On your busy resus shift you receive a EMS notification that a post arrest patient is en route, ETA 2 minutes. On arrival the patient is being actively bagged by EMS through an endotracheal tube placed in the field. He is hypotensive to 83/55 and tachy to 112. You confirm ETT placement with direct visualization and ETCO2 on the monitor reads 26. While you set up for central access for pressors, Respiratory therapy places the patient on the ventilator. What should your settings be? The paper below may guide your practice.

Favorable Neurocognitive Outcome with Low Tidal Volume Ventilation after Cardiac Arrest

Background: Out of Hospital Cardiac Arrest (OHCA) affects more than 400,000 patients per year in the US. Mechanical ventilation is standard post arrest care, however post arrest patient are more likely to suffer significant lung injury, including ARDS, and long term cognitive impairment. Currently there is no international consensus on tidal volume post arrest.

Question: Is there an association between lower tidal volume during early mechanical ventilation and neurological outcomes in out of hospital cardiac arrest patients? Authors hypothesized that lower tidal volumes would be associated with improved neurocognitive outcome at hospital discharge.

Design, setting, population: Retrospective cohort study, 2 major urban academic institutions in large cities, (UCLA in San Diego, Beth Israel Deaconess Medical Center in Boston). Population: Adults aged 18 and over, admitted after non traumatic OHCA, requiring mechanical ventilation for a minimum of 48 hours during hospital stay. (Patients were excluded if transferred from previous hosp, traumatic arrest, ICH, chronic mechanical ventilation, ECMO, or if vent data was not available).

Methods: Chart review by physician-investigators to ascertain eligibility, values for Vt were from preset and exhaled Vt in volume and pressure targeted modes. CPC score was determined via chart review by 2 physician-investigators blinded to Vt and other respiratory and illness measures.

Primary independent variable: Tidal volume measured in millimeters per kilogram predicted body weight (PBW) during the first 48 hours of ICU stay. Low tidal volume – (Vt less than or equal to 8ml/kg/PBW). High tidal volume – (Vt > 8 ml/kg/PBW).

Dependent variable: Neurological outcome at hospital discharge, defined as cerebral performance category (CPC).

Outcomes: 

Primary outcome: Lower Vt was significantly associated with favorable neurocognitive outcome (OR, 1.47; 95% CI, 1.12-1.92 per every 1ml/kg decrease in Vt, P = 0.005). 

Secondary outcomes: Lower Vt was associated with more ventilator free days (95% CI, 0.39-3.16 per every 1ml/kg decrease in Vt, P = 0.012), and more ICU free days (95% CI, 0.13-2.63 per every 1ml/kg decrease in Vt, P = 0.012)

The Take away: There was a concerning confounder; patients receiving therapeutic hypothermia happened to be more likely to be in the Low tidal volume group, also remember that you can never infer causation from an observational study. However, don’t throw the baby out with the bath water. It boils down to this: patients in the low tidal volume group had a higher CPC approximately 27% of the time; with just a mere 12% in the higher tidal volume group. That’s a NNT of 7, which is pretty darn good. There’s no harm in dialing it down.

References:

Beitler JR, et al. Favorable Neurocognitive Outcome with Low Tidal Volume Ventilation After Cardiac Arrest. American journal of respiratory and critical care medicine 2017.