To the Editor.—
During the continuous ventilatory support of patients with respiratory failure, it is customary to apply deep "sigh" breaths at periodic intervals. Certain commonly used respirators do not provide a mechanism for providing such deep breaths, and in these instances, the therapist may disconnect the machine and attach a hand-operated ventilation bag. This is done each hour and a number of deep breaths given by hand-squeezing the bag.The standard ventilation bag, if completely evacuated, will deliver 1,400 to 1,500 ml of gas. As used clinically, the amount delivered is variable but rarely exceeds 1,000 ml. Thus, the deep breath may be little more than the tidal volume on the respirator, and progressive atelectasis, with its attendant sequelae, may follow.The ideal deep breath should inflate the lungs to near the normal total lung capacity (TLC). This may be arrived at by dividing the predicted forced vital capacity
Nicholson D, Mazey RM. Artificial Sighs. JAMA. 1974;229(9):1166–1167. doi:10.1001/jama.1974.03230470018006
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