Invasive blood pressure monitoring:
Invasive (intra-arterial) blood pressure (IBP) monitoring is a commonly used technique in the Intensive Care Unit (ICU) and is also often used in the operating theatre.
This technique involves direct measurement of arterial pressure by inserting a cannula needle in a suitable artery. The cannula must be connected to a sterile, fluid-filled system, which is connected to an electronic patient monitor. The advantage of this system is that a patient’s blood pressure is constantly monitored beat-by-beat, and a waveform (a graph of pressure against time) can be displayed.
There are a variety of monitors with invasive blood pressure monitoring for Trauma, critical care and operating room applications. These include single pressure, dual pressure, and multi-parameter (i.e. Pressure / temperature).
The components of an intra-arterial monitoring system can be considered in three main parts:
- the measuring apparatus
- the transducer
- the monitor.
The measuring apparatus consists of an arterial cannula connected to tubing containing a continuous column of saline which conducts the pressure wave to the transducer.
The arterial line is also connected to a flushing system consisting of a bag of saline pressurised to 300 mmHg via a flushing device.
In the case of intra-arterial monitoring, the MEMSCAP transducer consists of a flexible diaphragm, connected to a MEMS piezoresistive element with an electric current applied across it. As pressure is applied to the diaphragm, it deforms and stretches the MEMS element, which resistance changes, altering the electrical output from the system.
Monitors amplify the output signal from the transducer, filter the noise and also display the arterial waveform in real time on a screen. They also usually give a digital display of systolic, diastolic and mean blood pressure.
IBP monitoring has numerous advantages.
The very first advantage is that it allows continuous ‘beat-to-beat’ blood pressure monitoring. This is useful in patients who are likely to display sudden changes in blood pressure (e.g. vascular surgery), in whom close control of blood pressure is required (e.g. head injured patients), or in patients receiving drugs to maintain the blood pressure.
A second advantage is that IBP technique also allows accurate blood pressure readings at very low pressures, for example in shocked patients.
A third advantage relies in the improvement of patient comfort, especially for those who are likely to need close blood pressure monitoring for a long period of time e.g. ICU patients. IBP monitoring avoids the trauma of repeated cuff inflations.
Other advantages include the fact that intravascular volume status can be estimated from the shape of the arterial pressure trace, and that the indwelling arterial cannula is convenient for repeated arterial blood sampling, for instance for arterial blood gases.