Chapter 91: Cardiac Output Measurement

KEY POINTS

INTRODUCTION

The management of hemodynamically unstable patients requires an assessment of the cardiac output (CO) and the patients’ intravascular volume status (cardiac preload). In most instances the absolute value of the CO is less important than the response of the CO to a therapeutic intervention. In limited circumstances, most notably in the perioperative setting, optimization of CO has been associated with improved patient outcomes. This chapter will review the role of CO monitoring in the ICU and operating room. The most common methods of monitoring CO will be reviewed followed by the utility of CO monitoring.

METHODS OF MEASURING CARDIAC OUTPUT

Pulmonary Artery Catheter

Adolph Fick described the first method of CO estimation in 1870.¹ Fick described how to compute an animal’s CO from arterial and venous blood oxygen measurements. Fick’s original principle was later adapted in the development of Stewart’s indicator-dilution method in 1897,² and Fegler’s thermodilution method in 1954.³ The introduction of the PAC in 1970 and its subsequent use in performing thermodilution measurements in humans translated the ability to measure CO from the experimental physiology laboratory to multiple clinical settings.⁴ The direct Fick method was the reference standard by which all other methods of determining CO were evaluated until the introduction of the PAC. Currently the PAC is considered the “gold standard” against which other devices are compared. Remarkably, the accuracy of the CO measurements as determined by the PAC has never been established. Furthermore, electromagnetometry and ultrasound using aortic flowprobes most closely represent a true “gold standard” for determination of CO but can only be performed in instrumented animals.^5,6,7 Despite the ubiquitous use of the PAC remarkably few studies have investigated the accuracy of the CO measurements as determined by thermodilution. A number of studies have compared the thermodilution CO with that measured by the Fick technique. These studies have reported a percentage error of between 56% and 83% (with < 30% being clinically acceptable).^8,9,10 Philips at al compared thermodilution CO with surgically implanted ultrasonic flow probes in an ovine model.⁵ The percentage bias and precision was –17% and 47%, respectively; the PAC under-measured dobutamine-induced CO changes by 20% (relative 66%) compared with the flow probe. This study found that the PAC was an inaccurate measure of CO and was unreliable for ...