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Apnea is a known response of concussive head injury. Hypoxic or ischemic brain injury has been documented in a high percentage of severe head injury deaths. The respiratory response after head injury remains poorly defined, however, and its contribution to hypoxic or ischemic mechanisms after severe head injury is unknown.Eighteen anesthetized but spontaneously breathing rats were subjected to fluid percussion head injury of varying severities. Respiratory rate and volume of air were recorded before and after injury with a Hans Rudolph/Varadyne pneumotach differential pressure transducer and graphically measured. Postmortem inspection of the brains was performed.Apnea and subsequent respiratory dysfunction are directly proportional to the magnitude of energy delivered to the brain. Higher energy results in a dysfunctional or absent respiratory response, probably attributable to failure or disorganized function of the medullary respiratory center.This study for the first time graphically depicts the respiratory response after head injury. Higher energy delivered to the brain directly correlates with a markedly abnormal respiratory response that probably contributes significantly to subsequent hypoxic or ischemic brain injury. The absence of the space-occupying hematoma or gross parenchymal disruption suggests that in the clinical setting many of these patients may otherwise have survivable head injury if rescued by early ventilatory assistance.