown in the figure; it includes the valve clicks to facilitate its measurement The measurement a includes the isovolumic acceleration and relaxation times The ratio of these to the ejection period, b, gives the Tei or myocardial performance index The higher this is, the worse the myocardial performance A reduction in the Tei index is seen with increasing gestational age, due to a reduction in isovolumic contraction and relaxation times Although it has been shown ineffective in animal models to reflect global myocardial function, it is sensitive to afterload changes and fetal conditions complicated by differential loading can be predicted and monitored using this technique.145,146 More recently, development of software has permitted an automated assessment of the Tei index, and this is a promising tool to permit more robust study of gestational changes.148 However, examination of the right ventricle is particularly challenging, and dual Doppler recordings may be more reliable than the automated version.149 Myocardial Strain and Strain Rate Definition Strain is the increase or decrease in length relative to its resting length when a force is applied, and strain rate is its rate of change Fetal myocardial shortening is represented by negative strain values during systole, whereas positive strain, or myocardial lengthening, occurs during diastole.150 Myocardial deformation is three dimensional with both longitudinal, radial, and circumferential components, and measurements of twist and torsion have been investigated in animal models and phantoms.150,151 Ventricular wall strain may be reported in two ways, natural and Lagrangian Natural or point strain is assessed by summating the small strain values calculated by automated software along the ventricular wall, and Lagrangian strain by summing the distances during the mechanical event and calculating strain along the whole length of the wall Lagrangian strain is likely the better methodology for the fetus, and more software packages now use this method However, both have been reported using the variety of technologies available,152,153 and conflicting gestational patterns reported in the fetus are likely the result of technical factors.154–156 Techniques Assessment of fetal strain is usually made offline using specialized software