Determination of cardiac output and of transcapillary fluid exchange by continuous recording of blood density.

We have tested the application of the continuous measurement of the density of the arterial blood for the calculation of the cardiac output and for the determination of the transcapillary fluid exchange. The density of arterial blood was continuously recorded in anesthetized dogs with a mechanical oscillator device (DMA 602 MW built by A. Paar KG, Graz). The time resolution of this device is less than 1 sec, the accuracy is 10(-6) g/ml. Simultaneously the arterial blood temperature was recorded with a thermistor probe. The intravenous injection of isotonic solutions yields temperature and density transients which are of a similar shape and proportional to the injected volume in amplitude. There is a good agreement between the cardiac output calculated from thermodilution transients and from density dilution transients. The injection of hypertonic solutions, e.g. 5% NaCl, 20% mannit or 14.4% urea, into a vein yields arterial density dilution transients which show marked differences and very characteristic features. They can be explained by the assumption of osmotic fluid shifts in the microcirculation of the lung. During the transient of a bolus of hypertonic NaCl or mannit, a transcapillary influx of about 0.170 X 10(-3) ml/sec per mosmols/l and per g wet tissue weight is generated. The effect of hypertonic urea is less by a factor of about 1/2, which is in agreement with the fact that urea diffuses rapidly into the erythrocytes. We conclude that the method of density dilution allows to record and quantify cardiac output and osmotic fluid shifts through the microcirculation of the lung of intact dogs.