Arterial vascular access allows the direct measurement of arterial blood pressure, and the sampling of arterial blood for blood gas measurements.
Planning Intravenous Access
The most suitable type of intravenous access for each patient needs to be considered. Factors to consider include the following:
- Choice of catheter, type, material, gauge, length
- Which vein to use, and correct preparation
- How easy/quick is it going to be to insert?
- Will the patient need sedation? If so, is it stable?
- Is this long-term access, or a short-term ‘fix’?
- What will be administered through the catheter?
The size of the catheter needs to be considered. Flow rates through the catheter are related to both its radius and length. Catheter radius has the greatest effect, the flow rate is related to the radius ‘to the power’ of 4. So, by doubling the diameter of a catheter you increase the flow rate by 16-fold. Increasing catheter length increases resistance. So where rapid fluid rates are required a wide and short catheter gives greatest flow rates. This is an important consideration, as often it is routine to stock longer catheters in the larger gauge.
The material a catheter is made from has an effect on how well it is tolerated. An ideal material is chemically inert so there is no vessel irritation. The catheter also needs to be relatively flexible so it is comfortable when the animal moves. Most catheters are made radiographically opaque by the addition of barium to the plastic. Recently, antibiotic impregnated catheters have become more common in the human field, usually central lines, where the long-term placement is a concern in case they act as a focus for infection such as methicillin-resistant Staphylococcus aureus (MRSA).
This usually refers to the way a catheter is placed or its shape:
- Butterfly catheters: needles with attached wings and extension tubing. They are suitable for collecting blood or thoracocentesis, but liable to damage any vein due to the sharp tip if left in place.
- Through-the-needle catheters: where the needle remains attached to the catheter but is secured and protected in a plastic guard outside the vein; these are usually bulky.
- Through-the-cannula (peel-away) catheters: where the catheter is placed through a peel-away sheath.
- Over-the-wire catheters, i.e. Seldinger technique: a guide wire is placed through a needle and the needle withdrawn, the catheter is then advanced over the wire into the vein and the wire withdrawn.
- Over-the-needle catheters: the most common catheter type used day to day in veterinary work, suitable for short to medium-term use. They are easy to place and complications are rare. They are available in a wide variety of gauges and lengths.
Peripheral Intravenous Access
In an emergency or as a short-term solution, peripheral veins are usually the most suitable as they allow rapid and effective catheterisation. Staff will be familiar with their location, and access can be achieved without sedation, and with only minimal restraint in most cases (see Table 3.1). Peripheral veins are adequate for administration of most fluids and drugs, and are ideal for most emergency cases. The cephalic vein on the forelimb is most commonly used, but we need to be aware and familiar with other sites in case injury dictates the forelimbs cannot be used. Examples include:
- Medial and lateral saphenous veins (see Figure 3.2)
- Dorsal common digital vein (over the metatarsal bones)
- Auricular veins, e.g. rabbits, Basset Hounds.
|Indications for use||Peripheral||Central|
|More time consuming|
Patient needs to be immobile
Sedation or GA often required
|Ease of use||Basic skills||Some training required|
|Serial blood sampling||Difficult||Easy|
|Obstruction flow: tolerated, etc.||Tolerated well|
Flow often obstructs if limb flexed
Position of animal does not affect flow
|Duration||Short term (up to 3 days)||Long term (week or more)|
GA, general anaesthesia.
When selecting the site consider any sources of catheter contamination or infection, e.g. vomiting, urination, diarrhoea or excessive salivation. Consider also whether fluid entering that vein will reach the central compartment, e.g. in the case of a gastric dilation and volvulus (GDV), any fluid administered through a hindlimb vein will not be effective due to effective obstruction of the caudal vena cava from the dilated stomach.
Placement of a Peripheral Intravenous Catheter
A large area of skin should be clipped to allow adequate aseptic preparation of the skin (see Figure 3.4). If the animal is long-haired then a 360° clip around the circumference of the leg may be necessary. Place the catheter as distally as possible. This allows further puncture at a more proximal site if necessary. Hands need to be washed before placement.
The catheter is usually placed directly through the skin, but in thick-skinned animals, or dehydrated patients, it can be useful to ‘nick’ the skin with a No.11 blade, this stops the catheter ‘bunching up’ at the tip. Advance the catheter through the skin at a 30–40° angle with the bevel up. Never pull the catheter back onto the stylet after advancing – you may damage the tip, or shear off part of the catheter (see Practical techniques at the end of the chapter).
Central Venous Catheterisation
A central venous catheter, or ‘central line’, is a one where the tip of the catheter lies within the cranial vena cava (or, less commonly, the caudal vena cava) (see Figure 3.5).
A central venous catheter is indicated where:
- Long-term fluid administration is likely
- Hypertonic medication, fluids or parenteral nutrition will be administered
- Measurement of central venous pressure is required
- Patient factors such as conformation or peripheral oedema/limb swelling mean maintaining a peripheral catheter would be difficult
- Regular serial blood samples are required.
Contraindications include coagulopathies, e.g. von Willebrand’s disease. Placement usually requires sedation or anaesthesia, this may not be ideal in an unstable patient. In these cases, initial stabilisation can be achieved via peripheral access, then a decision made regarding the need for a central line.
The jugular vein is the most common point for insertion of longer central catheters, the location is familiar to the clinician and easily accessible. Where access to the jugular is difficult (e.g. conformation, trauma), it is possible to use a peripherally inserted central catheter (PICC) technique, in this case a pre-measured long catheter is inserted in a peripheral vein, usually the saphenous, and threaded up to lie in the caudal vena cava.
Central Catheter Type Selection
Most central catheters are of the Seldinger/over-the-wire type, or of the through-the-needle/peel-away type; often the selection is made on personal preference. These catheters are available as single lumen or as multi-lumen types. In multi-lumen catheters, two, three or sometimes five ports will each have their own channel running right to the tip of the catheter, so fluids and drugs do not mix prior to entering the blood stream. Ports can be reserved for blood sampling only, or for parenteral nutrition (see Figure 3.6).