Low Tidal Volume Strategy for Patients without ARDS; Takeaways from PReVENT
EMS brings in a 67 year old male in a PEA arrest. ROSC is obtained after twenty minutes of downtime. He was intubated by EMS during transport. A colleague talks to the family and she lets you know that he was complaining of shortness of breath and chest pain for an hour before he had a witnessed cardiac arrest and that his PMH includes HLD and HTN. The respiratory therapist is asking for the ventilator settings.
A randomized clinical trial published in JAMA in November 2018 Effect of a Low vs intermediate Tidal Volume Strategy on Ventilator-Free Days in Intensive Care Unit Patients without ARDS investigated a ventilation strategy of 6 mL/kg versus 10 mL/kg. The primary outcome was ventilator free days at day 28.
961 patients were enrolled. 477 patients were randomized to the low tidal volume strategy and 484 patients were randomized to the intermediate tidal volume strategy. After 28 days, both groups had a median of 21 ventilator free days (mean difference, -0.27 [95% CI -1.74 to 1.19] P=0.71. Does this mean you can safely subject patients to higher tidal volumes? Well not exactly and this requires a deeper dive into the methods. Due to frequent pressure support trials in the low tidal volume group the mean tidal volume was actually 7 mL/kg. In the intermediate tidal volume group the investigators decreased the tidal volume if the plateau pressure exceeded 25 cm H2O, essentially using a lung protective strategy. The mean tidal volume in the intermediate group was 9 mL/kg. Limiting inspiratory pressure in the conventional group and allowing higher tidal volume in the treatment group all limit the impact of the intervention.
The largest percentage of patients that required mechanical ventilation in this trial was due to cardiac arrest. Returning to the respiratory therapist’s question for our patient above; which strategy will you use? We know from his history he does not have any pulmonary disease. The trial showed no harm from a low tidal volume strategy for patients without ARDS. Minute ventilation and PEEP did not differ significantly between groups. PaO2 and FiO2 did not differ between groups. Partial pressure of carbon dioxide was higher and arterial pH was lower in the low tidal volume group than the intermediate tidal volume group. In addition, patients in the low tidal volume group did not require additional sedation.
Based on the outcome of this study, which was the largest randomized clinical trial to investigate the role of tidal volumes in patients without ARDS it does not make physiological sense to subject patients to a higher tidal volume unless there was a concern and that can be made on a case by case basis such as patient-ventilator asynchrony as evidenced by severe dyspnea or a high minute ventilation is required to reach a safe pH.
Investigators, W., Simonis, F., Neto, A., Binnekade, J., Braber, A., Bruin, K., Determann, R., Goekoop, G., Heidt, J., Horn, J., Innemee, G., Jonge, E., Juffermans, N., Spronk, P., Steuten, L., Tuinman, P., Wilde, R., Vriends, M., Abreu, M., Pelosi, P., Schultz, M.(2018). Effect of a Low vs Intermediate Tidal Volume Strategy on Ventilator-Free Days in Intensive Care Unit Patients Without ARDS JAMA 320(18), 1872. https://dx.doi.org/10.1001/jama.2018.14280