A headrest assembly for supporting a face of a user of a massage device includes a resilient assembly and a support arm assembly. A circuit network is used to generate a modulation signal and a reference signal from a broadband signal such as a pseudo random bit sequence. The error in operation of the materials handling vehicle is repaired in response to receiving the wirelessly transmitted error event information obtained automatically from the CAN bus. Personnel are provided with access to the pipe string for applying a fastener to secure the control line to the pipe string. With simultaneous substituate bolus administration and reduction of blood flow rate, the composition of fluid flow is optimized for detection of recirculation, so that fairly large pressure changes result in venous and arterial branches, thereby improving sensitivity and reliability of the measurement method.