The investigators will generate pressure-flow loops and pressure-volume loops from aortic and left ventricular pressure waveforms and Doppler (desc. aorta) flow waveforms and compare left ventricular to arterial pressure-flow and pressure-volume loops as well as to cardiac power from the USCOM 1A device. The goal of this study is to test the hypothesis that non-invasive estimates of cardiac pressure-volume work (derived from ultrasound-based measurements) correlate with invasive estimates.
Extensive animal work by Suga et al. in the 1980s clearly demonstrate a relationship between left ventricular pressure volume area (PVA) and myocardial consumption of oxygen (MVO2).
PVA can be measured by combining radial artery pressures with ultrasound-derived estimates of aortic blood flow. Because the aorta and peripheral artery compartments are separated from the left ventricle by the aortic valve, the peripheral arterial pressures cannot perfectly approximate left ventricular pressures. However, because the majority of variation in the pressure volume loops is made up of changes in height and width (changes in the left ventricular end-diastolic curve are, by contrast, relatively small), both of which can be readily detected by changes in the peripheral arterial blood pressure tracing, this loss of information may be clinically insignificant.
USCOM, has developed a portable suprasternal Doppler probe (model 1A) capable of estimating left ventricular stroke volume; a unique feature of this device is its ability to utilize both stroke volume, heart rate, and mean arterial pressure in an attempt to measure cardiac power. This device has not been validated against invasive estimates of cardiac power.
Knowledge of MVO2 would be a useful clinical variable but is not widely available. The ability to non-invasively estimate MVO2 intraoperatively would give anesthesiologists the ability to measure the effect of hemodynamic interventions on myocardial consumption of oxygen and, when combined with stroke volume, estimate myocardial efficiency. Non-invasively estimates of MVO2 may also allow cardiologists a novel means of assessing the myocardium of patients with cardiovascular disease.
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