Page 8 - CBAC Newsletter 2015

the mediastinal aspect of the pericardial sac are fixed in physical terms, this means that due to recoil of the
space. This assures the normal reciprocation of atrial myocardium, the chamber cavity is expanding (recoiling)
and ventricular volumes during the cardiac cycle such faster than it can fill. One may ask: Where does the ener-
that the ejection fraction of the pericardial sac is about gy for suction come from? The answer is: systole. As the
5% — which means 850ml volume of the contents of ventricle ejects and overcomes the peripheral arterial
the sac decrease only by about 40-50 ml during the load, it simultaneously compresses intracellular (titin)
end-systolic phase of the cardiac cycle. The pericardial and extracellular (collagen, elastin, ECM, visceral
sac includes the entire myocardium, the roots of the pericardium…) elastic elements. This stored elastic
great vessels and the blood in the chambers. Because strain energy is released when enough of the cardiac
of conservation of volume - it is self evident, that for the muscle relaxes (crossbridges uncouple), and the elastic
left heart, if systolic ejection volume (into the aorta) elements ‘spring back’ and power the recoil of the
precisely equaled the pulmonary venous volume ventricle until the wall fully relaxes (i.e. diastasis is
entering the atrium (i.e. entering the pericardial sac) reached) and a relaxed equilibrium configuration is
as a result of the simultaneous descent of the closed achieved. Recall, at diastasis, there is no transmitral
mitral valve and aortic root – familiar to clinicians as the flow, the ventricle is in ‘equilibrium’ meaning it is not
pulmonary venous Doppler S-wave – there would be no moving. This means all the forces acting on the wall are
need for the diastolic pulmonary D-wave! But, D-waves balanced, but they are not zero!! After diastasis, in sinus
exist, thereby proving that LV ejection volume (leaving rhythm, atrial systole pulls up on the mitral annulus and
the pericardial sac through the aorta) is always greater pushes additional volume into the LV (Doppler A-wave)
than the simultaneously aspirated atrial S-wave i.e. atrial and passively distends the LV, while pushing a small,
filling volume from the lung. Steady state volume negligible volume of blood retrograde into the 4
conservation requires that the pulmonary vein D-wave pulmonary veins. If ventricular contraction is delayed
make up the difference, so that the cyclic sum of atrial relative to termination of atrial systole, (first degree A-V
filling, PV S- and D-wave, equal the systolic stroke block on the ECG), the distended LV starts to return to
volume ejected by the LV. The relationship between its equilibrium volume and therefore pushes blood back
mitral annular (peak) velocity (E’-wave) recorded by into the relaxed atrium (late diastolic mitral
Doppler tissue imaging (usually reported as the average regurgitation) – a benign phenomenon.
between lateral vs. septal aspects of annular (longitu-
dinal) motion) and the S- and D-wave amplitudes then To fill effectively, a ventricle must remain compliant
immediately becomes self-evident as a consequence during filling (not be too stiff), it must quickly eliminate
of near constant-volume physiology. Furthermore, the the ‘cramp’ that was the previous systole (effective
preferred longitudinal (rather than transverse) volume relaxation requiring Ca sequestration), and be able to
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accommodation attribute for the LV conveys nature’s physically accommodate (aspirate) the needed blood
optimized design for atrio-ventricular mass transfer volume. Thus global properties of stiffness and
(volume pumping) during diastole. Recall, that although relaxation comprise intrinsic ventricular properties that
longitudinal volume accommodation is nature’s determine, and can be used to quantitate diastolic
preferred spatial mode of LV filling, transverse volume function, while volumetric load (preload) represents
accommodation, although suboptimal, exists, and in the extrinsic parameter that modulates global diastolic
severe pathologic cases (IHSS, severe LVH) can be function.
viewed as a compensatory mode to facilitate survival by
maintaining cardiac output. (17) The difference between
‘volume-pumping’ and ‘pressure pumping’
Suction-pump physiology & diastatic equilibrium volume
During the cardiac cycle all four chambers fill and empty
It is established that at the instant of mitral valve open- once in a reciprocal manner. In order to fulfill the re-
ing, and initiation of the transmitral Doppler E-wave, LVP quirement of being able to increase cardiac output by a
continues to decline, while LV volume simultaneously factor of 4 or 5, while increasing heart rate by a factor
increases. {This is also true for the RV} In other words, of 3 to 3.5, there is the requirement of mass transfer
dP/dV<0 at, and for a little while after mitral valve (blood) from atrium to ventricle in a short interval at low
opening, until minimum LVP is reached. (Recall dP/dV pressure (to avoid pulmonary edema). Specifically, at
is the definition of chamber stiffness) This means that maximum exertion in a normal subject, at a heart rate
at mitral valve opening, blood does not get pushed into of 180, each cardiac cycle is about 330 msec, of which
the ventricle by the atrium, but is instead mechanically about 170 msec is diastole. During that short interval
sucked in, or aspirated, by the ventricle (7, 10, 21). In sufficient volume of blood needs to enter the LV to

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