Energy Expenditure in Patients Undergoing Targeted Temperature Management at 36°C After Cardiac Arrest, the Effect of Shivering and Ability to Maintain Goal Temperature

Abstract

Survivors of cardiac arrest undergo targeted temperature management (TTM) in the intensive care unit as a strategy to promote neurologic recovery and prevent further cellular injury from post-cardiac arrest syndrome (PCAS). During TTM, core body temperature is most often lowered to either 33°C or 36°C for approximately 24 hours. Since two landmark studies published in 2002 demonstrated a survival benefit of inducing hypothermia to 33°C, TTM has become a mainstay for post-cardiac arrest care. More recently, TTM at 36°C has gained popularity following a large randomized trial showing no difference in survival or neurologic outcome benefit for TTM at 36°C compared to that provided at 33°C. As TTM at 36°C is a newer strategy, less evidence exists related to the physiologic changes that occur at this “ultra-mild” hypothermic temperature. One of the effects of lowering body temperature is a reduction in energy expenditure (EE), which is believed to contribute to the protective effect of TTM. An important side effect of TTM is shivering, which can increase EE by 400% as a mechanism to raise body temperature. Both the increase in body temperature and energy demands may attenuate the benefits of TTM. TTM at 33°C has been shown to decrease EE 20-35%, but the change in EE at 36°C for post-cardiac arrest patients is not known. TTM can be initiated and maintained using surface cooling pads that circulate water within the pads placed on the body to regulate core temperature. The surface cooling device responds to a patient’s core temperature by either increasing or decreasing the temperature of the circulating water to maintain 36°C. The acceptable variability in the targeted temperature is ±0.5°C. Compliance with maintaining body temperature within the acceptable range has been shown to be more difficult at a target temperature of 36°C compared to 33°C, although the effect on outcomes is unclear. One reason for poor compliance is patient heat generation, such as occurs during shivering. To date, there is little evidence evaluating the impact of shivering on the ability to maintain 36°C during TTM. Additionally, the change in surface pad water temperature in response to changes in body temperature has not been well described. This dissertation aims to address key gaps in knowledge related to understanding the change in EE for patient receiving TTM at 36°C following cardiac arrest, and the effect of shivering on EE. Additionally, this dissertation describes the relationship between water temperature of the cooling device and body temperature, as well as relationship between temperature variability and shivering. We conducted a prospective observational case-series study of consecutive patients following out of hospital cardiac arrest to evaluate patterns of EE during the course of TTM 36°C. A total of three patients were included. We found that EE at 36°C was similar to that of other critically ill patients with and without brain injury. In addition, we observed that two patients with dynamic EE and temperature changes survived to hospital discharge with good neurologic outcome, and the patient with little temperature variability did not survive. This finding is in alignment with previous studies on the association with heat generation (increased EE) and improved outcomes. We then conducted a retrospective cohort study of patients with cardiac arrest who received TTM at 36°C using surface cooling pads at Harborview Medical Center in Seattle, Washington. A total of 186 patients were included. We found that there was a strong correlation between device water temperature and body temperature. For every 0.1°C change in body temperature, there was a 4.3°C change in water temperature. Additionally, we found that shivering was significantly associated with having an increased proportion of temperature measurements > 36 ±0.5°C, and significantly associated with survival to hospital discharge. This finding supports growing evidence supporting the relationship with the ability to shiver and improved outcomes. The studies within this dissertation aimed to add to the knowledge of caring for patients during TTM at 36°C.