Applied pressure of cooling gas: a neglected determinant for size of specimens in transbronchial cryobiopsy

BACKGROUND: Transbronchial cryobiopsy (TBCB) is an option to surgical biopsy for the diagnosis in interstitial lung diseases. Several impact factors have received wide attention, including the freezing time, cryoprobe size, and contact pressure. However, the effect of the applied gas pressure on the specimen size has not been well elucidated. The purpose of this study is to investigate the effect of the applied gas pressure on the TBCB specimen size. METHODS: Cryoprobes with a diameter of 1.9 mm were used to perform TBCB on 4 beagle canines under general anesthesia. TBCB was performed with a total of 16 time-pressure combinations that were randomly combined with 4 freezing times (3, 4, 5, and 6 s) and 4 gas pressures (40, 50, 55, and 60 bar). For each combination, 8 biopsies were performed. The size and quality of specimens, as well as complications, were evaluated. RESULTS: A total of 128 TBCB specimens were obtained. With the same freezing time, the specimen sizes obtained by different applied gas pressures were significantly different (P<0.05) and positively correlated with the gas pressures (r: 0.797-0.867). With the same gas pressure, the size of the TBCB specimens was positively correlated with the freezing time (r: 0.503-0.752). In the 40-bar group, no tissues were obtained when the freezing times were 3-5 s. In the 50-bar and 55-bar groups, qualified specimens were obtained when the freezing times were 5 and 6 s. In the 60-bar group, qualified specimens were obtained when the freezing times were 3-6 s. CONCLUSIONS: The TBCB specimen size was positively correlated with the applied gas pressure. The applied gas pressure contributed to the sample size and quality. To obtain qualified specimens with a 1.9-mm cryoprobe during TBCB, the lowest limit of the normal working gas pressure range should be increased to greater than 50 bar.