Introduction

Haemodynamic monitoring is essential in the cardiac critical care unit. Modern monitoring systems provide clinical staff with a plethora of physiological measurements that are used to diagnose and treat patients. Accurate measurements are imperative for patients to obtain the appropriate care. This chapter looks at the general principles involved and applies them to different types of measurement.

Invasive/noninvasive

Haemodynamic monitoring can be generally divided into invasive and noninvasive types. Noninvasive measurements are obtained without breaching the natural body armour (skin or mucosa) and include the electrocardiogram, pulse oximetry, temperature, capnography and blood pressure via a pressure cuff.

Invasive monitoring (obtained after breaching the natural body armour such as when inserting a catheter in an artery) is often preferred because it allows beat-to-beat measurements, greater accuracy of recorded values and the display of a continuous curve.

Pressure

Pressure is the force applied to a unit area of surface in a direction perpendicular to that surface. The internationally approved SI unit for pressure is the Pascal and 1 Pascal is 1 newton per square meter (1 Pa = 1N/m2). The standard atmosphere (atm) is an established constant and is approximately equal to typical air pressure at earth mean level and is defined as 1 standard atmosphere = 101,325 Pa.