A medical patch includes a substrate, an electrode, and circuitry. The substrate is configured to attach externally to a patient. The electrode is coupled to the substrate and is configured to sense electrocardiogram (ECG) signals from a heart of the patient, and to further sense electrical signals indicative of an impedance between the electrode and a probe in the heart. The circuitry is coupled to the substrate and includes a shared amplifier that is configured to simultaneously amplify the ECG signals and the electrical signals sensed by the electrode.
