The invention features a monitoring device that measures a patient's vital signs (e.g. blood pressure). The device features a first sensor configured to attach to a first portion of the patient's body that includes: i) a first electrode configured to generate a first electrical signal from the first portion of the patient's body; ii) a first light-emitting component; and iii) a first photodetector configured to receive radiation from the first portion of the patient's body after the radiation is emitted by the first light-emitting component and in response generate a first optical waveform. The device also features a second sensor that includes essentially the same components. An amplifier system, in electrical contact with the first and second electrodes, receives first and second electrical signals from the two sensors to generate an electrical waveform. A processor, in electrical contact with the amplifier system, receives the electrical waveform, the first optical waveform, and the second optical waveform. The processor runs computer code that processes the input waveforms with an algorithm to determine at least one of the patient's vital signs.