Maintenance of arterial pressure in infant rats during moderate and extreme thermal challenge.

It has recently been demonstrated in week-old rats that extreme cold challenges that lead to significant bodily cooling result in decreased cardiac rate. To determine whether pups are able to maintain arterial pressure in the face of decreasing cardiac rate in extreme cold, we measured blood pressure in unanesthetized week-old rats. Instrumented pups were thermally challenged and thermoregulatory and cardiovascular responses were monitored. Despite pronounced decreases in cardiac rate in the cold, pups were able to maintain mean arterial pressure (MAP), presumably by increasing peripheral resistance. At the lowest air temperature (17 degrees C) pups emitted ultrasonic vocalizations, and these emissions were accompanied by pulsatile increases in intraabdominal pressure (IAP) and MAP. We hypothesize that these pulsatile increases in IAP during extreme cooling reflect the use of the abdominal compression reaction to increase venous return during periods of diminished cardiac output.