Effects of prolonged supratentorial mass expansion on regional blood flow and cardiovascular parameters during the Cushing response.

To clarify the central nervous and the cardiovascular mechanisms mediating the Cushing response (CR) during intracranial hypertension, the effects of continuous and prolonged expansion of an epidural supratentorial balloon was studied in six dogs. Regional cerebral blood flow, cardiac output, extracerebral tissue blood flow and regional vascular resistance were measured with radioactive microspheres before and during the CR while heart rate, arterial blood pressure and ECG were continuously recorded. The results showed that balloon expansion led progressively to cerebrospinal ischemia beginning in the cerebrum and progressing to the pons, medulla oblongata and spinal cord in an orderly rostro-caudal fashion. This progressing ischemia was closely associated with patterned and stereotyped changes in cardiovascular indices. Thus, respiration ceased, heart rate decreased, blood pressure increased and cardiac output fell when the ischemia appeared in the lower pons. Further advancement of the ischemic front to the lower medulla oblongata led to an abrupt switch-over from bradycardia to tachycardia, which persisted until the experiment was terminated. Since bradycardia disappeared when ischemia reached the vagal cardiomotor nucleus, the initial bradycardia appeared to be the result of increased vagal discharge. When ischemia reached the pons, the mean arterial pressure rose to a maximum of 245 mm Hg (+/- 45) but then fell progressively when the ischemia reached the spinal cord. The decrease in cardiac output persisted throughout the CR and was associated with decreased flow in most extracerebral organs. Exceptions were the heart muscle and the adrenal medulla in which blood flow increased considerably. Changes in regional vascular resistance reciprocated the changes in flow. These results suggest that the vasomotor component of the CR is brought about entirely by peripheral vasoconstriction.