Introduction.
So now you can do an USCOM examination and get a good tracing, and from books one and two, you’ll have some idea of what the numbers are about, although learning what they mean is as much about practicing their application with real clinical cases as it is about understanding the theory. When you first started using the numbers at the bedside, you probably started with just cardiac output. It probably wasn’t long before you started looking at three or four other indices that you were confident with, to see how they changed and interacted with your interventions and the patient’s clinical course. The most common (and simple) framework is usually to start off looking at Cardiac Output (CO) (or Cardiac Index (CI)), Stroke Volume (SV) and SVR. If you’ve read book 3, “The USCOM and Inotropy” then you are probably looking a lot more at FTc.
For example, an adult patient in septic shock will usually have a low SVR, and a high CI, depending on how well preloaded they are. Since they usually need fluid, the SV and FTc are usually low. Watch what happens as you fill them with fluid; their SV and FTc increases, and as SV is raised CI increases also. Once they are adequately preloaded their FTc will be normal, but they may remain hypotensive, despite a high CI, because the SVR is low. Once you start the noradrenaline, you see the SVR increase, and you appreciate how nice it is to have indices to titrate your treatment against.
Similarly, you could look at a patient with cardiogenic shock using the four simple indices of CO/CI, SV/SVI, SVR and FTc. The CI is low resulting in a low blood pressure, and the body compensates to maintain perfusion by raising SVR. The FTc will probably be normal to high, warning you that extra fluid (preload) won’t help you here. Similarly the SV is likely to be low, but attempting to increase it further with fluid won’t raise the CI because the Starling curve is too flat (or even dipping). However when you start an inotrope, such as dobutamine, the CI increases, and the body responds by reducing the SVR. Again it is nice to understand the effect of therapy by measuring these simple haemodynamic parameters
Measuring these basic indices in real patients, simplifies clinical decision making and allows an appreciation of how easy circulatory management can be when you have real time objective information at your fingertips. Although we learnt these numbers at university, we by-passed them as there was no method of acquiring this information in clinical practice. It’s nice to welcome our old friends back! Once you’re comfortable using these basic numbers in clinical situations, it’s time to expand your repertoire and further eliminate the clinical guesswork. It’s time to read on…..
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