Comparative aspects of the strength of pulmonary capillaries in rabbit, dog, and horse.

This research article focuses on assessing the strength of pulmonary capillaries in rabbits, dogs, and horses. The researchers found that the thickness of the blood-gas barrier (BGB) and capillary radius play significant roles in determining the failure stress of the capillaries, however, additional factors might also contribute.

Background and Objectives

• The researchers previously demonstrated that high pressures across the wall of blood vessels (transmural pressures, Ptm) can lead to stress failure in the pulmonary capillaries of rabbits and dogs.
• They observed that the necessary Ptm for causing stress failure was higher in dogs as compared to rabbits and the total BGB thickness was also greater in dogs.
• This led to the hypothesis that stress failure may be connected to the thickness of the BGB.
• The Laplace relationship, which states that the stress on the wall of a vessel is proportional to its radius but inversely proportional to its thickness, also supports this hypothesis.
• In this study, the researchers aimed to test this hypothesis by comparing the BGB thickness and capillary radius in the lungs of rabbits, dogs, and horses. They additionally measured the Ptm required to cause stress failure in these animals.

Methods

• The lungs from 3 rabbits, 3 dogs, and 2 horses were perfusion fixed at a Ptm of approximately 30 cmH2O, and the BGB thickness as well as an index of capillary radius were measured.
• The thicknesses of the BGB were determined at random points on the capillary wall, while the capillary radii were obtained from mean measurements of major and minor axes on electron micrographs.
• Since horses are known to develop exercise-induced pulmonary hemorrhage, the pressure required to cause stress failure in horses was considered as the mean of the pulmonary arterial and left atrial pressures observed in galloping thoroughbreds. However, the authors note that the actual pressure required may be less.

Results

• The findings revealed that the BGB was thinnest in rabbits, intermediate in dogs, and thickest in horses, while the average capillary radii were 3.6, 3.4, and 3.2 microns for rabbits, dogs, and horses respectively.
• Calculated capillary wall stress values for the median total BGB thickness at a nominal Ptm of 30 cmH2O were 2.5 x 10(4), 1.7 x 10(4), and 1.5 x 10(4) N.m-2 for rabbits, dogs, and horses, respectively.
• The pressures necessary to cause stress failure corresponded with the species ranking — approximately 50, 90, and 130 cmH2O in rabbit, dog, and horse respectively.

Conclusions

• The differences in the capillary radius of curvature and BGB thickness were concluded to account for some of the observed differences in the necessary Ptm to cause stress failure.
• These differences also fit with species susceptibility to stress failure under high transmural pressures.
• However, the authors also acknowledge that other factors may be involved in determining the strength of the BGB, indicating the complexity of the process and suggesting the need for further research.