Described over a 100 years ago, pulse contour analysis was purported to be a surrogate for cardiac output. In this technique, the area under the systolic portion of the arterial pressure curve is calculated. This area changes during the respiratory cycle due to changes in intrathoracic pressure and subsequent changes in ventricular preload and afterload.
Fast forward to the modern era, and many of our perioperative and ICU hemodynamic monitors (PiCCO, LiDCO, FloTrac/Vigileo) rely on this principle to help us determine dynamic metrics like stroke volume variation (SVV), pulse pressure variation (PPV) and even give us an idea about fluid responsiveness at the bedside.
We have to also understand the limitations of these monitors. Arrhythmias, spontaneous respirations, an open chest, poor arterial line tracing and smaller tidal volumes can affect the accuracy of this technique; however, the ease by which we can obtain these dynamic measurements still makes these cardiac output monitors worth it under the right circumstances!