Diagnostic ultrasound at 7.6 MHz has been shown to induce pulmonary capillary hemorrhage (PCH) in rats anesthetized with ketamine and xylazine (Miller DL, Ultrasound in Medicine and Biology. 2012; 38; 1476). The objective of this study was to compare PCH results for ketamine anesthesia without xylazine, which can influence lung function. Female rats were anesthetized with ketamine alone (100 mg/kg) or ketamine (91 mg/kg) plus xylazine (9 mg/kg). The anesthetized rats were prepared by shaving the right thorax, and then mounted in a 38 °C water bath. A Phillips HDI 5000 (Philips Healthcare, Andover MA USA) with CL15-7 linear array was used to scan the right cranial or medial lung lobes in B mode at 7.6 MHz, 1 cm focus and 39 fps for 5 min in the water bath. The PCH was observed as comet-tail artifacts in the images and on the lung surface after euthanasia. For scanning with on-screen Mechanical Index (MI) values of 0.52, 0.7 and 0.9, PCH was found in 0 of 5, 2 of 5 (not significant) and 5 of 5 rats (P=0.001), respectively, which indicated a threshold at about MI=0.8 for ketamine without xylazine. This was substantially higher than the previous results for ketamine with xylazine anesthesia, which indicated at threshold at about MI=0.44. In a direct comparison of ketamine anesthetized rats at MI=0.7, 5 of 5 rats with xylazine given before scanning had PCH, while only 1 of 5 with xylazine given after scanning had PCH evident on the lung, a significant difference in occurrence (P<0.01). Xylazine, which produces moderate cardiovascular and respiratory depression, substantially reduced the threshold for PCH induced by diagnostic ultrasound. Physiological factors, in addition to physical exposure parameters, therefore may influence greatly the occurrence of PCH during pulmonary ultrasound.
