Critically evaluate the significance of tidal volume in the management of patients undergoing mechanical ventilation in Intensive Care.
There has been much interest in the use of low tidal volumes (eg. 6-9 mL/kg) in critically ill patients. The recent ARDSnet study confirmed a suspicion that the use of lower tidal volumes (6 vs 12 mL/kg) has significant benefits in those patients with Acute Lung Injury or ARDS (bilateral infiltrates and P/F ratio of < 300, within first 36 hours). Predicted body weight was used (calculated from height and sex). Many previous studies had not shown such a benefit (perhaps due to smaller differences in plateau pressures between groups ). In patients with “normal” lungs or those that do not meet entry criteria for the ARDSnet study, there is no evidence to suggest a benefit to the low tidal volume approach. On the contrary, the intermittent use of high tidal volumes (such as sighs or recruitment manoeuvres) has been shown to achieve short term benefits (improved P/F ratios, decreased shunt, open up collapsed areas) in patients with early ARDS or atelectasis. The global application of lower tidal volumes may well result in worse oxygen exchange unless counterbalanced with higher levels of PEEP (or intermittent recruitment manoeuvres).
The ventilation strategies in ARDS are discussed elsewhere.
However, this question is not just about ARDS.
The key issues are: