Animal Dependent Variables
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 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 subcutaneous port reservoir). (note: SAI has streamlined the process finding the right catheter for the right animal and vessel by designing catheters that are species and site specific).
Animal 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. Table 1 indicates which veins are typically used in large and small animals and when it is most appropriate:
Jackets, harnesses, tethers and swivels must also be configured to accommodate both the size and anatomical differences between species (see pages 72-81). 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, and construction to accommodate either the larger, stronger animals or smaller rodent species. These products are also subdivided by species for your convenience in the following sections of this manual. 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 also comes into 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. Other factors, including irritation, pressure and lack of blood flow at the reservoir site may also impact the functional longevity of the port. If you experience any of these complications with subcutaneous ports, please consult us for remedies and further guidance.
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 in 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.
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. Primates are generally housed in stainless steel cages as are rabbits. Mice and rats are housed 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 a more appropriate choice of system. 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 of sufficient length to allow the animal access to all corners of the cage attaches to the swivel. The swivel serves to relieve the rotational tension to keep the tether, and catheter within, from becoming twisted and/or kinked.
Swivels can be suspended over rodent shoeboxes or stationary 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 or anchored directly to the front of suspended caging with the tether running horizontally into the front of cage. Until recently, swivels were often connected to the fluid path by stretching the catheter and extension-line tubing over stainless steel cannulae which extend out of either of the swivel. The inner diameter (ID) of the tubing and the outer diameter (OD) of the swivel must be perfectly mated when these types of swivels are used. Disconnection and leaking will occur if even minor manufacturing variations occur in the diameters of either the swivel conduit, extension tubing or catheter material. These stretch or friction fittings also cause wear and tear on the flexible tubing which can cause perforations, leaks and backflow issues. Quick ConnectTM swivels, designed by SAI, alleviate all of these potential malfunctions. SAI has also invented Quick ConnectTM vascular access harnesses and tethers (with integral fluid lines) which eliminate most of these friction fit junctions and replaces them with secure, one size fits all, Luer Lock fittings. These innovative systems assemble in seconds and make disconnection for blood sampling, body weights and cage changes clean and easy (see pages 72-75).
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 the interaction of the animals wearing them with the catheters and surgical sites to a minimum and work well when animals are alone. With more than one animal in a cage, the companion has easy access to the other’s infusion apparatus and destruction of the system is common. Totally implantable systems, (i.e. Alzet pumps) however, do not share this requirement.