Apparatus for monitoring arterial pulse waves includes at least one separator ring receivable on at least one digit of a limb effective to tether a feed tube supplying a measuring device to a digit. Each separator ring is of a compliant material, of a non-interrupted tubular configuration, designed to avoid interference of blood supply to or from the digit to which it is applied, and of sufficient resilience and thickness to enable it to distance each adjacent digit from a digit mounted tubular socket probe so as to prevent contact between the tubular socket probe and adjacent digits, without interference to blood circulation, irrespective of the thickness of the digit on which the separator ring is received.
