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NC3Rs: National Centre for the Replacement Refinement & Reduction of Animals in Research
Guidance

Blood sampling: Vascular catheters

Catherisation or cannulation in laboratory animals can be considered for long-term sampling or multiple sampling over a relatively short time period.

Introduction

When correctly implanted, catheters can reduce the stress and discomfort associated with multiple sampling techniques such as repeated restraint and needle stick. In addition to blood sampling, catheterisation can also be considered for repeated substance administration or continuous ambulatory infusion. Catheters can be used to access deep, normally inaccessible vessels (e.g. the hepatic portal vein) and temporary cannulas can also be considered for superficial veins. Technical developments have made automated blood sampling possible in animals with implanted catheters. 

The ethical review of research protocols proposing to use implanted catheters should carefully weigh the benefits against the costs of the surgical procedures required to implant catheters (e.g. potential pain, discomfort and distress) and the risk of long term complications, such as infection, thrombosis (which dependent on the site can result in stroke) and reactions to implant materials.

The main challenges to successful long-term vascular access are:

  • Preventing thrombosis, which can lead to catheter and vessel blockage (occlusion).
  • Using appropriate catheter material, design and construction.
  • Preventing catheter-related infection and managing cutaneous exit sites.
  • Restricting animal interaction with the catheter.

Good planning and experimental design are essential to ensure optimum research outcomes from animals with vascular catheters.

Preventing thrombosis

Circulating blood has the capacity to clot (form a thrombus) at sites of injury, foreign materials or where blood flow is either slowed or irregular (due to stasis or turbulence).

A thrombus can arise on the blood vessel wall or on the surface of a catheter and in time, due to cellular factors being deposited, undergo transformation into a fibrin sheath. A thrombus within the vessel lumen around a catheter tip can cause partial blockage permitting infusion of substances while preventing withdrawal of blood. If extensive, the thrombus may cause total blockage (occlusion). Another risk of thrombosis is thromboembolism. Minimising the potential for thrombus formation can be achieved by:

  • Selecting materials with low thrombogenicity to minimise thrombus formation on the catheter surface, such as silicone or polyurethane.
  • Using atraumatic surgical technique and suitably flexible ('soft') catheters with low surface friction to minimise vessel trauma.  Vessel trauma can lead to thrombosis around the implant.
  • Positioning the catheter tip in a flowing blood stream can help prevent thrombosis due to stasis. Some types of catheter have a tip configuration which acts as a valve preventing ingress of blood and luminal thrombosis (e.g. a Groshong catheter).
  • Good catheter use and management will also minimise the possibility of thrombosis. This involves minimising blood residues in the catheter lumen by careful flushing and locking (see below). 
  • Be aware that thrombosis risk is greater in arterial catheters due to higher pressure having the potential to force blood into the catheter lumen. Use of such catheters requires careful management.

Catheter locking and access

During catheter access it is important to minimise the time in which blood remains stationary in the lumen.

Immediately after the blood samples have been collected, the catheter lumen should be flushed thoroughly before blood begins to clot. During the time periods when a catheter is not being accessed, the lumen must be filled with a suitable solution to exclude blood and prevent clotting - this process is called locking. Locking technique and solutions are also important in preventing infection. A wide range of lock solutions and techniques are used and have been described (also see Table 1 below). The choice of lock solution should be made to ensure that its composition minimises experimental interference (e.g. heparin may be undesirable if blood clotting mechanisms are being studied and taurolidine is an antibacterial compound). 

Table 1. Catheter lock solutions

Lock solution

Advantages

Disadvantages

Notes

0.9% saline

  • Enables filling of the catheter without exogenous chemical matter.
  • Cheap and easily obtained.
  • Low flow rate 0.9% saline may be used in infusion studies.
  • Provides no thrombosis prevention.
  • No antibacterial activity.
  • Continuous low rate infusion (keep vein open [KVO] setting on infusion pumps) is effective in maintaining catheter patency.

0.9% saline with heparin (20-500 IU/ml)

  • The most widely used material, with heparin providing thrombosis prevention.
  • This solution provides no antibacterial action.
  • Suggested heparin concentration is empirical - low concentrations for frequent flushing and smaller animals.
  • Higher concentration for less frequent flushing and larger animals.

40% dextrose (glucose)

  • The high osmolarity prevents bacterial colonisation and multiplication.
  • The high osmolarity contributes non-specifically to clot prevention/disruption.
  • Dextrose occurs naturally in blood and is rapidly metabolised.
  • Available cheaply as a pharmaceutical formulation.
  • Crystallisation is a risk.
  • The viscosity of the solution helps to prevent blood accessing the catheter lumen (but makes the lock solution slightly more difficult to withdraw).
  • Adding heparin (100-500 IU/m) will provide additional prevention against thrombus.

50% sucrose (saturated)

  • As for 40% dextrose.
  • Cheap and readily obtained.

 

  • Usually prepared and sterilised in the laboratory by autoclaving (so risk of contamination/pyrogens).

Glycerol

  • High viscosity, high osmotic pressure.
  • Can be difficult to aspirate.

 

Polyvinylpyrrolidone (PVP)

  • High viscosity, high osmotic pressure.
  • Can be difficult to aspirate.

 

Sodium citrate

  • A cheap antithrombotic which can be used where heparin is contraindicated.
  • Heat labile - therefore cannot be sterilised by autoclave.
  • No antibacterial action.

 

Taurolidine citrate solution 6.7%

  • Taurolidine is an effective and convenient antibiotic.
  • Citrate salt provides specific antithrombotic action.
  • Expensive.
  • Supplied commercially for catheter locking in preclinical research.

Catheter design

The biological, chemical and physical properties which are required for optimal vascular catheter design and construction are listed in detail in the list below. There are many considerations, for example:

  • Vascular catheters come in contact with blood and vascular endothelium, as well as other body tissues such as skin and connective tissue. The intravascular implants can rapidly become coated in biofilm derived from the animal's circulating blood. The biofilm acts as a substrate for thrombosis and microbial colonisation. The nature of the catheter material and any surface coatings applied influences the quality and quantity of biofilm that forms.
  • Catheters should be compatible with the chemical compounds or solvents administered during experiments and must not bind substances of interest during blood withdrawal.
  • Catheter strength and durability are also important properties.

It is therefore recommended to purchase catheters, and to take advice, from commercial suppliers.

Desirable properties of vascular catheter materials:

Biological

  • Non-irritant - provokes minimal inflammatory response
  • Non-carcinogenic - low tendency to cause neoplasia
  • Non-thrombogenic - low tendency to cause blood clotting
  • Non-toxic
  • Resists microbial adhesion
  • Resists biofilm deposition

Physical

  • High tensile strength
  • Resists compression - maintains lumen patency
  • Optimum flexibility
  • Low friction coefficient
  • Dimensional stability
  • Tolerates physical sterilisation methods (e.g. heat, steam, irradiation)
  • Ease of fabrication (e.g. heat forming or welding)
  • Non-permeable (water, gases, solvents)
  • Radiopacity - ability to image catheter with X-rays

Chemical

  • Absence of leachable additives (e.g. catalysts and plasticisers)
  • Stable during storage
  • Stable on chemical sterilisation
  • Stable on implantation (non-biodegradable)
  • Permits adhesives in fabrication (possibility of bonding dissimilar materials)
  • Accepts surface coatings (e.g. hydrogel, antithrombotic, antibacterial)
  • Compatibility with chemical compounds and solvents (absence of absorption and chemical reaction)
  • MRI (Magnetic Resonance Imaging) compatible

Given the many biological, chemical and physical properties which are required for optimal vascular catheter design it is perhaps not surprising that only a few naturally occurring and synthetic materials have proved suitable for constructing vascular catheters. In practice, there is no single material that can be used for all applications and therefore catheter materials need to be selected based on an assessment of the intended application. For example:

  • Flexible catheters can reduce endothelial injury which can lead to thrombosis, but they are more difficult to insert. The training and experience of the surgeon are vital factors in optimising catheter implantation and ensuring successful outcomes.
  • Surface coatings modify catheter properties such as thrombogenicity, friction coefficient or antimicrobial properties, but in experimental surgery it must be remembered that coatings applied to implants may be biologically active and capable of influencing data. Pilot studies may be required to generate data characterising changes caused by such materials and consideration should be given to suitable controls in experiments.
  • The physical shape of a catheter tip can play a significant role in reducing endothelial trauma. Many commercially available catheters have a rounded tip (Figure 1) which is considered to be less traumatic than square cut tubing or bevel ended tubing, although the latter is easiest to insert.
Detail of a rounded tip of an intravascular catheter: this design helps to minimise trauma to the vascular endothelium.
Figure 1. Detail of a rounded tip of an intravascular catheter: this design helps to minimise trauma to the vascular endothelium.

Preventing infection

Catheter-related infection is a major risk factor which can compromise vascular catheter studies.

Literature reviews provide only a small number of publications reporting implant-related infections in laboratory animals and how to prevent or manage them. However, there is a vast number of publications concerning human vascular catheter-related infection, and also many reports of animal models of implant infections showing that animals are not resistant to surgical and implant infections. As in vivo experiments become more complex and animals with vascular catheters are kept for much longer periods than in the past (i.e. weeks and months as opposed to hours or days), evidence of infection is likely to increase.

Microbial infections can originate externally, via the catheter lumen at the skin/catheter interface, and internally, via haematogenous spread from distant sites. All surgically-implanted foreign materials can potentially cause infection. Steps that can be taken to reduce infection include:

  • Using a strict aseptic technique to reduce the risk of infection.
  • Prophylactic antibiotics may help reduce the risk of infection, but should not be used to cover for deficiencies in aseptic technique; it is recommended that veterinary advice be sought before using antibiotics.
  • Even where implantation has been performed under strict aseptic conditions, catheter infection from the animal's skin or blood borne from distant sites (e.g. wounds) is possible. A pre-operative health examination should be performed in all animals undergoing any experimental surgery to identify possible infection risks and exclude unhealthy animals from procedures.

Exit sites through skin represent a special challenge as the smooth, non-adherent surfaces of catheter tubing that are desirable for the intravascular portion will create a potential opening where it passes through the skin.

  • Applying materials such as polyester meshes or velour to the catheter at the catheter exit site in the skin promotes the ingrowth of body tissue, and in this way can help to provide resistance to microbial infection.
  • The use of subcutaneous flanges around catheters may also help to stabilise the skin and limit movement around the catheter exit site facilitating better sealing of skin around the catheter.
  • Titanium materials also provide good biointegration at skin-to-implant interfaces and are worth considering when designing or choosing a vascular catheter.

Vascular access ports

As skin exit sites pose a major risk of catheter infection, totally-implanted systems using vascular access ports (VAPs) were developed and became adopted for laboratory animal use (Figure 2).

Vascular access ports consist of an implantable chamber attached to the catheter. The chamber has a thick polymer septum which is capable of withstanding repeated punctures, and provided that specially designed (Huber) needles are used, it has a self-sealing action (Figure 3).

Puncturing the skin overlying the VAP is necessary to access the lumen. Since the septum of a VAP is very thick it gives stable support to the needle and it is possible to leave the needle in situ for many hours or days if required, provided it is suitably protected. Following needle withdrawal, no special care is required and infection rates associated with the use of VAPs are very much lower than for external catheters.

In addition to reducing infection rates and the need for protective dressings or jackets, VAPs can allow animals to be group-housed as the risk of damage to external implants is removed.

Potential problems associated with VAP implantation are dehiscence of overlying skin and displacement or inversion of the device if it is not securely anchored during surgery. Good surgical technique can help to prevent these problems and selecting small, low profile VAPs may help to prevent skin dehiscence.

A typical vascular access port with attached vascular catheter, suitable for use in larger laboratory animals.
Figure 2. A typical vascular access port with attached vascular catheter, suitable for use in larger laboratory animals.
Cross section of a vascular access port; the septum has been punctured with a right-angled Huber needle.
Figure 3. Cross section of a vascular access port; the septum has been punctured with a right-angled Huber needle.

Catheter locking

  • Catheter locking has already been mentioned in connection with preventing thrombosis. Locking technique and solutions are also crucial in minimising infection.
  • Whatever solution is used to lock a catheter, a strict aseptic technique must always be used.
  • Adding antimicrobial agents to catheter lock solutions is possible but the potential influences of these substances on experimental outcome must be considered. It is inappropriate to use antibiotics reserved for clinical use in catheter locking solutions as this may give rise to bacterial populations with antibiotic resistance. 

Experimental design

There are many factors and details that should be addressed to ensure a satisfactory outcome of vascular catheter implantation. It is also necessary to consider the details of specific experiments. The exact nature of implants and their anatomical placement will be influenced by the species used and the purpose of the study; for example, the desired blood sampling route and regimen, the location of infusion, and blood pressure measurement.

It is essential that preliminary in vitro tests are performed to establish that compounds and solvents are compatibile with catheters, including any connecting tubing, infusion pumps and reservoirs.

Particular problems that may be encountered with infusion studies include:

  • Absorption of the compound by tubing and other materials.
  • Crystallisation of the compound.
  • Compounds precipitating out of solution at low flow rates in narrow bore catheters and tubing.

The presence of foreign material within the vascular system and exposure to any eluting coatings from catheters coupled with the compounds used in locking solutions can all give rise to biological or pathological changes that may confound experimental data (e.g. microthrombi may arise and be deposited in the lung during infusion toxicology studies). Access to good background data and well-designed control groups will assist with data interpretation.

Glossary

Ambulatory infusion

  • An animal is freely moving without need for a tether to connect with the catheter. This is normally only possible with larger animals that can be fitted with jackets to carry an infusion pump and compound reservoir. Totally implanted pumps can sometimes be used in rodents but have size limitations.

Atraumatic

  • Minimal tissue injury is caused during the procedure.

Bicompatibility

  • Good toleration of implants by animal tissues after implantation.

Biofilm

  • A coating which develops on implanted materials derived from the animal's own tissue fluids and cells.

Catheter

  • Flexible tube inserted into body cavities or organs for medical or experimental procedures.

Dehiscence

  • Bursting open or splitting along natural or sutured lines.

Endothelium

  • The layer of cells lining blood vessels.

Friction coefficient

  • Surface friction influences how easily a catheter can be inserted into a blood vessel and any injury to the vessel lining (endothelium).

Haematogenous spread

  • Spread of microbial infection through the blood stream.

Thrombogenic

  • Property of causing or promoting blood clotting (thrombosis).

Thrombosis

  • The process of blood clotting in which normally fluid blood.

Lumen

  • The cavity within a catheter through which blood is withdrawn or compound infused.

Septum

  • The thick polymer diaphragm allowing sealing a vascular access.

Vascular access port

  • An implantable chamber for intermittent access of catheters by puncture via a self-sealing septum with a special (Huber) needle.

Venepuncture

  • Insertion of a hypodermic needle through skin into a vein to withdraw blood samples or administer compounds.

References and resources

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  3. LASA (2010). Guiding principles for preparing for and undertaking aseptic surgery: A report by the LASA Education, Training and Ethics section
  4. Luo YS et al. (2000). Comparison of catheter lock solutions in rats, 51st Annual Meeting of the American Association of Laboratory Animal Science, San Diego, 6 November.
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