The promptly, efficient results were observed - All of alive patients of 3 group had value of BEecf returning promptly to normal range in samples M2, M3, M4 except the value of fetal group 1 that increased continually very much in the samples M2, M3 and M4.Conclusion: BEecf was valuable, helpful, efficient index in evaluation and treatment of acidosis in severely ill patients with acute respiratoy failure.