Noninvasive cardiac output monitoring in cardiothoracic surgery patients: available methods and future directions

The monitoring and optimization of cardiac output (CO) are central components of perioperative hemodynamic management in patients undergoing cardiothoracic surgery. Until recently, echocardiography and invasive indicator dilution methods have been the mainstays of CO monitoring in these patients. However, completely noninvasive methods to estimate CO have become available during recent years. In this review, the physical measurement principles, limitations, and measurement performance of the different techniques for continuous noninvasive CO estimation in the setting of cardiothoracic surgery are described. Methods to estimate CO in a completely noninvasive manner include noninvasive pulse wave analysis (using a finger cuff method or automated radial artery applanation tonometry), thoracic electrical bioimpedance and bioreactance, pulse wave transit time, and partial carbon dioxide rebreathing. All these technologies have been evaluated in cardiothoracic surgery patients, but the validation studies describing the measurement performance in comparison with invasive reference methods have shown inconsistent and, in part, contradictory results. In addition, all technologies have major limitations with regard to the applicability during routine clinical care in the operating room or the intensive care unit. Therefore, the methods for noninvasive CO estimation described in this review still require technological improvements with regard to measurement performance and clinical applicability before they can be recommended for routine perioperative hemodynamic management of cardiothoracic surgery patients outside of studies.