Page 9 - CBAC Newsletter 2015

maintain stroke volume (cardiac output) and blood of filling, diastolic intervals consist of early rapid filling
pressure. This is achieved by the LV being a E-waves followed by diastasis followed by atrial systole
volume- (suction) pump in diastole, while being a generated A-waves. E- and A-wave contours were
pressure/volume pump in systole. The RV is always simplified as triangles. To this day, triangle-based
a volume pump, because in systole it raises (mean) indexes, including the peak heights of the E- and
pressure only slightly from ≈5 in the RA to ≈15 mmHg A-waves and their ratio, (E peak , A peak , E peak /A peak ), the
in the PA. The atria (assuming sinus rhythm) are always deceleration time and duration of the E-wave (DT, E ),
dur
volume pumps – they never generate a significant and the velocity time integral (VTI) of the E- and A- waves
increase in pressure –their job is to move the volume (Figure 1A), are routinely measured. The use of a
along and pre-stretch the LV prior to systole. Once the triangular approximation to the E-wave shape is
‘volume-pumping’ and ‘pressure-pumping’ paradigm is convenient, especially when one considers the quality
appreciated, the Frank-Starling mechanism becomes of images available using the ‘old’ technology. However,
obvious as the physiologic solution to the requirement as a result of the improved temporal resolution and
that assures time-averaged, equal volume pumping of image processing capabilities of current technology, the
the left heart vs. the right heart and it provides a curvature of E-wave contours can clearly be discerned
functional and physiologic explanation for the observed and the information they convey can be determined.
shape and cyclic motion of the L and R sided chambers. (See figure 1B). Despite these advances, physiologically
important curvature features of the E-wave are usually
An interesting and useful question to ask is “Why does disregarded. However, there is benefit in understanding
the heart fill in two phases (E-and A-waves) but empty the physical and physiologic principles that govern the
in a single phase?” In other words what is the purpose curvature, and we discuss this in more detail below
of the atrial kick? That is, what is the advantage of (PDF Model).
normal sinus rhythm rather than (rate controlled) atrial
fibrillation or atrial standstill? The primary evolutionary Technological advances allow the measurement of even
purpose of atrial systole cant be to deliver filling volume smaller (i.e. tissue) velocities. This allows measurement
since some folks are totally asymptomatic while in (rate of the longitudinal displacements of the mitral annulus.
controlled) atrial fibrillation. In the normal heart E-wave Once again, the shapes of mitral annular velocity
volume is 85% of stroke volume and A-wave filling contours are usually considered as triangles, with
volume is 15%. In middle age and later the proportions determination only of the peak, labeled E’. In selected
change such that by age 65, 70% E-wave and A-wave patient populations E’ correlated with measured
filling volumes are about the same. It turns out that end-diastolic pressure (10). Improved technology has
what atrial systole achieves is an additional 10% stretch revealed further details regarding the curvature and
of myocytes (sarcomeres) resulting in increasing peak oscillations of the E’-wave, and we discuss these
+dP/dt for the subsequent systole. In other words the detailed insights at greater length below.
benefit of pre-systolic stretch is increased ability to
overcome load in the next cycle. This aspect of muscle Additional novel imaging capabilities include techniques
physiology is familiar to everyone who has tried to jump such as speckle tracking that allows for strain and
to achieve maximum height –as in a basketball game strain-rate measurements. Speckle tracking is a recent
when jumping for the ball. Maximum height is achieved example of technological progress, because it relies
if the muscles are slightly pre stretched –as in the slight on the information content inherent in the seemingly
bending of the knees to pre-stretch the thigh muscles random arrangement of bright speckles present in all
prior to jumping! echocardiographic images (5). Importantly, strain and
strain rate indexes are ‘local’ indexes that characterize
How to measure diastolic function wall motion function only in the echo slice being imaged.
In contrast, the E-wave is an index that reflects global,
Both physiologists and clinicians have proposed rather than segmental diastolic function and is therefore
numerous indexes of filling function (39). The nature of more sensitive and specific in being able to characterize
these indexes depended critically on technology and the global, rather than regional or segmental diastolic
ability to invasively or noninvasively characterize filling function. It is global function that best reflects the
function. As echocardiography technology advanced overall function state of the heart and its clinical
from m-mode, to two-dimensional (2-D) to Doppler to correlation with, for example, exercise tolerance or
color Doppler, early work focused on the features of shortness of breath.
pulsed-wave Doppler measured transmitral flow velocity
E-wave contours, approximated as triangles. In terms While the various echo-based imaging modalities

CBAC Center Heartbeat | 3