Mass Transfer and Control.
emphasize that the following notes do not attempt to describe our
intellectual property accurately or completely. They are intended to
give a flavour of our approach. The numbers quoted are indicative
and not suitable for design purposes.
give a brief description of how the human respiratory system works.
We then go on to explain the requirements for a system that can
reproduce this performance when the lung is severely damaged.
discharges oxygenated blood to the arterial system. The arteries
deliver the blood to the various organs that require energy (for
example, muscles). In these organs, carbon-hydrogen-oxygen compounds
react with oxygen from the blood stream to generate energy. The
reaction is akin to a controlled burning. (Indeed, the calorific
value of foodstuffs is estimated by burning the food under controlled
conditions). The reactions can be illustrated by equations such as
with carbohydrate. Example, glucose:
+ 6O2 → 6CO2
with hydrocarbon (saturated fat). For example, decane:
case of carbohydrates, one mole of carbon dioxide is produced per
mole of oxygen consumed. In the case of saturated fats, about 0.65
moles of carbon dioxide is produced per mole of oxygen consumed. The
quantity of carbon dioxide produced per mole of oxygen consumed
depends on the mixture of food that we eat. Typical values are 0.8
to 0.9 moles carbon dioxide per mole of oxygen.
dioxide produced is absorbed into the blood. Thus, at each step of
supporting life, the blood becomes depleted in oxygen and enriched in
carbon dioxide. The depleted blood is collected in the veins and
returned to the lungs.
remove carbon dioxide from the blood and recharge it with oxygen.
exchange takes place through membranes making up the alveoli in the
lungs. This mass exchange process is governed by standard
mass-transfer equations. Thus, the rate of mass transfer is given
is mass transfer rate, for example in kg/s.
is the mass transfer coefficient, for example in m/s.
is the mass transfer area, for example in m
is the concentration difference driving
the mass transfer, for example in kg/m
(1) applies both to oxygen transfer and to carbon dioxide transfer.
For concentrations in molar units (for example, mole oxygen per mole
gas), the percent concentration is numerically equal to the partial
pressure in kPa.
is the gas-phase concentration of
is the equilibrium concentration of
oxygen over blood. On the “in” breath,
is the atmospheric concentration of oxygen, namely 21% or 21kPa. On
the “out” breath, the exhaled air has an oxygen
concentration of about 16% (16kPa). The equilibrium partial pressure
over the blood increases from about 6kPa (venous blood input) to
about 12kPa (arterial blood output). It is emphasized that the
equilibrium concentration of oxygen over blood is not equal to the
concentration within the blood.
A typical relationship between blood
oxygen saturation and equilibrium partial pressure is illustrated in
The curve depends on the blood carbon dioxide
concentration. For lower blood carbon dioxide levels, the oxygen
partial pressure is less (thus, for a given pressure, the blood
oxygen saturation is higher). Conversely, for higher blood carbon
dioxide levels, blood absorbs oxygen less efficiently. As blood
passes through the lungs, the carbon dioxide level falls, so that, at
the arterial blood outlet, the oxygen concentration is near
saturation. Notice that the equilibrium line becomes steep near to
100% saturation. Thus, the blood oxygen saturation becomes
insensitive to the partial pressure of oxygen over the blood.
is the equilibrium concentration of carbon dioxide over blood and
is the atmospheric concentration of carbon dioxide. The equilibrium
partial pressure over the blood decreases from about 6.5kPa (venous
blood input) to about 5.5% (arterial blood output). On the “in”
breath, the atmospheric partial pressure of carbon dioxide is
negligible. It increases to about 4kPa in the exhaled air.
response to increased metabolic rate, the respiratory demand
increases, the heart rate (and blood circulation rate) increases and
breathing becomes deeper and faster. Fresh air reaches deeper into
the lungs, which decreases the resistance to mass transfer and hence
increases the Mass Transfer Coefficient (U). At the same
time, the effective mass-transfer area increases. Hence, referring
to equation (1), the mass transfer rates for both oxygen and carbon
employs carbon dioxide as the main indicator with which to control
respiration rate. If the blood carbon-dioxide concentration drops
significantly, the respiration rate is higher than needed to meet the
respiratory demand. Conversely, if the blood concentration rises
significantly, the respiration rate is too low. Thus, to an
approximation, the body has a set-point blood carbon dioxide
concentration. If the concentration is lower than the set point,
respiration rate decreases. If the concentration is higher than the
set point, respiration rate increases.
The Haemair respiratory aid and
feature of the Haemair approach is that it is aimed at conscious
mobile patients. To this end, we match oxygen and carbon dioxide
mass transfer rates to the respiratory demand of the patient.
Furthermore, we employ a flow of natural air to provide oxygen and
remove carbon dioxide.
three main variants of our device. The simplest to employ consists
of a mass exchanger, as illustrated in figure 2. It takes
deoxygenated blood, extracted from a main vein, removes carbon
dioxide, replaces it with oxygen, and returns the oxygenated blood to
the body. The second variant places the mass exchanger within the
body to eliminate the hazard of taking a significant blood flow
outside the body. The final version is a prosthetic lung, as
illustrated in figure 3.
three variants, mass transfer is controlled so that performance
mimics that of natural lungs. In this way, the natural respiratory
control mechanism controls heart rate etc, and control is fully
integrated with the natural respiratory system.
external device will be deployed first. It is easily reversible and
major parts are available for maintenance. The easy reversibility is
important in treating emergency and acute cases for which the device
might be needed for no more than hours or weeks. Once we have
established that long maintenance-free operation is possible, we can
move on to the intermediate device. The clinical procedure to
“plumb” the device into the blood circulation system is
more complex and maintenance is more difficult. However, the
engineering is simpler. The only significant external item required
is a small air pump, or fan. This device is more suited to patients
who will need it for months – for example, as a bridge to
transplant. It should enable patients to leave hospital and continue
treatment at home. The final variant, a prosthetic lung, serves as
an alternative to a lung transplant. This variant is illustrated in
figure 3. It cannot be deployed until we have extensive favourable
experience with the reversible devices. However, it offers hope to
those currently excluded from transplant waiting lists – for
example, most terminal emphysema sufferers.
interested in the technology, our published PCT Patent Application
No. W02005/118025 is available. Details of subsequent applications
can be provided against suitable signed confidentiality agreements.
Please contact Haemair explaining your interest. We are pleased to
share information with those who share our goal of improving the
lives of sufferers from lung disease.