Embark on a journey through the intricate workings of the heart with this comprehensive guide. From the fundamental principles of cardiac work to the nuanced interplay between pressure, volume, and flow dynamics, this resource leaves no stone unturned. Dive deep into the concept of the triple produ...
The triple product - Cardiac work
From physics, we know that force × displacement = work, but then, given that pressure is
force/surface area, pressure × volume = work () ⋅ *+, = -).
In the volume-pressure graphs, each point in the
plane represents a product ) ⋅ *+, and therefore a
certain value of energy (the area of the rectangle
from the origin to that point). During the ejection of
blood, the heart produces external work, which can
be computed as the product of the pressure times
the change in volume: essentially, the area under the
ejection part of the curve. Actually, some work is
given back to the ventricle when it is filled by venous
and atrial pressures (the area below the preload
curve during diastole), but the overall work required
from the heart during one cycle can be
approximated by a rectangle with height = mean
systolic pressure, width = stroke volume, and area
(pressure-volume area) equal to average systolic
pressure × stroke volume: ./0 = 1. ⋅ 23.
The stroke volume, on average, equals the venous return; still, acutely and momentarily it may change:
rising from the bed causes the sympathetic system to contract the veins, and this prevents the head from
becoming hypo perfused and thus fainting due to low pressure. A contraction or an increase in tone in
big veins will displace some blood from the veins to the heart and, thus momentarily, increase blood
venous return.
Another very interesting situation of increased entry of blood into the heart in a single cycle (thus an
increase in venous return) is during breathing: when we inhale, we reduce the thoracic pressure by
expanding the cavity, the air enters the lungs and, simultaneously, blood coming from superior and
inferior venae cavae, increasing the momentary filling of the heart, whereas exhalation induces a positive
pressure on the thorax and thus reduce the stroke volume and the amount of blood flow.
The work produced in one minute is PVA times heart frequency: 4/!"#$%& = 1. ⋅ 1/ ⋅ 56. This is
defined as the “triple product”. A significant amount of energy, however, is “wasted” when the heart
relaxes isometrically (more blood could be ejected, if there were no arterial pressure): this potential
energy (PE) is depicted by the “triangular” area to the left of the line of isovolumic relaxation. Overall,
considering the PE, the energy used during isovolumetric contraction (that does not produce external
work) and the heat released in intracellular movements, the efficiency of the heart (work produced /
energy used) is about 7 ≈ 20-25%; still oxygen consumption (and need), *82, is essentially proportional
the external work produced by the heart, and therefore to the triple product: 7 ⋅ *82 = () ⋅ (* ⋅ 9:.
The concept of heart efficiency is important, it must not be confused with contractility. If the heart
contractility is enhanced – due to a change in preload (and stretch of the fibers), due to an increase in
atrial pressure, or to the sympathetic system or to a cardiac stimulant drug – the force of contraction is
increased, so is the work performed by the heart, and inevitably so is the oxygen consumption. If the
contractility is enhanced, the efficiency is not. Rather, the efficiency typically decreases (it does
upon sympathetic activation), which means that not only the work increases, but the ratio external work/
oxygen consumption decreases and so the need for oxygen increases even more.
The triple product is an important concept, because if there are problems in the perfusion of the heart
– ischemic heart disease (IHD) from coronary troubles (therefore the heart doesn’t receive enough
oxygen) – the need of oxygen by the heart must be reduced (so that the small quantity of oxygen it
receives is enough for its function) by reducing the blood pressure, the stroke volume or the heart
frequency.
• Blood pressure can be reduced by acting on peripheral resistances, on blood volume or on heart
performance.
59 Body At Work II
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