First, let’s understand the point of non-invasive cardiac output monitoring. Patients who are entering septic, cardiac, or what-have-you shock have inadequate tissue perfusion. Tissue perfusion is dependent on cardiac output, which in turn depends on stroke volume.
You can get an idea of how well tissues were being perfused by looking at the lactate. You could try to see if a person was fluid-responsive by bolusing them a small amount and reassessing. But if you wanted more precise methods? Just a decade ago, the only way to monitor cardiac output was with a pulmonary artery catheter (PAC) using thermodilution. While thermodilution is still the gold standard, we have a couple of other options now.
Here are some terms that are useful to know:
- SVI (stroke volume index): amount of blood being pumped with each beat, indexed to body surface area (normal is 35-40 ml//M2 )
- CI (cardiac index): normal is 2.5- 4.0 L/min/M2
- SVV (stroke volume variation): percentage of variability in the stroke volume between inspiration and expiration. Heart rate variability will make the SVV less reliable
The NICOM has been clinically validated as a tool for non-invasive cardiac monitoring. It sounds like something out of a sci-fi novel: using the amount of time it takes for an electric current to pass through the chest as a reference point, the NICOM translates this time into flow into SVI. Increased time=increased stroke volume.
The Vigileo monitor can be use to monitor continuous cardiac output (with the Flotrac) or continuous central venous oxygen saturation (using the PreSep triple lumen oximetry catheter). It can provide data on SVI, SVV, and CI. The advantage of the Flotrac is that you can quickly see if the SVI is rising with volume. Unlike the NICOM, the Vigileo probably shouldn’t be used if the patient has an arrhythmia or vasospasm or vasoplegia (as sometimes post-operative patients who have gotten anesthesia are).
- most accurate when patient has normal lung compliance and regular heart rate
- an SVV of >13% suggests the patient is dry and you can try giving fluids
- if the SVV is <13% but the patient has non-compliant lungs, you can still try giving fluids
- SVV may not be reliable in cases of: arrhythmias, low ejection fraction, noncompliant lungs, other modes of ventilation besides assist control.
Does it matter which one you choose? The ICU nurses I’ve worked with pledge allegiance to one or the other, but there is evidence that NICOM and the Vigileo have similar monitoring capabilities.