Infusion devices Flashcards
NOTE: last few slides not made into cards- re review
Types of infusion devices:
* Drop rate counter
* Syringe-type cassette
* Peristaltic mechanism
* Syringe drivers and TCI pumps
- Drip counter: Early designs of infusion devices relied on gravity for fluid delivery. IV giving set generates droplets of a constant size for any given fluid. Administration rate controlled by adjusting a roller clamp
- Syringe-type cassette: next developed. Known as ‘volumetric pumps’ to differentiate from earlier design. Become less common dure ot improved accuracy of modern peristaltic pumps.
- Peristaltic mechanism: modern pumps mostly use peristaltic mechanism to push fluid along, known as ‘infusion pump’. Use simple IV fluid-administration sets.
- Syringe drivers: traditionally used for more accurate control of smaller volumes of infusion. aka ‘syringe pump’. Microprocessers allow drug delivery on ‘unit drug, per unit weight, per unit time’. Used in PCAs
- Anaesthesia pump aka TCI pump: latest generation of syringe drivers, which incorporate pharmacokinetic models that provide drug dosing based on target drug concentration in plasma or at theoretical effect site.
Syringe-type cassette: describe the mechanism
- A valve operating in harmony with the piston directs flow from the infusate bag to the syringe resevoir or from the syringe resevoir to the patient
- Cassette volume is typically 5ml
- Down stroke time for the piston (during which no drug is infused to the patient) about 1s
NB originally called ‘volumetric pump’. Become less common due to the improved accuracy of modern peristaltic pumps that use simple IV giving sets.
Stepper motors: function in both syringe drivers and peristaltic pumps, flow rate possible
- Found in all modern electrically powered infusion devices, both syringe drivers and peristaltic pumps
- Designed so that, within certain load limits, it will rotate a set amount (typically 1.8 degrees) for each pulse of electricity delivered to it by the driving circuit -> therefore a microprocessor can accurately control the rate and amount of rotation of the motor (c.f. conventional electric motor where speed varies with current supplied and mechanical load faced).
Syringe drivers:
* stepper motor drives a ‘lead screw’. Carriage sitting on the lead screw grasps the syringe plunger to drive it forwards.
* A sensing arms detects the barrel diameter to identify different syringe sizes so that volume delivered is correct. (see figure)
* Flow rate 0-1200ml/h, depending on syringe size. Note usual maximum infusion rate of 1200 ml/h is significantly reduced when using smaller syringes as they require the plunger to travel further.
Peristaltic pump:
* stepper motor acts through series of cams to compress the tubing of the infusion set
* flow rate 0-999ml/h
Peristaltic pump: describe the mechanism
Tubing of giving set is compressed by either:
* series of rotating rollers [more commonly used for enteral feeding]
* wave of **mechanical fingers (cam followers) **[most common for IV administration] - see diagram
Tubing must be hard wearing, of known and consistent internal volume, and have no memory after compression (so fills easily on being released). Often made of precision silicone tubing
Elastometric pumps: accuracy, what may affect this
Flow rate is typically within 10-20% of given rate
May be affected by:
* Over or underfilling of the resevoir can affect flow rate
* Temperature, particularly at the flow restrictor. Change of 10 degrees C in temp of water-based fluids -> altered viscoscity -> 20-30% change in flow rate. [therefore v important to follow manufacturers instructions re positioning of the device e.g. close to patients skin vs outside clothing]
Elastometric pump: mechanism, advantages
Mechanism:
* Components: fill port, resevoir balloon, flow restrictor, +/- compressible resevoir
* Powered by energe stored by stretching the elastomer that forms the resevoir of drug (resevoir balloon)
* Due to v low flow rates through flow restrictor (usually max 5-10ml/h), flow is effectively independent of downstream resistance and of the upstream pressure of the emptying resevoir baloon
Advantages:
* Low cost
* Enable patient to be. mobilised or discharged from hospital with analgesic or anaesthetic drug infusion still in progress
* Can be used for PCA or regional anaesthesia
Can work as PCA device: by placing a compressible resevoir downstream of the flow restrictor. ‘Lock out time’ until a repeat bolus can be administered is governer by the refilling of this compressible resevoir and hence the flow rate through the flow restrictor i.e. overdosing should not be possible
Different functionalities of infusion devices required:
* Ward
* Anaesthesia
* PCA
* TCI
One basic infusion pump chassis can effectively function as different devices according to the microprocessor configuration.
Ward:
* Frequently restricted to function only in ml/h mode
* Inbuilt drug library is valuable so staff don’t have to make own calculations of correct administration rate
Anaesthesia:
* Able to deliver in terms of drug per unit patient weight per unit time, with choice of units
* Discretion in choice of units sometimes limited to anaesthetists only
PCA:
* Syringe drivers designed so commands from a control handle, operated by the patient, activate the delivery of an additional pre-programmed dose of analgesic
TCI:
* Inbuilr processor can contain algorithms for pharmacokinetics of particular drugs -> drug dosing based on theoretical desired patient plasma or effect site concentration
Line pressure monitoring: mechanism, how required
Function: to detect obstructed or extravasated catheters
Mechanisms include:
* In-line pressure monitor, made up of a separate chamber with a flexible membrane (as part of infusion tubing) that presses against a pressure transducer housed on the device
* Syringe pumps: Force transducers placed around the drive mechanism acting on the plunger so that pressure is calculated indirectly. e.g. piezoresistor bonded onto metal chassis which carries the lead screw. Distortion of the steel plate caused by force acting on the lead screw causes a change in resistance which can be calibrated to be read as a pressure within the syringe and giving set
* Linear peristaltic pumps: sensing piston that presses on the infusion line immediately downstream of the pumping chamber can be calibrated to measure line pressure indirectly (see picture)
Occlusion pressure monitoring: how set
Pressure at which device senses an occlusion condition -> stops the infusion
* Should be set as low as possible to protect patients from extravasation injuries
* Generally set at highest limit in anaesthesia: long narrow-bore infusion lines with high internal resistance, bolus doses delivered at high rates, >=2 infusions through one narrow-bore peripheral cannula, close monitoring
Notes: some devices also have disconnection alarm when detect a sudden drop in pressure
Air-in-line detection alarm: when required, mechanism
- All peristaltic infusion pumps must have air embolus protection capable of detecting bubbles as small as 0.1ml in volume - because with an empty drip set, theoretically a peristaltic infusion pump could infuse an unlimited amount of air.
- Usually an ultrasonic system operating across the infusion line
- Not thought to be necessary for syringe drivers
TCI systems: proprietary vs open-label
Proprietary:
* First commercially developed TCI system was Diprifusor by AstraZeneca in 1996 for propofol (then still under patent). Any pump with a Diprifusor chip behaves in exactly the same manner, because the same proprietary algorithm controls the infusion pump.
Open-label: do not require use of specific tagged proprietary drug
* After expiry of patent on propofol, other manufacturers incorporated TCI algorithms into their syringe drivers
* in 2011: 4 manufacturers producing open label TCI pumps in UK: Arcomed, Braun, CareFusion, Fresnius Kabi
* Has markedly increased potential for confusion and error: differnet pharmacokinetic models available, manufacturers may implement their own versions of the various pharmacokinetic models available, non-standardized user interfaces and pump set up procedures
The Open TCI Initiative is an attempt by workers in the field of drug pharmacokinetics and pharmacodynamics to agree on a single unifying model for each drug which would be the basis for all dosing by TCI.
Differences between TCI models:
* will all models give same clinical effect for given target concentration?
* do same models in different pumps give same clinical effect for given target concentration?
- Given target concentration calculated using one pharmacokinetic model does not necessarily translate to same dose given (and thus clinical effect) when that target is delivered by a different PK model
- E.g. Marsh model (used by Diprifusor) and Schnider model will deliver different amounts of propofol for same target concentration because the two models predict different concentrations following a different dose
- Additionally, manufacturers may tweak the PK models with own interpretations of pharmacokinetic data. e.g. Fresnius pump uses a Marsh model with a much shorter half-time for equilibration of the effect site concentration than other manufacturers’ pumps that also use a Marsh model
Picture shows various propofol delivery:
* top = Fresnius Kabi Marsh effect site model (ke0 altered to assume much more rapid equilibration of effect and plasma compartments). Note that therefore a much smaller overshoot of plasma drug concentration is needed to achieve the desired effect site concentration, so reduced initial amount of propofol is delivered
* middle = Diprifusor Marsh effect site model
* bottom = Diprifusor March plasma site model (note no plasma overshoot)
TCI models in obese patients: which models cannot be used
Use of James equations for the calculation of lean body mass
* Used by Schnider model (propofol) and Minto (remifentanil)
* Generate decreasing values above a certain body weight, depending on height and gender -> therefore Fresnius pumps will not progress with TCI for BMI >42 for males or >25 for females (i.e. the inflection points on the James equation graphs after which the LMB starts to decrease), and **Alaris pumps ** will not allow entry of a greater weight than the corresponding figure for these BMIs
Diagram illustrates Lean Body Mass as calculated for a range of body weights for a 168 cm female. The inflection point (yellow circle) represents a BMI of 35
Common deadspace and multiple infusions when >1 drug is delivered through a single intravenous device
- Alterations in the administration rate of one fluid temporarily affect the infusion rate of the other fluids
- If one infusion stops, a considerable drop in drug concentration of second drug may be expected. After restarting the infusion, a bolus of the second drug is immediately administered -> peak in drug concentration
Figure shows possible effect on the blood concentration of propofol following changes in the flow rate of a secondary infusion
Degree of peturbation is a function of the shared volume of the fluid pathways: therefore common pathway/deadspace must be kept to minimum
Risk of reflux when more than one drug is delivered through a single intravenous device
If there is a distal obstruction -> drugs from pumped lines may reflux up an attached gravity-fed line. Therefore imperative that such lines have antireflux valves
Note: occurs frequently when drugs are injected into side ports of IV catheters or simple giving sets with blocked intravascular catheters -> when obstructed catheter is flushed, bolus of drug is then given (e.g. paralysed awake patient in recovery)
Risk of siphoning when more than one drug is delivered through a single intravenous device
If syringe drive positioned above patient and plunger is not held but only pushed by drive mechanism: possibility of free flow of drug because of the differenc ein hydrostatic pressure
Prevention:
* Modern drivers clamp the plunger and have detector to protect against incorrectly mounting syringe plunger
* Do not pole mount driver significantly higher than the patient
* Purpose build giving sets incorporate anti-siphone valves (essentially a high resistance valve)
