The importance of flow pulsatility for the rate of transvascular fluid filtration in lungs

Does Roller Pump-Induced Pulsatile CPB Perfusion Affect Microvascular Fluid Shifts and Tissue Perfusion?

BACKGROUND

Pulsatile versus nonpulsatile cardiopulmonary bypass (CPB) perfusion remains debated. Beneficial effects on tissue perfusion, inflammation, and microvascular fluid exchange have been linked to pulsatile perfusion by some investigators and denied by others. This study evaluated fluid extravasation and tissue perfusion during nonpulsatile or pulsatile roller pump-induced CPB perfusion.

METHODS

Fourteen pigs underwent roller pump-induced pulsatile (n = 7) or nonpulsatile CPB perfusion (n = 7) for 90 minutes. Fluid input/losses, colloid osmotic pressures (plasma/interstitium), hematocrit, serum electrolytes, serum proteins, tissue perfusion, and total tissue water content were measured, and plasma volume and fluid extravasation were calculated.

RESULTS

Fluid additions/losses, plasma volume, and fluid extravasation changed similarly in both groups during CPB with no between-group differences. Neither was between-group differences observed for tissue perfusion and total tissue water content, with one exception. Total tissue water content of the right (3.92 ± 0.26 versus 4.32 ± 0.28 g/g dry weight) and left ventricle (4.02 ± 0.25 versus 4.33 ± 0.24 g/g dry weight) was lowered in the pulsatile group.

CONCLUSIONS

No important differences were found between pulsatile and nonpulsatile CPB perfusion for microvascular fluid balance and tissue perfusion.

Isoflurane in Contrast to Propofol Promotes Fluid Extravasation during Cardiopulmonary Bypass in Pigs

Background:

A highly positive intraoperative fluid balance should be prevented as it negatively impacts patient outcome. Analysis of volume-kinetics has identified an increase in interstitial fluid volume after crystalloid fluid loading during isoflurane anesthesia. Isoflurane has also been associated with postoperative hypoxemia and may be associated with an increase in alveolar epithelial permeability, edema formation, and hindered oxygen exchange. In this article, the authors compare fluid extravasation rates before and during cardiopulmonary bypass (CPB) with isoflurane- versus propofol-based anesthesia.

Methods:

Fourteen pigs underwent 2 h of tepid CPB with propofol (P-group; n = 7) or isoflurane anesthesia (I-group; n = 7). Fluid requirements, plasma volume, colloid osmotic pressures in plasma and interstitial fluid, hematocrit levels, and total tissue water content were recorded, and fluid extravasation rates calculated.

Results:

Fluid extravasation rates increased in the I-group from the pre-CPB level of 0.27 (0.13) to 0.92 (0.36) ml·kg−1·min−1, but remained essentially unchanged in the P-group with significant between-group differences during CPB (pb = 0.002). The results are supported by corresponding changes in interstitial colloid osmotic pressure and total tissue water content.

Conclusions:

During CPB, isoflurane, in contrast to propofol, significantly contributes to a general increase in fluid shifts from the intravascular to the interstitial space with edema formation and a possible negative impact on postoperative organ function.

Effect of cardiac output on pulmonary hemodynamics

The purpose of this study was to test the hypothesis that there is a direct relation between pulmonary arterial input resistance and compliance in seven anesthetized cats. Cardiac output was altered by varying the direction and flow through an arteriovenous fistula with a roller pump. Pulmonary arterial input impedance was calculated from the Fourier analysis of the pressure and flow waveforms. Input resistance, pulmonary arterial compliance and characteristic impedance were estimated from the impedance spectra with a lumped parameter model. We found that pulmonary arterial compliance increased with cardiac output (P less than 0.002) but characteristic impedance was independent of flow. The observed response has the effect of reducing the hydraulic power lost by the pulmonary vasculature due to wave reflection. In three additional cats we measured the diameter of the main pulmonary artery by sonomicrometry. We found a linear relation between mean pulmonary arterial pressure and pulmonary arterial diameter during alterations of cardiac output. This result suggests that the increase of pulmonary arterial compliance with cardiac output is due to geometric factors rather than changes in vascular elastic properties.

Macromolecule transport across the pulmonary microvessel walls

In summary, it has been difficult to assess the permeability properties of the pulmonary capillary membrane. However, new mathematical and experimental techniques have recently been developed which are of sufficient sensitivity and specificity to begin to evaluate the complex mechanisms responsible for many forms of lung pathology. While future work will undoubtedly need to address problems associated with heterogeneity of pulmonary blood flow, and with an equally heterogeneous population of vascular permeability and fluid formation sites, we currently need to focus on using the correct experimental approaches for assessing vascular permeability. The appropriate techniques are described in the text and indicate that the measurements of reflection coefficients using lymph obtained at high vascular pressures, filtration coefficients obtained from both isolated and intact lungs, and two-pore models are useful in assessing vascular permeability.