Time course of blood volume changes in an isolated lung lobe after venous pressure elevation

The elevation of venous pressure (Pv) in isolated perfused organs causes organ weight to increase in a biphasic manner. The initial rapid phase results primarily from an increase in blood volume (BV), whereas the second slower phase is generally considered to reflect fluid filtration. Recent studies have suggested, however, that BV may continue to increase during the slow weight gain phase. To address this question, we made serial measurements of circulating BV by indicator dilution with indocyanine green dye in a canine isolated perfused left lower lung lobe (LLL) preparation during 40 min of Pv elevation. Pv was raised to approximately 18 Torr in six LLLs beginning an average of 28 min after the start of perfusion. After an initial rapid increase, BV continued to increase at a slower rate for approximately 30 min. The increase in BV observed between 3 and 40 min of Pv elevation [4.3 +/- 0.3 (SE) ml] was 47.9 +/- 9.1% of the weight gain that occurred during this period. In six additional LLLs, Pv elevation was delayed until approximately 70 min after the perfusion was started. In these LLLs, BV generally achieved constancy 3 min after Pv was elevated. These data indicate that the dynamics of the BV response of this preparation to Pv elevation is time dependent and that gravimetric determinations of the rate of fluid filtration may substantially overestimate the true filtration rate in the presence of continuing increases in BV. The increases in BV observed in the first group of LLLs appear to be due to vascular recruitment rather than stress relaxation.