Single breath measurement of pulmonary blood flow in man: the causes of variability.

Abstract

Theoretically, pulmonary blood flow can be measured by the uptake of a 'soluble' gas (e.g. freon-22) compared with an 'insoluble' gas (e.g. argon). As with all indirect methods, a number of uncertainties exist. Inhalation of a freon-22 and argon mixture (3.5%, 10%, 35% O2 balance N2) was made, with a subsequent, slow steady exhalation. The complete manoeuvre is termed the single breath technique (SBT). The repeatability of the SBT was assessed over four repeat measurements at rest, the mean difference between paired measurements lying in the range -0.09 to 0.04 l min-1, excluding the first trial measurements. Various features of the SBT were investigated to assess their influence on the reliability of the technique. It is shown: that the initial volume inhaled should be close to vital capacity; that a brief period of apnoea greater than 9 s following inhalation is necessary; that the subsequent exhalation should be about 10 l min-1; and that difficulties associated with recirculation of freon-22 are not encountered, presumably due to its wide distribution in body water. Under these highly controlled circumstances the technique is reliable.