The occlusion pressure indicated at the end of the line represents the peak pressure that the pump is able to generate before the leadscrew/friction bearing starts to slip and the occlusion alarm is activated by the optical sensing circuitry. In use there is bound to be a pressure gradient along the line when there is a flow and any check valves or antisyphon devices will have pressure-drops across them. A certain amount of force will also be required to overcome stiction in the syringe. The force required to overcome these losses can be equated to a loss of pressure measured at the output of the system and the rest of the applied force produced by the syringe driver actually does work in delivering fluid and overcoming the opposing forces in the patient's circulation. This is the pressure measured at occlusion.
Putting it very simply: approaching occlusion then maximum syringe force = force to overcome losses + force opposing the occlusion pressure. Some of the losses in the system occur due to syringe stiction (sticking plunger) and friction in the mechanics, i.e. leadscrew, halfnut, plunger clamp, tube and motor/gearbox assembly. The less the friction in the mechanics, stiction in the syringe and pressure drops in the line then the greater the pressure indicated at occlusion alarm, measured at the cannula.
My point is the physical occlusion limit, i.e. mechanical adjustment of the occlusion setting nut, may be identical but if different lines and syringes are used for wet setup or there are fluctuations due to mechanical wear then the indicated occlusion pressure will change accordingly as the losses vary. The danger to the patient is the maximum force that the pump can generate, irrespective of the largely uncontrollable losses in the syringe/line and fluctuations in the patient (e.g. extension/flexion of limbs used for peripheral infusions).
The size of syringe, dimensions of the line, viscosity of the fluid and mechanical condition of the pump can influence the pressures obtained at occlusion in practice. Thus a wet setup performed at one hosptial maintenance department may produce different occlusion alarm thresholds, i.e. measured pressures, than those measured at other departments when testing the same syringe driver. The biggest influence is likely to be the syringe stiction, line length, delivery rate (causes pressure gradients) and the presence of inline valves.
Use different syringe types and sizes along with different length extension lines and you'll see that the occlusion alarm pressure using a wet setup will vary considerably. Even with a consitent setup and a repeatable method then you have to be careful to use a new syringe or a lubricated syringe where stiction is negligible. New syringes are lubricated for one pull, to draw up drugs and then a delivery, by the way.
It is clear in syringe manufacturing standards that the range of pressure required to overcome static and dynamic friction (read as losses, which translate to pressure variation at the output of the system) are very wide - not far off the limits for some pumps. 3300s using pre-filled BD 50 syringes have to be set to within a few Kgf of the upper limit to avoid nuisance occlusion alarms where I work. I've attributed the problem to the fact that the lubricant has been dispersed when the syringes have been drawn and sat with drug in them).
Removing these sources of pressure variations from the system, i.e. by not performing wet oclusion alarm setup with syringes and lines during calibration, removes uncertainties in the losses. This allows consistent, i.e. more repeatable plunger forces to be measured. Consistent alarm limits, based on these repeatable measurements, can be obtained every time the machine is calibrated.
Unfortunately, with the occlusion alarm's reliance on mechanics, the Graseby 3XXX range are susceptible to wear and need to be fitted correctly. Fit a new leadscrew and other components including a motor/gearbox, friction bearing, etc, etc and set the occlusion alarm limit. I can guarantee you that after a few hours working under load that the occlusion alarm limits will be significantly higher than they were at calibration. This is because of the reduction in frictional losses as the leadscrew mechanics "bed-in" during use.
We use the Cardinal Medical (ALARIS Products) force gauge to ensure that the basic calibration of the Graseby 3XXX occlusion alarm is repeatable, i.e. that we've run the pump under backpressure after changing components or calibrating to ensure variations in mechanical losses have worked their way out of the system and are therefore minimised. At least if the mechanics are calibrated reliably to within Graseby's specifications then when the variable parts of the system, that are beyond our control, are fitted, then the pumps should alarm, to indicate problems, whether they involve the syringe, lines or the infusion site, consistently.
Losses will still vary in use on the patient as will the conditions at the infusion site (thus the occlusion alarm pressure will vary) but the maximum force generated by the syringe driver, hence maximum theoretical occlusion pressure, is fixed to an acceptable absolute maximum force measured by removing unknown sources of error in the measurement. This accurate setting of the force by removing variables, is more effective in protecting the patient than assuming all syringes used for calibration have low stiction and other factors in wet setup are negligible.
Even using the force gauge we find that as the length of the plunger is varied then the occlusion force measured varies along the leadscrew. We test with the plunger clamp at three points along the leadscrew with the force gauge to ensure the occlusion alarms within the specification. I am certain that if replacement mechanical components have not been given time to "bed-in" that the force indicated at occlusion will he higher than when the machine was calibrated within a few hours from putting the device back into use.
This has been learned in practice since the courses I attended were in the late 80's/early 90's and this was not discussed then. We were using the ALARIS test gauges for Grasebys when ALARIS first made them available for welmed/PX000 calibration purposes. Prior to that on factory visits to Graseby I'd observed that the standard service method of checking/setting occlusion alarm limits was using weights on a piece of dowelling with the pumps up-ended.
One other thing that needs to be remembered is that the case needs to be torqued-up to the appropriate values - not just to ensure accurate syringe sizing and a good seal that prevents case-cracking but I have seen occlusion alarm-limit values change with how tightly the case is fastened together.
