Introduction

Minimally invasive haemodynamic monitoring devices are increasingly beingused in cardiac critical care. Their advantages include ease of set-up, continuous measurement of stroke volume (SV) and cardiac output (CO) and the ability to provide other haemodynamic data to assist circulatory management.

Arterial pressure monitors

LiDCO and PiCCO monitors use a modified form of the Fick principle to analyze the arterial pressure waveform and calculate specific data based on this. They both allow calculation of CO and preload responsiveness, a useful measure of intravascular volume.

Cardiac output

THE LIDCO

The LiDCO (Lithium Dilution Continuous Cardiac Output monitor) detects changes in the concentration of a fixed dose of lithium. Lithium is chosen because of its low toxicity and high signal-to-noise ratio. Lithium is injected via a central venous line and sensed via a radial or other systemic arterial line every time the monitor is calibrated. This allows measurement of CO using the Fick principle and calibrates the instrument for further readings. Calibration is recommended when the monitor is first connected to a patient, and then every 24 hours. This method uses a characteristic of the arterial pulse waveform known as the pulse power. Pulse power analysis makes use of the law of mass conservation, and assumes that within a fixed circulatory system, any acute changes in that system are due to changes in SV input.