Biomedical Engineering Reference
In-Depth Information
VP
E
Δ
η
=
(4.33)
input
Efficiency can also be defined in terms of power as the ratio of total power
output to the total power input. The anatomy of the heart allows a relatively direct
calculation of the efficiency of the heart muscle. The energy of the heart is primary
resourced from the oxidation of glucose, delivered to the heart muscle by the blood
flow. Using the Weir equation (4.39), every milliliter (at standard temperature and
pressure, STP) of oxygen used releases 20J of energy. Thus, one needs to estimate
the rate of oxygen utilization by the heart to calculate the total energy consumed
by the heart. Since the heart is supplied with oxygen by coronary arteries, oxygen
usage is determined by the product of coronary blood flow and oxygen used per
unit of blood. Normal coronary blood flow of an adult resting male is about 0.250
L/min (4.167 cm
3
/s), the oxygen concentration in the arterial blood entering the
heart is 0.20 cm
3
O
2
/cm
3
blood and venous blood leaving the heart muscle has 0.05
cm
3
O
2
/cm
3
blood.
Thus, the rate of O
2
usage
=
(4.167 cm
3
/s)*(0.20
−
0.05) cm
3
O
2
/cm
3
blood
=
0.625 cm
3
oxygen/s.
Total energy used
=
(0.625 cm
3
O
2
/s)*(20 J/cm
3
O
2
)
=
12.5 J/s
=
12W
P
1.691[
W
]
η =
out
=
=
0.15 or 15%
P
12[
W
]
in
Thus, if the efficiency of cardiac muscle is defined as the efficiency of convert-
ing ATP to power, then the efficiency of the cardiac muscle is about 30%. This is
significantly higher compared to the efficiency of the best automobiles built on
internal combustion engines, which is about 15%.
4.5.3 Pumps in Series and Parallel
In many disease states (e.g., congestive heart failure), where parts of the heart do
not function properly, the required flow rate and power cannot be achieved by
the heart. Artificial pumps can be used to support functioning of one chamber of
the heart: some examples of these are: left ventricular assist devices (LVAD) and
right ventricular assist devices (RVAD); left ventricular assist systems (LVAS); or
two chamber support [e.g., biventricular assist device (BiVAD)] or the entire heart.
While designing these pumps, some design criteria have to be considered in bio-
medical applications include the material with which they are made of (discussed in
Chapter 6), size compactness so that the patient can move around safely, negligible
vibration and noise level of the pump so that the patient is at ease, and the continu-
ous power source. Once the pumps are developed, they can be used either in series
or parallel. If two identical pumps are placed in series, their flow rate will be equal,
and each will provide half of the power.
