Indicator dilution measurement of lung water considerations of the method

Objective: Pulmonary oedema is life threatening. A method is needed for reliable measurement of lung water in man in order to clarify aetiology, pathophysiology, prognosis, and to evaluate treatments against pulmonary oedema.

Method: The heavy water-indocyanine green technique for measurement of cardiac output and lung water was applied in two human and two experimental studies. In parallel the thermo-dye technique was applied in one of the experimental studies. The results from these four studies together with results presented in the literature were analysed according the criteria set forth by The National Institute of Health, USA 1985, for the measurement of lung water in man, that is versatility, reproducibility, sensitivity, and accuracy.

Results for the heavy water-indocyanine green method. Versatility: No adverse reaction was recorded in the two human studies. On-line optical detection of heavy water and indocyanine green did not expose the subjects to radio-labelled isotopes and the blood loss was low compared to timed blood volume collection. Reproducibility: The coefficient of variation was below 10 % for cardiac output and central blood volume, and 10-17% for lung water. Sensitivity: Changes in lung water below the level of gas exchange disturbance could be detected in a group of subjects. Accuracy: Cardiac output, mean transit time and lung water for heavy water were linearly related to independent reference measures. Evidence for barrier limitations for heavy water in the clinical setting were weak. The method is unreliable at shock states and pulmonary embolism.

Results for the thermo-dye method. Versatility: The method did not expose the subjects to radio-labelled isotopes and there was no blood loss but requires a thermistor equipped fibre-optic catheter placed in the aorta. Reproducibility: The coefficient of variation was approximately 10 % for cardiac output and lung water. Sensitivity: In healthy lungs small differences in lung water can be detected. In pulmonary oedema the large increase in deviation cripples the sensitivity of the method. Accuracy: Recovery of cold and the error in mean transit time were dependent on the distribution volume for cold. This undermines reliable calculation of cardiac output and lung water.

Conclusions for the heavy water-indocyanine green method: The method is an improvement with regard to radio-labelled methods with the same reproducibility. The methods allows for detection of change in lung water under the range where gas-exchange disturbance occurs. The method provides values close to gravimetric lung water values over a wide range, independent of flow and volume, with the exception for shock states and severe vascular obstruction.