Estimating the diameter of the left main bronchus.

Two hundred and six standard PA chest X-rays were measured. The mean diameter of the left main bronchus (LMB) was 12.6 +/- 1.9 mm. The mean diameter of the 92 male LMB was 13.0 +/- 2.6, and for females 11.8 +/- 1.6. These data correspond closely to that reported by others. The correlation between the diameter of the LMB and that of the trachea was 0.74 but was not precise enough to be useful as an estimate for clinical use. Similarly there was good correlation between the diameter of the right main bronchus and that of the left (r = 0.75) but not precise enough to be clinically useful. In contrast to previous claims, only direct measurement of the left main bronchus has sufficient precision to define the appropriate size of left double-lumen tube to be selected for lung separation.