Heart's Oxygen Supply Sufficient in Hypothermia

A new study may have ruled out insufficient oxygen supply to the heart as the critical variable in whether a mammal's heart survives while in a hypothermic state.

Researchers from the University of Tromsø (Norway) and the University Hospital of North Norway (Tromso) divided anesthetized rats into three groups: two hypothermic groups and a control group. They exposed one hypothermic group to 15°C for one hour before rewarming. They exposed the second group also to 15°C, but for five hours. The control group was kept at normal temperature, 37°C. The researchers followed the hemodynamic changes during hypothermia and rewarming and later examined the rats' heart cells.

When core body temperature was lowered to 15°C, both hypothermic groups experienced a similar reduction in circulating blood volume. Both showed that the physiologic mechanisms that help transport oxygen and help unload oxygen from hemoglobin were working. From that, the researchers concluded that oxygen supply was not a limiting factor for survival in the experiments. And while the rats in the one-hour group spontaneously normalized their blood pressure, cardiac output, and heart rate after they were rewarmed, the rats in the five-hour group experienced substantial lowering of cardiac output and other cardiac functions, and were more likely to experience fatal heart failure during rewarming. The study was published in the July 2006 issue of the American Journal of Physiology-Heart and Circulatory Physiology.

"We conclude that the heart failure we see after rewarming is not due to a lack of oxygen supply or oxygen transportation because unloading of oxygen at the cell level from hemoglobin is functioning,” said lead author Dr. Torkjel Tveita, an anesthesiologist who treats victims of hypothermia. He added that it appears that the problem comes as a result of the lower cardiac output.

Rewarming of victims of severe hypothermia almost always causes heart failure of varying severity, but little is known about why that happens. Doctors typically cease cardiopulmonary resuscitation (CPR) efforts in a normothermic patient after 30 minutes, but based on this study, the authors recommend that CPR should continue on hypothermic patients until after they have returned to normal temperature.



Related Links:
University of Tromsø
University Hospital of North Norway