Basic measurement concepts

Precise measurement of physiological parameters during anaesthesia is vital, and enables clinicians to deliver safe and appropriate care to patients. Monitoring devices are essential tools in the clinical environment, and accurate records of the measurements provided by these monitors must be kept. The physical parameter to be measured is known as a measurand. Examples of measurands in clinical practice include temperature, voltage and pressure. In the process of measurement, a measurand should be quantitatively compared with a predefined standard. The fundamental components of a basic measurement system consist of a sensor, signal, display at a human interface and a feedback loop. The signal or surrogate marker of a measurand is often processed before display. It is important that the output of a measurement system accurately reflects the value of the measurand. Challenges presented by measurement systems involve the maintenance of precision and accuracy. The input relative to output of a transducer is ideally linear in nature but is subject to hysteresis and drift making regular calibration essential. Outputs of measuring devices will also depend on their ability to respond to static or dynamic change. The readings obtained from measurement devices are also subject to human error.