Limited effect of mild therapeutic hypothermia on outcome after prolonged resuscitation

Mild therapeutic hypothermia interferes with multiple cascades of the ischaemia/reperfusion injury that is known as primary mechanism for brain damage after cardiac arrest. First resuscitation attempts and the duration of resuscitation efforts will initiate and aggravate this pathophysiology. Therefore we investigated the interaction between the duration of basic and advanced life support and outcome after cardiac arrest in patients treated with or without mild therapeutic hypothermia.

This retrospective cohort study included patients 18 years of age or older suffering a witnessed out-of-hospital cardiac arrest with presumed cardiac cause, which remained comatose after restoration of spontaneous circulation. The basic and advanced life support 'low-flow' time, categorized into four quartiles (0-11, 12-17, 18-28, ≥ 29 min), was correlated with neurological outcome.

Out of 1103 patients 613 were cooled to a target temperature of 33 ± 1 °C for 24h. In the three quartiles with 'low-flow' time up to 28 min cooling was associated with >2-fold odds of favourable neurological outcome. In the fourth quartile with 'low-flow' time of ≥ 29 min cooling had no influence on neurological outcome (OR: 0.73; 95% CI: 0.38-1.4, test for interaction p<0.01).

The duration of resuscitation efforts, defined as 'low-flow' time, influences the effectiveness of mild therapeutic hypothermia in terms of neurologic outcome. Patients with low to moderate 'low-flow' time benefit most from this treatment.