Critically evaluate the significance of tidal volume in the management of patients undergoing mechanical  ventilation  in Intensive Care.

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College Answer

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).

Discussion

The ventilation strategies in ARDS are discussed elsewhere.

However, this question is not just about ARDS.

The key issues are:

  • Tidal volume factors into minute volume, and determines CO2 removal
  • Tidal volume also determines the degree of lung inflation and recruitment of atelectatic lung
  • Adequate tidal volumes are important in ensuring patient comfort and decreasing sedation requirements
  • Low tidal volume (6ml/kg) works well for ARDS patients, prevents volutrauma and improves survival; however its disadvantages are hypercapnea and need for higher levels of sedation.
  • As a ventilation strategy, there is no advantage to its use in patients whose lungs have a normal compliance
  • In fact in patients with normal lung compliance low tidal volumes may lead to atelectasis and deterioration of gas exchange