Correspondence – ‘A study of noninvasive positive-pressure mechanical ventilation in the treatment of acute lung injury with a complex critical care ventilator. Is it the best choice?’

Dear Sir,

We are grateful for Dr Claudett’s attention to this article, in which he has highlighted some constructive points.

In our study, the findings suggest that there were more satisfactory clinical outcomes following the treatment of patients with acute lung injury when noninvasive positive pressure ventilation (NIPPV) was delivered, using a complex critical care ventilator compared with a conventional mini-ventilator. ¹

Noninvasive positive pressure ventilation can be delivered using various types of ventilatory equipment and interfaces. ² Full-service intensive care unit (ICU) ventilators, portable bilevel pressure generators and devices specifically designed to be used for NIPPV are available. ² Our team has achieved satisfactory results in the treatment of acute lung injury using NIPPV delivered via a complex critical care ventilator in clinical practice, and therefore undertook this study, with the conclusions summarized above. However, as Dr Claudett has pointed out, NIPPV with a critical care ventilator is not the ideal choice in the management of acute lung injury. In contrast to invasive ventilation, NIPPV uses an open-circuit design that is inherently leaky. Although ventilation can often be assisted even in the presence of sizeable leaks, ³ different ventilators and ventilator modes may be more (or less) able to compensate for air leakage. This compensatory capability may be important in optimizing the success of NIPPV.

As Dr Claudett mentioned, modern noninvasive mechanical ventilation systems have automatic compensation for leakage, whereas double-circuit critical care ventilators (such as the one in our study) have a limited capacity to compensate for leaks. ICU ventilators were originally designed to ventilate intubated patients, with minimal or no leakage. ⁴ However, when they are used to deliver NIPPV, air leaks are inevitable. Air leaks have generally caused triggering problems with ICU ventilators but, with the growing popularity of noninvasive ventilation, some newer ICU ventilators (such as the Hamilton G5, Hamilton Medical AG, Bonaduz, Switzerland) have ‘noninvasive modes’ among their standard inventory of ventilatory patterns. ⁴ When the noninvasive modes are activated, the ventilator automates leak compensation, adjusting bias flow and trigger sensitivity, and deactivates some nuisance alarms. ⁴ It has been reported that although leaks interfere with several key functions of ICU ventilators, noninvasive ventilation modes can correct part or all of this interference. ⁵ On the basis of this, we did not test for air leaks and did not provide data on the highest levels of pressures used or the percentages of leak.

Our study consisted of a prospective part and a retrospective part. In the prospective part of the study, there were no significant differences between the three groups at baseline (see Table 1¹). However, given that this was a prospective observational study, selection bias cannot be strictly ruled out. In the retrospective part of the study, the demographic and baseline clinical characteristics showed no significant differences between the two groups of patients with acute respiratory distress syndrome (see Table 2¹). Because these two groups were not treated during the same time period, the results may have been affected by bias.

Overall, the sample size was relatively small, so the evidence should be considered with caution. Our study may be considered as a pilot study that serves as support for calculating the sample size required for future studies. Further research to investigate the potential usefulness of complex critical care ventilators in the delivery of NIPPV in the treatment of acute lung injury should be undertaken in large, multicentre, prospective trials.