A surgical instrument includes an impedance sensing system for monitoring the position of the tip of the surgical instrument relative to the pericardial space. The surgical instrument consists of a guidewire or needle including a conductive core terminating at a distal tip of the guidewire or needle. The distal tip has a conductive surface exposed to patient tissues and/or fluids. The impedance sensing system includes a first electrode formed by the exposed surface of the guidewire or needle, and at least a second electrode isolated from the conductive core of the guidewire or needle. The second electrode may be formed on an outer surface of the guidewire or needle, or may be a pad electrode applied to the patient skin. The impedance sensing system also includes an impedance analyzer for measuring impedance and phase using one or more frequencies.
