The frequent measurement of a patient’s arterial blood pressure (BP) is standard care in anesthesia. The guidelines state that BP must be measured, but how it is measured is up to you. Is the presence of a pulse an indicator of adequate BP? No, not really. But that was how it was done in the distant past. In some modern situations, however, a finger on a pulse is all you may have to work with until help comes. Is a sphygmomanometer an acceptable way to monitor BP? Yes, it is, if the operator is capable of reliably using one. The problem with sphygmomanometers is that they are labor intensive. But up until the mid 1980s, that is just how most BPs were taken when a patient was under an anesthetic.
Fortunately for us, there are now ways to measure BP that require less work on our part. Noninvasive BP monitoring (NIBP) relies on an oscillometer to give us an accurate BP reading. All we have to do is place the BP cuff on the patient, press “start,” and tell the monitor how often we want it to measure BP. The other method of measuring BP is with an intraarterial catheter. This is an example of invasive monitoring versus the other ways we mentioned, which are classed as noninvasive monitoring.
Of course, the kind of BP monitoring is chosen for use in an anesthetic is a complicated subject. Much of the decision depends on the kind of case and the physical status of the patient. But we will not get into that here. We want you to understand how both invasive and noninvasive BP monitoring works.
Invasive Blood Pressure Monitoring
Depending on the rotation you are on or the type of patient you anesthetize regularly, you may use a pressure transducer every day. Invasive BP monitoring, central venous pressure (CVP) measurements, and pulmonary artery catheter readings all require a transducer somewhere between the patient and the monitor display. (Okay, you could use a columnar manometer, but that would be really inconvenient.)
You can go into as much physics with pressure monitoring and transducers as you want. It would take an electrical engineer or physicist to understand pressure transducers inside and out. Don’t worry; we aren’t engineers or physicists either. So if we can understand it, so can you. The bottom line is that we use these things very frequently, but we have no idea how they work. Yes, it is complicated, but we will try to make it as simple as possible so we all can understand this extremely important but underappreciated marvel. But we will have to talk about some physics to get to where we want to go.