Pressure-regulated volume control ventilation as a means of improving lung-protective ventilation

Dear Editor,

We read with interest your recent articles on lung-protective ventilation^1,2 and would like to commend the authors of both publications on their excellent work. We would like to add to the conversation with a summary from our own audit at Sheffield Teaching Hospitals.

In August 2016, our general intensive care unit (ICU) introduced new ventilators and changed the initial mandatory ventilation mode from pressure control (PCV) to pressure-regulated volume control (PRVC) in order to limit large tidal volumes. Potential benefits of PRVC include use of a decelerating flow pattern whilst guaranteeing a set tidal volume despite changing lung compliance. We aimed to assess whether this practice change led to an overall improvement in the delivery of lung-protective ventilation (delivery of tidal volumes of 6–8 ml/kg predicted body weight (PBW) and peak pressures less than 30 cmH₂O).

We retrospectively reviewed data from two cohorts of undifferentiated ICU admissions before and after introduction of PRVC. Patients were included if they had both at least 28 h of mechanical ventilation and two days admission to ICU. Hourly tidal volume and peak inspiratory pressure measurements were directly recorded by our electronic clinical information system (MetaVision, iMD Soft) from the 4th to 28th hour of admission. Anonymised data were collated in MS Excel and groups were compared using two-tailed t-testing. A p-value of < 0.05 was deemed statistically significant. Two arterial blood gas results at 4 and 16 h from initiation of mechanical ventilation were assessed.

In total, we analysed 116 patients in cohort 1 (mainly PCV) and 110 patients in cohort 2 (mainly PRVC). Amongst the first group, 81% of 2773 total hours were spent on PCV with a mean tidal volume per PBW of 8.4 ml/kg and a mean peak pressure of 22.1 cmH₂O. By contrast, the second group spent 82% of 2662 total hours on PRVC, resulting in a mean tidal volume of 8.0 ml/kg PBW and mean peak pressure of 20.7 cmH₂O. Both reductions in tidal volumes and peak pressures were statistically significant (p < 0.05).

More importantly, the percentage of hours spent with excessively large tidal volumes (above 10 ml/kg) fell from 19% to 9% and the percentage of hours with peak pressures exceeding 30 cmH₂O fell from 13% to 5%. This was accompanied by a reduction in the percentage of patients with a respiratory alkalosis (pH>7.45 and PCO₂ < 4.67) on the 4-h gas from 14% to 3% and 16-h gas from 10% to 3%.

We acknowledge the obvious limitations of the single centre and retrospective nature of our audit. Interestingly, mean tidal volumes in our cohorts have reduced (albeit modestly) after introduction of a volume-guarantee mode. This is in keeping with the findings by Newell et al., who reported lower mean tidal volumes in units favouring volume control modes as compared to units using pressure control modes. ¹ It is important to note that our patients were undifferentiated ICU patients, and therefore aggressive tidal volume limitation was not necessarily pursued.

Whilst we await the outcome of further studies examining tidal volume limitation in non-acute respiratory distress syndrome patients, ³ our audit adds additional data on the promising role of PRVC in avoidance of excessive tidal volumes. This is particularly relevant in a critical care environment where constant and immediate ventilatory input by nursing or medical staff may not be practical.

We are encouraged by the excellent work on standardisation and improvement of ventilation practice in critical care.^4,5 In addition to educational interventions and regular audit activity, we would recommend consideration of PRVC mode as an instrument to prevent excessive tidal volumes and its potential deleterious sequelae.