Estimation of Cardiac Output and Lung Tissue Volume in Closed-Circuit (Rebreathing) Systems

Abstract

In 1959, Cander and Forster [1] showed how the rebreathing of a gas of moderate solubility in blood could be used to measure pulmonary tissue volume and pulmonary capillary blood flow. There have been many attempts to improve the technique since then by focussing on one or more details of the method [2–5], but the basic principles are still used today as presented by Cander and Forster. These principles are illustrated in Fig. 1. The left-hand side shows the experimental system consisting of the lung attached to a bag of known volume (V bag). The lung is considered to be homogeneous and is characterized by the variables gas volume, V lung; tissue volume, V tissue, and blood flow, Q. The bag is filled with a gas mixture such as 1% helium, 1% acetylene, 21% O2 and 77% N2 to the volume V bag, and at time zero the lung inhales the gas mixture from functional residual capacity (V lung). For some 10–15 s after the initial inhalation, the lung rapidly exhales and inhales from the bag, the system being closed as in Fig. 1.