Other Techniques for Circulatory and Perfusion Monitoring

Other methods for detecting adequacy of the circulating blood volume or end-organ perfusion have focused on measurement of circulating blood volume. The double indicator dilution method is one technique for assessment of blood volume at the bedside. With the use of equipment for transpulmonary thermodilution measurement of cardiac output, cold indocyanine green dye is injected intravenously, and the dye dilution and cold dilution curves are recorded with an arterial catheter inserted in the femoral artery and positioned in the descending aorta. The dye serves as an intravascular marker, whereas the cold diffuses and is convected into the extravascular space. The monitoring computer derives the intrathoracic blood volume from these measurements.^[798] Early investigations suggested that intrathoracic blood volume is a better measure of cardiac preload than traditional measurements such as CVP or PAWP are.^{[798] [799] [800] [801] [802] [803]}

Pulse dye densitometry is another method that may provide a bedside indication of the circulating blood volume that was previously measured only with complex radioactive isotope techniques. This method involves the intravenous injection of indocyanine green dye and detection of the dye with a peripheral pulse dye densitometer that uses two-wavelength light absorption, similar to a pulse oximeter.^{[804] [805] [806] [807]}

Other monitoring techniques aimed at detecting inadequate tissue blood flow have focused on the splanchnic circulation as a site that may provide an early indication of hypoperfusion. Gastric tonometry is a recently automated technique that allows measurement of gastric intramucosal PCO₂ and calculation of intramucosal pH with a semicontinuous air tonometer.^{[808] [809] [810]} Some investigations have already noted the superiority of this end-organ monitor as an indicator of the adequacy of tissue perfusion and a predictor of perioperative complications or death.^{[756] [810] [811]} Unfortunately, it is not known whether therapy directed at correcting such splanchnic hypoperfusion will confer clinical benefit.^{[812] [813]} These nontraditional cardiovascular monitoring techniques, such as measurement of intrathoracic blood volume and gastric intramucosal pH, offer new variables for cardiovascular monitoring that may augment or even supplant some current monitors.