A new method for noninvasive bedside determination of pulmonary blood flow

Abstract

A method is presented for determining the pulmonary blood flow from measurements of the time-averaged end-tidal pCO₂ and the CO₂ output.

The novel technique is based on a formula that is derived from Fick's principle in such a way that it allows a direct calculation of the lung perfusion from simultaneously measured changes in end-tidal pCO₂ and CO₂ output.

These changes are induced by altering the ventilation pattern of the patient for short (30s) periods of time. Different ways of doing this are discussed and it is shown that a bidirectional change in ventilation, involving hyper- and hypoventilation patterns, most adequately corresponds to the formula derived.

The method has been validated by comparison with cardiac output data obtained by thermodilution. Forty-two measurements were performed during mechanical ventilation on five dogs and six patients with essentially healthy lungs. Lung perfusion was in the range 0·4–6·5l/min. We found that Q_{CO 2}=0·97 Q_thermo with a s.d.=18%. The reproducibility of individual measurements was better than 0·3l/min.