Cardiac Time
Interval
Cardiac time interval are regulated
precisely by the mechanics and functions of the myocytes; hence, these
intervals are good measure of cardiac function. TDI is well suited for
determining the timing of myocardial events. The precise timing of these events
is helpful in understanding the mechanism of myocardial relaxation and
myocardial suction during early diastolic filling. In healthy heart, in which
efficient myocardial relaxation is used effectively to suck blood from LA into
the LV during
early diastole, the time of onset of mitral inflow (E) coincides with what of
myocardial early diastolic motion (relaxation) of the mitral annlus (Ea).
However, in hearts with delayed myocardial relaxation and increased filling
pressure, diastolic filling (onset of the E wave) depends more on the increased
LA pressure and occurs earlier than the onset of the early diastolic motion of
the mitral annulus. Therefore, the time interval between the onset of the
mitral E velocity and that of the mitral annulus diastolic motion (Ea) increases,
and this increased interval has been proposed as a new variables to assess LV filling pressure.
A limitation of measuring cardiac time
intervals by pulsed wave Doppler echocardiography is nonstimultaneity because
different cardiac cycles are usually needed to measure various intervals which
in turn are used together. One solution is to have the capability of obtaining
multiple pulsed wave Doppler recordings simultaneously. Another creative means
to measure cardiac intervals from a single cardiac cycle is to use tisuue
Doppler anatomic colour M-Mode from the anterior mitral leaflets. From this
technique, isovolumic contraction time, isovolumic relaxation time, and LV ejection time can be
measured reliably from a single cardiac cycle.
Mechanical dyssynchrony is measured by
time intervals between peak ejection systolic velocities or peak strain of
multiple myocardial segments.
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