Introduction
Over the past several years there has been a growing interest in understanding the diastolic properties of the heart in children. In adults, numerous studies have tried to unravel the complexity shrouding diastolic properties and in the process, the field of "diastology" has been established. However, perhaps due to a lower incidence of diastolic dysfunction in children, the field of diastology has made slower progress than that in adults. Traditionally diastole has been subdivided into four components: (1) isovolumic relaxation, (2) rapid filling phase, (3) slow filling phase (diastasis) and (4) atrial contraction. Although this subdivision provides an elegant physiologic model, from the practical standpoint a more realistic subdivision of diastole consists of two major components: (1) relaxation and (2) diastolic filling.
Left ventricular (LV) relaxation starts with the closure of the aortic valve and continues until the mitral valve opens. As both the aortic and mitral valves remain closed during relaxation, the LV volume remains unchanged and filling of the LV does not occur. However, relaxation prepares the LV for the process of filling. It causes myocardial fibers to decompress and untwist and results in recoil of the fibers like an unloaded spring. At the subcellular level, relaxation occurs during detachment of actin-myosin cross-bridges as a result of removal of Ca2+ from troponin-C, by the sarcoplasmic reticulum. During the process of relaxation the LV pressure declines extremely rapidly. Therefore, a slower decline in LV pressure results from impaired relaxation. Although relaxation is an isovolumic event and actual filling of the LV does not take place during this phase, it prepares the LV for the event of filling. Therefore, a continuation of the process of relaxation is necessary for early filling period of diastole [1] , a period during which the majority of cardiac filling occurs.
Section snippets
Invasive indices of LV relaxation
The process of relaxation has been studied extensively both in the research as well as in the clinical setting. Invasive indices derived by high-fidelity micromanometric catheters positioned in the LV, have been used as gold standards. In order to interpret noninvasive indices of relaxation, we need to first understand the principles behind the invasive indices. This is especially important as some noninvasive indices of relaxation are used as surrogates for the invasive ones. Two invasive
Isovolumic relaxation time (IVRT)
IVRT has been used as a noninvasive index of relaxation. It differs from tau, in the sense that tau describes the rate of drop of LV pressure during relaxation, whereas IVRT measures the duration of relaxation. IVRT can be calculated by various techniques, including a combination of a phonocardiogram, to detect the closure of the aortic valve and M-mode echocardiogram, to detect opening of the mitral valve. In the present era phonocardiograms are used somewhat infrequently. However, the Doppler
Transmitral Doppler profile
The mitral inflow Doppler profile is characterized by an E and an A wave. The deceleration time of the E wave is the time interval from the peak of the E-wave to the actual or extrapolated baseline crossing of the decelerating E wave. In children and young adults the mean deceleration time is 100 ms (range: 48–139 ms) [11] (Table 1). The phenomenon of E and A wave reversal that has been described in adults, consists of a lower E wave velocity and a higher A wave velocity with prolonged
Pulmonary venous Doppler profile
When the mitral inflow Doppler has a ‘normal’ appearance, pulmonary vein Doppler can provide additional insight into diastolic function. Pulmonary vein Doppler waves can be obtained in younger children quite easily by transthoracic echocardiogram from the upper pulmonary veins using a foreshortened apical 4-chamber view, with the sample volume placed one cm inside the mouth of the pulmonary vein [15]. However, since the pulmonary veins are in the far field of the ultrasound beam, optimal
Non-invasive determination of minimum dP/dt and tau
In adults mitral regurgitation jet as well as aortic regurgitation jet has been used to calculate tau. Chen et al. [18] have evaluated tau by the Doppler technique and compared it with invasive measurement of tau, using a micromanometric catheter, in a canine model of mitral regurgitation. By digitizing the Doppler profile of the regurgitant jet, the instantaneous pressure gradient between the LV and LA throughout systole, was calculated using the simplified Bernoulli equation. This resulted in
Future directions
In order for noninvasive indices to be successful in predicting abnormalities of relaxation and diastolic dysfunction in the clinical arena, they should be simple to use and offer the same specificity as that offered by invasive gold standards. Two noninvasive indices, color M-mode Doppler and Tissue Doppler imaging (TDI) have shown considerable promise in studies performed in adults. However, in children further studies need to be performed.
Conclusions
Diastolic dysfunction of the LV is characterized by two major components, impaired relaxation and decreased compliance. Impaired relaxation is often the first abnormality noted in diastolic dysfunction. Noninvasive indices of relaxation are often dependent on age, heart rate and loading conditions. Therefore, accurate interpretation of each of these indices is necessary, especially when applied to children. However, by utilizing a combined approach, integrating many of the noninvasive indices
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