In human anaesthesia Total Intravenous Anaesthesia (TIVA) is now well established and depending on the preference of the anaesthetist either inhalational anaesthesia or TIVA will be the first option. This was possible by the development of rapidly metabolised drugs with suitable pharmacokinetic and pharmacodynamic properties. This evolution promoted the further development of syringe pumps to enable practical simultaneous intravenous application of different drugs necessary to obtain a balanced anaesthesia.
Syringe pumps in essence guarantee precise delivery of a drug. Some devices will function only with a well defined type of syringe: a defined brand and a defined size (e.g., brand X and only 20 ml). Other devices accept (recognise automatically) different brands and sizes. The desired rate of delivery can be introduced by the anaesthetist. Mostly this is in ml/time unit. The device will warn for presence of an obstruction or the presence of air. The approach of the end of available drug in the syringe will be indicated. The total dose delivered can mostly be checked at any moment (either in ml or in mg). The devices can work on build-in batteries so that they can easily be transported with the patient. The administration of a bolus of drug can be somewhat user-unfriendly in basic versions but very easy in others.
More elaborate options
The flexibility in entering infusion rate depends on the type of syringe pump and the software. Possibilities include entering in ml, mg or microgram; pro minute or pro hour; more elaborate software enables the introduction of body weight, the desired rate in mg or microgram per hour or per minute, the concentration of the drug. Calculation of the rate in ml is then done automatically.
A further evolution in human anaesthesia is the possibility of preprogramming desired infusion schemes of a drug. Several pumps can function simultaneously and independently and some manufacturers offer a central numerical and graphic display of each pump's performance.
For human anaesthesia the concept of target controlled infusion has been developed in the first place for propofol anaesthesia. The special syringe pump disposes of special dedicated software (Diprifusor). The results of numerous pharmacokinetic measurements of propofol injections/infusions in humans have been incorporated in the special software. The relation of plasma levels of propofol and anaesthetic depth on the other hand has also be extensively studied and is known. This creates the possibility to enter body weight and the desired plasma level (an indicator of anaesthetic depth and a common number for those anaesthetists who work with the system) of propofol. The programme of the pump will automatically infuse propofol at changing rates so that this plasma level is (in theory) achieved. The program takes also in account the supplementary boli that the anaesthetist might decide to administer based on his clinical and monitoring information. The principle of target controlled infusion is under development for other drugs like the opiate remifentanil and for use in different types of patients and conditions. In veterinary medicine it is under development for propofol in dogs in the first place.
Examples of the use of syringe pumps in veterinary medicine
Continuous rate infusion (cri) is to often be preferred to intermittent bolus injection of an (anaesthetic) drug. The plasma (anaesthetic) level will be much more equalised with cri avoiding temporary sharp peak plasma levels caused by bolus injection.
a) To administer intravenous anaesthetics
After induction with a bolus of propofol, anaesthesia is easily continued with a continuous rate infusion of propofol, the rate of which can be guided by clinical signs and possibly the feedback of supplementary electronic monitoring. Dosage is very different depending on many factors not at least the presence of analgesics or alpha2-agonists (0.1-0.5 mg/kg/min). For instance TIVA using propofol and an opiate like fentanyl or alfentanil is a good alternative for inhalation anaesthesia in the dog and solves the problem of pollution of the theatre.
b) To administer perioperative analgesics
Administration of fentanyl during surgery is a classical example. Continuous infusion is more practical then intermittent injection of boli. Other drugs are ketamine and lidocaine are more and more used in balanced anaesthesia protocols and most practically administered also as a continuous infusion.
c) To accurately dose inotropic support during anaesthesia
Administration of dopamine or dobutamine are sometimes necessary as a part of measures to support/improve cardiovascular performance in some anaesthetised patients. This is difficult with "drop counting" from classical infusion sets especially in very small patients. Usual doses of both drugs are in the order of 2-10 microgram /kg/min.
d) To administer fluids /blood products to very small patients
With classical means the danger of overdosing fluids to very small patients (like pups, rodents, birds, etc...) is real. Syringe pumps are a reliable alternative.
e) All drug administration in the perioperative period
A classical example is the postoperative analgesic treatment in selected cases. Infusions of opiates, ketamine or lidocaine are in common use. Furthermore a lot of drugs used in the perioperative intensive care field are candidates to be accurately and reliably administered with syringe pumps; (antiarrhythmics, some antibiotics, cytostatica, special infusions,...) sometimes over a long time period.
f) Epidural drug administration
Epidural catheters are now used in veterinary medicine in selected cases to provide spinal analgesia (opiates, local anaesthetics). As an alternative to intermittent epidural injection and in order to obtain a more equal level of analgesia the drugs can be administered as cri using the epidural catheter via syringe pumps.
Syringe pumps are without doubt excellent tools for veterinary medicine as nowadays they are reliable and accurate. They come in basic versions as well as sophisticated devices to meet individual needs and can be used in a lot of different settings not limited to anaesthesia as such.