Biomedical Engineering Reference
In-Depth Information
introduction of dialysis therapy more than 50 years ago,
patient mortality dropped from 90% to 50%. However,
mortality in the intervening years has not improved from
the 50% level. In ICU patients presenting with other co-
morbidities, the mortally rate is between 50% and 80%.
Although the etiology of ARF varies, it is often associated
with a reduction in urine output and a drop in systemic
blood pressure. Waste products accumulate in the blood,
electrolyte levels become abnormal, pH usually drops,
and edema sets in. Initially, ARF was treated in the same
manner as chronic renal failure, with intermittent he-
modialysis. This approach quickly removes the metabo-
lite, electrolyte, and fluid imbalance. However, these
levels vary widely because the body continuously gener-
ates their production between dialysis sessions. Conse-
quently, there has been a move toward continuous renal
replacement therapy (CRRT) (Galletti et al., 1995). In
this mode, renal function is supplied continuously while
the patient recovers. There are several different CRRT
modalities. Historically, arterial circulation was used to
provide the force to move blood across a dialyzer car-
tridge. Because of the need for arterial access and in-
herent complications and the technical improvement in
venovenous therapies, these methods have been largely
abandoned. The venovenous modalities differ primarily in
the methods of clearance. Slow continuous ultrafiltration
(SCUF) is a method employed to remove volume by
taking off fluid filtered by a dialysis membrane by con-
vective force. Continuous venovenous hemodialysis
(CVVHD) provides clearance using diffusive clearance
by running dialysate across the membrane. Continuous
venovenous hemofiltration (CVVHF) removes fluid by
convection (as in SCUF) and then provides replacement
fluid back to the patient. Finally, continuous venovenous
hemodiafiltration combines the clearance properties of
CVVHD and CVVHF. These approaches are provided by
bedside machines incorporating the required pumps,
control instrumentation, filters, and fluids.
Figure 4.2-5 Abbot Plum XL infusion pump.
Infusion pumps and controllers employ a variety of
electromechanical mechanisms to regulate fluid flow to
the patient. These include peristaltic or diaphragm
pumps, syringe devices, and variable pinch clamp
mechanisms that control gravity-driven flow. Usually, the
tubing sets used by these devices are dedicated and not
interchangeable. Infusion pumps with a wide range of
rates (1-999 cc/hour) have been available for decades.
Modern infusion devices have a wide range of pro-
grammable infusion rates and extensive alarm capabil-
ities. Features include proper set placement detection,
pressure limits, proximal and distal occlusion alarms, end
of infusion alarm, and programmable keep-vein-open
(KVO) rates. Additional safety features to prevent
unrestricted free flow of IV fluid to the patient are in-
corporated as well. In order to avoid medication errors,
systems that will associate pharmacy requisitions, the
infusion device, and patient identification are being
developed. Additional intelligence in such systems will
identify patient sensitivities, incompatibilities between
infused medications, and violation of nominal drug con-
centrations and infusion rates.
Clinical engineering and the ICU
Perhaps the most complex and technically challenging
hospital environment is the ICU. Traditionally, CEs have
been involved in medical device evaluation, inspection,
maintenance, layout, design, and integration of the various
instrumentation systems. As the devices' and systems'
complexity has increased over the years, CEs have been
looked to as repositories of technical knowledge that is
beyond that of the users (i.e., the nurses and physicians).
This requires maintaining a substantial level of expertise in
the physics and physiology of measurements and thera-
pies; knowledge of computer operating systems; networks
and communications protocols; and the peculiarities of
many different instruments as well as an appreciation of
Dialysis: kidney and organ support
The sickest ICU patient will begin to manifest failure in
one or more organs. Acute renal failure (ARF) is not un-
common in the ICU setting. ARF occurs in approximately
5% of all hospitalized patients and, historically, has been
associated with a high risk of mortality. Following the
