End-tidal carbon dioxide concentration, carbon dioxide production, heart rate, and blood pressure as indicators of induced hyperthermia.

In 4 spontaneously breathing, barbiturate-anesthetized dogs, hyperthermia was induced with 2,4-dinitrophenol while rectal temperature, heart rate, mean blood pressure, end-tidal carbon dioxide, and carbon dioxide production (milliliters per minute) were measured continuously. The latter was determined with a pneumotachygraph (to obtain respired volume) and an infrared carbon dioxide analyzer that measured inspired and expired carbon dioxide concentration. Of the five physiologic measurements, the increase in carbon dioxide production preceded the increase in rectal temperature by more than 120 seconds. End-tidal carbon dioxide was an unreliable indicator in the spontaneously breathing animal of approaching hyperthermia during spontaneous breathing due to a transient tachypnea, which decreased end-tidal carbon dioxide. The carbon dioxide production (milliliters per minute) increased immediately and reached three to five times the control value. Blood pressure and heart rate were insensitive indicators of approaching hyperthermia.