which incorporates a sensor in the catheter or insertion trocar needle in this case electrical which will immediately report a change in the sensed parameter in this case inductance resistance capacitance etc indicating that the vessel has been entered this same reading can then them be monitored continuously as the instrument is manipulated e g the catheter is slid over the trocar needle into the vessel to ensure that the instrument does not migrate during manipulation and remains within the desired space another embodiment uses heat differentials to guide a catheter needle to the appropriate space tissue for example by placing a cold pack on the skin over the femoral artery a temperature differential will exist with the warmest location being in the intravascular space a temperature sensing catheter can be guided to the warmest location which would be inside the vessel this sensing technique may be employed with virtually any invasive instrument to ensure correct placement via detection of changes in any of the aforementioned parameters e g physical chemical thermal electrical acoustic vibratory optical or other parameter capable of being sensed with the only requirement being that the target tissue or space within the body must have a sufficiently distinct sensor reading that it may be distinguished from its surrounding tissues these invasive instruments may include but are not limited to instruments catheters or devices.