Measurement of dynamic lung fluid balance in the mechanically ventilated dog. Theory and results.

An expression (LN) is presented for the net fluid leakage from the intravascular to the extravascular space in the lung. It is based on a new dog model and is the sum of rate of change in extravascular lung water content (EVLW), thoracic lymph flow, and pleural fluid formation. The rate of change of EVLW (delta EVLW) in ml/kg/h was calculated from repeated measurements of EVLW with a double-indicator dilution technique (dye/cold) and corrected according to the relation between EVLW measured by this technique and gravimetry. LN was studied in lung-healthy mechanically ventilated dogs during a prolonged period of mechanical ventilation with and without the application of a positive end-expiratory pressure of 10 cmH2O (1.0 kPa). During mechanical ventilation, LN was found to be 0.3 ml/kg/h in the basal condition, increasing to 0.5 ml/kg/h (P less than 0.01) after a mean period of 7 h. After the application of a positive end-expiratory pressure (PEEP) of 10 cmH2O (1 kPa) for 0.5-2 h, LN was found to increase significantly, from a mean of 0.3 ml/kg/h to 0.9 ml/kg/h (P less than 0.01). We conclude that LN is a useful quantitative expression in experimental studies on lung fluid balance.