This chapter covers the background physics that is not otherwise covered in the relevant chapters associated with the equipment discussed. It is a useful introduction to some loosely related, but widely applicable concepts. Any measurement made by the anaesthetist can be simplified and made easier to understand if it is represented by its basic dimensions. Dimensions are the basic components of equations and are independent of the units used. For example a common term in physics and medicine is velocity, which has the dimensions of length per unit time, written dimensionally as [L][T]−1. This means that, independent of the measurement system used, the measurement of velocity requires that the numerical value of a length be divided by the numerical value of a time. Equations or graphical axes can be predicted and their validity can be checked by dimensional analysis. Each side of an equation can be represented in basic dimensions and both sides should balance. The dimensions needed to describe most events are mass [M], length [L], time [T], and temperature [θ]. An example of dimensional analysis is described later in this chapter but first units must be discussed. All measurements need to have their correct unit and symbol attached to them. Equations have a unique language and the syntax must be correct for communication within the international community. In the past many different systems of units were used; one was Imperial, another c.g.s. The Systéme International (SI) of units was established in 1960 and is now the recognised system of measurement communication. For completeness the base quantities are shown and some useful derived physical quantities are shown in Table 3.1 In the SI system the combination of basic units involves multiplication and division but multiplication is shown as a space and division is shown as a negative superscript. For example, velocity, in metres per second, is m s−1. Prefixes to the name of each unit are usually in multiples of 103 and 10−3. There are a few non-SI units that are still used in medicine, (not just anaesthesia) and these seem resilient to change. One is the millimetre of mercury (mmHg) for intravascular pressures (100 mmHg = 13.3 kPa), and total and partial gas pressures.
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