When you are using a Millar Pressure-Volume catheter with the PowerLab and LabChart software, a good way to determine the location of your catheter is to monitor the shape of the pressure waveform. You will be able to see a change in the waveform shape as soon as you enter the ventricle, as shown in the image below:
How can I be sure I have advanced my Pressure-Volume catheter far enough?
Once the pressure waveform looks more typical of a ventricular measurement, then you can open the XY View (from the Window menu in LabChart), and plot the pressure signal versus the volume signal to ensure the loops look correct. If the loops look like the screenshot below, you will know your catheter is placed correctly:
If your loops appear to be leaning, if they seem to have a tail at one of the corners, or are shaped more like a figure-eight, usually the problem is with the placement of the volume electrodes. The volume electrodes should be at the very edges of the ventricle (from the apex to the valve). This means that the animal you're measuring from should have a ventricle length equal to the distance between the volume electrodes. If your pressure signal looks correct but the loop still looks off, try adjusting the catheter placement to see if the volume signal can be improved. If you are not able to get good loops despite adjusting placement, make sure your catheter's volume electrode spacing is appropriate for the animals you are using.