How does a syringe driver benefit your patients?
Many modern drugs have a short duration of action. When the clinically beneficial effects of these drugs are required over a longer period (e.g. pain relief or anaesthesia) the only ways to extend their duration of action would be either to increase the dose (potentially of limited effect and possibly resulting in an escalation side effects) or administering further doses. This can be achieved in numerous ways, from a simple repeated bolus to administering the drug within a carrier fluid (such as crystalloid solutions) by giving-set or fluid pump, to the most reliable and accurate method: the syringe driver.
In human anaesthesia the use of syringe drivers for total intravenous anaesthesia (TIVA) is now well established and, in some scenarios, may be chosen in preference to inhalational anaesthesia1. The delivery of drugs, in particular opioid analgesics for palliative care, using syringe drivers is also now routine.
Accurate, easy to use, portable syringe driver devices with features such as an occlusion alarms, air bubble alarms, and disconnect alarms have all been developed in the human field and proved helpful in ensuring unrivalled reliability.
The superiority of a syringe driver over other methods of delivering drugs, often very small volumes over a prolonged period, can be demonstrated by examining basic drug pharmacokinetics in comparison to a bolus administration regime.
After a drug has been administered as single bolus, the plasma concentration will rise rapidly and then fall over time. The fall in plasma concentration is due partly to redistribution of the drug to tissues, and partly to drug metabolism and elimination2, this requires the drug to be re-administered to maintain its effect. Bolus dosing leads to a ’peak and trough’ effect in plasma concentration: whenever the drug is re-administered there will be a peak, and a trough occurs each time the drug effect wanes before the next dose. This effect is particularly important when we consider the common applications of analgesia and anaesthesia where lack of drug effect may mean that the patient experiences pain or a decrease in anaesthetic depth (Figure 1).
As can be seen from Figure 1, while it is possible to maintain plasma drug concentration within the therapeutic range for most of the time using bolus administration, the peaks and troughs involved may cause the patient to experience increased side effects (such as respiratory depression) as the drug concentration can exceed therapeutic levels shortly after each bolus, and then a lack of efficacy (light anaesthesia or breakthrough pain) as the drug concentration falls below therapeutic levels, all within the same treatment episode. With constant rate infusion (CRI) however, the drug concentration remains much more stable, remaining within the therapeutic range. It should be noted that although the plasma concentration usually reaches therapeutic concentration soon after initiation, it does not plateau until 3-5 half-lives after the infusion has begun (e.g. fentanyl can take 13.5 – 22.5 minutes to reach steady state)3. Therefore, for many drugs, a loading dose (initial bolus) is used to increase the speed at which an effective plasma concentration is reached2.
Elimination half-life is used to describe the pharmacokinetics of a single administration of drug and to determine when to re-dose, but it is not very useful for CRIs. Once a CRI is stopped, plasma concentration will fall depending on redistribution of the drug from other compartments as well on the elimination of the drug. To better describe this the concept of a context-sensitive half-time is useful. This concept depends on the redistribution characteristics of each drug (which may make some drugs unsuitable for continuous infusion)2. The context in this case is the time of the infusion. Practically speaking, the use of a drug with a short context-sensitive half-time will result in a fast recovery independent of the duration of the infusion.
Administering intravenous anaesthetics TIVA or PIVA
After administration of a bolus of induction agent (e.g., Alfaxan), anaesthesia may be maintained with a continuous rate infusion (CRI) of the same drug4, either alone (TIVA) or in combination with inhalant drugs (partial intravenous anaesthesia or PIVA). The rate can be adjusted based on clinical signs and the feedback from electronic monitoring.
For additional information please see:
- Isoflurane Out of Stock: TIVA or not to TIVA
- Alfaxan for Induction and Maintenance of Anaesthesia User Guide
- Dose Guide for CRI and Intermittent Bolus
- Alfaxan for Maintenance of Anaesthesia FAQ
Syringe drivers provide more stable drug plasma levels and fewer complications than intermittent bolus (as detailed above), simple drips (where counting drops to work out administration rates can be confusing and result is errors), or fluid pumps. They also alleviate the wastage that will occur if the drugs are added to bags of fluid to facilitate delivery via simple drips or fluid pumps.
Administering peri-operative drugs
Several drugs suitable for controlling intra- and post-operative pain and have short enough context-sensitive half-times to be used in a CRI, these include ketamine, lidocaine and opiods like fentanyl, remifentanil and morphine.
The delivery of several different drug classes can be markedly enhanced by using a syringe driver device, these include:
- Inotropic support, drugs such as dobutamine or dopamine can used to support cardiovascular function in some patients need to be accurately administered1
- Fluids to small patients via the intravenous or intraosseous route
- Epidural drug administration
This can be particularly useful in small patients or in very compromised patients where a slower rate of delivery is desired.
PRACTICAL CONSIDERATIONS - GETTING TO KNOW YOUR DEVICE
Some devices only work with specific brands of syringes, while some will automatically recognise a range of syringes. It is vital that you have a selection of the correct syringes in stock.
Starting/stopping an infusion and priming
This is simple but vital information. Consider preparing a sheet or flow chart for your specific syringe driver so that anyone in the practice has a guide to using it. Priming (with the drug) the entire infusion system is essential to ensure that the correct dose is administered from the start of the infusion.
What units it uses (ml/min, ml/hr)
Most devices will use units of ml/min or ml/hr. Some devices use mm, these are generally older and require calibration with a specific syringe to calculate delivery rates, they are usually not suitable for drug delivery in practice. Devices are available that are calibrated in ml/24hr, these are also unsuitable for anaesthesia (although they may be used for long-term drug delivery of analgesia).
In some more advanced models, doses in mg/hr and other information like patient weight and drug concentration can be entered and the correct dose rate is automatically calculated.
How to load, unload and change a syringe
Devices will have different ways to lock syringes into place, most are simple enough but are designed so that they cannot be activated unintentionally.
Setting a dose
Devices have generally been designed to ensure that setting a dose is as easy and clear as possible
How to change a dose intra-operatively
Some models come with a key-lock function (which may slow down the process of altering the dose) to prevent accidental or unintentional altering of the dose rate. This is a particular problem with palliative drug administration in humans, and since many of the syringe drivers are old medical devices the key-lock function is not uncommon.
How to deliver a supplementary bolus
Some drivers have this function, if not, a supplementary syringe of drug should be kept at hand for the purpose, to ensure that an extra dose can be quickly delivered if needed.
THINGS TO REMEMBER
Set up and prime the device (and tubing) before use
As with most of anaesthesia, preparation is key to success. Once the decision has been made to use a CRI the device should be set up ready to go before the animal is induced to minimise disruption once the procedure has begun.
Check the batteries
In the case of a power failure most devices have a battery back-up
Use low dead space luer lock tubing and luer lock syringes
Luer lock connectors will minimise the chances of leakages and disconnects during patient manipulation, and low dead space giving sets minimise wastage of drugs used.
Make sure of cannula placement
Some more modern syringe drivers contain sensors that will alarm if there is an increase in resistance resulting from a blockage or from accidental perivascular administration. One of the advantages of the syringe driver devices is that the pressures generated can overcome minor postural obstructions.
Check connectors and cannula placement regularly
Connectors and cannulae can easily become misaligned especially if the device is to be used post-operatively outside of the controlled environment of theatre.
The delivery of contaminated drugs intravenously over a sustained period would pose a serious risk to patients, and since the devices are set up in advance, care should be taken to maintain sterility while loading and to ensure that accidental contamination does not occur subsequent to set up.
Intubate the patient (unless clinically inappropriate) and supply supplemental oxygen
Supplying supplemental oxygen should be considered essential given the respiratory depressive effects of many of the commonly used combinations of drugs used for anaesthesia. Intubation is also extremely helpful in enabling ventilatory support (e.g. IPPV) if required.
Originally published: Wednesday, 6th March 2019
Last updated: Thursday, 7th March 2019
- Moens Y. (2004) Syringe Pumps for Anaesthesia/Analgesia: Toy or Tool? World Small Animal Veterinary Association World Congress Proceedings
- Zoff A, Bradbrook C, Constant rate infusions in small animal practice. Companion Animal. September 2016, 21 (9): 516-522
- Sano T., Nishimura R., Kanazawa H. et al. (2006) Pharmacokinetics of fentanyl after single intravenous injection and constant rate infusion in dogs. Vet Anaesth Analg 33: 266–73
- Ambros B., Duke-Novakovski T., Pasloske K.S. (2008) Comparison of the anesthetic efficacy and cardiopulmonary effects of continuous rate infusions of alfaxalone-2-hydroxypropyl-β-cyclodextrin and propofol in dogs. American Journal of Veterinary Research. 69(11): 1391-1398.
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