Animal
Dependent Questions
1. What is the intended species?
The main differences between species, as they pertain to infusion
system design, are the variations in size and anatomy. The size of the
animal and therefore the vessel or other access site varies greatly with
species. One of the components most affected by these size variations is
the catheter. In relatively small animals, such as a rat or mouse, the
vessels and other potential administration sites can be extremely small
so a catheter with a small outer diameter (OD) must be used. For
example, the femoral vein of a 200 gram rat will accept a catheter with
an OD of .040 inches. Smaller catheters will fit more easily however too
small a catheter may restrict flow of infusate or allow even a small
fibrin plug to stop flow altogether. The length of the catheter also
must be adjusted to account for the distance between where the tip is to
reside and the cutaneous exit site (or port reservoir). SAI has taken
the guess work out of finding the right catheter for the right animal
and vessel by designing catheters that are species and site
specific.
Size, also has an impact on which vessel is chosen for access.
Peripheral vessels are easily accessed in large animals whereas the tail
vein is the only peripheral vessel commonly accessed in rodents. Please
note, test formulation may also have a significant effect on which
vessel is chosen. Click on Table 1 for an indication of
which veins are typically used in large and small animals and when its
most appropriate.
Jackets, harnesses, tethers and swivels must also be configured to
accommodate both the size and anatomical differences between species.
Jacket size variations are an obvious difference in all systems. With
tethered systems however, both the tether and swivel must be of the
appropriate size, weight and durability to accommodate either the
larger, stronger animals or smaller rodent species. These products are
also subdivided by species for your convenience in the sections that
follow. Ambulatory systems require no tethers or swivels but the animal
must be large enough to carry both the pump and volume to be delivered,
comfortably.
Differences in the thickness of the skin between species comes in to
play when choosing the appropriate subcutaneous access port to implant.
Rodents and primates have relatively thin skin and therefore best
results are achieved when ports with low profiles are used. Low profile
ports are also recommended for pigs and sheep since their skin is
sensitive and tight to the underlying musculature (unlike dogs and
rabbits). Ports that are too large are more likely to erode through the
skin at the reservoir implantation site. Local infection and erosion may
also occur if the portal is accessed too often or continuously for long
durations in thin skinned animals. If you experience these
complications, please contact us. Other factors such as chemical
irritation and surface pressure may also have an impact.
2. What size is the animal?
More often than not, the size of the
individual animals within the species also vary greatly. Small, medium
and large jackets (or harnesses) may need to be purchased to accommodate
the range of sizes encountered within the same study. Jackets and
harnesses may also require changing or resizing during the course of the
study. Check sizing on a regular basis especially when animals are young
and significant growth is likely.
Investing extra time to size jackets
and harnesses to individual animals and properly acclimate them to the
apparatus will pay huge dividends. Be sure whichever apparatus you
choose is fit securely to the animal to keep them from slipping out but
not so tightly as to restrict blood flow or cause the animal discomfort.
Acclimating the animals to outerwear is also important to help limit
animal interaction with the infusion apparatus. Allowing them time to
adjust to the jacket or harness, before surgery, will reduce
complications once the animals are fully instrumented.
3. How will the
animals be housed?
Dogs, pigs and sheep can be housed in runs or pens
although dogs are often housed in stainless steel cages, as are
primates. Rabbits also are usually in stainless steel cages with mice
and rats in either suspended stainless steel or polycarbonate “shoebox”
caging.
The style of housing used will dictate how the swivel and tether
are anchored, the length of the tether and extension lines, and if
ambulatory (no swivel or tether) may be more appropriate. Large animals
tethered in stainless steel caging will have the swivel anchored to the
cage bars, or inserted and anchored through a hole cut in either the
cage wall or ceiling. A tether allowing the animal to reach all corners
of the cage attaches to the swivel to keep the tether, and catheter
within, from becoming twisted.
Swivels can be suspended over rodent
shoeboxes, stationary or on counterbalance arms, with the tether running
vertically through the bars. Swivels can also be suspended over holes
cut through the ceilings of stainless steel suspended cages. More
commonly, swivels are anchored directly to the front of suspended caging
and the tether run horizontally into the cage.
Although animals with
ambulatory systems can socialize for significant periods under
supervision, it is recommended that subjects of infusion studies,
wearing external apparatus, be singly housed. The jackets, harnesses and
tethers are configured to keep animal interaction with the catheters and
surgical sites to a minimum. With more than one animal in the cage, the
companion can get at the infusion apparatus more readily than the one
wearing it and destruction of the system is common. Totally implantable
systems, (i.e. Alzet) however, do not share this requirement.
Test
Formulation Dependent
1. What is administration site?
Test
formulations can be infused by many routes at many sites. The most
common route of infusion is intravenous but intrathecal, epidural,
subcutaneous, intracerebral ventricular, intraduodenal among others, are
also used. The determination of the route of administration depends on
many factors related to the test article and formulation. How the test
article is absorbed, distributed, metabolized and excreted play a major
role in this determination. Pharmacologic effect as well physical and
chemical characteristics of the formulation also may have a direct
influence. For the most part, discussions here will be limited to
administration of intravenous solutions.
As a rule, whenever possible, all solutions intended for IV infusion
should be adjusted for both pH and tonicity prior to infusion. Solutions
with very high or very low pH can cause serious local irritation
especially in peripheral veins. Solutions that are very hypotonic or
hypertonic may not be hemocompatible and may also cause serious
complications.
2. What are the interval, period and duration of
infusion?
There are many veins that can be accessed for
intravenous infusion (Table 1). The vein that is chosen for the infusion
often depends on the period, interval, duration and frequency of
dosing.
By definition, the period is the length of time the infusion is being
administered each day, (seconds to 24 hours). The interval relates to
intermittent infusions and is the measure of time between infusions.
Frequency is the number of infusions (per unit time) and duration is the
length of time between the beginning of the first infusion and the end
of last. Short periods of infusions with relatively long intervals
between doses allow the use of peripheral veins. Easy access and the
ability to restrain the animal for short periods make peripheral veins
ideal for infusions up to four hours. Peripheral vessels can be accessed
multiple times with and sites can be rotated or moved to avoid chronic
injury. Percutaneous catheters can also be secured with adhesive backed
tape to provide temporary anchoring and thereby reduce the number of
“sticks”.
In general, however, when dosing periods are longer,
it is advantageous to anchor the catheter more securely in a larger
vessel. Anchoring reduces the chance that the animal will dislodge the
catheter while unrestrained, allowing the animal more freedom of
movement during longer infusion periods, even though a surgical
procedure may be necessary. For example, to administer infusions of one
to two hours, multiple times a day in large animals, a peripheral
vein-catheter can be taped down and accessed, for up to 48 hours,
allowing the animal untethered freedom of movement. Over a span of many
days it is more reliable to anchor to a larger vessel (i.e. with suture
beads) and subcutaneously tunnel to an appropriate exit site.
Infusion durations up to 2 weeks, however, can be accomplished
utilizing an 18ga, 10cm polyurethane catheter (see Clini-cath page 49)
in the jugular vein of a free roaming canine, following a non-invasive
surgical procedure. Longer durations are not recommended due to the
close proximity of the vessel insertion and percutaneous access sites.
This close proximity shortens the distance bacteria must travel down the
surface of the catheter to gain entry into the bloodstream. The longer
the catheter remains in place the better the chance bacteria has to
travel that distance and cause potentially serious complications.
In rodents, the peripheral vein of choice is the tail-vein. This vein
is easily accessed with a needle or tail-vein catheter while the animal
is restrained for up to 4 hours. Using a tail vein catheter, tail-cuff,
tether and swivel, this access can be maintained up to two weeks without
invasive surgery.
Central venous catheters are usually placed in
the jugular or femoral veins and tunneled either to the mid-scapular
region or the base of the tail. Best results are obtained with jugular
cannulations when the tip of the catheter extends close to (not into)
the atrium. The turbulence with the catheter tip at this location helps
keep it clear of thrombi however, if the catheter is inserted further,
into the heart, it can cause cardiac irritation.
Jugular
cannulas are easy to place and are well suited for infusion and blood
sampling up to two weeks. Although many researchers prefer jugular vein
access due to the relative ease of implantation, in our experience,
femoral vein catheters are more reliable in long-term applications. Best
results with femoral vein catheters are achieved when the catheter is
inserted into the posterior vena cava so the tip is between the femoral
and renal bifurcations.
Central veins should also be employed
when the test article or formulation is potentially caustic or
irritating to the vessel. The higher level of dilution provided by
larger vessels will help minimize these effects. The reduced size of
peripheral veins also contraindicates their use when administering large
volumes at high flow rates.
3. What materials are compatible with the test formulation?
The chemical properties of the test formulation must be compatible
with the materials that come in contact with the fluid path. Test
formulations can breakdown these materials resulting in malfunctions
and unintended exposures to your test subjects. Test articles
(especially proteins) can adhere to the inside of these components
resulting in lower than intended exposures. The only sure way to find
out if your system is compatible is to run the test formulation through
it, analyze what comes out and inspect the system afterwards.
Components that are negatively affected at that point, can be
substituted with those of alternate material composition. If it appears
that the test article is sticking to the catheter or tubing, often,
prefilling (“charging”) them with the test formulation and storing them
(usually at refrigerator temperature) for a short time, to bind up the
available adsorption sites, will solve this problem.
The most common leachate researchers should be aware of is
Di(2-ethylhexyl)phlthalate or DEHP. DEHP is a plasticizer that is
typically added to polyvinyl chloride (PVC) medical devices (IV bags,
extension lines, catheter tubing….) to increase the flexibility of the
polymer. The amount of DEHP that will leach out increases with
temperature, lipid content, and duration of contact. A wide-range of
adverse effects have been seen in laboratory animals, the most
concerning on the development of the male reproductive system.
The best way to avoid concerns regarding DEHP is to use polymers
other than PVC like polyurethane, polyethylene, silicone or simply a
non-DEHP PVC. Fortunately, there are many non-DEHP PVC’s available, in
fact all of SAI’s PVC extension lines are non-DEHP.
Material tracking issues, chemical compatibility, Good Manufacturing
Practices and reliability are by far the best reasons to use FDA and
ISO 10993-1, medically approved for human use components. Medically
approved devices must adhere to strict guidelines for quality and
maintain extensive records to track all of the components that are used
in their manufacture. These records can be queried whenever serious
questions arise.
4. What volume and rate are being delivered?
One of the most controversial questions encountered in infusion related
research is: what is the highest volume or rate that can be delivered
without adverse effects? The actual question should be: what volume of
test article formulation can be delivered such that test article related
effects are not masked or exacerbated by the volume administered? As a
general rule , since low volumes will have less effect than high
volumes, low volumes and rates are recommended. Unfortunately, in
preclinical research, the highest dosage attainable is often determined
by the solubility of the test article and higher volumes must be used.
Infusing at very low rates is also very difficult to do for practical
reasons. Verifying very low flow rates is very difficult and dead
volumes can often exceed intended volumes of delivery.
The
volume and rate of delivery will have a great impact on pump selection.
For the most part, you must choose a pump that can accurately deliver
the test volume at the rate or rates required. A quick scan of the
specifications listed in the pump section this manual will indicate
which pump or pumps are suitable. Note, the relatively large
volumetric bench-top/cage-mountable pumps provide a wide range of
volumes and rates of delivery. Syringe pumps also provide a wide range
of delivery rates but are limited by the range of standard syringe
sizes. When high rates are called for, multiple syringe changes may be
necessary. They also tend to be relatively large and may need added
space long to accommodate a fully withdrawn syringe plunger. For these
reasons syringe and volumetric pumps are most suitable for restrained
and/or tethered infusion systems.
Rate of delivery becomes more
important when choosing an ambulatory pump since currently there are no
ambulatory pumps which can deliver at both the very high and very low
ends of the range. Reservoir capacity is also very important to avoid
excessive change-outs.
Extremely small catheters may also
restrict flow if very high flow rates are chosen. Be sure to check the
flow capacity of your catheter of choice when these circumstances are
encountered since lowered flow rates or blown catheters may result.
Choosing a pump with pressure sensors and/or arms will help identify
this problem before serious consequences result.
Other Things to Consider...
Ambulatory vs. Tethered Infusion:
The choice between an ambulatory or tethered infusion system became possible when infusion pumps that were small, accurate and rugged enough to accomplish this feat became available. Prior to then, tethered systems using volumetric or syringe driver pumps were necessary.
Ambulatory systems allow animals unrestrained and untethered freedom of movement at all times. Long-term tethered infusion in large animals has been virtually banned in some European countries due to the belief that untethered systems put less stress on the animal subjects. New advancements in pump technology, jacket design, catheter material, and surgical techniques, and the improved demeanor of the animals have made this the system of choice for many researchers.
In most countries guidelines pressuring researchers to use ambulatory systems with large animals do not exist therefore, tethered systems are still common. No guidelines exist anywhere regarding ambulatory versus tethered systems in rodents. It is not practical to perform long-term ambulatory infusion studies in small animals, unless the delivery volumes remain relatively small, since the animal must carry both the pump and test formulation.
Tethered systems allow constant physical contact with the pump. This is advantageous when reservoirs require changing and makes it easier to monitor pump operation. Ambulatory systems require capture and temporary restraint of the animal to change reservoirs and constant monitoring can only be accomplished using telemetry or remote alarm sensors. Tethered systems allow for delivery of higher volumes since the animal is not required to carry the reservoir. On the other hand, ambulatory systems allow the animal untethered freedom of movement which gives the appearance of a happier and less stressed animal. In short, there are advantages and disadvantages to both systems.
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