·Pressure Transducers communicate with the Pump Controller via an ADInstruments Bridge Amplifier. These need to be calibrated using a known pressure source such as a manometer or column of water. With the MLT844 transducers, 120mmHg will relate to approximately 5mV in LabChart. A two point calibration should be done with known values below and above your expected output (eg 0 and 150mmHg if you are expecting max values of 120mmHg). The values can be checked after the experiment or even during if you have doubts about the reliability of the readout in LabChart by placing a manometer on a side arm of a 3-way tap and switching the input from the perfusion line to the manometer.This will show you that the values you are seeing from your perfusion line are correct or not.
·The minimum to maximum flow rate of a peristaltic pump as controlled by the STH pump controller should read 0-5V in LabChart with the Units conversion off.
·If you are using Black/White tubing this would relate to approximately 0-25ml/min. This means that if you are setting a flow rate of 12ml/min, this will be using the mid range of the pump, giving enough scope above and below this value to increase or decrease the pump speed if the pressure changes.
RELATIONSHIP BETWEEN FLOW AND PRESSURE
In Constant Flow mode:
·Increase Flow will increase the pressure
·Decrease the flow will decrease the pressure
·At a constant flow rate the pressure will increase with increased pressure in the system (eg vasoconstriction or blockage in the tubing or cannula). Pressure will fall with decresing pressure in the system (eg vasodilatation).
In Constant pressure mode:
·If the pressure increases in the system, the flow rate will fall to keep the pressure constant
·If the pressure in the system decreases there will be an increase in the flow to keep this constant.
·If you adjust the pressure level with the pressure adjust the flow rate will vary until it finds the correct level necessary to maintain the new set pressure.
If the relationship seems incorrect and all of the above factors have been checked, change mode (from constant flow to constant pressure or vice versa) to see if the heart behaves in the expected manner or if there are still issues between the expected and achieved values.
As well as the technical considerations, there are a number of physiological parameters that would need to be ruled out:
·Oxygen level, is this sufficient to maintain a healthy heart? Also is the gas mixture sufficient to maintain physiological pH?
·pH – is this at the expected value
·Temperature – Is this too high or too low?
·Buffer - Does this have the correct concentration of compounds?
·Traces of drugs or other elements in the system from previous experiments?
·Bacterial growth due to excess use where the buffer is not sufficiently cleaned out.
·Physical blockages in the system preventing sufficient flow
·Aorta length, if this is too short the valve could be blocked by the end of the cannula preventing perfusate entering the coronary vessels.