Figure 4. Diagram of the Fick method for measurement of cardiac output. The catheter is positioned in the pulmonary artery (PA), and blood samples are withdrawn simultaneously from a peripheral artery and the PA as room air is inspired. The volume of expired air is measured, the inspired volume is calculated, and the difference between the oxygen content of inspired air and that measured in the expired air can then be used to calculate oxygen uptake at the lungs per minute. See text for further discussion. (Reproduced with permission from West JB: Best and Taylor's Physiological Basis of Medical Practice, 12th edition. Baltimore, Williams and Wilkins, 1990, Figure 2, 140 Section 2, Chapter 13, p 245.)