An intraosseous (IO) catheter is a hollow needle which is placed into the intramedullary cavity to administer medications and fluids (Figure 1). IO catheterisation is often discussed during veterinary nurse training. The route can be used for fluid administration and for many drugs and veterinary nurses are taught about this route as an option in all of their patients. However, once qualified, in the author's experience this route of administration is rarely used in general practice. This is probably because the majority of patients seen in small animal practice have veins of a sufficient size for peripheral intravenous (IV) catheterisation. For this reason, IO catheters are usually reserved for the critical patient where cardiovascular collapse may mean that IV access is difficult. They are most commonly used in very small patients, such as neonates or exotic species, in which venous access is impossible (Lind, 2016). Any fluid that can be given intravenously (e.g. blood, balanced electrolyte solution, glucose) can also be given by the IO route (Macintire, 2008), making this route invaluable in debilitated patients. Administration of crystalloid and/or colloidal fluids through the intramedullary cavity has the same efficacy and an equivalent absorption rate to the intravenous route. In fact, over 50% of the administered IO fluid passes into the central circulation within 30 seconds of it being administered to the patient (Lamberski et al, 1991). Fluids can be given in short regular bolus rates or alternatively, if tolerated, by constant rate infusion (CRI) using a syringe driver or infusion pump.
The placement of IO catheters can be considered an act of veterinary surgery under The Veterinary Surgeons Act 1966, however schedule 3 amendment does accept veterinary nurses can perform minor surgery provided they are not entering a body cavity. The bone cortex could be considered a body cavity and therefore the IO catheter should be placed by a qualified veterinary surgeon.
Exotic species
Sadly, many exotic patients are critically ill by the time they reach a veterinary clinic. This is probably due to the fact that many of these animals are prey species. As an evolutionary survival strategy, prey species hide signs of pain and illness to escape the risk of predation, meaning that owners tend to pick up illness at a much later stage in the disease process. Therefore, by the time they are presented to a veterinary surgery, many are dehydrated, have been suffering long periods of anorexia, weight loss and are sometimes in a collapsed state. For this reason, it is important to rehydrate and support them by providing fluids. This then poses the issue of which is the most effective and practical route to use for fluid and drug administration, especially if the patient is very small.
Due to their small size or sometimes species-specific anatomy, intravenous fluid administration is not possible in many exotic species; it is usually only a common route in small mammals such as rabbits or ferrets and larger birds such as waterfowl and larger parrots.
The oral and subcutaneous routes are instead often relied on during these smaller patient's hospital stays. However, the oral route is generally a slow route of administration and would be especially so in a small herbivore patient suffering secondary gut stasis due to a period of anorexia, a patient that is seen commonly in general practice. If the patient is suffering from severe blood loss or acute haemorrhage, the enteral route is not a suitable route for volume resuscitation (Donohoe, 2012). Contraindications for subcutaneous fluid therapy include patients that are severely volume depleted because of dehydration or hypovolaemia and hypothermic patients. In these situations, blood is shunted away from the subcutaneous vasculature, leading to poor and inconsistent absorption of administered fluids (Mensack, 2008). Subcutaneous and oral routes therefore do not support the circulating blood volumes of fluids or the central venous pressure (CVP) as much as the IV or IO route. It is also impossible to administer blood products or colloids via these routes. For this reason, IO catheters should be considered in debilitated exotic patient, in which IV catheter placement or maintenance is not a viable option.
Limitations
Due to the very nature of placing a needle into the bone, placement of an IO catheter is moderately uncomfortable, and should always be performed with analgesia and, in some cases, anaesthesia depending on patient condition (Lennox, 2008). IO catheters can be used primarily for short-term vascular volume expansion, or perhaps until an IV catheter site can be obtained (Hawkins, 2007). IO catheters can theoretically remain in place for 72 hours (Briscoe, 2004), but may be removed before this time, if thought to be restricting joint movement or if the animal appears painful during drug administration. Complications of these catheters include extravasation of fluids, iatrogenic fractures and osteomyelitis; these complications can be reduced by ensuring an aseptic technique is used and the catheter is placed by a competent individual. Catheters should not be placed into fractured bones, patients with sepsis or those with metabolic bone disease (MBD) (Lamberski et al, 1991). This can pose a problem with reptiles due to the high proportion of patients seen with MBD, so if in doubt the patient should be radiographed prior to placement of an IO catheter.
Placement sites
Mammals
The most common sites used for IO catheters in small mammals are the proximal femur or proximal tibia (Figure 2), although other long bones can be used if necessary. Due to the size and variation of many of these patients, some knowledge of the species anatomy is advisable to avoid undue damage to the bone or surrounding joints from misplacement of the needle.
Birds
In avian species, two sites are frequently used for IO fluid administration. A catheter can be introduced into the distal aspect of the ulna or into the proximal tibiotarsus (Dubé et al, 2011). Both the femur and humerus are pneumatic, which means that they are hollow and are directly connected to the air sac systems. Placing fluid into these bones would have disastrous consequences as the bird's respiratory system would be flooded with the fluids infused. Care should always be taken when placing an IO catheter in a new species for the first time as some may have physiological differences to others. If in doubt, then it is best to consult an experienced exotic veterinarian or research the species anatomy in more detail beforehand.
Reptiles
IO catheters are most useful in lizards, although can be used in small crocodilians and turtles (Mader et al, 2006). The preferred site for IO catheters in lizards is in the tibial crest as with small mammals (Figure 3). Chelonians may have a needle placed into plastrocarapacial junction (bridge) (Figure 4) or gular region of the plastron, but this is often considered difficult to place. In recent studies neither in the bridge nor the gular injections provided systemic vascular distribution and at 22 minutes only 50% of the injected bolus had been absorbed into the venous system (Young, 2012). The femur or humerus are alternatives in this case, but for most chelonians the jugular vein has been shown to be most effective for fluid resuscitation if accessible. Snake species unfortunately do not possess the long bone features appropriate for IO catheterisation, and for this reason usually have a central venous catheter placed into the jugular vein if necessary.
Placement of an IO catheter
Spinal needles are preferred over hypodermic needles, which can become plugged with cortical bone (Powers, 2006), although either can be used in smaller patients. Guidelines for fluid volume and rates have historically been based on those for traditional pet species, although ongoing work is aimed at adapting these protocols for use in exotic pet species (Lichtenberger, 2007). The rate at which the fluids can flow is directly associated with the diameter of the needle much like that with IV catheters, however movement and position will also change flow rates.
To place any IO catheter, the following equipment will be required:
See Step-by-step guide for placement of the IO catheter.
Nursing considerations and management of an IO catheter
Besides occlusion, the most common complication is extravasation of fluids subcutaneously and/or into the surrounding soft tissue. Extravasation of fluids commonly occurs when the needle is misplaced, or when several attempts are made during needle insertion, resulting in enlargement of the entry site in the bone relative to the diameter of the needle (Lennox, 2008). Muscle necrosis can occur in these cases if certain drugs have been administered into the site recently. Confirmation of placement is difficult and only confirmed on a radiograph; two views should ideally be taken to ensure the needle is within the cortex in both planes (Figure 6). Flushing gently can test patency. When flushing an IO catheter in the ulna, it should be possible to see the basilic vein blanching as fluid is absorbed into the venous system (Figure 7).
Step-by-step guide to placement of the IO catheter:
Pain can be associated with administration of drugs and fluids into IO catheters and this has been documented in human medicine in responsive patients (Cooper et al, 2007). There is some evidence suggesting that infusing lidocaine through the catheter prior to administration of medications can reduce this pain (Llicki et al, 2016); care should always be taken not to overdose the patient with lidocaine as many of these species are very small.
Catheter care
As with IV catheters IO catheters should always be checked regularly for signs of swelling and redness. The dressing should be removed and the area cleaned on a daily bases (Ballard et al, 2016). This is to reduce the risks of infection and osteomyelitis in the bone developing. The site should be checked for any signs of leakage or any signs of infection present such as discharge. If the patient appears to be interfering with the site or there are any other concerns, removal is recommended. Often catheters are only left in situ for less than a day or until further vascular access can be obtained if at all possible. In some patients or in limbs such as wings, it may be necessary to immobilise the limb during the time the catheter remains in situ, this will help prevent patient interference, pain associated with movement and reduce the chance of blockage or displacement of the catheter.
Conclusion
IO catheters should always be considered as an alternative route of fluids and drugs. They are a useful route for fluid administration in exotic species, especially those patients that are very debilitated or collapsed with no viable intravenous access. If placed correctly and monitored carefully they can be extremely advantageous and potentially lifesaving in these patients. They should, however, be monitored carefully once in place.