1. An implantable monitoring device to monitor a patient's arterial blood pressure and configured to be implanted subcutaneously, the device comprising: subcutaneous (SubQ) electrodes configured to obtain a subcutaneous electrocardiogram (SubQ ECG) that is indicative of electrical activity of the patient's heart; a plethysmography sensor configured to obtain a plethysmography signal indicative of changes in the patient's arterial blood volume; and an arterial blood pressure monitor configured to detect a predetermined feature of the SubQ ECG signal indicative of ventricular depolarization, detect a predetermined feature of the SubQ ECG signal indicative of ventricular repolarization, detect a predetermined feature of the plethysmography signal, and determine a time t 1 from the predetermined feature of the SubQ ECG indicative of ventricular depolarization to the predetermined feature of the plethysmography signal, determine a time t 2 from the predetermined feature of the SubQ ECG indicative of ventricular repolarization to the predetermined feature of the plethysmography signal, and determine at least one value indicative of the patient's arterial blood pressure based on the times t 1 and t 2 determined from the SubQ ECG signal and the plethysmography signal.