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; an arterial blood pressure monitor configured to detect one or more predetermined feature of the SubQ ECG signal, detect one or more predetermined feature of the plethysmography signal, and determine at least one value indicative of the patient's arterial blood pressure based on at least one said detected predetermined feature of the SubQ ECG and at least one said detected predetermined feature of the plethysmography signal.