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May - 2017

   

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Favorable Neurocognitive Outcome with Low Tidal Volume Ventilation after Cardiac Arrest

J Beitler, T Ghafouri, S Jinadasa, et al Am J Resp Crit Care Med, 2017, 195(9);1198-1206

Comment

There is increasing interest about ventilatory management following out of hospital cardiac arrest (OHCA), with trials underway or proposed looking at CO2 and )2 management. This propensity adjusted two-centre retrospective analysis examines the association of tidal volume and neurocognitive outcome after OHCA.

They identified 256 adults with OHCA (76% witnessed, 51% VF/VT), who required MV for greater than 48-hrs, from 2008-2014, and calculated time-weighted average Vt over the first 48 hours of MV. The primary outcome was favourable hospital outcome at hospital discharge (CPC 1,2), ascertained by blinded physician investigators. The exposure of interest, tidal volume, was analysed as a continuous variable, and a dichotomised variable (less than or greater than 8 ml/kg PBW), with a propensity score developed using logistic regression to estimate the risk of receiving high or low Vt.

They report;
- Mean VT over the first 48 hours was 7.9 ml/kg PBW (range, 4.9–14.3 ml/kg PBW).
- 38% had average VT greater than 8 ml/kg PBW over the first 48 hours, 4% received an average VT less than or equal to 6 ml/kg PBW during this time.
- In unadjusted analysis, lower VT was significantly associated with favorable neurocognitive outcome (OR, 1.47; 95% CI, 1.12–1.92 per 1-ml/kg PBW decrease in VT; P = 0.005).
- In the prespecified primary analysis, after adjusting for propensity score, lower VT remained significantly associated with favorable neurocognitive outcome (OR, 1.61; 95% CI, 1.13–2.28 per 1-ml/kg PBW decrease in VT; P = 0.008).
- The association between lower VT and favorable neurocognitive outcome was robust to method of covariate adjustment and handling of the independent and dependent variables of primary interest.
- In the propensity-matched sensitivity analysis, each patient receiving high VT was successfully matched with a patient receiving low VT (n = 97 per group), with balance achieved between groups for all available biologically relevant covariates. In this cohort, lower VT again was significantly associated with favorable neurocognitive outcome (OR, 1.68; 95% CI, 1.11–2.55 per 1-ml/kg PBW decrease in VT; P = 0.014).
- Reanalysis using ordinal logistic regression, entering CPC as an ordinal dependent variable and adjusting for propensity score, confirmed that the association between VT and neurocognitive outcome was not dependent on dichotomizing CPC scale (OR, 1.30;95% CI, 1.02–1.68 per 1-ml/kg PBW decrease in VT; P = 0.038; score test for proportional odds assumption P = 0.073). Reanalysis entering VT as a dichotomized variable and adjusting for propensity score similarly found an association between lower VT and favorable neurocognitive outcome (OR, 2.95;?95% CI, 1.22–7.12; P = 0.016).
- In terms of secondary outcomes lower Vt was associated with more ventilator-free days (β = 1.78; 95% CI, 0.39–3.16 per 1-ml/kg PBW decrease; P = 0.012) and shock-free days (β = 1.31; 95% CI, 0.10–2.51; P = 0.034). Vt was not associated with hypercapnia (P = 1.00).

Overall this retrospective study reported lower Vt after OHCA was independently associated with favorable neurocognitive outcome, more ventilator-free days, and more shock-free days. These findings suggest low-Vt ventilation after cardiac arrest is beneficial. Although it is limited by methodology, it is difficult to imagine the justification for a prospective study with a "high Vt" arm. So should we just practice low Vt in OHCA?

 

Abstract

Rationale: Neurocognitive outcome after out-of-hospital cardiac arrest (OHCA) is often poor, even when initial resuscitation succeeds. Lower tidal volumes (Vts) attenuate extrapulmonary organ injury in other disease states and are neuroprotective in preclinical models of critical illness.
Objective: To evaluate the association between Vt and neurocognitive outcome after OHCA.
Methods: We performed a propensity-adjusted analysis of a two-center retrospective cohort of patients experiencing OHCA who received mechanical ventilation for at least the first 48 hours of hospitalization. Vt was calculated as the time-weighted average over the first 48 hours, in milliliters per kilogram of predicted body weight (PBW). The primary endpoint was favorable neurocognitive outcome (cerebral performance category of 1 or 2) at discharge.
Measurements and Main Results: Of 256 included patients, 38% received time-weighted average Vt greater than 8 ml/kg PBW during the first 48 hours. Lower Vt was independently associated with favorable neurocognitive outcome in propensity-adjusted analysis (odds ratio, 1.61; 95% confidence interval [CI], 1.13–2.28 per 1-ml/kg PBW decrease in Vt; P = 0.008). This finding was robust to several sensitivity analyses. Lower Vt also was associated with more ventilator-free days (β = 1.78; 95% CI, 0.39–3.16 per 1-ml/kg PBW decrease; P = 0.012) and shock-free days (β = 1.31; 95% CI, 0.10–2.51; P = 0.034). Vt was not associated with hypercapnia (P = 1.00). Although the propensity score incorporated several biologically relevant covariates, only height, weight, and admitting hospital were independent predictors of Vt less than or equal to 8 ml/kg PBW.
Conclusions: Lower Vt after OHCA is independently associated with favorable neurocognitive outcome, more ventilator-free days, and more shock-free days. These findings suggest a role for low-Vt ventilation after cardiac arrest.

May


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