Threshold estimation of ultrasound-induced lung hemorrhage in adult rabbits and comparison of thresholds in mice, rats, rabbits and pigs

Abstract

The objective of this study was to assess the threshold and superthreshold behavior of ultrasound (US)-induced lung hemorrhage in adult rabbits to gain greater understanding about species dependency. A total of 99 76 +/- 7.6-d-old 2.4 +/- 0.14-kg New Zealand White rabbits were used. Exposure conditions were 5.6-MHz, 10-s exposure duration, 1-kHz PRF and 1.1-micros pulse duration. The in situ (at the pleural surface) peak rarefactional pressure, p(r(in situ)), ranged between 1.5 and 8.4 MPa, with nine acoustic US exposure groups plus a sham exposure group. Rabbits were assigned randomly to the 10 groups, each with 10 rabbits, except for one group that had nine rabbits. Rabbits were exposed bilaterally with the order of exposure (left then right lung, or right then left lung) and acoustic pressure both randomized. Individuals involved in animal handling, exposure and lesion scoring were blinded to the exposure condition. Probit regression analysis was used to examine the dependence of the lesion occurrence on in situ peak rarefactional pressure and order of exposure (first vs. second). Likewise, lesion depth and lesion root surface area were analyzed using Gaussian tobit regression analysis. Neither probability of a lesion nor lesion size measurements was found to be statistically dependent on the order of exposure after the effect of p(r(in situ)) was considered. Also, a significant correlation was not detected between the two exposed lung sides on the same rabbit in either lesion occurrence or size measures. The p(r(in situ)) threshold estimates (in MPa) were similar to each other across occurrence (3.54 +/- 0.78), depth (3.36 +/- 0.73) and surface area (3.43 +/- 0.77) of lesions. Using the same experimental techniques and statistical approach, great consistency of thresholds was demonstrated across three species (mouse, rat and rabbit). Further, there were no differences in the biologic mechanism of injury induced by US and US-induced lesions were similar in morphology in all species and age groups studied. The extent of US-induced lung damage and the ability of the lung to heal led to the conclusion that, although US can produce lung damage at clinical levels, the degree of damage does not appear to be a significant medical problem.

Figures

Fig. 1

Attenuation coefficient regression functions (linear regression lines ± 95% confidence band for the regression line) as a function of frequency for the intercostal tissues of New Zealand White rabbits at 37°C. The lower frequency attenuation coefficient data were derived from 16 rabbit chest walls and the higher frequency data were derived from 10 rabbit chest walls.

Fig. 2

(a) Lesion occurrence, (b) Lesion depth and (c) Lesion surface area as a function of the in situ peak rarefactional pressure. The dashed lines are straight lines connecting the mean values and are intended to provide graphical guidance. Error bars are the standard errors of the mean (n = 20 for each exposure condition except 6.65 MPa, for which n = 18).

Fig. 3

Plots of 95% confidence intervals for the estimated ED05s (solid horizontal lines with the ED05 estimates located at the centers) and ED50s (dotted horizontal lines with the ED50 estimates located at the centers) based on (a) lesion occurrence, (b) lesion depth and (c) lesion area in rabbits, rats and mice. The x-axis indicates the in situ peak rarefactional pressure, and the y-axis displays the species and study name. The two vertical lines in each box represent the weighted averages of the ED05 and ED50 estimates, respectively, with the average values at the base of each vertical line. For each of the four studies, a common exposure regime was used: 5.6-MHz center frequency, 1-kHz PRF, 10-s ED and 510-μm beamwidth.

Fig. 4

Summary of ED05 occurrence thresholds in terms of pr(in situ) (MPa) (bars; left axis) and MI (lines, right axis). * denotes the studies that are evaluated in Tables 2, 3 and 4, and Fig. 3. Error bars are SEMs. These data, by groupings from left to right, are derived, respectively, from Zachary et al. (2001a), O'Brien et al. (2001b), O'Brien et al. (2003b), O'Brien et al. (2003a), O'Brien et al. (2003a), and this study.

Fig. 5

Summary of occurrence thresholds in terms of pr(in situ) (MPa) (bars; left axis) and MI (lines, right axis) from studies not using our experimental techniques and statistical approach. These data, by groupings from left to right, are estimated, respectively, from Child et al. (1990), Raeman et al. (1993), Frizzell et al. (1994), Raeman et al. (1996), Holland et al. (1996), Baggs et al. (1996), Dalecki et al. (1997a) and Dalecki et al. (1997b).

Fig. 6

Global summary of occurrence thresholds in terms of in situ peak rarefactional pressure as a function of frequency for four species (see Figs. 4 and 5) The solid line is the threshold equation derived from pre-2000 data (AIUM 2000).

Fig. 7

Global summary of occurrence thresholds in terms of the MI as a function of frequency for four species (see Figs. 4 and 5). The solid line denotes an MI of 1.9, the FDA regulatory limit (FDA 1997).