Effect of induced hypothermia on respiratory parameters in mechanically ventilated patients

Introduction

Therapeutic hypothermia is applied to reduce hypoxia-induced organ injury. In the past decades, the use of hypothermia has increased in critically ill patients who are mechanically ventilated (MV). Data on the effect of hypothermia on gas exchange and lung mechanics in these patients are limited. In this retrospective study, we describe the effect of induced hypothermia and rewarming on respiratory parameters in patients after a cardiac arrest.

Methods

Patients with a Glasgow Coma Scale <8 after resuscitation for a cardiac arrest in whom hypothermia was applied (32 to 34°C), were enrolled. Patients with PaO₂/FiO₂ ratio <200 or MV with positive end-expiratory pressure (PEEP) >15 cmH₂O were excluded. Ventilator settings and arterial blood gasses were retrieved from the electronic patient database during hypothermia and at every °C grade increase during rewarming. Statistics include z paired t test.

Results

From a cohort of 98 patients, 35 patients were excluded, leaving 62 patients for analysis. During hypothermia, arterial pCO₂ decreased, while end tidal (et) CO₂ was low at unchanged minute volume ventilation (Figure 1). Hypothermia increased the P/F ratio from 255 ± 55 to 283 ± 12 at unchanged PEEP (P < 0.05), while fluid balances were positive in all patients (2.5 ± 1.6 l). After rewarming, arterial pCO₂ was unchanged while etCO₂ increased. The P/F ratio after rewarming was unchanged compared with the start of hypothermia, while lower PEEP levels were applied (7.0 ± 0.4 cmH₂O vs 6.1 ± 0.3 cmH₂O, P < 0.05).

Conclusions

Induced hypothermia improved ventilation and oxygenation in critically ill patients. Hypothermia may be considered in patients with acute lung injury, in whom low minute ventilation results in severe hypercapnia.